physics and chemistry of partial discharge and corona. recent...

IEEE "kansactions on Dielectrics and Electrical Insulation Vol. 1 No. 5, October 1004

1994 WHITEHEAD MEMORIAL LECTURE

761

Physics and Chemistry of Partial Discharge and Corona

Recent Advances and Future Challenges

R. J. Van Brunt National Institute of Standards and Technology,

Gaithersburg, MD

ABSTRACT Results of recent research on physical and chemical processes in partial discharge (PD) phenomena are reviewed. The terminology used to specify different types or modes of PD are discussed in light of a general theory of electrical discharges. The limitations and assumptions inherent to present theoretical models are examined. The influence of memory propagation effects in controlling the stochastic behavior of PD is shown. Examples of experimental results are presented that demonstrate the nonstationary characteristics of P D which can be related to permanent or quasi-permanent discharge-induced modifications (aging) of the site where the PD occur. Recom- mendations for future research are proposed.

1. INTRODUCTION ITHIN the past fifteen years there has been a rapid W growth of research activity concerned with partial

discharge (PD) phenomena. A PD is generally thought of as a highly localized or confined electrical discharge within an insulating medium between two conductors, and in some cases PD is the precursor to a complete electrical breakdown or fault. The occurrence of PD can be the cause of electrically-induced aging of insulating materials manifested, for example, by formation of cor- rosive gaseous byproducts, erosion, sputtering, and 'tree' formation. PD, despite its localized nature, is an enormously complex phenomenon that often exhibits chaotic, nonstationary, or fractal type behavior with seemingly unpredictable transitions between different modes that exhibit distinctly different time-dependent characteris-

tics. The existence of many different observed modes of PD behavior has caused some confusion and inconsis- tencies in the terminology used to define these modes, and this has, in turn, led to attempts a t clarifying the terminology [I]. One source of confusion in the search for generalizkd descriptors of PD phenomena arises from the infinite variety of geometrical and material conditions under which PD can occur.

The recent upsurge of research on PD phenomena has been driven in part by development of new fast digital and computer-based techniques that can process and an- alyze signals derived from PD measurements [2]. There seems to be an expectation that, with sufficiently sophis- ticated digital processing techniques, it should be possible not only to gain new insight into the physical and chemical basis of PD phenomena, but also to define PD

1070-9878/94/ $3.00 @ 1994 IEEE

762 Brunt: Physics and Chemistry of Partial Discharge and Corona

‘patterns’ that can be used for identifying the characteristics of the insulation ‘defects’ a t which the observed PD occur. Among the various digital techniques that have been applied recently to PD measurements are 1. precise pulse-shape and pulse-burst characterizations

using broad-band detection [3-51, 2. recording of phase-resolved PD pulse-height distribu-

tions using various types of digitizers or multichannel analyzers [6-81,

3. quantification of PD pulse-to-pulse or phase-to-phase memory propagation effects using stochastic analysis [9,10], and

4. removal or reduction of noise using digital filtering techniques [ 111. The impressive results from use of these measurement

techniques have been compared with predictions from computer simulations, e.g., of discharge pulse shape [4,12] and stochastic behavior [13,14].

In addition to the introduction of new signal processing techniques applied to PD measurement, recent investigations have been undertaken also into effects that P D have on the properties of the materials in which they occur. Included in these investigations are measurements of 1. gaseous byproducts from the discharge [15,16], 2. removal of solid material or other local changes in mor-

3. changes in solid dielectric surface resistivity [18,19], 4 . surface acidification, oxidation or deposition of mate-

5. growth of voids, cracks, or trees within a dielectric

It has been noted [18,27] that the PD-induced changes in the physical and chemical characteristics of the discharge site will in turn cause changes in the PD behavior that make the phenomenon inherently nonstationary. The susceptibility to nonstationary behavior complicates the interpretation of PD signals. In general, PD phenomena are described theoretically by a set of coupled nonlin- ear differential equations from which it can be shown that feedback and memory effects are important and can be responsible for complex stochastic behavior. The problems of quantifying, unraveling, interpreting, and understanding the complicated behavior of PD are the challenges of present and future research.

PD has been the subject of numerous published re- views [l, 28-32] some of which were presented in previous Whitehead Memorial Lectures [33-351. The focus of the present review will be on relatively recent work published within the past fifteen years dealing primarily with the basic physical and chemical processes responsible for, or associated with the discharge phenomenon. Particular emphasis will be given to work carried out in our laboratory a t NIST using examples of results obtained for air,

phology I171,

rial [20-221, and

[23-261.

SF6, and SF6/02 gas mixtures in point-plane or point- dielectric barrier gaps.

2. DEFINITIONS AND TERMINOLOGY A persistent difficulty that has plagued the field of gas-

discharge physics is that of imprecision and confusion in the terms used to define different types of electrical- discharge phenomena. There continues to be disagree- ment about the meaning and appropriateness of the words that have been applied as designators of discharge type. The definitions used for PD phenomena have been discussed in recently published papers [1,4,28]. As has been noted in these works, part of the difficulty with finding precise definitions is a consequence of the multitude of different types of discharge behavior that have been observed. Another issue confronting those who seek meaningful definitions is that concerning the regions of transition between different types of ,discharge behavior. An example is the case of the transition from a pulsating negative corona (Trichel pulse [36]) to a glow discussed in the work of Cernak and Hosokawa [37]. A Trichel pulse can be viewed as a discharge current pulse associated with a transient glow discharge or as a glow discharge that tries to develop but is extinguished by accumulating space charge.

The purpose of this Section is to clarify the meaning of the terms used here to designate discharge phenomena. It is not intended as a recommendation to standardize definitions. It should always be kept in mind that different words are sometimes used to identify what are in reality simply different modes or manifestations of the same phenomenon. One is confronted with a situation that is not unlike that of describing the phenomena of weather. It is easy to distinguish ‘clear’ or ‘cloudy’ days but ‘part- ly cloudy’ or ‘mostly sunny’ days present a problem of imprecision in definition.

As will be discussed in Section 3, PD phenomena can be described in general by a set of coupled differential equations, and different discharge types may simply correspond to different solutions of these equations for different boundary conditions. Ideally the ‘mathematical description’ provides the only meaningful ‘definition’. In practice, because of the complexity of a complete mathematical treatment, one is forced to rely on qualitative descriptors that cannot have precise definitions.

The term ‘partial discharge’ will refer here to a class of discharge phenomena that satisfy a set of restrictions on such parameters as size, intensity, and temperature. Included in the category of PD are various types of discharges that have been given different names such as corona, constricted glows, electron avalanches, localized Townsend discharges, streamers, and dielectric-barrier

IEEE !Tkansactions on Dielectrics and Electrical Insulation

Table 1. gas Iiumuer oensity (cm-a) electron number density (cmW3) positive ion number density ( ~ m - ~ ) negative ion number density ( ~ m - ~ ) ionized coefficient (cm-l)

electron attachment coefficient (cm-l) electron-ion recombination coefficient ( cm3 /s) ion-ion recombination coefficient

ion diffusion coefficients (cm2/s) electron charge (C) permittivity of free space dielectric permittivity (i = 1,2) surface charge density (C/cm2)

(cm3 /SI

surface conductivity (A/Vcm)

surface current density (A/cm2) electric field strength (V/cm) transverse electric field component

electron mean free path (cm-l) electron velocity (cm/s) electron kinetic energy (eV) electron mass (kg)

(V/cm)

efficiency factor (electrons/photon)

photon frequency (s- ' ) electron energy distribution function

electron energy distribution function

ionization cross section (cm')

discharges. PD is assumed here to be a gas discharge that may or may not occur in the presence of a solid (or liquid) dielectric. The presence of the solid (or liquid) dielectric can affect the behavior of PD by providing a source of secondary electrons or by modifying the local field due to accumulation of surface or bulk charge.

The actual discharge zone of PD is assumed to be restricted to a portion of the insulating gap between two conductors to which voltage is applied. The discharge zone is defined here to be that region in which the electric-field strength is sufficiently high to allow release of electrons by various collision processes either in the gas or on a surface. The examples considered here

Vol. 1 No. 5 , October 1994

attachment cross section (cm') momentum transfer cross section (cma) reaction rate coefficient (cm3/s) amplitude of nth PD pulse (C) time separation between nth and (n - l)th pulses limiting maximum corona pulse amplitude (C) unconditional PD pulse amplitude distribution conditional P D amplitude

ion transit time in gap (5)

gap spacing (cm) integrated pos. or neg. PD charge (C) phase of ith neg. P D pulse (rad) unconditional integrated charge distribution unconditional integrated charge distribution unconditional phase distribution conditional phase distribution rates of electron release(s-')

surface work functions (eV) frequency of applied voltage (rad/s) corona discharge current (A) concentration of chemical species X (mol) space charge contribution to local field (V cm-l) ion mobility (cm2/Vs) normalized production rates (mol/C/SFs mol) sum of normalized production rates (mol/C/SFs mol)

763

are mainly restricted to P D generated in point-to-plane type electrode gaps where the plane electrode may or may not be covered by a dielectric surface. The discharge zone in this case is confined to the immediate vicinity of the point electrode. Although, there may exist regions outside of the discharge zone in which charge transport occurs, they are not considered to be part of this zone because the field strength is not high enough to support ionization. On the other hand, the existence of charge carriers outside of the discharge zone can have an influence on activity within this zone.

P D is also assumed to be a relatively 'cold discharge' in the sense that the mean energy of electrons is always


considerably higher than the mean energy of the molecules in the gas, i.e., the discharge is not very effective in heating its surroundings, and there is a lack of equilibri- um between charged and neutral species.

The tcrm ‘corona’ applies to a special category of PD that occur in cases where solid insulating surfaces are absent or are far removed from the discharge zone. The des- ignations of positive and negative to corona refer to the polarity of the voltage applied to the most highly stressed electrode assuming that this can be defined. In the case of a point-plane gap, the point is obviously the most highly stressed electrode. In the case of a point-point gap, the polarity designation losses its meaning. In general, the behavior of positive and negative corona can be quite different and can exhibit distinctly different modes. For example, regular Trichel pulse corona I361 occurs only a t a negatively stressed point electrode. The differences between positive and negative corona discharge behavior are, in part, a consequence of the different predominant sources of initiatory or secondary electrons [28,38].

PD can exhibit either pulsating or nonpulsating characteristics. The pulsating behavior generally results from the buildup of surface charge or space charge a t the discharge site which causes the local elect,ric-fie!d etr--?AL to drop below the level necessary to sustain ionization. The pulsating P D are said to be self-quenching. Pulsat- ing behavior is particularly common when PD occur in proximity to a solid insulating surface, e.g., in a dielectric cavity or in a point-dielectric gap. Pulsating discharges that occur in gaps in which one or both of the electrodes is covered with a dielectric surface are sometimes referred to as dielectric barrier discharges or silent discharges [39].

A particular type of nonpulsating discharge that will be considered in the present work is a glow discharge. Glow discharges are sometimes designated as either positive or negative depending on whether they occur near the anode or cathode. The specific type of glow discharge considered in this work is a constricted negative glow corona that occurs near a negatively stressed point electrode. In electronegative gases like air and SFG, this type of glow is preceded a t lower gap voltages by the pulsating (Trichel-pulse) corona mentioned above. It is constricted in the sense that the region of ionization and light emission (discharge zone) is confined to the immediate vicinity of the point electrode.

Glow discharges can occur under a wide range of conditions and can exhibit different voltage-current and time- dependent behavior. At the present time, there seems to some confusion and disageement about the proper definitions and terminology that should be applied to this type of discharge [l, 2,19,40-421. The present discussion will be restricted to steady glows for which the spatial

and temporal characteristics are determined largely by an equilibrated ion-electron space-charge density distribution that significantly modifies the electric field within the discharge zone. The glow is ‘steady’ in the sense that, for a given applied voltage, the discharge current is constant or changes very slowly with time. In general, ‘glow discharges’ can be unstable, especially near conditions identified with transitions to other modes, and can exhibit fluctuating, oscillatory, or pulsating behavior [43,44]. The different observed modes of glow-type discharge behavior have been described using a variety of terms such as ‘abnormal glow’, ‘pulsating glow’, ‘pseudoglow’, and ‘swarming microdischarge’. The meanings of these terms will not be explored here.

There is a weaker class of discharges that is often placed in the category of PD, namely electron avalanches and diffuse Townsend discharges. These discharges are ‘weak’ in the sense that their development is not significantly affected by space charge, i.e., the electron and ion densities are not high enough to perturb the applied electric field. An electron avalanche is an electron multiplication process that is usually very localized in its development and appears as a pulse. Its growth is usually restricted by geometrical constraints, and its pulsating behavior is mainly due to an inability to regenerate, i.e., its intensity as measured in terms of number of ions produced and radiation emitted is too weak to produce a sufficient number of secondary initiatory electrons needed for regeneration. An electron avalanche can be considered as a special case of a Townsend discharge which may also be diffuse, i.e., it can occur over a relatively broad area between parallel plates, and can be regenerative, i.e., self-sustained. The electron-avalanche or Townsend discharge descrip- tions are applicable to the early stages of breakdown or PD development when the numbers of electrons and ions are relatively low. When the local space-charge density reaches a level high enough to significantly perturb the local field, the discharge can propagate in the gas by the streamer mechanism discussed in earlier works [28,35]. It is generally believed that photoionization accounts for the high speed of streamer propagation [45,46]. Under some conditions, pulsating PD may involve streamer formation in the development stage of the discharge.

Most of the PD phenomena considered here, as well as those of most practical interest, are self-sustained, i.e., their maintenance does not require exposure of the gap to external radiation. Discharges known as sparks, arcs, or flashover are considered to be complete electrical breakdown processes that are too intense or too hot to be categorized as PD. There are some authors [40,41], however, who include intermittent or self-quenching sparks among PD phenomena.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 5 , October 1994 785

3. GENERAL T H E O R Y OF PARTIAL DISCHARGE

PD phenomena can be described mathematically by a set of coupled differential equations. A solution to these equations, if it can be found, will yield observable characteristics of the discharge such as the time dependence of the discharge current. It can be argued that all types of discharges such as glow-type corona or diffuse Townsend discharges satisfy the same set of equations. The differences in discharge type should arise from differences in assumed initial conditions (defined, for example, by gas content, pressure, and dielectric surface conductivity), and assumed boundary conditions (defined, for example, by electrode geometry, thickness of a dielectric, and applied voltage). In general, the discharge model should include both physical and chemical processes. Examples of physical processes are: ionization by various mechanisms, electron attachment, diffusion of neutral and charged species, and deposition or sputtering at surfaces. In the category of chemical processes are various chemical reactions that can occur in the gas phase or on surfaces and may involve neutral or charged species. In general, the physical and chemical processes cannot be treated as independent, e .g . , the rate of ion formation by electron collision affects the rate of reactions between ion and neutral molecules that may be responsible for formation of observable stable or quasi-stable molecular byproducts.

The coupling between various types of discharge processes is illustrated by the block diagrams in Figure 1. The normal physical processes that affect the initiation, growth, and extinction of a discharge are indicated in Figure l(a), and physical and chemical processes associated with discharge-induced 'aging' are indicated in Figure l (b) . Aging processes involve modifications of gas composition and electrode or dielectric surface conditions and generally have a much longer time frame than the processes responsible for development of a single discharge event. Advantage can be taken of this large dif- ference in time scales to simplify discharge models that consider both the discharge physics and related discharge chemistry [39,45,46].

An example of a set of coupled equations similar to that used by Morrow [12,47] to model negative-corona pulses in oxygen is given below. These equations include only the physical processes occurring in the discharge, i.e., they correspond to the situation shown in Figure l(a). In a somewhat generalized version of Mor- row's corona model, the electron, positive ion, and negative ion densities are assumed to satisfy the continuity equations

Initiation lnitlatlon Mechanism 92

- r) Mechanism # 1

(Ion Impact) (Photoelectrons))

-r $.

f

* Secondary Electrons

r r

Growth (or Decay) Electric Field + 01 Electron and b Distribution Ion Densities

Surtace Charge

Ion Stable By-products Chemistry

Modiflcallons

Figure 1.

(a) Block diagram of a model for discharge initiation, growth, and decay that applies in a short time frame in which PD-induced aging can be ne- glected. (b) Block diagram of a discharge model that includes physical and chemical changes due to PD-induced aging that applies in a long time frame.

Here N e , N p , and Nn are respectively the electron, positive-ion, and negative-ion number densities and U , , u p , and U, are the corresponding electron, positive-ion, and negative-ion drift velocities. In general, the densities and drift velocities depend on position in the gap. The parameters a , ~ , and 01, are respectively the electron- impact ionization, attachment, and electron-ion recombination coefficients which also depend on position. The positive and negative ion diffusion coefficients and ion-ion


recombination coefficients are denoted by Dpl D,,, and 02 respectively. The spatial variations of the above coefficients are determined primarily by their dependence on the local electric field-to-gas density ratio, E(F')/N, where N, without a subscript, denotes the neutral gas density, generally assumed for PD conditions to be much greater than the charged-particle densities. The vector r' denotes position in the discharge gap measured from an arbitrary origin.

0 IO 20 30 40 ( i l l T i ( n s )

1 I I I I

1 ton current -- I f W " " i?

2 a n-1 I I - Y i j , 150 200 2%J 300 3 SO

(1)) Time ( ns ) Figure 2.

Calculated PD current pulses for a 0 .5 mm gap in an air-like gas mixture at atmosperic pressure for: (a) high overvoltage and (b) low overvoltage 141.

Equations (1) to (3) are coupled to the three-dimensional Poisson's equation for the electric field

V . d(F') = Z[Np(F') - Nn(i;? - Ne(p3] (4)

where e is the electron charge, and eo is the permittivity of free space, which is assumed to apply in the gas. If it can be assumed that ionization, attachment, recombination, and diffusion are the only significant processes, then the electrical characteristics of the discharge can be modeled by numerically solving the above set of simultaneous equations subject to boundary conditions that apply, for example, at the electrode or dielectric surfaces [12,50]. By using a model based on the above equations, Morrow was able to predict the shape of the first negative-corona pulse in oxygen [12,49]. Similar models have been used to calculate PD pulse shapes for small gaps or gaps in

€ 0

which solid dielectrics are present [4,50]. Lui and Raju [51] have recently developed a Monte-Carlo method to simulate corona current pulses in SFs. Figure 2 shows examples of PD current pulses calculated by Novak and Bartnikas [4] for a 0.5 mm gap in a model gas (air-like mixture) for high and low overvoltages. Indicated in this Figure are the total current and the ion current component. The calculations are consistent with observations [4,52,53] which show that the electron component of PD pulses typically have a fast rise time and a width that is less than 50 ns. It is also seen from the example shown in Figure 2 that details of the pulse shape can depend significantly on the conditions of the gap in which the PD is formed.

When solid dielectrics are present, the surface boundary conditions can be quite complex and may exhibit a spatial and temporal dependence. The electric field at a dielectric boundary depends on the instantaneous surface charge density U according to the requirement [50,54]

where_ 5 i! a unit vector normal to the surface and €2, €1

and E-, E+ are the corresponding permittivities and fields that apply on either side of the boundary. The value of U

at any time is determined by the fluxes of charged particles (ions and electrons) at the surface, by the probabilities that ions either stick to the surface or are reflected, and by the probability that charged particles are ejected from the surface by ion bombardment or other mechanisms.

Assuming that the bulk conductivity of the dielectric is much less than its surface conductivity I', it is necessary from charge conservation that the surface charge density satisfy a continuity equation of the form

where I? is the surface current density and s is a source term. Assuming that r is independent of the field, the surface current density is related to !he tangential component of the electric-field strength Et a t the surface by

which gives

au J?*. v r + r v . gt + - - s = o at allowing that r can depend on T' . In the absence of the source term, the solution to Equation (8) leads to a local decay of the charge density [55,56]. In general, the source term depends on the incident ion flux, which can, in turn, depend on the field at the surface, e.g., at a SUE- ciently high charge density, the incoming ions can simply

. ..

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 5, October 1994 76 7

be repelled from the surface. The dynamics of surface charging when a dielectric is present, as represented by Equation (a), must be considered in a complete model of PD.

Both dielectric and metallic surfaces in the discharge gap are sources and sinks for charged particles. Sec- ondary processes for release of electrons at surfaces by ion or photon bombardment , quenching of metastable excited species, or field emission are important in initiating and sustaining PD. Under conditions of high electrical stress, such as near a sharp point electrode, the secondary processes may depend on the local electric-field strength. If we consider only the release of photo-electrons from a cathode, the derivative aNe/at within approximately one mean-free path A, of an area element d A of the cathode surface will be given by

where @ is the time-dependent photon flux (number of photons per second per unit area) a t d A for a. photon of frequency v , T p h is the efficiency factor (number of photoelectrons released per incident photon), and V, is the mean photoelectron velocity. In general, Yph can depend on the location of dA, i.e., the properties of the cathode surface need not be uniform. On a longer time scale, yph can have a time dependence due to discharge- induced aging effects. The flux @ must be computed from radiation transport theory that takes into account the electron-impact excitation coefficients, absorption coefficients, and cascading effects [12].

Equations (1) to (3) are valid only if the coefficients a, r,~, and are constants that are independent of time and depend only on the local reduced field strength E / N . In general, a complete description of the electron transport in a discharge requires the determination of the phase- space distribution function f(Ce, ?, t ) , which can be found either from a solution of the Boltzmann transport equation [57]

or from a Monte-Carlo simulation [58]. In Equation ( lo ) , me is the electron mass and I ( f ) is a source-sink term, usually referred to as the collision integral. This term accounts for the transport of particles into and out of a volume of phase space due to collisions. The transport equation is essentially a continuity equation in phase space, and various numerical methods have been developed to solve this equation [59-611 under different conditions.

For conditions of steady-state and spatial uniformity of the field where

V f = O (11) af at - E O

the solution of Equation (10) yields a field-dependent velocity distribution function that can be converted to an electron kinetic-energy distribution function F ( E , E / N ) , using

The E/N-dependent electron-impact ionization and attachment coefficients are then given respectively by [62,63]

and

where Q i ( c ) and denote the total ionization and attachment cross sections. The drift velocity can also be computed from the kinetic-energy distribution function using the integral expression

112 dF f i e = 7 (6) $ /Q- ’ -dc 0 de (15)

where Q M is the momentum transfer cross section.

Equations (13) and (14) imply an average over a large number of electrons. I t can thus be argued [35] that the ionization and attachment coefficients as defined by these equations have no meaning in the early stages of discharge growth when there are only a small number of electrons in the discharge gap. Moreover, the steady- state condition implied by Equation (11) breaks down if there is a sufficiently rapid change in the electric field with time or with position. The meaning of the coefficients is therefore also questionable in the immediate vicinity of a sharp electrode where there can be a large gradient in the electric field. It is necessary to keep these limitations in mind when ionization and attachment coefficients are used to model discharges [64].

It should be realized that the forms of Equation (1) to (3) imply simplifications and assumptions that are not always clearly stated. For example, it is tacitly assumed in writing Equation (3) that only one type of negative ion is formed in the discharge. As is shown in Table 2 for the case of pure oxygen, there is the possibility of forming more than one type of negative ion. Moreover, there are reactions that convert one negative ion into another, and


Table 2 . Examples of reactions involving formation, destruction, and conversion of negative ions in oxy-

n.

e + O z + M -+ 0 2 - + M [M = O2, OI 0 2 ( l A g ) ]

e + 0 2 -+ 0 2 - + hv e + 0 2 -+ O - + O + + e e + 0 2 4 0 - + o e + 0 2 ( ' A g ) + M -+ 0 2 - + M

0- + 0 -+ 0 2 + e 0- + 0 2 + -+ 0 + 0 2

0- $ 0 2 ( ' A g ) -+ @ + e e + 0- -+ 0 + 2 e 0- + M -+ O + M + e

0 2 - + 0 2 ('E:) -+ 2 0 2 + e 0- + 0 3 -+ 03- + 0 0 2 - 4- 0 3 -+ 0 3 - + 0 2

0 2 - + 0 -+ 0- + 0 2

[M = 0 2 , 0, 0 2 ( l 4 ) I

[M = 0 2 1

destruction of negative ions can occur by processes other than ion-ion recombination. To be more complete, Equa- tion (3) could be replaced with a set of coupled equations for the different negative ions represented by

1

where Nn;( i = 1,2, f . .) is the number density of the i th negative-ion species and the added term in brackets is a source-sink term that allows for ion conversion and negative-ion destruction by collisional detachment. Here k j k of the first term in brackets is the reaction rate coefficient for conversion of the j t h ion species into the i th species by reaction with a molecular species of density N k . Similarly, the second term corresponds to conversion of the i th species into another species and the last term corresponds to collisional detachment by electron impact. In some cases, it may also be necessary to include additional negative-ion processes such as photodetachment and detachment by heavy particle collisions [65,66].

When the multitude of different reactions is considered, it can be seen that the formulation of the problem takes on a significant added complexity. Generally it can be expected that most of the many possible processes are

relatively unimportant and can usually be safely neglect- ed. Ideally one should start with as complete a model as possible and eliminate processes only after performing a sensitivity analysis that determines their relative importance [48].

Simplified models such as based on Equations (1) to (4) are usually deterministic in the sense that they always yield the same result for a given set of boundary conditions, e.g., the predicted PD current pulse amplitude and shape can have no statistical variability. In reality, PD current pulses exhibit significant statistical variation. The statistical distributions of PD pulse parameters such as amplitude, shape, and time-of-occurrence can be derived when the statistical natures of the initiatory electron release and initial electron avalanche growth are considered [13,28]. Statistical variations in PD behavior are often largely determined by the effects of pulse-to-pulse memory propagation whereby residual space charge, etc. left behind by a PD pulse will influence the probabilities for initiation and development of subsequent pulses. The importance of memory effects in determining the statistical properties of pulsating PD is shown in Section 4.

Second PD Pulse

Development of 1st PD

,--. CCZ-'! - -*- --- - \,--,/-. .- Neg Ion

Space 4 Charge\/z'\ 1 ---,I

+ 11

+ ti + At

Figure 3

+ f2

Motion of ion space associated with pulsating negative corona generated by a point-plane electrode configuration in an electronegative gas. In- dicated are successive times, t l , tl + At, and t z after extinction of the first corona pulse up to the initiation of the second pulse ( A t < t 2 - t l ) .

4. STOCHASTIC BEHAVIOR AND MEMORY EFFECTS 4.1. dc DISCHARGES

It is known that pulsating PD phenomena can exhibit complex stochastic behavior in which 'memory effects' play an important role [28]. The importance of

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 5, October 1994

memory effects was first established from the measurement of various conditional and unconditional P D pulse- amplitude and time-separation distributions for negative point-plane Trichel-pulse corona [9,67,68]. In the case of Trichel pulses, the memory effect results from the influence of space charge and metastable excited species generated during a discharge pulse on the initiation and growth of subsequent discharge events as illustrated by the diagram shown in Figure 3 that was suggested in the work of Lama a.nd Gallo [69].

The pulsating nature of negative corona that occurs in electronegative gases like 0 2 and SF6 is a consequence of the accumulation of a negative-ion space charge in the gap that causes a reduction of the electric-field strength below the level needed to sustain the discharge. The next discharge pulse occurs only after the ion space charge has moved sufficiently far away from the point electrode to allow restoration of the field to the level required for discharge initiation and growth. However, the presence of negative-ion space charge 'clouds' in the gap from previous events will tend to reduce the field strength near the point and thus affect the environment in which the next pulse develops. The discharge model discussed in Sec- tion xx which was used by Morrow [12,49] to describe Trichel pulses in oxygen only applies to the first pulse because it does not consider the modifications of gap conditions due to previous pulses.

Because the initial growth of a discharge pulse depends exponentially on the local field strength, it can be expected that the 'size' of a discharge pulse, as measured for example by its amplitude or integrated charge, will be sensitive to the small reductions in the field a t the point electrode due to the presence of space-charge clouds in the gap. The significant effect of space-charge clouds on the growth of Trichel pulses was demonstrated [67] from measurement of conditional pulse amplitude distributions pl(q,)At,-l), which give the probability that the nth discharge pulse will have an amplitude between qn and qn + dq, if its time separation At,-, from the previous event is restricted to lie within a narrow window between At,-, and At,-, + 6(At,-l).

Figure 4 shows examples of normalized conditional negative-corona pulse amplitude distributions measured by Kulkarni and Van Brunt [70] for different indicated values for At,-l. These results were obtained for a point-plane gap in an SFs/OZ mixture containing 10% SFB for an absolute pressure of 100 kPa, a point-plane gap of 1.7 cm, and a gap voltage of 14.0 kV. The expectation values defined by

QnPl(qnlAtn-l)dqn (17)

769

1.0

0

-1.0 - am E U

a -2.0 . -

-3.0 < Q

OI

- -4.0 4

-5 .0

20 40 60 80

q (PC) Figure 4.

Normalized conditional pulse-amplitude distributions for negative point-plane corona pulses in a Oz/lO% SFs mixture for the indicated fixed time separations At,-t, compared with the corresponding measured and calculated normalized unconditional pulse-amplitude distributions [61]. The calculated distribution (dashed line) was obtained from the measured pi(q,lAt,-i) and p o ( A t , - l ) using Equation ( l a ) . The solid lines are Gaussian fits to the pl (g , )At , - l ) distributions.

show a pronounced dependence on At,- 1 , namely (q,,(Atn-l)) increases with increasing At,-l. This is an expected trend since the greater the time separation from the previous discharge event the greater will be the distance that the negative ion space-charge clouds will have moved away from the point electrode, and consequent- ly the greater will be the local field in which the next discharge event develops.

The conditional amplitude distributions are compared in Figure 4 with the corresponding unconditional amplitude distribution po(qn), measured under the same condition. The fact that po(qn) # P I ( q n l A t n - 1 ) for an arbitrary At,-l implies that there exists a correlation between the random variables qn and At,-,. Because of the dependence of qn on At,-l, the distribution po(qn) is related to the time separation distribution function,


p,(At,-1), by the integral 00

P o ( q n ) = J P o ( A t n - l ) P l ( q n l A t n - l ) d ( d t n - l ) (18) At,

where At, is the minimum pulse-to-pulse time separation required for restoration of the local field to a value that is high enough to allow the growth of a discharge pulse. The result of performing this integration using the corresponding measured time-separation distribution is shown by the dashed line in Figure 4 which is in reasonable agreement with the measured po(qn) distribution. From this analysis, it is clear that the statistical spread in corona pulse amplitudes is determined largely by pulse-to-pulse memory propagation.

1.0 1

*mm

-8: 0.2 .a AtC

0 I L I I I I

50 100 150 200

' 1:; 0

At(@ Figure 5.

Normalized pulse amplitude and corresponding pulse time separation distributions for negative point-plane corona pulses in a 02/10% SFe mixture obtained with and without ultra-violet irra- diation of the point [61].

It can be inferred from Equation (18) that any changes which occur in the time separation distribution of the corona pulses will necessarily produce changes in the corresponding pulse-amplitude distributions. This is illustrated in Figure 5 which shows corresponding unconditional amplitude and time separation distributions measured for point-plane negative corona pulses in a O,/lO% SFe gas mixture with and without exposure of the point to ultra-violet radiation [60]. When radiation is present, it artificially enhances the rate for release of initiatory photoelectrons from the point and thus reduces the mean time between pulses. It is seen that when the radiation is on, the distribution po(At,) is narrower and shifted to lower At, values than when the radiation is absent. The corresponding p o ( q n ) also becomes narrower and yields a lower mean value for qn when the radiation is on. In the absence of radiation, po(qn) exhibits a sharp peak at a value denoted in Figure 5 as qlim. This is the limiting maximum mean value of the amplitude for corona pulses that occur in a gap free of space charge and corresponds to time separations from the previous pulse that are greater than about 80 ps which is the approximate minimum time required for a negative-ion cloud to make a transit across the gap.

The gap transit time At, for negative ions can be estimated using the expression

where z d is the point-to-plane gap distance, E is the unperturbed axial electric-field strength, and p is the ion mobility which is assumed to be independent of E. Thus, there should be a loss of memory due to space charge from previous corona pulses if At,-l > At , . Successive pulses that satisfy this condition are essentially equivalent to the 'first' pulses treated in Morrow's model.

A more complete stochastic analysis of Trichel pulses [67] has shown that both the amplitude of a pulse and its time separation from a previous pulse are, in general, correlated with the amplitudes and time separations of all previous pulses, i.e., the Trichel-pulse phenomenon is a non-Markovian process in which memory can extend indefinitely back in time. Moreover, the complete analysis reveals that there are other sources of memory propagation such as that associated with metastable excited species produced during previous PD events which affect not only the growth of subsequeht events through a local enhancement of the ionization coefficient, but also the discharge initiation probability through electron ejection by metastable quenching on the surface of the point electrode. The ion space charge dominated memory effect evident from the conditional pulse-amplitude

IEEE Zlansactions on Dielectrics and Electrical Insulation Vol. 1 No. 5 , October 1994 771

distributions shown here for SFs/OZ gas mixtures is consistent with results obtained for other electronegative gas mixtures such as Nz/Oz and Ne/Oa [67,71].

100.0 I I I I , I , , ,

Va = 10.1 kV D = 7.5cm d -0.4cm

80.0

I

20.01 I I I I 1 I I 1 I

0.0 20.0 40.0 60.0 80.0 100.0 Atn-1 (PSI

Figure 6. The expectation value (q,,(At,,-~)) vs. At,,-, for dc negative PD pulses generated at a point electrode separated in air from a 7.5 cm diameter PTFE surface by a distance of 0.4 cm [63].

When a solid dielectric such as polytetrafluoroethylene (PTFE) is placed on the surface of the plane electrode in a point-plane gap, it is found [72] that Trichel pulses still occur when the point is negative, even a t relatively small gap spacings. Moreover, for most point-dielectric spacings a t which Trichel pulses occur, the dominant memory effect is still that due to the motion of negative-ion space charge. At sufficiently small spacings, there is evidence of a memory effect due to the accumulation and decay of charge on the dielectric surface. This is illustrated by the data in Figure 6 which which shows a plot of (q,(At,-1)) vs. At,-l for negative corona pulses generated using a point electrode located 0.4 cm from a PTFE surface in air. Also marked on this Figure are the critical minimum time between successive pulses At,, and the gap transit time At!, estimated using Equation (19). It is seen that even for At,-, > Atl there is an increase of (q,,(At,-l)) with At,-l, albeit a slower increase than for At,-, < At(. It is speculated that for At,-l > Atl, the memory effect is due predominantly to changes in the electric field at the point that result from a relatively slow dissipation or redistribution of charge deposited on the PTFE surface by previous discharge events. At a small enough gap distance, it is found [72] that the discharge will cease entirely after the occurrence of a relatively small number of pulses. The cessation of the discharge can be attributed to the accumulation of enough quasi-permanent dielectric surface charge be- neath the point electrode to keep the local field below

the level required for discharge development. The initiation of subsequent discharge activity will depend on the rate of surface-charge dissipation. Under dc conditions, it can therefore be expected that the mean time between PD events which occur within a dielectric cavity can be quite long, i.e., > 5 h.

0 Q 0 0 Figure 7.

Illustration of a sequence of PD pulses generated at a 'crack' in the insulation of shielded cable when an alternating voltage is applied.

4.2. ac DISCHARGES The cessation of PD due to the accumulation of per-

manent or quasi-permanent charge on dielectric surfaces which happens for constant applied gap voltage need not occur for alternating voltages. This is because the surface charge deposited during any half cycle of the applied voltage will enhance the local field during the subsequent half cycle when the polarity reverses. An example of the situation that can be encountered for alternating voltages is illustrated in Figure 7. For the case shown in this Fig- ure, PD are assumed to occur a t a crack in the insulation of a shielded cable. The first PD might occur near the peak of the applied voltage on either half cycle once the local field strength exceeds the inception level denoted by E,. The charge deposited on the insulator surface by this event will influence the phase-of-occurrence of the next event which is shown to appear near the zero crossing. Because of the deposited charge, the local field strength in the crack will exceed the field E, that would occur in the absence of this charge as the polarity of the applied voltage changes sign. This explains why the first PD on the next half cycle can occur at or near the zero crossing.

This picture suggests that the predominant memory effect for ac-generated PD which occur on or near insulating surfaces is that due to surface charging. The significant influence of surface charging on phase-to-phase

7 72

+- U 00 +I + U -- 1.2 5- a‘ i .a 0 w J

0.e

3 0.E a

0.4

0.;

C (

Brunt: Physics and Chemistry of Partial Discharge and Corona

PO i = 1,8,16

3 1.0 1.1 1.2 Qi (rad. / n) Figure 8.

Normalized conditional and corresponding unconditional phase-of-occurrence distributions of the first, eighth, and sixteenth PD pulses to appear in the negative half cycle for a 1.5 mm point- to-dielectric discharge gap in air at 3.0 kV rms and 200 Hz. The dashed lines correspond to the unconditional distributions and the open and closed points correspond respectively to phase distributions conditioned on the indicated restricted ranges for Qt of the previous half cycle ~ 4 1 .

memory propagation for ac-generated PD was first exper- imentally demonstrated [73] by measuring various conditional probability distributions such as p l ( 4 f IQ+), where 4%: is the phase-of-occurrence of the ith negative PD pulse on an arbitrary negative half cycle of the applied voltage and Qt is the net integrated charge associated with positive PD on the previous positive half cycle, i.e.,

Examples of measured conditional and unconditional phase-of-occurrence distributions of negative PD pulses previously report.ed [14] for a point electrode slightly displaced from a PTFE surface are displayed in Figure 8. Given in this Figure are results for the first, eighth, and sixteenth pulses on the negative half cycle. The fact that the mean phase of the first pulse occurs near the zero crossing, i.e. (4;) 21 A, suggests the significance of surface charge in enhancing the local field. Moreover, the fact that the unconditional and conditional distributions are not equal even for the sixteenth pulse, i.e.

Po($,) # pl (dr61Q+) , implies a correlation between the variables 4a: and Q t . The results clearly demonstrate, as expected, that the 1a.rger the amount of charge Q+, the sooner in phase will be the occurrence of negative pulses on the subsequent negative half cycle, i.e. as Qt increases, (4%; (Q+)) will decrease, where

27

The unconditional phase-of-occurrence distributions can be constructed from the conditional distributions using the integral expression

m

P 0 ( 4 ; ) = J p o ( ~ + ) p l ( g ; ~ ~ + ) d ~ + (22) 0

where p o ( Q + ) is the unconditional distribution of integrated positive PD charge in a half cycle.

It is important to point out that the phase-resolved PD pulse-height distributions previously reported by others [7-9,741, do not contain information about memory propagation. Although phase-resolved distributions are ‘conditional’ in the sense that they restrict the phase interval of observation, they are not conditional in the sense of either selecting individual pulses according to their order of occurrence or in imposing restrictions related to the behavior of PD that occurred at earlier times (or phas- es). A physical interpretation of phase-resolved PD data requires an unraveling of memory effects as discussed in the recent work of Van Brunt and coworkers [14]. The complexity of the unraveling process makes it extremely difficult to interpret phase-resolved pulse-height distributions in terms of discharge mechanisms. It can thus be argued that the measurement of selective PD amplitude or phase distributions that are conditioned on a particular past history of the discharge process can provide greater physical insight than the usual phase-resolved distributions.

The PD memory effects considered in this Section are those associated with residuals produced by previous discharge events such as space charge that can dissipate with time. It is this kind of memory effect that largely controls the stochastic behavior of PD a t any given time. This effect is also distinct from ‘memory effects’ associated with aging processes that produce permanent changes of materials or geometrical configuration from which the discharge gap cannot recover. The aging produced by PD contributes to the nonstationary behavior in the stochastic properties of PD that is discussed in Section 5.

IEEE Bansactions on Dielectrics and Electrical In

5. NONSTATIONARY BEHAVIOR Nonstationary behavior of PD has been observed under

a variety of experimental conditions [18,19,26,27,75] . By the term nonstationary behavior it is meant that the statistical characteristics of PD such as the pulse height or phase distributions have a dependence on time. It has previously been argued [ 141 that , because the statistical characteristics of PD are determined largely by memory effects, it can be expected that these characteristics will be susceptible to nonstationary behavior. The susceptibility to nonstationary behavior can be understood from a consideration of the implications of memory effects. If it is found, as shown in Section 4, that the amplitude of a PD pulse depends strongly on its time separation from the previous pulse, then anything that changes the pulse time separation distribution will necessarily, according to Equation (18) , also change the amplitude distribution. It will be shown below that the time (or phase) distribution of PD is primarily controlled by the rate of initiatory electron release into the gap, which is, in turn, affected by the local field strength at or near the emitting electrode (or dielectric) surface.

To show the connection between the PD pulse time- separation distribution, po(Atn-l) , and the rate R: of the electron release from a surface, it will be assumed that the probability for occurrence of a PD between times t and t + dt is proportional to Rf and given by

r t 1

p i ( t ) d t = R:(t) 1 - &(t’)dt’ dt (23) 1 6 I where the f designation refers to the polarity of the applied voltage at the most highly stressed electrode, e.g., the point electrode. The time differential probability function satisfies the requirement

1 p; (t’)dt’ < 1 (24)

The term in brackets on the right-hand side of Equa- tion (23)is the probability that a PD has not occurred up to time t.

In general, R$(t) depends on time, where, as shown below, the time dependence in a short time frame is most likely governed by the time dependence of the local field, and in a long time frame by aging processes such as result from discharge-induced modification of surfaces that produce changes in the effective work functions. Assum- ing that t is restricted to a small increment of time 6 t in which Rf is assumed to be constant, it can be shown

isulation Vol. 1 No. 5 , October 1884 773

that the function which satisfies Equation (23) and (24) has the form

p ; ( t ) = R: exp(-Rft) (25)

where t 6 6t . The probability that a PD occurs in the time interval 6 t is then given by

6 t

P f ( W = J P ; W d t ’ (26) 0

which gives

pf(6t) = 1 - exp(-Rf6t) (27)

Using a field emission model for electron release from surfaces [13] gives a time dependence for R$(t) of the form

where C: and C,f are factors that are related to the anode, cathode work functions w+ and w- by

R f ( t ) = c;IE(t)l2 exP(-C:lIE(t)l) (28)

Cl f - - b:lO(bzw;/a) (29)

and

Here b l , bz, and b3 are positive constants.

w , the local field is given by For the case of a sinusoidal applied voltage of frequency

E(t + 6 t ) = Eosin[w(t +at)] - AE,(t + at) (31)

where the second term on the right-hand side is the contribution to the local field by space charge or surface charge. For a point electrode on or close to the surface of an insulator, the changes in AE, associated with the deposition of surface charge for a PD event are considered to be abrupt provided 6t is comparable to or smaller than the intrinsic width of the PD pulse. This means that AE, behaves like a step function where

AE,(t + 6 t ) = AE,(t) (32)

(33)

if no PD occurs within 6t, and

AE,(t + 6 t ) = AE,(t) 7 AEj’

if the j t h positive or negative PD occurs within 6t. Here AE: is the change in local field due to charge deposited on the surface by the j t h PD pulse. The behavior of the local field represented by Equation (31) to (33) will, when substituted into Equation (28), give the time dependence for R:(t).

The PD time-separation distribution can be computed numerically by discretieing each pulse time separation into multiples of 6 t , i.e.,

At , - , = tn - tn- l = m6t (34)


where m is an integer and At,-l >> 6t. The probability that the nth pulse will be separated from the (n - 1)th pulse by a time At,-l is then given by

po(At,- l)d( At,- I ) = m- 1

{ exp [ -R: ( ~ t ) 6 t ] } { 1 - exp [ - ~ f ( d t ) a t ] } k = l

(35) which is obtained using Equation (27) and the law of probabilities. Of particular relevance in the present discussion is the strong dependence of R$ on E ( t ) and also on the surface work functions implied by Equations (28) to (30). The dependence of R$ on E ( t ) when w+ and w- are constant produces the type of ‘stationary’ memory effect discussed in Section 4, i.e., the statistical distributions represented by po(qn) and p,(At,-l) do not vary with time. However, time variations in either w+ or w- can introduce nonstationary behavior that is reflected in a time dependence of the distributions po(qn) and po( At,- 1 ) . The numerical procedure discussed above has been used in the formulation of a Monte-Carlo simulator of PD [ lo , 13,14,18] from which various conditional and unconditional PD amplitude, phase, or time distributions can be extracted.

Figure 9.

Measured phase-of-occurrence distributions of the first negative PD pulse and unconditional amplitude distributions of the corresponding positive PD pulses vs. distance of a point electrode from a flat PTFE surface in air. Also indicated by the dashed lines are the corresponding mean values ( b ; ) and ( q : ) , i 2 1 [67].

Results of recent experimental investigations indicate that there can be several sources of nonstationary behavior in the statistical characteristics of PD. For example, there is evidence that small changes in the geometrical

configuration of a point-dielectric gap such as could result from mechanical motion or removal of material during the discharge, as discussed in Section 6, will produce large changes in the behavior of PD [76]. The sensitivity to gap spacing for PD generated by applying an alternating voltage to a point electrode over a PTFE surface in air is illustrated by the experimental results displayed in Figure 9. This Figure shows measured phase- of-occurrence distributions of the first negative PD pulse and unconditional amplitude distributions of the corresponding positive P D pulses vs. point-to-dielectric gap spacing. Also indicated by the dashed lines are the corresponding mean values (4,;) and ( q r ) , i > 1. The most dramatic change occurs as the gap spacing is increased from 0 to 0.75 cm. The statistical characteristics of PD for the case where the point touches the dielectric are distinctly different from the case where there is a small spacing of 0.5 mm. As the gap spacing increases, the characteristics change from those of a surface discharge to those of a corona discharge, i.e., the presence of the dielectric surface becomes increasingly less important. For gap spacings greater than about 1.20 cm, the positive discharge ceases entirely, and the negative discharge takes on the appearance of a phase-restricted train of Trichel pulses.

Another source of nonstationary PD behavior is that associated with discharge-induced changes in dielectric surface conductivity. It was argued in an earlier work by Rogers [77] that the ‘self-extinction’ of pulsating PD in a dielectric cavity can be attributed to an increase in the conductivity of the cavity walls that occurs when the discharge is active. More recently, Hudon and coworkers [19] have reported the transformation of a pulsating discharge into a pulseless glow discharge in a gap formed by parallel epoxy resin planes. They showed that this transformation is related to an increase in the surface conductivity of the epoxy. Possible mechanisms for the increase in surface conductivity will be mentioned in Sec- tion 6.

Additional evidence of the effect of changing surface conductivity is provided from recent work on point-epoxy gaps [18,27,78]. Figure 10 shows examples of integrated charge distributions po(Qt) and p , ( Q - ) , measured a t different indicated times for discharge pulses generated by applying a 50 Hs, 3.0 kV rms alternating voltage to a sharp point electrode that touches a flat cast epoxy surface. In this case, the epoxy contained Ala03 filler. Also shown in this Figure are the measured mean numbers of positive and negative PD pulses ( ( T I + ) and (n-)). The significant feature of these results is the disappearance of positive PD pulses after - 1.5 h. In the early stages of discharge activity, there is an approximate charge bal- ance, i.e. (Q+) - (Q- ) , thus suggesting (from charge


1 0

0 8

0 6

0 4

02

00 0 4 -

5 02 ; 00

p 00

3 0 4 q B 02

0 4 ( n ) = 2 5 0

02

00

1 = 1 6 h

02 0.4 I

-1.2 0.0t.. 4 I I I I I J L I I I

0 200 400 800 8001oO0 100 200 300 400 500 600 700 CHARGE (pC) CHARGE (pC)

Figure 10. Positive and negative integrated-charge distributions measured for PD generated by a point electrode touching a flat epoxy surface at the indicated times after a 50 Hs voltage was applied. Also shown are the corresponding mean numbers of positive and negative PD pulses-per-cycle and the mean charge (vertical arrows) [IS].

conservation) that all of the discharge current can be accounted for by the observed PD pulses. However, as time progresses, an obvious imbalance occurs which implies that some of the current is not associated with the discharge pulses.

Subsequent measurements (see Section 6) showed that there was a significant increase in epoxy surface conductivity near the discharge site consistent with the observations of Hudon and coworkers [19]. With a modified version of the previously discussed Monte-Carlo simulator, it was possible to show that the disappearance of positive PD is consistent with an increase in the local surface conductivity [18]. Using Poisson's equation coupled with Equation (6) for s = 0, and assuming a locally uniform surface conductivity r gives the following expression for surface charge density

a. - - -ro at

If it can be assumed that AE, in Equation (31) is proportional to U at the discharge site, then the solution to Equation (36) leads to an exponential decay in a time

interval 6 t given by

AE,(t + at) = AEB(t)exp(-[r6t) (37)

where [ > 0 is a proportionality constant. In the Monte- Carlo simulation, only the negative surface charge is al- lowed to decay. The numerical procedure described above is then modified so that if AE,(t) < 0 and no PD event occurs in the time 6 t , Equation (32) for AE,(t + 6 t ) is replaced with Equation (37). Equation (32) still applies if AE,(t) > 0 and Equation (33) still applies if a PD event occurs in 6 t independent of the sign of AE,(t) .

In the original model [13], the effective work function w+ of the dielectric surface was assumed to be constant, i.e., independent of the local field. To account for the disappearance of positive PD, it was also necessary to assume that the positive PD initiation probability decreases with decreasing negative surface charge density. This was accomplished by giving w+ a time dependence of the form

for AE,(t) < 0, where ao,al, and a2 are positive constants. The form implied by Equation (38), although empirical, is considered to be physically reasonable, since the probability for removing an electron from a negatively charged insulating surface should increase with increasing negative-charge density.

Using the model described above, positive PD are found to disappear if [18]

tr b W I T (39)

When this condition is satisfied, the charge deposited by PD during the negative half cycle will have almost completely disappeared when the next positive half cycle begins.

Another contribution to nonstationary behavior that might be especially important under static-gas conditions such as within a cavity of a solid dielectric is that associated with discharge-induced changes in the chemical composition of a gas. An example of nonstationary behavior due to this effect is illustrated by the positive PD pulse-height distributions for SF6 shown in Figure 11. In this case, the PD were generated using a point-plane gap in an enclosed vessel containing SF6 a t an absolute gas pressure of 200 kPa [79]. In pure SF6 under these conditions, the point-plane positive corona exhibits a dis- tinctive pulse-burst characteristic [80]. However, after - 4 h of operation, the PD burst characteristic is found to be replaced by a continuous chain of pulses that have a relatively lower amplitude, and this is reflected by the change in the pulse-height distribution such as shown


L 121 . 5 a l d 7 A 1.1110 l/s

f.2860 l l s

t.5 min

. ..

. ..

0 10 20 30 40 50 60 DISCHARGE LEVEL (pC)

Figure 11 Time variation of pulse-height distributions for positive-dc-corona pulses in SFs at 200 kPa. Al- so indicated are the corresponding average corona currents I and observed burst pulse repetition rates f [70, 711.

in Figure 12. That the changes seen in Figure 11 were indeed due to changes in the gas composition was verified by replacing the gas in the vessel and restarting the discharge. When this was done, it was found that the amplitude distribution again exhibited a shape similar to that which had appeared initially in the previous test and the same change in PD behavior occurred with increasing time-of-operation.

There is convincing evidence in this case that the observed rapid change in the positive-corona characteristics of SF6 is due to a buildup of water vapor in the gas during the discharge. Although the source of the water vapor is unknown, it is speculated that it comes from discharge-induced desorption from the electrode surfaces.

P = 300 kPa V = 22.0 kV

I 1

I I , I

5 10 15 20 25 0 0

DISCHARGE LEVEL lpCl Figure 12.

Measured pulse-height distributions of positive- dc-corona pulses for pure and Hz 0-contaminated SFs at a gas pressure of 300 kPa [72].

The change in amplitude distribution seen in Figure 11 can be simulated by adding a trace amount of water vapor to SF6 as shown in Figure 12 [81].

It is now known that discharge initiation near a positive point electrode can be significantly enhanced by the presence of small quantities of H20 [82]. The initiation of a corona discharge near a positive point electrode is expected to be due largely to the release of electrons from negative ions by a collisional electron detachment process. It has been shown [83] that in pure SF6, the only collisional-detachment process which can occur with a reasonable probability is that involving the negative ion F- , namely

F- + SF6 ---+ F + SF6 + e (40)

Because of the high threshold energy for this process (8.0 eV), and because F- is likely to be a minor ion in pure SF6, the probability of initiation by process (40) is


expected to be quite low for E / N above the critical value needed for ionization growth where (Y 2 Q. On the other hand, when H20 is added, other types of negative ions such as OH- can appear [84] that will undergo detachment with orders-of-magnitude higher probability than F- . It can thus be expected that low levels of water-vapor contamination can have a disproportionately large effect in initiating and maintaining positive corona in SFs. On a longer time frame, there are other chemical changes in SFG discussed in Section xx that could influence the behavior of PD such as through modification of the ionization or attachment coefficients.

The tendency for PD to exhibit nonstationary behavior complicates any attempts to define meaningful statistical patterns that can be used in proposed pattern- recognition schemes based on neural networks, etc. for possible identification of the defect sites a t which PD occur [85-871. Nevertheless, the time development of statistical PD patterns may provide additional useful information for identification purposes.

6. PD-INDUCED AGING When discharges occur in an insulating medium, they

can be expected to cause physical and chemical changes of the surrounding material, and in many cases, these changes will be detrimental to the performance of the materials as an electrical insulator. The material changes resulting from exposure to PD are either directly or indi- rectly a consequence of bombardment of energetic electrons that can have mean energies in excess of 10 eV under breakdown conditions. From a practical point-of- view, the problem of insulation aging has been of para- mount concern and there has been extensive research into the mechanisms of aging. In this Section, the focus will be on a few selected aging processes that are relevant to the types of PD phenomena discussed above.

It was pointed out in Section 5 that the exposure of cast epoxy resin surfaces to PD under alternating voltage conditions causes changes in the surface characteristics which can, in turn, influence the behavior of PD. Micro- scopic examinations of the epoxy surfaces after exposure to PD in air revealed that material had been removed at the discharge site. Results of typical surface profile measurements in the vicinity of a discharge site are shown by the profilograms in Figure 13 [17]. The locations of the point electrode used to generate PD are indicated by the vertical arrows. The epoxy materials used in this case contained an A1203 filler. The discharges were generated in room air by applying a 4.0 kV rms (50 Hz) voltage to the point electrode for a period of 22 h. In all cases, it was found, as seen in the Figure, that the surface roughness increased significantly a t the center of

1 I 1 Ilm 1mm ’

Figure 13. Profilograms of epoxy surfaces after exposure to PD in air. The vertical arrows indicate the location of the point electrode [17].

discharge activity. The measurements before and after PD exposure showed that there was a volume decrement indicating that material had been removed. For longer PD exposures, narrow channels appeared that are per- haps the precursors to tree formation [26,75]. Changes in surface profiles such as seen in Figure 13 can also change the local electric-field strength as well as the ability of the surface to trap charged particles. Removal of dielectric material by PD can be safely considered as a ‘permanent’ alteration or aging effect.

12 16 20 0 4 8 DISTANCE (mm)

Figure 14.

Measured changes in epoxy surface resistance vs. distance from the center of the PD site for different times since the termination of discharge exposure in air [17].

It was noted that PD also cause increases in the local surface conductivity of epoxy. Figure 14 shows the results from measurements of the epoxy surface resistance


vs. distance from the center of the discharge site for different indicated times since the termination of discharge exposure. The discharge conditions were similar to those that produced the surface damage shown in Figure 13. The resistances were measured by moving small parallel wires across the epoxy surface as described previously [17,18]. The change in surface conductivity at the discharge site appears to be 'quasi-permanent' in a sense that the conductivity will, given a sufficiently long time after PD exposure, decrease to a value closer to that observed before initiation of the discharge. This is illustrated by the trend shown in Figure 14. Moreover, wiping the surface with a damp cloth tended to restore the conductivity to the pre-discharge value [17].

The precise nature of the PD-induced physical and chemical changes of epoxy surfaces that give rise to an increase in surface conductivity remain unknown. The fact that the predischarge condition can be largely re- stored after wiping suggests that the changes are likely due to formation of deposits. There is evidence from the work of Hudon and coworkers [22] that droplets of glycol- ic and formic acid are formed on epoxy surfaces during exposure to PD. These acids are known to be far more conductive than the epoxy resin matrix.

A necessary condition for the formation of PD is that the mean energy of electrons should be sufficiently high to cause a net growth in the ionization of the gas. Specif- ically for electronegative gases, this requires that CY > q. Under this condition, the electron energies are also high enough to produce molecular dissociation leading to the formation of highly reactive free radicals such as 0, OH, F, and SF5 by processes like

e + 0 2 + O + O + e (41)

(42)

and e + SF6 -+ SF, + (6 - n ) F + e

where n is an integer satisfying the restriction 1 < n < 5. These reactive fragments can then lead to the formation of other reactive species. In the case of SFe/O2 mixtures, the discharge byproducts are known to include SO2, SOFz, S02F2, SOF4, SF4, S2F10, SzOFio, SzOzFio, and HF [15, 161. Examples of fast neutral reactions in the active discharge zone that occur in SF6 corona when 0 2

and H20 are present are listed in Table 3. Species like F and HF will attack solid insulating surfaces and contribute to the removal of material.

As seen from Table 3, the gas-phase chemistry associated with PD phenomena can be very complex. The decomposition of SF6 in corona discharges has been extensively investigated in recent years because of the wide- spread use of SF6 as an insulating medium in HV appara-

Table 3. Examples of important fast reactions that occur in the discharge Bone of a negative corona in SFa containing Ha0 and 0 2 . M = third body. -

Reaction

F + H 2 0 -+ O H + H F

SF5 + O + M -+ SOF5 + M

SF5 + 0 -+ S O F 4 + F

-+ S O F 4 + H F OH + SF5

SOF5 + S F 5 -+ SOF4 + S F 6

SF3 + 0 -+ SOF2 + F SF5 + F + M -+ SF6 + M SF3 + F + M -+ SF4 + M SF5 + SF5 + M -+ S2Flo + M SF3 + SF3 -+ SF2 + SF4 SF3 + SF5 -+ SF6 + SF2

Rate coeff. cm3/s

9.0 x 10-12 1.1 x 10-l2 1.0 x lo-" 1.0 x 1 0 - l ~ 2.0 x 10-l' 1.0 x 10-12 1.5 x 2.4 x lo-" 1.4 x

5.0 x lo-'' 2.2 x 10-12

tus and related concerns about toxicity and corrosiveness of the discharge byproducts [15,16,88-901.

Figure 15 shows examples of data on the production rates for the major sulfur-containing oxyfluoride by-products from negative point-plane corona discharges in SF6 [91]. These results were obtained using a continuous constant-current negative-glow corona operated at 40 pA for a total absolute gas pressure of 200 kPa. Shown in Figure 15 are the charge rates of production normalized to the SF6 concentration as given by

r ( X ) = - Zd[:Fe] (F) (43)

where Zd is the discharge current, [XI is the quantity in moles of the by-product of interest, and [SFs] is the absolute quantity of SF6 in moles. The total oxyfluoride production given by

rt = r(SOF4) + r(SOF2) + r(SOaF2) (44)

is compared in Figure 15 with the estimated maximum possible SF6 decomposition rate rt(max). The estimate of the maximum rate is based on using the calculated rate for reaction (42) [61] in a model of the discharge [47,48], and assuming that every sulfur-containing frag- ment (SF,) converts to an observable by-product.

The tendency of rt to fall considerably below rt(max) is a consequence of the fact that recombination reactions such as

SF5 + F + SF6 -+ 2SF6

are fast at moderately high gas pressures and therefore very efficient in reforming SFG after it has been dissociat- ed. The increase in rt with increasing oxygen content is primarily a consequence of the diluting effect of 0 2 which

(45)


N Calculated rt (max)

- SFe/Op mixture -

Y 0 SOF4 Q) -

E" A SOpFp 2 1 0 3 A r t - (SOF4+SOF2+SO2F2) -

o SOFp -

0 0

0

0

101 I 1 1 I 1 0 20 40 80 80 100

PERCENT 0 2 Figure 15.

Measured charge rates-of-production of the oxyfluorides SOFl, SOFz, and SOzFz from negative dc point-plane glow-type corona in SFe/02 mixtures [El]. Also shown are the net oxyfluoride production rate T t , and the calculated maximum SFa destruction rate in the discharge Tt(max). The production rates have been normalised to the SFe concentration. The discharge current was 40 pA and the total pressure was 200 kPa.

can inhibit the recombination reactions such as process (45)-

Under the discharge conditions that yielded the results shown in Figure 15, it was found that other gaseous byproducts can be formed which are not derived from SF6 dissociation, i.e., they contain neither sulfur nor fluo- rine. Among these is the gaseous compound COz. Shown in Figure 16 are examples of the measured yields of CO2 vs. net charge transported given by the product of discharge current and time. The data in this Figure were obtained for a 40 pA glow-type negative corona in the three indicated SF6/02 mixtures a t an absolute total gas pressure of 200 kPa. For these experiments, a stainless steel point electrode was used. It is speculated that the CO2 is

NEG. CORONA AT 40 )rA CO2 from SF, /02 MIXTURES P = 200 kPe 0 SFs A SF611%02

SFS 110% 0 2

B 10

0 1.0 2.0 3.0 4.0 5.0 6.0 0

NET CHARGE 'TRANSPORTED (c) Figure 16.

Measured yields of CO2 vs. net charge transported in a 40 pA negative glow corona in SFe and indicated mixtures of SFs with 0 2 .

derived from reactions of oxygen with carbon contained in the steel electrode. The rate for CO2 production is seen to increase with oxygen content as expected.

The production of COz, even in relatively pure SF6, may be a good indication of so called 'electrode condi- tioning'. During a negative-corona discharge, the point electrode can be eroded by ion bombardment. This is manifested by the appearance of increased surface roughness or micro-pits a t the tip of the electrode. The discharge- enhanced roughness of the electrode surface can cause it to become a more efficient emitter of electrons [80] and can cause an increase in the catalytic destruction of some relatively unstable byproducts such as S2Flo [go].

The production rates for SF6 byproducts from corona or PD implied by the results shown in Figure 15 sug- gest that under practical conditions where the volume of SF6 is relatively large, the buildup of byproducts from the discharge will be too slow to significantly affect the ionization, attachment, and recombination coefficients of the gas. On the other hand, if the decomposition occurs in a small volume such as in a dielectric cavity, it is possible that a large fraction of the gas can become decomposed in a short time, and this could significantly modify the discharge behavior. In general, if the gas consists of more than. one component, then the ionization coefficient given by Equation (13) must be replaced by


the sum [63]

_ - - - I -

O

where Qij is the ionization cross section of the j t h component of density Nj, and

Similar modifications would need to be made in 7 and other coefficients. Hence, if Nj changes with time due to discharge-induced modifications of the gas, then so will the ionization coefficients, etc., and likewise also the discharge characteristics.

To date, there have not been many attempts to model the chemistry in PD. Only recently has a success- ful model been formulated for SFs, and this is restricted to negative-glow type discharges in point-plane gaps [47,48]. Some success has also been achieved in modeling the chemistry of pulsating dielectric barrier discharges in oxygen [39]. The task of modeling the chemistry in PD is quite formidable as can be appreciated when it is realized that Tables 1 and 2 list only a small fraction of the many hundreds of possible chemical processes that may need to be considered.

7. CONCLUSIONS AND RECOMMENDATIONS

VIDENCE is provided here of significant progress E that has been made in achieving a better understanding of the physical and chemical bases of PD phenomena. If anything is to be gained from reading this work, it should hopefully be an appreciation of the fact that PD phenomena are enormously complex. In attempts to ex- plain PD behavior, caution must be exercised in making simplifying assumptions about the relevant mechanisms.

As is often the case with basic research, the increase in knowledge about PD raises more questions and places ever greater demands on the quality of fundamental data needed to carry us up to the next level of understanding. In pursuit of this next level, it is recommended that emphasis be given to the following avenues of research: (1) extend computer simulations of discharge growth to include the modifications of the local environment by residuals from prior discharge activity; (2) develop models that can predict the transition from a pulsating corona to a glow discharge; (3) develop models that predict the size of constricted glows in highly nonuniform fields; (4) perform experiments and calculations to give more information about the influence of high local fields on secondary electron emission from surfaces due to impact of photons,

ions, and metastable species; (5) develop better experimental and theoretical techniques to investigate the dynamics of dielectric surface charging during PD activity; (6) acquire more information about changes in PD behavior during electrical tree formation in solids; (7) develop better diagnostics to investigate permanent and quasi- permanent changes in the composition and morphology of solid surfaces exposed to PD; (8) extend measurements of PD-induced decomposition of dielectric gases to lower currents and pulsating PD conditions; (9) develop spec- troscopic or other nonintrusive methods for measuring temperature profiles in PD; and last but not least, (10) give special attention to determination of the effects of absorbed and gas-phase HzO on PD behavior. In reality, there is a much greater number of avenues than suggested here for future basic research on PD that is at least as great as the multitude of different modes of discharge activity. There is yet much more to be learned about PD, and as more is learned, this information can hopefully be put to good use in constructing better diagnostics of insulation performance. I caution against undertaking new large scale efforts to develop practical PD diagnostics without a good understanding of the phenomena being observed.

J. B. Whitehead, for whom this lecture series is named, was fascinated and concerned with corona discharge phenomena in insulation, and conducted significant pioneer- ing research on this topic [92]. It can be argued that the following comments offered in the conclusions of his 1910 paper [92] on the dielectric strength of air are relevant today:

“Corona formation and the laws which regulate it are of the first importance for electrical engineers since corona is to be prevented both as a source of loss and as an enemy of insulation. The field is replete with fascinating problems. Their solution however involves the use of methods and apparatus generally not familiar to the physicist. ”

ACKNOWLEDGMENT This work was performed in the Electricity Division,

Electronics and Electrical Engineering Laboratory, Na- tional Institute of Standards and Technology, Technol- ogy Administration of the U. S. Department of Com- merce. The author is grateful for support provided by the National Institute of Standards and Technology, the U. S. Department of Energy, the U. S. Nuclear Regula- tory Commission, and the Electric Power Research Insti- tute, and for major contributions from S. V. Kulkarni, M. C. Siddagangappa, A. V. Phelps, T. Las, E. W. Cernyar, P. von Glahn, H. Slowikowska, J. T. Herron, K. L. Strick- lett, J. K. Olthoff, and M. Misakian.

Y. Shibuya et al.: Electromagnetic Waves from Partial Discharges and their Detection using Patch Antenna

1070-9878/10/$25.00 © 2010 IEEE

862

Electromagnetic Waves from Partial Discharges and their Detection using Patch Antenna

Yoshikazu Shibuya, Satoshi Matsumoto, Masayoshi Tanaka,

Shibaura Institute of Technology 3-7-5 Toyosu, Koto-ku, Tokyo, 135-8548, Japan

Hirotaka Muto and Yoshiharu Kaneda Mitsubishi Electric Corporation

8-1-1 Tsukaguchi-Honmachi, Amagasaki City, Hyogo, 661-8661, Japan

ABSTRACT The electromagnetic (EM) wave generated by partial discharge (PD) is analyzed using electric dipole model. The pulse electric and magnetic fields are described by analytical functions. The interaction of patch antenna or microstrip antenna with the EM wave is investigated on EM theory, which leads to a simple transmission-line model. Then, the response of patch antenna to the PD-initiated EM wave is numerically analyzed as a circuit transient. It is found that the generated EM fields and the response of patch antenna strongly depend on how fast PD develops. The sensitivity of patch antenna to PD is expressed in a simple chart. The analytical results are verified by an experiment in which the EM wave from a short-gap discharge is detected by the patch-antenna PD sensor. The patch-antenna system is expected to be used for detecting PDs occurring inner parts such as winding’s inter-turn.

Index Terms — Partial discharges, UHF sensors, Microstrip antennas, Microstrip resonators, Electromagnetic transient propagation, Electromagnetic transient analysis.

1 INTRODUCTION

THE partial discharges (PDs) have been studied for quite a long time in connection with the reliability of high voltage apparatus, in which the detection of PD is particularly important. The apparent charge is traditionally regarded as the measure of harmfulness, which is obtained in a test observing the current or voltage induced in the coupled circuit [1, 2].

Electromagnetic (EM) wave is reported to generate from corona or from short gap discharges in high voltage power lines [3, 4]. Those studies were concerned about the high voltage equipment’s possible interferences to radio communications. The frequency spectra are found to be quite wide extending beyond GHz.

The idea of detecting PD utilizing EM wave is first conceived in gas insulated apparatus in the form of so-called ‘UHF sensor’, which is essentially a capacitive coupler [5, 6]. The technique has already developed into a level to be used in on-line monitoring of gas insulated equipment [7, 8].

Making use of EM wave in detecting PD from other equipment has been also proposed [9, 10]. Both dipole and loop antennas have been tested. The detection frequency being under 300 MHz (i.e. VHF range), there is the problem of severe interferences from the public broadcast and other sources. Since the noise level decreases at higher frequencies, a highly sensitive antenna which works reliably in UHF range (0.3–3 GHz) is desired.

The microstrip antenna (patch antenna) is composed of a conducting strip separated from a ground plane by a dielectric substrate. Thanks to the simple structure, this technology made a rapid progress in 1970s and 1980s in the area of communication devices [11]. Now, it is widely used as the transmitter of cellular phones, etc.

The idea of using the patch antenna for PD sensor is first proposed in middle of 1990s [12, 13]. However, it is quite recent that practical UHF sensors of this type are developed [14-19]. The characteristics of patch antenna are relatively well understood in communication technology, but they are for continuous wave conditions. In the case of PD sensor, the patch antenna has to respond to the pulse EM wave. On the other hand, the receiving characteristics are not so well understood compared to the transmitting characteristics. The objective of this paper is to provide a series of simulation methods by which the transient response of patch antenna to the EM wave by PD can be analyzed based on realistic assumptions.

In the first part of this paper, a simple model of PD is proposed after reviewing the transient PD currents reported in literature. Secondly, the EM fields are formulated on the physical model. Then, the excitation of patch antenna by the pulse EM wave is analyzed based on transmission-line model. Most of the calculation is conducted using analytical functions. Lastly, the simulation is verified by an experiment that the patch antenna is illuminated by a short-gap discharge. Manuscript received on 24 November 2009, in final form 19 January 2010.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 3; June 2010 863

i0(t) d

C0

V(t)

C0

d i0(t)

V(t)

(a) Void discharge (b) Gap discharge

Figure 2. Charge movements in PD.

2 PD CURRENT AND EM WAVE A short transient current associated with PD generates EM

wave. After reviewing the PD currents reported, a simple dipole model of PD is proposed from the generalized current waveform. Then, EM fields are analyzed using the model.

2.1 PD CURRENT WAVEFORMS PDs occur in various occasions such as (a) void discharge,

(b) surface discharge, (c) point-plane gap, (d) discharge involving conducting particles [20]. A similar discharge takes place in (e) short-gap discharge which has been being studied in conjunction with electrostatic discharge (ESD) [21].

Although the actual PD currents are difficult to observe, careful measurements have revealed a waveform shown in Figure 1. The observed current peak Ip is smaller than the true value, the degree depending on the coupling of measuring circuit. However, the waveform should be of true PD current. The half-amplitude width Th shown in Figure 1 is taken as the parameter of waveform. The value Th may be broken down to Th1 and Th2 to be more precise.

Table 1 lists typical parameters read from PD current waveforms reported in literature. The values of Ip observed in void or point-plane are much smaller than those in particles or short gaps. The reason is not only the true current is smaller in the former, but also the coupling is weaker. There is a tendency of Th1 < Th2 for larger gap conditions, however, the two are roughly same for short gaps. It is also noticed in Table 1 that the half-amplitude width Th becomes extremely small, even smaller than 0.1 ns, for contacting gap discharges simulating ESD [31].

DIPOLE MODEL The true PD current of shorter gaps is approximated by the

following Gaussian waveform using half-amplitude width Th [28, 32].

2log2whereexp)( 0

2

000

hTt,

ttIti =

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛= − (1)

The true charge amount Q is expressed by

000 )( ItπdttiQ == ∫∞

∞− (2)

As depicted in Figure 2, the true charge Q is supposed to move by a distance d which corresponds to the void length for void discharge or the equivalent charge separation length for gap discharge. This movement defines the time-varying electric dipole expressed in the form:

dQItdπp

ttpdt

ttIdtp

t

⋅==

⎥⎥⎦

⎤

⎢⎢⎣

⎡=

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−⋅= ∫

00

00 00

Δwhere

erf2Δexp)(

2

(3)

Since this is considered to be the quantity interacting with outside circuits, the apparent charge measurable by conventional method is related to Q·d, rather than the true discharge Q.

The gap voltage is expressed in a similar equation:

00

Δwhereerf2Δ)( C/, QV

ttVtV =

⎥⎥⎦

⎤

⎢⎢⎣

⎡= (4)

Here, ΔV is the voltage change in the process, C0 being the equivalent capacitance of the gap.

2.2 EM FIELDS GENERATED BY DIPOLE The EM wave generated by time varying electric dipole p(t)

is analyzed using Hertz vector:

rπc/rt

0ε4)−

=(p

Π (5)

Here, ε0 and c are the permittivity and velocity of light in free space, respectively. Variable r is the distance between dipole and observation point. According to EM theory, the fields are given by the following equations [33].

t∂∂

==Π

Π, curlεcurlcurl 0HE (6)

Since p(t) is given in an analytical function in equation (3), the fields E and H can be obtained in analytical functions

time

Ip

Ip/2

Th1 Th2

Th

Figure 1. General waveform of PD currents observed.

Table 1. PD Current Parameters Observed in Various Conditions.

time (ns) PD type gap conditions Ip

(mA) Th Th1/Th2 ref.

0.1 mm void in PE 0.64 mm void in PE

34 20

0.87 5.3

0.44/0.44 1.5 / 3.8 [22]

0.05mm void in PTFE 0.5mm void in PTFE

6.7 12

0.37 2.4

0.19/0.19 0.5 / 1.9 [23]

void in epoxy resin 27 2.4 1.0 / 1.4 [24]

(a)

2 mm void in PMMA 6 1.4 0.4 / 1.0 [25] pt.-plane in 0.4 MPa SF6 7 0.8 0.3 / 0.5 [26] pt.-plane in 0.1 MPa SF6 4.3 0.49 0.16/0.32 [27] (c) pt.- plane in 0.1 MPa SF6 3 0.27 0.05/0.22 [28]

(d) 2 mm dia. Al. particle in 0.38 MPa SF6 1300 0.07 0.035

/ 0.035 [29]

0.66 mm gap in air 16000 2.0 1.0 / 1.0 [30] (e)

contacting electrodes in air n.a. 0.047 0.023 / 0.023 [31]

Y. Shibuya et al.: Electromagnetic Waves from Partial Discharges and their Detection using Patch Antenna 864

Ex p(t)

x

y

z

Hz

electric dipole

observation point

y

Figure 3. Electric dipole and observation point.

Figure 4. Example of peak electric fields as a function of observation distance. The current-induced and electrostatically induced components are effective in short ranges only.

Ex

EM wave

x

(x, y, z)

z Hz

dipole Δp Th D

y

Figure 5. Parameters of far field generated by PD.

Figure 6. Function f(τ) used in expressing EM fields.

using equation (6). Consider the case, without losing generality, that p(t) is at the origin aligned to x axis as shown in Figure 3, then the Hertz vector Π is given in a simpler form:

0erfε8

Δε4 000

=Π=Π⎥⎦

⎤⎢⎣

⎡ −=

−=Π zyx ,r/c

rπp

rπr/ctp

tt)(

(7)

If an observation point is chosen on y axis (the direction of maximum EM radiation) as shown in Figure 3, using equation (7) in equation (6), yields the following expressions, other components Ey, Ez, Hx, Hy being zero.

( )

⎥⎦

⎤⎢⎣

⎡ −−

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

η−

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

−μ=

03

0

2

02

023

2

03

023

0

erfε8Δ

exp4

Δ

exp2Δ

tt

tt

ttt

c/|y|yπ

p

c/|y|ytπ

p

c/|y||y|tπc/|y|p

Ε

/

/x

(8)

( )

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛ −−+

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

−−=

2

02

023

2

03

023

exp4

Δ

exp2Δ

tt

ttt

c/|y|ytπ

p

c/|y|ytπcc/|y|pH

/

/z

(9)

Where, μ0 and η denote the permeability and impedance of free space, respectively.

The first and second terms in equations (8) and (9) indicate the radiation and the current-induced fields, respectively. The third term in equations (8) is the electrostatic field [33]. Since the relation Ex=−η Hz holds in the first two components, EM fields can be represented by the electric field. The presentation method is followed in the rest of this study.

The second and third terms of Ex in equation (8), proportional to 1/y2 and 1/y3, respectively, are effective only in the vicinity of source, therefore, they indicate near field. The first term or radiation, being proportional to 1/y, prevails in longer ranges, therefore, it is far field. Figure 4 shows how the amplitudes of those three components decay with the observation distance y in the case of Th=0.35 ns and Δp=913 pC·mm (a value chosen so that Epeak by radiation becomes 1 V/m at y= 1 m). It is noticed that the EM fields can be treated as far field if the observation distance is larger than 0.1 m.

2.3 FAR FIELD The far field or radiation field is defined by the first term of

equations (8) and (9). Shifting the origin by the observation distance D as shown in Figure 5, and assuming

222 zxD y +>> + , the EM fields at (t, x, y, z) can be expressed as follows:

( ) ( )220

230

0

exp2π22Δ

where

) ,(

τ−τ=τ=

η−=⎟⎟

⎠

⎞⎜⎜⎝

⎛ −=

ef,Dt e

pμE,

EH,

ty/ctfEz,yx,tE

/p

xzpx (10)

Here, f(τ) is a derivative of Gaussian function shown in Figure 6. Value Ep is the peak of electric field.

The EM fields from PD given by equation (10) are of plane wave, since they are function of t and y only. The waveform, in both time and space, is expressed by function f(τ). Since the electrical field peak Ep is inversely proportional to square of t0 or Th, the radiated field strength strongly depends on how fast the source dipole changes.


feed

ξ

ζ η

ξ0

output

L

h

H

E incident EM wave

patch conductor

ground plane

dielectric substrate (dielectric const. ε r )

W

(a) Configuration

Z0 output

Re Ce Ce Re

L transmission line excitation

Z, v ξ0

(b) Transmission-line model

Figure 7 Construction of patch antenna and transmission-line model.

Table 2. Parameters of Antennas I, II, III Analyzed.

εr dimensions (mm) L W h ξ0

Z (Ω)

v (m/μs)

Ce (pF)

Re (Ω)

I 1.0 80.4 56.5 2.0 12.0 13.3 300 0.35 781

II 2.0 58.1 40.8 2.0 6.2 13.4 218 0.368 1489

III 4.0 41.9 29.4 2.0 3.2 13.5 158 0.436 2881

Figure 8. Frequency characteristics of output impedance for three antennas.

3 RESPONSE OF PATCH ANTENNA The transmitting characteristics of patch antenna or

microstrip antenna have been extensively studied [34], however, its receiving characteristics have not been looked over enough. Particularly, the behavior in pulse operating condition is not clear. Here, the transient response of patch antenna to the far-field pulse EM wave from PD is investigated using a transmission-line model.

3.1. RESONANCE CHARACTERISTIC The patch antenna is composed of a patch conductor on a

dielectric substrate as shown in Figure 4a. Let (ξ, η, ζ) be the coordinate system fixed with antenna, independent of the coordinates (x, y, z) used describing EM fields. When illuminated by the EM wave, the patch conductor behaves as a resonator. Considering the transverse electromagnetic mode along ξ axis, i.e. along the length L, a voltage is induced distributed along the direction. The antenna output is usually taken from a point displaced by ξ0 from the center, denoted as ‘feed’ in Figure 7a [34].

The resonance is most easily understood by the transmission-line model shown in Figure 4b. The characteristic impedance Z and propagation velocity v can be calculated from patch width W, the substrate’s dielectric constant εr and thickness h [35]. The equivalent resistance Re and capacitance Ce can be also determined from the antenna dimensions [36].

The frequency characteristic of patch antenna is of narrow band. The selection of resonance frequency fr is important in designing a PD sensor with this antenna. The resonance frequency of 1.8 GHz is chosen for its application to the on-line monitoring system of rotating machines, in order to avoid noises [16]. Following this practice, the condition fr=1.8 GHz is presumed for the antennas analyzed in this paper.

There are many ways to design a patch antenna satisfying the condition fr=1.8 GHz. Three examples are given in Table 2, in which substrates with different εr are used. The patch length L is shortened for larger εr to give the same fr, and the width W is set proportional to L. The calculated transmission-line

parameters Z, v, as well as the line-end constants Re, Ce are listed in Table 2.

The impedance Z0, seen from the feed or output point is easily obtained from the equivalent circuit of Figure 7b. The feed points ξ0 in Table 2 are determined so that the impedance Z0 turns out to be exactly 75 Ω at the frequency fr=1.8 GHz. Figure 8 shows the frequency characteristics of Z0 for Antennas I, II, III. The narrow-band frequency characteristic comes from the resonance in antenna. Comparing the characteristics of three antennas, it is seen that the band width is narrower for larger dielectric constant.

3.2. EXCITATIONS BY EM FIELDS The incoming EM wave excites an electrical disturbance in

patch antenna. Although this phenomenon is shown to be analyzed quite well by the finite differential time domain (FDTD) technique [37], the procedure is somewhat cumbersome. It is desired that the excitation phenomenon is analyzed more easily and in more comprehensible manner if possible.

In the analysis of receiving antenna, it is required to take into account the currents induced in the antenna [38]. In the present situation depicted in Figure 7a, the effect of currents in ground plane should be considered. The boundary conditions impose the constraints that only the perpendicular electric field and the parallel magnetic field exist in the vicinity of patch antenna surface.

The perpendicular electric field Eζ induces an electromotive force (EMF) ee, as shown in Figure 9a. Considering the substrate’s dielectric constant εr and thickness h, the voltage ee is evaluated by:

( ) ( )r

e,tEh

,teε

ξ−=ξ ζ (11)


h

ξ dξ

Eζ

ξ

C dξ

re ε

Ehe ζ−=

rεE

fieldelec. ζ=

εr

Hη

h

ξ dξ

ξ

ξ⋅μ= η dHh2fluxmag.

0

Emdξ

tH

h2E 0m ∂

∂μ−= η

Ldξ

(a) Effect of electric field (b) Effect of magnetic field

Figure 9. Voltages induced in patch antenna by EM fields.

Figure 10. Eight different antenna positions in respect to incoming EM wave. Thick arrow shows the longitudinal direction.

(a) Antenna position: face-on A (b) Antenna position: side-on A

Figure 11. EMFs calculated for Antenna II subjected to EM wave of Th=0.35 ns, Ep= 1 V/m.

Let Hη be the lateral magnetic field component of the incident EM wave. The field should increase to 2Hη, when illuminated from top as shown in Figure 9b, by the effect of currents in ground plate. However, the field should be eliminated when illuminated from back. It is further assumed that the field should remain the same when the EM wave comes sideways or in parallel to the ground plane. Then, the electromagnetically induced EMF (expressed by the electric field directed to ξ axis) is evaluated by:

( )

( )( )

( )( )

( )⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

⎨

⎧

∂

ξ∂μ−

∂

ξ∂μ−

=ξ η

η

backfromdilluminate0

sidefromdilluminate

topfromdilluminate2

0

0

t,tH

h

t,tH

h

,tEm (12)

These equations (11) and (12) give analytical expressions if the relation between coordinates (x, y, z) and (ξ, η, ζ) is fixed specifying the antenna position. There are 8 independent antenna positions as shown in Figure 10. In the figure, EM wave comes horizontally (i.e. in y direction). The thick arrows on patch indicate the direction of resonance (ξ axis). Other possible intermediate positions can be treated as a superposition of some of those cases.

Evaluating the EMFs in the 8 cases on the above rules, only two cases:

‘face-on A’, and ‘side-on A’

are found to give EMFs, whereas EMFs for other 6 cases diminish. Figure 11 show these two EMFs calculated for the condition that Antenna II is subjected to the EM wave of Th=0.35 ns, Ep= 1 V/m. Since they are functions of t and ξ, EMFs are displayed in three-dimensional graphics.

In Figure 11a, the case of ‘face-on A’, electrically induced EMF ee is zero. This means the patch antenna works as pure magnetic field sensor. In Figure 11b, the case of ‘side-on A’, both electrical and magnetic EMFs are present.

The EMF waveforms for other antennas (i.e. Antenna I or III) can be analyzed giving similar results. An increase in dielectric constant εr leads to decrease in the electrically induced ee. On the other hand, because the dielectric constant does not influence the magnetically induced Em, The differences of Em among Antennas I, II, and III are marginal. For this reason, the following analysis is done only for Antenna II regarding it as a typical example.

3.3. TRANSMISSION-LINE MODEL Figure 12 shows an expedient method to bring the two

kinds of EMFs into the equivalent circuit of transmission-line model. The length Δξ represents a section of patch conductor, in which corresponding sources are introduced.

As for the electrically induced EMF ee, a current source Δi is introduced as shown in Figure 12a. From the relation that the voltage across CΔξ has to be equal to ee, Δi is determined as follows:

( )tteCi e

∂ξ∂

ξΔ=Δ, (13)


LΔξ

CΔξ

Δξ

Δi ee

Δξ Em Δξ

CΔξ

Δe

LΔξ/2

(a) Equivalent current source (b) Equivalent voltage source

Δi

Δe Δξ/2

Z, v Z, v

Δξ/2

Δi

Z, v Z, v

Δξ/2 Δe

Δξ/2

(c) Transmission-line model

Figure 12 Construction of equivalent transmission-line model with currentand voltage sources representing induced EMFs.

Ce Re e1 e0 e2

ξ = −L/2 0 −ξ0 ξ0 L/2

Z, v Z, v Z, v Z, v Z, v Z, v Re Ce

L/2− ξ0 2 ξ0 L/2− ξ0

i02 z0 v0

i1 i01 i2

V2 V1

Figure 13. Three-division transmission-line model.

Where, C is the distributed patch capacitance per unit length. Its value is obtainable from the transmission-line parameters by

ZvC 1

= (14)

As for the electromagnetically induced EMF Em, a voltage source Δe is introduced as Figure 12b. From the relation that the voltage along the section Δξ should be equal to Em Δξ, Δe is determined as follows:

( ) ξΔξ=Δ ,tEe m (15) A transmission-line model thus constructed is shown in

Figure 12c. The transmission-line model with a number of such sections along the length of patch length might be a canonical model. However, introducing many sources in the model unnecessarily complicates the numerical analysis.

A greatly simplified model shown in Figure 13 is obtained when the entire length L is divided into three sections:

2,,

2 0000LL

≤ξ≤ξξ≤ξ≤ξ−ξ−≤ξ≤−

The current sources i1, i01, i02, i2 are positioned at division boundaries, and the voltage sources e1, e0, e2 at division midpoints. Consulting equations (13) – (15), those sources are evaluated by integrating the EMFs in relevant intervals as given below.

( ) ( ) ( )

( ) ( )

( ) ( ) ( ) ( )

( ) ( )∫∫∫

∫∫

∫∫

ξ

ξ

ξ

ξ−

ξξ=

ξξ=ξξ=

==

=ξ∂

ξ∂=

−−

+

+

−−

−−

−

ξ

ξ

ξ

ξ

2/

02

0

00

0

2/1

2/

2/2

2/

002

0

2/01

2/

2/1

,

,,,,

;"1,"1

,"1,,1

2

2

2

2

0

0

0

0

L

m

mm

L

L

L

L

L

L

dtEte

dtEtedtEte

Zvti

Zvti

Zvtid

tte

Zvti

L

e

(16)

Since the incident EM fields are given by an analytical function in equation (10), all the time-resolved waveforms of equation (16) can be evaluated analytically using equations

(11) and (12). The exact mathematical expressions are obtainable if antenna conditions are specified. Although the expressions are complicated, the numerical calculation can be easily executable with the aid of computer. This is an advantage of the present method based on simplified physical models.

3.4. SIMULATION OF RESONANCE The circuit of Figure 13 is analyzed using the

Electromagnetic Transients Program (EMTP), or more specifically EMTP/ATP [39]. For analysis, the following parameters have to be given:

(a) Far field EM wave parameters Th, Ep (b) Antenna properties L, ξ0, Z, v, Re, Ce (c) Antenna position (see Figure 10) (d) Output impedance z0 Figure 14 in next page shows the results in the case that

Antenna II is subjected to EM wave of Th=0.35 ns, Ep= 1 V/m. The output impedance is set as z0=75 Ω. The two cases: (a) and (b) correspond to those of Figure 11.

The first row in Figure 14 shows the incident EM field. The second and third rows show the waveforms of current and voltage sources which are calculated using equation (16) a priori. Those currents and voltages are imported to EMTP as externally determined functions. The last row shows the EMTP results, in which the antenna output voltage v0, the voltage across z0, is shown together with the patch-end voltages V1 and V2.

Although the initial excitation of current and voltage sources lasts for about 1 ns, a longer resonating oscillation persists in the output v0 as well as in the voltages V1 and V2. Comparing (a) and (b) of Figures 14, it is interesting to note that the output amplitude for face-on position is by far larger than that for side-on in spite of zero current sources.

The oscillation in the output voltage v0 is the radio frequency (RF) signal of 1.8 GHz (detectable by conventional rectifier in actual circuit). Let VRF be the initial zero-peak amplitude as indicated in the figure. The value of VRF depends on the simulation conditions.

From the simulations of various Th, it is found that the RF signal VRF is greatly affected by the EM waveform parameter Th. Figure 15 shows the sensitivity characteristic in which the coordinate is normalized as VRF/Ep. The value VRF obtained from EMTP decreases to an extremely small inaccurate level in the range Th > 0.8 ns. Therefore, the curves are extended according to the 1.8 GHz component of excitation waveform estimated by Fourier transform, in which the waveform is approximated by the second derivative of Gaussian function.

In Figure 15, the RF output voltage VRF decreases by more than three decades when the EM wave half-amplitude time Th increases from 0.35 to 1 ns. This drastic change in antenna


Figure 15. Antenna RF output as a function of incident EM wave parameterTh for Antenna II, z0=75 Ω.

-1

0

1

-1 0 1 2 3 4 5

(V/m)

incident field Ex(t, 0)

-80

50

-1 0 1 2 3 4 5

(μA)

current sources

i1, i01, i02, i2 =0

-2

4

-1 0 1 2 3 4 5

(mV)

voltage sourcese1 e0 e2

-12

0

12

-1 0 1 2 3 4 5

(mV)

t (ns)

patch voltages v0 V1 V2

VRF

-1

0

1

-1 0 1 2 3 4 5

(V/m)

incident field Ex(t, 0)

-80

50

-1 0 1 2 3 4 5

(μA)

current sources

i1 i01 i02 i2

-2

4

-1 0 1 2 3 4 5

(mV)

voltage sourcese1 e0 e2

-12

0

12

-1 0 1 2 3 4 5

(mV)

t (ns)

patch voltages v0 V1 V2

VRF

(a) Antenna position: face-on A (b) Antenna position: side-on A

Figure 14. Simulation results of Antenna II’s response to EM wave of Th= 0.35 ns, Ep= 1 V/m for output impedance z0=75 Ω.

filter/ rectifier Vout (dc) VRF (RF)

patch-antenna PD sensor

output

EM field Th, Eｐ

parameters: Q･d, Th

PD D

Figure 16. Patch-antenna PD detection system.

sensitivity with half-amplitude time Th is the consequence of the narrow band characteristic of patch antenna as shown in Figure 8.

4 DETECTION OF PD The PD detection system using patch antenna, or patch-

antenna PD sensor is constructed as shown in Figure16 [16, 19]. The device yields dc output Vout according to the RF signal VRF through a band-pass filter and rectifier. Here, the

sensitivity to PD of this system is discussed based on the analytical results of foregoing sections. Lastly, an experimental result is shown for verification.

4.1 ANTENNA SENSITIVITY TO PD As shown in Section 2, PD can be modeled by the electrical

dipole Δp or Q·d which changes polarity in time Th. The incident EM field Ep at the distance D is expressed by equation (9). For a given value of Th, Ep is proportional to Δp/D or Q·d/D. Since the value VRF/Ep is determined by Figure 15, the proportional relation between VRF and Q·d/D can be obtained once the antenna conditions are fixed. This is the antenna sensitivity for a given Th.

Figure 17 shows the antenna sensitivity for the case that the EM wave arrives at Antenna II in ‘face-on A’ position. Five values of Th in the range 0.35–1.15 ns are chosen. This is a simple chart showing the sensitivity of Antenna II to PDs in various conditions. Here again, it is noticed that the sensitivity of patch antenna is strongly dependent on the parameter Th. The dotted line shows the estimated noise level in the experiment shown below.


noise level

Figure 19. PD sensor output voltage levels for 8 antenna positions. The distance from discharge is D=0.5 m.

monitoring electrode potentials

Vout

discharge

sensor output (200mV/div)

noise level 400ns

Figure 18. PD sensor output waveform when it is illuminated by a discharge in ‘face-on A’ position at distance D=0.5 m. Upper two traces show potentials of probe electrodes.

Figure 17. Normalized chart of antenna sensitivity showing relation betweenRF output and PD parameters for Antenna II, ‘face-on A’ position, z0=75 Ω. Dot and dotted line are from experiment.

4.2 EXPERIMENT USING SHORT-GAP DISCHARGE A discharge between insulated two sphere electrodes of 20

mm diameter with 1 mm gap is used as a substitute for PD [40]. This can be seen equivalent to a gap-discharge type PD of Figure 2b. The following parameters are estimated from the dimensions and preliminary experiments:

mmpC10537)(

pC5490mm713pF371kV44

0

⋅×=Δ=⋅

====Δ

.pdQ

,Q,.d,.C,V

The parameter Th is not measured, but thought to be nanosecond range consulting the literature data in Table 1.

The patch-antenna PD sensor commercially available [16, 19] is used in the experiment. The patch-antenna construction is similar to Antenna II in Table 2. Therefore, the simulation results described in Section 3.4 including Figures 14-17 are assumed to be applicable.

In the experiment, the final output Vout in Figure 16 is recorded together with the electrode potentials monitored by adjacent ‘probe electrodes’. Figure 18 shows the waveforms observed when the sensor is illuminated in ‘face-on A’ position at the distance D=0.5 m. The time delay of about 150 ns between the source discharge and sensor output signal is probably due to the delay in the electronic circuit which includes not only a band-pass filter but also a logarithmic amplifier. Since the output noise level is well-defined, it is possible to identify the signal and determine the value Vout as indicated in Figure 18.

Similar observations are repeated 10 times for each of the 8 antenna positions cited in Figure 10. The average values of Vout are summarized in Figure 19. As expected from the result of Figure 14, the output level of ‘face-on A’ is highest followed by that of ‘side-on A’. Smaller outputs are observed in other cases, although they should be zero in the simplified theory in Section 3.2.

Dependence of output voltage on source distance is examined to check the correspondence between analytical and experimental. The antenna RF output is estimated from dc output Vout using the rectifier system’s input/output characteristic obtained in continuous wave condition. Figure 20 shows the relation between thus estimated VRF and the distance D obtained from ‘face-on A’ experiment. The dBm value of VRF is for the output impedance of 75 Ω. In theory, VRF is inversely proportional to D, therefore, the relation should be expressed by a declining straight line as shown in the log-log plot of Figure 20. This tendency is barely identified in the experimental curve.

Those correspondences seen between the experimental results of Figures 16 and 17 and the analytical deductions support the credibility of the present method.


-95

-90

-85

-80

-75

-70

-65

0.1 1

estim

ated

VRF

(dBm

)

D (m)

experimental noise leveltheoretical

0.2 0.3 0.5 (log10 D)

Figure 20. Estimated antenna output VRF as a function of distance Dcomparing with the theoretical relation (position: face-on A).

5 CONCLUDING REMARKS A simple dipole model is found to be useful in describing

the EM fields PD generates. The Gaussian current assumption is convenient since the pulse EM fields are expressed by analytical functions. A simple transmission-line model is proposed for simulating the patch antenna response to PD-generated EM fields. The current and voltage sources are calculated using the analytical functions defined by EM theory. The physical models will contribute to the better understanding of the pulse EM wave generation by PD and its detection by patch antenna.

The simulation of the transmission-line model is successfully executed using EMTP. The results are summed up in the normalized chart (Figure 17), by which the sensitivity of patch antenna to PD can be estimated in various conditions. The sensitivity greatly depends on the PD current’s half-amplitude width Th or how fast PD develops. Faster the PD, higher the sensitivity.

The experiment using a patch-antenna PD sensor shows the tendencies predicted by analysis about the effect of antenna positions and the observation distance. However, the experimental result in Section 4.2 may give an impression that the sensitivity of PD sensor is not high enough for the discharge of mmpC10537 4 ⋅×=⋅ .dQ . The value of VRF=−75.3 dBm for D=0.5 in Figure 17 corresponds to 0.065 mV in 0-peak value, which is plotted as a dot in Figure 17. Thus, the half-amplitude width Th may be in the range 0.85 - 1 ns for the short gap used in experiment. The sensitivity is expected to become higher for discharges with smaller Th or faster PDs.

In the conventional PD measurement, apparent charge is regarded as standard index [2]. There is a naive hope that the outputs of new type PD sensors are to be calibrated by the apparent charge [8] (for UHF sensor), [17] (for patch antenna). However, either in UHF sensor or in patch antenna, the calibration will be difficult because their sensitivity varies with the PD waveforms due to the inherent resonance characteristics. The calibrating the patch-antenna PD sensor by apparent charge will be particularly difficult since its frequency characteristic is of the narrow frequency bandwidth as shown in Figure 8.

As pointed out earlier, the sensitivity of patch antenna increases in the case PD develops fast. The PD occurring at inter-turn of inverter-fed motor windings may be the example of fast PD, considering that the PD’s spectrum is observed to extend to GHz range [41]. The patch-antenna PD sensor is expected to be well-suited to detect this kind of inner PDs, which are normally difficult to be detected by the conventional PD measurement due to reduced coupling.

ACKNOWLEDGMENT The authors wish to express gratitude to Mr. K. Umezu of

ECG Electronic Co. who first poised the question ‘how a patch antenna behaves in the EM wave from PDs’. That has lead to the present study. They are also grateful to Professors T. Kawamura and Y. Murooka, (Prof. Emeriti of Tokyo University, and National Defense Academy of Japan, respectively), for their useful advices and constant encouragements given during the research. Their thanks are extended to Mr. K. Kasajima and other students in Shibaura Institute of Technology who have joined the discussions.

REFERENCES [1] F. H. Kreuger, Paratial discharge detection in high voltage equipment,

Butterworths, London, 1989. [2] International Electrotechnical Commission, “High-voltage test

techniques – Partial discharge measurements”, IEC Standard 60270, 3rd ed., 2000.

[3] W. E. Pakala, and V. L. Chartier, “Radio noise measurements on overhead power lines from 2.4 to 800 KV”, IEEE Trans. Power App. Syst., Vol. 90, pp. 1155-1165, 1971.

[4] K. Arai, W. Janischewskyj, and N. Miguchi, “Micro-gap discharge phenomena and television interference”, IEEE Trans. Power App. Syst., Vol. 104, pp. 220-232, 1985.

[5] S. A. Boggs, G. L. Ford and R. C. Madge, "Coupling devices for the detection of partial discharges in gas-insulated switchgear", IEEE Trans. Power App. Syst., Vol. 100, pp. 3969-3973, 1981.

[6] B. F. Hampton, and R. J. Meats, "Diagnostic measurements at UHF in gas insulated substations", IEE Proc. Generation, Transmission and Distribution, Vol.135, pp. 137-145, 1988.

[7] J. S. Pearson, O. Farish, B. F. Hampton, M. D. Judd, D. Templeton, B. W Pryor, and I. M. Welch, "Partial discharge diagnostics for gas insulated substations", IEEE Trans. Dielectr. Electr. Insul., Vol.2, pp. 893-905, 1995.

[8] R. Kurrer, and K. Feser, "The application of ultra-high-frequency partial discharge measurements to gas-insulated substations", IEEE Trans. Power Del., Vol. 13, pp. 777-782, 1998.

[9] M. Lauersdorf; and K. Feser, "Detection and suppression of corona discharges during PD-measurement by means of RI-reception", IEEE Intern .Symp. Electr. Insul. (ISEI), pp. 310-313, 1994.

[10] M. Hikita, H.Yamashita, T. Kato, N. Hayakawa, T. Ueda, and H. Okubo, "Electromagnetic spectrum caused by partial discharge in air under AC and DC voltage application", IEEE 4th Int. Conf. on Properties and Applications of Dielectric Materials, Vol.2, pp. 570-573, 1994.

[11] K. R. Carver, and J. W. Mink, "Microstrip Antenna Technology", IEEE Trans. Antennas Propag., Vol. 29, pp. 2-24, 1981.

[12] T. Miyata, and H. Otani, “Microwave sensor for detecting a discharge occurring in an electrical apparatus”, US Patent No.5,726,576, 1998

[13] T. Miyata, and H. Otani, “Microwave sensor for discharge faults”, US Patent No.5,903,159, 1999

[14] Y. Kaneda, M. Yoshimura, H. Muto, E. Iwanaga, and O. Hamamoto, “Monitoring system of partial discharge for high-voltage motors using GHz electromagnetic wave detection”, Annual National Convention of IEE of Japan, 2-029, 2005 (in Japanese).

[15] Tang Ju, Xu Zhongrong, Zhang Xiaoxing, and Sun Caixin, "GIS partial discharge quantitative measurements using UHF microstrip antenna sensors", IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp. 116-119, 2007.


[16] H. Muto, Y. Kaneda, H. Aoki, and O. Hamammoto, "On-line PD monitoring system for rotating machines using narrow band detection of EM wave in GHz range", Intern. Conf. Condition Monitoring and Diagnosis, Beijing, China, pp. 1093-1096, 2008.

[17] T. Choi, J. R. Jung, M. S. Kim, J. B. Kim, and H. S. Lee, "Development of the on-line iPDM system for the PD diagnosis for 25.8kV C-GIS", Intern. Conf. Condition Monitoring and Diagnosis, Beijing, China, pp. 420-423, 2008.

[18] K.-S. Lwin, K.-J. Lim, D.-H. Shin, N.-J. Park, D.-H. Park, K. B. Cho, and H.-D.g Kim, "Off-Line PD diagnosis for stator winding of Rotating Machines using UWB sensors", Intern. Conf. Condition Monitoring and Diagnosis, Beijing, China, pp. 155-158, 2008.

[19] Y. Ohki, "News from Japan", IEEE Electr. Insul. Mag., Vol.25, No.3, pp. 60-61, 2009.

[20] L. Niemeyer, “The physics of partial discharges”, Int. Conf. on Partial Discharge, Leicester, UK, pp. 1–4, 1993.

[21] P. F. Wilson, and M. T. Ma, "Fields radiated by electrostatic discharges", IEEE Trans. Electromagn. Compat., Vol.33, pp. 10-18, 1991.

[22] J. M. Wetzer, and P. C. T. van der Laan, "Prebreakdown currents: Basic interpretation and time-resolved measurements", IEEE Trans. Electr. Insul., Vol.24, pp. 297-308, 1989.

[23] E. H. R. Gaxiola, and J. M. Wetzer, "Partial discharge modelling and measurements on micrometer voids in solid dielectrics", 7th Intern. Conf. Dielectric Materials, Measurements and Applications, pp. 322-325, 1996.

[24] G. C. Stone, H. G. Sedding, N. Fujimoto, and J. M. Braun, "Practical implementation of ultrawideband partial discharge detectors", IEEE Trans. Dielectr. Electr. Insul, Vol.27, pp. 70-81, 1992.

[25] T. R. Blackburn, Z. Liu, B. T. Phung, and R. Morrow, "Investigation of partial discharge mechanisms in a void under AC electric stress", 11th Intern. Symp. High Voltage Engineering, Vol. 4, pp. 364-368, 1999.

[26] R. Baumgartner, B. Fruth, W. Lanz, and K. Pettersson, "Partial discharge. X. PD in gas-insulated substations-measurement and practical considerations", IEEE Electr. Insul. Mag., Vol. 8, No.1, pp. 16-27, 1992.

[27] H. Okubo, N. Hayakawa, and A. Matsushita, "The relationship between partial discharge current pulse waveforms and physical mechanisms", IEEE Electr. Insul. Mag., Vol.18, No. 3, pp. 38-45, 2002.

[28] A. J. Reid, M. D. Judd, B. G. Stewart, and R. A. Fouracre, "Partial discharge current pulses in SF6 and the effect of superposition of their radiometric measurement", J. Phys. D: Appl. Phys., Vol. 39, pp .4167-4177, 2006.

[29] M. D. Judd, and O. Farish, "High bandwidth measurement of partial discharge current pulses", IEEE Intern. Sympos. Electr. Insul. (ISEI), Vol.2, pp.436 - 439, 1998.

[30] H. Tomita, “Effect of charge in the time of charging voltage on spark discharge from a metal disk”, IEE Japan Trans. Fundamentals and Materials, Vol.128, No.9, pp. 577-584, 2008 (in Japanese).

[31] K. Kawamata, S. Minegishi, A. Haga, and O. Fujiwara, “Measurement of radiated electromagnetic field strength sue to Microgap discharge in voltage below 1 kV”, Trans. Institute of Electronics, Information and Communication Engineers Vol.J92-B, No.2, pp. 506-508, 2009 (in Japanese).

[32] S. A. Boggs, and G. C. Stone, "Fundamental limitations in the measurement of corona and partial discharge", IEEE Electr. Insul. Mag., Vol.17, No.2, pp. 143 - 150, 1982.

[33] J. S. Stratton, Electromagnetic theory, IEEE Press, John Wiley & Sons, 2007 (Originally, McGraw-Hill, 1941).

[34] G. Garg, P. Bhartia, and I. Bahl, Microstrip antenna design handbook, Artec House, 2001.

[35] H. A. Wheeler, "Transmission-line properties of parallel strips separated by a dielectric sheet", IEEE Trans. Microw. Theory Tech., Vol.13, pp. 172–185, 1965.

[36] A. Derneryd, "Linearly polarized microstrip antennas", IEEE Trans. Antennas Propag., Vol.24, pp. 846 - 851, 1976.

[37] J.-P. Seaux, A. Reineix, B. Jecko, and J. H. Hamelin, "Transient analysis of a space-borne microstrip patch antenna illuminated by an electromagnetic pulse", IEEE Trans. Electromagn. Compat., Vol. 33, pp .224 - 233, 1991.

[38] E. Roubine, and J. C. Bolomey (Trans. Meg Sanders), Antennas Volume 1, North Oxford Academic Publishers, 1987.

[39] L. Prikler, and H. K. Høidalen, “ATPDRAW version 3.5 Users' Manual”, SINTEF Energy, 2002.

[40] M. Tanaka, S. Matsumoto, Y. Shibuya, Y. Iwase, and K. Kasajima, “Net charge calibration of partial discharge simulated by spark gap in air”, Annual Conf. Fundamentals and Materials Society IEE Japan, O-96-II, 2009 (in Japanese).

[41] A. Cavallini, D. Fabiani and G. C. Montanari, "A novel method to diagnose PWM-fed induction motors", IEEE Trans. Dielectr. Electr. Insul., Vol.15, pp. 1313-1321, 2008.

Yoshikazu Shibuya was born in 1941. He graduated from Kyoto University in 1964. He received the MS degree from Kyoto University and the Ph.D. degree from the University of Salford, UK, in 1966 and 1974, respectively. He was with Central Research Laboratory, Mitsubishi Electric Corporation from 1966 to 1999. His field is insulation techniques relating to high voltage power apparatuses such as GIS and transformer. He was a professor at Shibaura Institute of Technology from 1999 to 2008, and is

now Professor Emeritus. Dr Shibuya is a fellow of IET and a member of IEE of Japan.

Satoshi Matsumoto (S’81-M’84-SM’02) was born in Tochigi, Japan in 1955 He received the MS and the Ph.D. degrees from the University of Tokyo in 1981 and 1984, respectively. He was with Heavy Apparatus Engineering Laboratory of Toshiba Corporation from 1984 to 2007. His research interests are high voltage insulation and high voltage testing. He was a visiting professor at Kyushu Institute of Technology from 1999 to 2003. He is a professor at Shibaura Institute of

Technology since 2003. Dr. Matsumoto is a senior member of IEE of Japan

Masayoshi Tanaka was born in 1984. He graduated from Shibaura Institute of Technology in 2008. He is pursuing postgraduate degree at the Graduate School of Engineering, Shibaura Institute of Technology from 2008. He is a member of IEE of Japan.

Hirotaka Muto was born on 6 February 1961. He received the B.S., M.S. and doctoral degrees in electrical engineering from Nagoya University. Currently, he is a manager at Advanced Technology R&D center, Mitsubishi Electric Corporation, Japan, where he has been since 1985. He has been engaged in development of insulation technology for various high voltage apparatus such as GIS, power transformer and motor.

Yoshiharu Kaneda was born in 1947. He has been active in the field of electrical insulation research for about 30 years. Since joining Mitsubishi Electric Corporation in 1962, He has been engaged in diagnostic testing of electrical insulation for high voltage rotating machines and the development of several on-line diagnostic systems. He is a member of IEE of Japan.

IEEE !lkansactions on Dielectrics and Electrical Insulation Vol. 1 No. 5, October 1994 781

REFERENCES M. G. Danikas, “The Definitions Used for Partial Discharge Phenomena” , IEEE Trans. Elec. Insul.,

B. H. Ward, “Digital Techniques for Partial Dis- charge Measurements-A Report on the Activities of the Working Group on Digital Analysis of Par- tial Discharges”, IEEE Trans. Power Delivery, Vol.

M. Cernak, T. Kaneda and T. Hosokawa, “First Negative Corona Pulses in 70% N2 + 30% SF6 Mix- ture”, Japanese J. Appl. Phys., Vol. 28, pp.

R. Bartnikas and J. P. Novak, “On the Character of Different Forms of Partial Discharge and their Related Terminologies”, IEEE Trans. Elec. Insul.,

T . V. Blalock, A. L. Wintenberg and M. 0. Pace, “Low Noise Wide-band Amplification System for Acquiring Prebreakdown Current Pulses in Liquid Dielectrics”, IEEE Trans. Elec. Insul., Vol. 24,

F. H. Kreuger, E. Gulski and A. Krivda, “Classi- fication of Partial Discharges”, IEEE Trans. Elec. Insul., Vol. 28, pp. 917-931, 1993. T . Okamoto and T. Tanaka, “Novel Partial Dis- charge Measurement Computer-aided Measure- ment Systems”, IEEE Trans. Elec. Insul., Vol.

B. Fruth and L. Niemeyer, “The Importance of Sta- tistical Characteristics of Partial Discharge Data”, IEEE Trans. Elec. Insul., Vol. 27, pp. 60-69, 1992. R. J . Van Brunt and S. V. Kulkarni, “Method for Measuring the Stochastic Properties of Corona and Partial-discharge Pulses”, Rev. Sci. Instrum., Vol.

R. J . Van Brunt and E. W. Cernyar, “System for Measuring Conditional Amplitude, Phase, or Time Distributions of Pulsating Phenomena”, J . Res. Nat. Inst. Stand. and Tech., Vol. 97, pp. 635- 672, 1992. V. Kopf and K. Feser, “Possibilities to Improve the Sensitivity of PD-Measurements by Using Digital Filters” , Proc. Int. Symp. on Digital Techniques in HV Measurements, Toronto, Canada, pp. 2-27-

R. Morrow, “Theory of Negative Corona in Oxy- gen”, Phys. Rev. A, Vol. 32, pp. 1799-1809, 1985.

Vol. 28, pp. 1075-1081, 1993.

7, pp. 469-479, 1992.

1989-1996, 1989.

Vol. 28, pp. 956-968, 1993.

pp. 641-647, 1989.

21, pp. 1015-1016, 1986.

60, pp. 3012-3023, 1989.

2-31, 1991.

[13] R. J . Van Brunt and E. W. Cernyar, “Stochas- tic Analysis of ac-generated Partial-discharge Puls- es from a Monte-Carlo Simulation”, 1992 Annu- al Report-Conference on Electrical Insulation and Dielectric Phenomena, IEEE, NY, pp. 427-434, 1992.

[14] R. J. Van Brunt, E. W. Cernyar and P. von Glahn, “Importance of Unraveling Memory Propagation Effects in Interpreting Data on Partial Discharge Statistics”, IEEE Trans. Elec. Insul., Vol. 28,

[15] R. J . Van Brunt, “Production Rates for Oxyfluo- rides SOF2, S02F2, and SOF4 in SF6 Corona Dis- charges”, J . Res. Nat. Bur. Stand., Vol. 90, pp.

[16] F. Y. Chu, “SF6 Decomposition in Gas-insulated Equipment”, IEEE Trans. Elec. Insul., Vol. 21,

[17] H. Slowikowska, T. Las,, J . Slowikowski and R. J. Van brunt, “Modification of Cast Epoxy Resin, Surfaces During Exposure to Partial Discharge”, Proc. 7th Int. Symp. on Gaseous Dielectrics, Plenum, NY (in press, 1994)

[18] R. J. Van Brunt, P. von Glahn and T . Las, “Non- stationary Behavior of Partial Discharge During Discharge-induced Aging of Dielectrics”, IEE Proc. -Sci., Meas., Tech., (in press, 1994).

[19] C. Hudon, R. Bartnikas and M. R. Wertheimer, “Spark-to-glow Discharge Transition Due to In- creased Surface Conductivity on Epoxy Resin Specimens”, IEEE Trans. Elec. Insul., Vol. 28,

[20] N. Foulon-Belkacemi, M. -P. Panaget, M. Goldman and A. Goldman, “Influence of Electrode Materials and Coverings on the Gaseous By-products Gener- ated by Air Corona Discharges”, Proc. 7th Int. Symp. on Gaseous Dielectrics, Plenum, NY (in press, 1994)

[21] J . P. Borra, A. Goldman and M. Goldman, “Gen- eration of Condensation Nuclei by dc Electrical Discharges in Point-to-plane Configuration”, J . Aerosol Sci. (in press, 1994).

[22] C. Hudon, R. Bartnikas and M. R. Wertheimer, “Analysis of Degradation Products on Epoxy Surfaces Subjected to Pulse and Glow-type Dis- charge”, 1991 Annual Report-Conference on Elec- trical Insulation and Dielectric Phenomena, IEEE,

[23] T. Okamoto and T. Tanaka, “Prediction of Tree- ing Breakdown from Pulse Height of Partial Dis- charge on Voltage-phase Angle”, Japanese J . Appl.

pp. 905-916, 1993.

229-253, 1985.

pp. 693-726, 1986.

pp. 1-8, 1993.

NY, pp. 237-243, 1991.

Phys., Vol. 24, pp. 156-160, 1985.


[24] J. T. Holboll and M. Henriksen, “Partial Discharge [35] A. Pedersen, “On the Electrical Breakdown of Patterns Related to Surface Deterioration in Voids Gaseous Dielectrics”, IEEE Trans. Elec. Insul., in Epoxy”, Conf. Record - 1990 IEEE Int. Symp. Vol. 24, pp. 721-739, 1989. on Elec. Insul., IEEE publ. CH2727- 6/90, pp.

[25] T . Ito, K. Jogan, T. Saito and Y. Ehara, “Phase Analysis of Discharge Magnitude Distributions In- side a Small Void and Its Application to Diagnosis of Deteriorating Insulations”, 1989 Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, IEEE, NY, pp. 111-116, 1989.

[26] F. Komori, N . Nishiguchi, M. Hikita and T . Mizu- tani, “Detection of Electrical Tree Initiation and Assessment of Shape of the Developing Tree Using Pattern Recognition of Partial Discharge Occur- rence Phase Distribution”, IEE Conference Pro- ceedings No. 378, pp. 68-69, 1993; “Construction of Expert System Prototype for Degradation Di- agnosis and Measurement System Using Personal Computer: Application for Degradation Diagnosis of PD Statistical Parameter Using Phase Informa- tion”, Trans. IEE Japan, Vol. 113-A, pp. 406-415, 1993.

[27] R. J . Van Brunt, P. von Glahn and T . Las, “Partial Discharge-induced Aging of Cast Epoxies and Re- lated Nonstationary Behavior of the Discharge Sta- tistics”, 1993 Annual Report - Conference on Elec- trical Insulation and Dielectric Phenomena, IEEE,

[28] R. J. Van Brunt, “Stochastic Properties of Partial- discharge Phenomena”, IEEE Trans. Elec. Insul.,

[29] R. Bartnikas, “Detection of Pariial Discharges (Corona) in Electrical Apparatus”, IEEE Trans. Elec. Insul., Vol. 25, pp. 111-123, 1990.

[30] R. S. Sigmond, “Corona Discharges”, inElectrical Breakdown in Gases, ed. J . M. Meek and J . D. Craggs, John Wiley and Sons, New York, pp. 319- 384, 1978.

115-119, 1990.

NY, pp. 455-461, 1993.

Vol. 26, pp. 902-948, 1991.

[36] G. W. Trichel, “Mechanism of the Negative Point- to-plane Corona Near Onset”, Phys. Rev., Vol.

[37] M. Cernak and T. Hosokawa, “Initial Stages of Negative Point-to-plane Breakdown in Helium”, Japanese J . Appl. Phys., Vol. 26, pp. L1721- L1723, 1987.

[38] R. J. Van Brunt and M. Misakian, “Mechanisms for Inception of dc and 6O-Ha AC Corona in SFs”, IEEE Trans. Elec. Insul., Vol. 17, pp. 106-120, 1982.

54, pp. 1078-1084, 1938.

[39] B. Eliasson and V. Kogelschatz, “Modeling and Applications of Silent Discharge Plasmas” , IEEE Trans. Plasma Sci., Vol. 19, pp. 309-323, 1991.

[40] R. Bartnikas and J . P. Novak, “On the Spark to Pseudoglow and Glow Transition Mechanism and Discharge Detectability”, IEEE Trans. Elec. In-

[41] M. G. Danikas, R. Bartnikas and J. P. Novak, “Discussion-on the Spark to Pseudoglow and Glow Transition Mechanism and Discharge Detectabili- ty”, IEEE Trans. Elec. Insul., Vol. 28, pp. 429- 431, 1993.

Sd., Vol. 27, pp. 3-14, 1992.

[42] Y. Kondo and Y. Miyoshi, “Pulseless Corona in Negative Point to Plane Gap”, Japanese J. Appl.

[43] D. B. Ogle and G. A. Woolsey, “Current Pulses in SF6 Glow Discharges”, J . Phys. D: Appl. Phys.,

[44] T . Ishida, Y. Mizumo, N. Nagao and M. Kosa- ki, “Computer Aided Partial Discharge Analyz- ing System for Detection of Swarming Pulsive Mi- crodischarges” , IEE Conference Proceedings No.

Phys., Vol. 17, pp. 643-649, 1978.

Vol. 22, pp. 1829-1834, 1989.

378, pp. 99-100, 1993. [31] M. Golch-” and A. Goldman, “Corona Dis-

charges”, in Gaseous Electronics, Vol. 1, Academic [45] S. K. Dhali and P. F. Williams, “Two-dimensional

Studies of Streamers in Gases”, J . Appl. Phys., Vol. Press, NY, pp. 219-290, 1978. 62, pp. 4696-4707, 1987.

[32] J . H. Mason, “Discharges”, IEEE Trans. Elec. In-

[33] J. C. Devins, “The Physics of Partial Discharges in Solid Dielectrics”, IEEE Trans. Elec. Insul., Vol.

[34] R. Bartnikas, “A Commentary on Partial Dis- charge Measurement and Detection”, IEEE Trans. Elec. Insul., Vol. 22, pp. 629-633, 1987.

d . , Vol. 13, pp. 211-238, 1978.

19, pp. 475-495, 1984.

[46] J . Liu and G. R. Govinda Raj,, “Streamer Forma- tion and Monte-Carlo Space-charge Field Calcula- tion in SFe”, IEEE Trans. Elec. Insul., Vol. 28, pp.

[47] R. J . Van Brunt and J . T . Herron, “Fundamental Process of SF6 Decomposition and Oxidation in Glow and Corona Discharges”, IEEE Trans. Elec. Insul., Vol. 25, pp. 75-94, 1990.

261-270, 1993.

IEEE !Zkansactions on Dielectrics and Electrical Insulation Vol. 1 No. 5, October 1994 783

[48] R. J . Van Brunt and J . T . Herron, “Plasma Chem- ical Model for Decomposition of SF6 in a Negative Glow Corona”, Physica Scripta, Vol. 50 in press,

[63] R. J . Van Brunt, “Common Parameterizations of Electron Transport, Collision Cross Section, and Dielectric Strength Data for Binary Gas Mixtures”,

1994. J. Appl. Phys., Vol. 61, pp. 1773-1787, 1987.

[49] R. Morrow, “Theory of Stepped Pulses in Negative Corona Discharges”, Phys. Rev. A, Vol. 32, pp.

[50] W. Egli and B. Eliasson, “Numerical Calculation of Electrical Breakdown in Oxygen in a Dielectric- barrier Discharge”, Helvetica Physica Acta, Vol.

[51] J . Liu and G. R. Govinda Raju, “Simulation of Corona Discharge. Negative Corona in SFe”, IEEE Trans. Diel. Elec. Insul., Vol. 1, pp. 520-529, 1994.

[52] B. Gravendeel, F. J. De Hoog and M. A. M. Schoenmakers, “Fast Photon Counting in Nega- tive Corona Discharges in the Trichel Regime”, J .

[53] D. A. Scott and G. N. Haddad, “Negative Corona in Nitrogen-oxygen Mixtures”, J . Phys. D: Appl.

3821-3824, 1985.

62, pp. 302-305, 1989.

Phys. D: Appl. Phys., Vol. 21, pp. 744-755, 1988.

Phys., Vol. 20, pp. 1039-1044, 1987.

[64] T. Taniguchi, H. Tagashira and Y. Sakai, “Validity of a Boltzmann Equation Method for Predicting the Effect of Ionization and Attachment”, J. Phys.

[65] B. C. O’Neill and J . D. Craggs, “Collisional De- tachment of Electrons and Ion-molecule Reactions in Oxygen”, J. Phys. B: Atom. Molec. Phys., Vol.

[66] C. Doussot, F. Bastien, E. Marode and J . L. Moruzzi, “A New Technique for Studying Ion Conversion and Detachment Reactions in Oxygen and in O z / S 0 2 and 0 2 / N z Mixtures”, J . Phys. D:

[67] R. J . Van Brunt and S. V. Kulkarni, “Stochas- tic Properties of Trichel-Pulse Corona: A Non- markovian Random Point Process”, Phys. Rev.

D: Appl. Phys., Vol. 10, pp. 2301-2306, 1977.

6, pp. 2625-2633, 1973.

Appl. Phys., Vol. 16, pp. 2451-2461, 1982.

A, Vol. 42, pp. 4908-4932, 1990.

[54] J . B. Whitehead, Lectures on Dielectric Theory and Insulation, Mcgraw-hill, New York, pp. 10-12, 1927.

[55] I. W. McAllister, “Electric Fields Associated with Transient Surface Currents”, J . Appl. Phys., Vol. 71, pp. 3633-3635, 1992.

[56] I. Somerville and P. Vildand, “Surface Spreading of Charge due to Ohmic Conduction”, Proc. Royal Soc. London, Vol. A399, pp. 277-293, 1985.

[57] G. H. Wannier, Statistical Physics, Wiley, New York, 1966. Editore, Padova, pp. 227-230, 1988.

[68] J . P. Steiner, Digital Measurement of Partial Dis- charge, PhD Thesis, Purdue University, 1988.

[69] W. L. Lama and C. F. Gallo, “Systematic Study of the Electrical Characteristics of The ‘Trichel’ Cur- rent Pulses from Negative Needle-to-plane Coro- nas”, J. Appl. Phys., Vol. 45, pp. 103-113, 1974.

[701 s. v. Kulkarni and R. J. Van Brunt, UStochas- tic Properties of Negative Corona Trichel Pulses in SFG/Oz Mixtures”, Proc. 9th Int. Conf. on Gas Discharges and Their Applications, Benetton

[58] E. E. Kunhardt, Y. Tzeng and J . P. Boeuf, “Sto- chastic Development of an Electron Avalanche”, Phys. Rev. A, Vol. 34, pp. 440-449, 1986.

[59] E. E. Kunhardt, “Electron Macrokinetics in Par- tially Ionized Gases: The Hydrodynamic Regime”, Phys. Rev. A, Vol. 42, pp. 803-814, 1990.

[60] N. J . Carron, “Energy-diffusion Calculation of the Electron-swarm Distribution Function”, Phys. Rev. A, Vol. 45, pp. 2499-2511, 1992.

[71] R. J. Van Brunt, K. L. Stricklett, J . P. Steiner and S. V. Kulkarni, “Recent Advances in Partial Dis- charge Measurement Capabilities a t NET”, IEEE Trans. Elec. Insul., Vol. 27, pp. 114-129, 1992.

[72] R. J . Van Brunt, M. Misakian, S. V. Kulkarni and V. K. Lakdawala, “Influence of a Dielectric Barrier on the Stochastic Behavior of Trichel-Pulse Coro- na”, IEEE Trans. Elec. Insul., Vol. 26, pp. 405- 415, 1991.

[61] A. V. Phelps and R. J . Van Brunt, “Electron- 1731 R. J. Van Brunt and E. W. Cernyar, “Influence of transport, Ionization, Attachment, and Dissocia- Memory Propagation on Phase-Resolved Stochas- tion Coefficients in SFe and Its Mixtures”, J . A p tic Behavior of ac-generated Partial Discharges”, pl. Phys., Vol. 64, pp. 4269-4277, 1988. Appl. Phys. Lett., Vol. 58, pp. 2628-2630, 1991.

[62] M. F. Frechette and J. P. Novak, “Boltzmann- [74] C. G. Jenkinson and J. P. Reynders, “Partial Dis- Equation Analysis of Electron Transport Proper- charge Measurements Using Tailored Excitation ties in CC12FZ/SFs/Nz Gas Mixtures”, J . Phys. Waveforms”, IEEE Trans. Elec. Insul., Vol. 28, D: Appl. Phys., Vol. 20, pp. 438-443, 1987. pp. 1068-1074, 1993.


[75] N. Hozumi, T . Okamoto and H. Fukagawa, “Simul- taneous Measurement of Microscopic Image and Discharge Pulses a t the Moment of Electrical Tree Initiation”, Japanese J . Appl. Phys., Vol. 27, pp.

[76] R. J . Van Brunt, E. W. Cernyar, P. von Glahn and T. Las, “Variations in the Stochastic Behavior of Partial-discharge Pulses with Point-to-dielectric Gap Spacing”, Conf. Record-1992 IEEE Int. Symp. on Elec. Insul., IEEE publ. 92 CH3150-0,

(771 E. C. Rogers, “The Self-extinction of Gaseous Dis- cha.rges in Cavities in Dielectrics”, Proc. IEE, Vol.

[78] P. von Glahn and R. J. Van Brunt, “Performance Evaluation of a New Digital Partial Discharge Recording and Analysis System”, Conf. Record- 1994 IEEE Int. Symp. on Elec. Insul., IEEE

[79] R. J . Van Brunt and D. A. Leep, “Corona-induced Decomposition of SFs”, Gaseous Dielectrics 111, Proc. 3rd Int. Symp. on Gaseous Dielectrics, ed. L. G. Christophorou, Pergamon Press, NY,

[80] R. J . Van Brunt a.nd D. A. Leep, “Characterization of Point-plane Corona Pulses in SFs”, J. Appl. Phys., Vol. 52, pp. 6588-6600, 1981.

[81] R. J . Van Brunt, “Effects of HzO on the Behavior of SF6 Corona”, Proc. 7th Int. Conf. on Gas Dis- charges and Their Applications, Peter Peregrinas,

[82) R. J . Van Brunt, “Water Vapor-enhanced Electron- avalanche Growth in SF6 for Nonuniform Fields”,

[83] J. K. Olthoff, R. J . Van Brunt, Y. Wang, R. L. Champion and L. D. Doverspike, “Collisional Elec- tron Detachment and Decomposition Rates of SF, , SF,, and F- in SF6: Implications for Ion Trans- port and Electrical Discharges”, J . Chem. Phys.,

572-576, 1988.

pp. 349-353, 1992.

104 A, pp. 621-630, 1958.

publ. 94 CH3445-4, pp. 12-16, 1994.

pp. 402-409, 1982.

pp. 255-258, 1982.

J. Appl. Phys., Vol. 59, pp. 2314-2323, 1986.

Vol. 91, pp. 2261-2268, 1989.

SF6 Corona. Part 11: Influence of Water and SF6 Neutral By-products”, J . Phys. D: Appl. Phys.,

[85] E. Gulski and A. Krivda, “Neural Networks as a Tool for Recognition of Partial Discharges”, IEEE Trans. Elec. Insul., Vol. 28, pp. 984-1001, 1993.

[86] N. Hozumi, T. Okamoto and T. Imajo, “Discrimi- nation of Partial Discharge Patterns Using a Neu- ral Network”, IEEE Trans. Elec. Insul., Vol. 27,

[87] A. A. Mazroua, R. Bartnikas and M. M. A. Sala- ma, “Discrimination Between PD Pulse Shapes Us- ing Different Neural Network Paradigms”, IEEE Trans. Diel. Elec. Insul. (in press, 1994)

[88] M. Piemontesi, R. Pietsch and W. Zaengl, “Analy- sis of Decomposition Products of Sulfur Hexafluo- ride in Negative dc Corona with Special Emphasis on Content of H2O and 0 2 ” , Conf. Record-1994 IEEE Int. Symp. on Elec. Insul., IEEE publ. 94

1891 A. M. Casanovas, J . Casanovas, F. Lagerde and A. Belarbi, “Study of the Decomposition of SFtj under the Negative Polarity Corona Discharge (Point-to- plane Geometry): Influence of the Metal Consti- tuting the Plane Electrode”, J. Appl. Phys., Vol.

[go] R. J . Van Brunt, J . K. Olthoff and M. Shah, “Rate of SzFlo Production from Negative Corona in Compressed SFs”, Conf. Record-1992 IEEE Int. Symp. on Elec. Insul., IEEE publ. 92 CH3150-0,

[91] M. C. Siddagangappa, R. J . Van Brunt and A. V. Phelps, “Influence of Oxygen on the Decomposi- tion Rate of SF6 in Corona”, Conf. Record-1986 IEEE Int. Symp. on Elec. Insul., IEEE publ. 86

[92] J. B. Whitehead, “The Electric Strength of Air”, Trans. AIEE, Vol. 29, pp. 1159-1187, 1910; “The Electric Strength of Air II”, Trans. AIEE, Vol. 30,

Vol. 25, pp. 774-782, 1992.

pp. 550-555, 1992.

CH3445-4, pp. 499-503, 1994.

72, pp. 3344-3354, 1992.

pp. 328-331, 1992.

CH2196-4-DEI, pp. 225-229, 1986.

pp. 1857-1965, 1911.

[84] I. Sauers and G. Harman, “A Mass Spectometric Study of Positive and Negative Ion Formation in an

Manuscript was received on 22 July 1994.

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