design of systems of covered overhead conductors by means of electric field calculation

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005 807

Design of Systems of Covered Overhead Conductorsby Means of Electric Field Calculation

Joze Pihler, Member, IEEE, and Igor Ticar, Member, IEEE

Abstract—Recently, in medium voltage overhead line systems,previously bare conductors are being replaced with conductorscovered with a layer of insulation. Covered conductors enablean economical increase in operational reliability. At the currentstage of development, there are several ways of replacing bareconductors with covered ones. Such a replacement also requireschanging the accompanying line accessories, insulators, and lineprotection devices.

The selection of the optimum combination of covered conduc-tors and equipment has been done by means of electric field cal-culations. The optimization parameters have included the use ofexisting equipment to the maximum possible extent, new elementsof equipment that have to be added, and the prevention of distur-bances and failures in operation. Theoretical findings have beenconfirmed with laboratory tests on prototype combinations of cov-ered conductors and the accompanying equipment.

Index Terms—Covered conductor, electric field, finite elementmethod, medium voltage overhead line, partial discharges.

I. INTRODUCTION

I N THE early 1960s, utilities in the USA began replacingbare conductors in medium voltage, overhead distribution

networks with covered ones. This was also being done at almostthe same time in Australia. In the area where a conductor isfixed to the insulator, it was necessary to remove insulation fromthe conductor, which caused very intensive corrosion. This facteventually led to termination of their use.

In the early 1970s, covered conductors became attractiveagain due to solutions to the corrosion problem as well as otherproblems. Some Nordic countries (Finland and Sweden) wereearly users. Nowadays such conductors are used in CentralEurope. It has been found that the covered conductors are lowerin long-term cost than bare conductors because of better oper-ational reliability. As a bonus, the new technology is friendlierto people, flora, fauna, and the entire environment. In spite ofslightly higher capital costs, the overall costs are lower due to asignificantly reduced number of failures.

In each of the countries where these conductors have beenintroduced, a specific method of construction and design ofaccompanying equipment was developed. The users aim atkeeping the existing equipment in use after the replacementof the conductors. This caused certain disturbances in theoperation of the system. Some authors [1], [2], pointed outthese problems as early as the mid 1980s. There are various

Manuscript received February 11, 2003; revised June 23, 2004. Paper no.TPWRD-00060-2003.

The authors are with the Faculty of Electrical Engineering and ComputerScience, University of Maribor, 2000 Maribor, Slovenia (e-mail: [email protected]; [email protected]).

Digital Object Identifier 10.1109/TPWRD.2004.839210

factors mentioned in the analyses of these disturbances, amongwhich the most problematic are radio-frequency disturbances,the burning down of conductors, the phenomenon of audibleand visible corona, etc.

Due to the abundance of steep slopes between high hills anddeep valleys in Central Europe, it was necessary to develop spe-cial suspension clamps for holding the insulated conductor. Thisled to additional problems due to the series connection of dif-ferent dielectric materials. Such connections induce strong elec-tric fields that cause the above mentioned disturbances.

This paper gives the results of research that has been doneby means of electric field calculations on the basis of the fi-nite element method (FEM). Various systems of covered con-ductors have been simulated to establish the causes of possiblefailures and the reason for corona. On the basis of these simula-tions, the optimum combination of covered conductors and ac-companying equipment has been chosen. The results have beenconfirmed with measurements of corona discharges on identicalcombinations of devices.

II. REPRESENTATION OF A SYSTEM OF COVERED

OVERHEAD CONDUCTORS

The insulation material used for the covering (polyethylene,XLPE, etc.), and its thickness (2 to 3 mm) are important.A covered conductor can be affixed using a special clamp[Fig. 1(a), (b), and (c)] that is connected to the insulator(Fig. 3), or can be affixed directly to the insulator hardwareas shown in Fig. 4. The materials of the clamps shown inFig. 1(a) and (b) are either metal or rigid, different typesof insulation materials. The shape of the lower part of bothclamps is the same whether of metal or organic insulation. Theupper part of the insulation clamps may also be as is shownin Fig. 1(c). Whenever the part of the clamp which grabs thecable is made of insulating material, the clamp is designatedas an insulated clamp. The most frequently used insulators areglass pin insulators (SPIs) (Fig. 2) and porcelain pin insulators(PPIs) (Fig. 3). PPIs and SPIs are conventional insulators, alsointended for use in systems with bare wires; thus, their differ-ences are at first sight negligible. In the case of glass insulators,leakage paths of electric current are 20% longer. These twotypes of insulators essentially differ in regard to their electricfield distribution. Porcelain insulators are made of one piece,while glass insulators consist of glass disks, fixed togetherby a cement-based compound. This compound representsanother insulation material with a different dielectric constantthan the basic material. In the cement-based compound or atthe junction between glass and cement-based compound, airholes may appear. Cycloaliphatic and composite insulators are

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808 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

(a)

(b)

(c)

Fig. 1. Front and side views of (a) a representative conductor suspensionclamp, (b) a representative conductor clamp, (c) the upper part of representativeinsulation clamp.

Fig. 2. Glass pin insulator (SPI).

manufactured using the latest technology, and therefore requirea completely new design of accessories for connecting theconductors to insulators.

III. ELECTRIC FIELD CALCULATION

Maxwell’s equations (1)–(4) are the starting point for elec-tric field analysis. These are partial differential equations thatdescribe the relations between fields of the following vectors:(electric field gradient), (electric flux density), (magneticfield strength), (magnetic flux density) and (electric currentdensity). is the scalar of volume density of the charge andis Hamilton’s operator [3].

(1)

(2)

(3)

(4)

Fig. 3. Porcelain pin insulator (PPI) with upper part of clamp of Fig. 1(a).

Problems in electric field that can be described by such a net-work of relationships are nowadays often solved by the FEM.This method is based upon a division of a certain region ofa problem into small subregions, the so-called finite elements.The method is used for solving 2-D and 3-D problems. In theformer case, the finite elements are usually triangular or quad-rangular, while in the latter case tetrahedrons, prisms or hexa-hedral elements are mostly used.

For our FEM calculations, we used the FEM2D computerprogram package, developed at IGTE TU in Graz (Austria). Thispackage enables calculations of 2-D electric and magnetic fieldproblems for rotationally symmetrical and planar cases.

The program requires entering the basic two-dimensionalstructures, boundary conditions, kinds of material and the fieldsources. Due to the use of isoparametric elements, eight nodalelements of second order enable good geometrical substitutionof the actual problem with finite elements.

After the geometrical data of the problem has been entered, itis necessary to proceed to the boundary conditions. Dependingon the nature of the actual problem, it is possible to choosebetween Direchlet’s and Neumann’s boundary conditions. Onthe inner edges of the structure it is also possible to set specialboundary conditions.

In the next step, the material properties that appear in theproblems have to be entered into the program (dielectric con-stant, etc.). It is possible to choose between linear or nonlinear,as well as between isotropic or nonisotropic materials, ac-cording to the nature of the problem to be solved.

Finally the peak value of the line-to-ground power frequencyvoltage (20 kV), or the zero-to-peak value of impulse testingwithstand voltage (125 kV) is entered and the program is run.

The final structure of entered data is a geometrical represen-tation of the optimized element and its environment (Fig. 4).

IV. RESEARCH AND DEVELOPMENT OF A

SYSTEM OF COVERED CONDUCTORS

In certain combinations, a system of covered conductors canbe very problematic, due to the series connection of insulatingmaterials with very different dielectric constants. In such cases,the voltage stress across the material with a lower dielectricconstant can become much higher than the material with ahigher dielectric constant. The above mentioned computer pro-gram has been used for calculating electric field strengths forvarious combinations of conductors, insulators and suspension

PIHLER AND TICAR: DESIGN OF SYSTEMS OF COVERED OVERHEAD CONDUCTORS BY MEANS OF ELECTRIC FIELD CALCULATION 809

Fig. 4. A 24-kV porcelain and SPIs with conductor, clamp, and supporter.

(a) (b) (c)

Fig. 5. Vector directions and regions for determination of electric fieldstrength. (a) Around conductor. (b) Between screws of the clamp. (c) Betweensupporter and conductor.

clamps. On the basis of this value, it is possible to see reasonsfor possible failures, as well as to change the design (shape) ofthe electrodes and geometrical distances between conductiveand insulation parts of the system of covered conductors toobserve the effects on the magnitude and distribution of theelectrical stress. Two 24-kV pin insulators, a glass and a porce-lain one, have been treated. They have been equipped withmetal supporters, and the covered conductor has been affixedusing a metal and insulated clamp (Fig. 4).

In calculating the electric field strength, the area of obser-vation has been limited to the area around the conductor, thearea between screws of the clamp, and the area between theconductor and the supporter (Fig. 5). In these areas, the elec-tric field strength in and directions has been calculated atthe following potentials: 20 kV (peak voltage) and 125 kV (im-pulse testing withstand voltage).

After the data is entered in the preprocessor that enables inputof data on material structure and shape, sources and boundaryconditions, we obtain a model of the conductor, clamp andinsulator.

For the 2-D problem solution, we determine the stress in the xand y directions. The shape has been as close as possible to theactual structure. Such an electric problem can be solved underthe given boundary conditions. The edges of the supporter andclamp have been limited by Direchlet’s boundary conditions,while the edge of the insulator has been limited by Neumann’sboundary conditions.

Fig. 6. Metallized insulated clamps. (a) Insulated clamp with a metaled outeredge; (b) entirely metaled insulated clamp.

Fig. 7. Curve of electric field strength in conductor insulation for porcelaininsulator at 20 kV (peak value).

V. RESULTS OF ELECTRIC FIELD CALCULATIONS

Several simulations of the electric fields have been done withthe elements described in Section IV:

• covered conductor in a metal clamp with porcelain andglass insulator at voltage potentials (peak values) 20 and125 kV;

• covered conductor with defective insulation in the metalclamp with porcelain and glass insulator at voltage poten-tials (peak values) 20 and 125 kV;

• covered conductor in an insulated clamp with porcelainand glass insulator at voltage potentials (peak values) 20and 125 kV;

• covered conductor with defective insulation in the insu-lated clamp with porcelain and glass insulator at voltagepotentials (peak values) 20 and 125 kV.

There is also a new combination of a covered conductor in ametallized insulated clamp, which is suitable for SPIs (Fig. 6).

A. Covered Conductor in a Metal Clamp

The electric field strength in the areas shown in Fig. 5 hasbeen calculated for the porcelain and glass insulator and forthe previously mentioned levels of voltage. The distribution ofthe electric field around the conductor is shown in Fig. 7. The


TABLE IVALUES OF ELECTRIC FIELD STRENGTH AROUND THE

CONDUCTOR IN METAL CLAMP

Fig. 8. Electric field strength in X direction (above the metal supporter) and inY direction (between the conductor and metal supporter) at 20 kV (peak value).

top part of the picture shows the conductor’s geometry andthe electric field strength inside the insulation (the thicknessof insulation is 2.3 mm). The bottom part of the picture showsthe distribution of electric field strength in the direction of the

-axis. The highest magnitudes of electric field strength are atthe conductor’s surface, and it decreases toward the insulationsurface. Both parts of the picture are plotted in actual units ofmeasurement.

The highest values of electric field strength in the insulationof the conductor that is fixed by a metal clamp in a porcelain orglass insulator are given in Table I.

The next area of electric field strength calculation is thearea between the clamps of the conductor and metal supporter[Fig. 5(c)]. The distribution of electric field strength is given inFig. 8.

As is evident in Fig. 8, the values of the electric field strengthin X direction (top part of the picture)—curve “X direction”(bottom part of the picture), and in Y direction (top part of thepicture)—bold curve “Y direction” (bottom part of the picture),are significantly below the values of around the conductor.The electric field strength in the cement compound is four

Fig. 9. Distribution of the electric field and curve of electric field strength Efor the glass insulator and defective conductor insulation in a metal clamp at20 kV (peak value).

times higher than in the glass (curve Y direction). The substan-tial increase of electric field strength on the curve “X direction”is in the area immediately above the edge of the metal supporter.It is a consequence of the numerical computations. Both partsof the picture are plotted in the actual units of measurement.

B. Covered Conductor With a Defect in the ConductorInsulation in the Metal Clamp

The clamping of the covered conductor may be expected toweaken the dielectric both by thinning it and by causing the for-mation of cavities between the inner surface of the covering andthe outer surface of the conductor (Fig. 9). The rate at whichsuch cavities are formed is expected to be accelerated by temper-ature excursions to high levels as may result conductor heatingfrom overloading.

As is seen in Fig. 9, the electric field strength in the area of theair cavity (between the conductor and conductor insulation) istwice as large as that in the nondefective conductor insulation.The length of the cavity (Fig. 9) in the longitudinal directionwas sufficient to maximize the stress in the gap in the radialdirection. The width (circumferential) of the cavity was 6.8 mm,and the depth was 0.32 mm.

The highest values of electric field strength in the air cavityof conductor insulation for the same voltages as in the previouscases are given in Table II.

The electric field strength in the immediate vicinity of themetal supporter and between the conductor and metal supporterin such a case decreases. The electric field strength in the cementcompound is higher, in the system with a glass insulator, than inthe glass.

C. Covered Conductor in Insulated Clamp

It is characteristic for an insulated clamp that electric fieldstrength is lower around the conductor than in the case of the


TABLE IIELECTRIC FIELD STRENGTH IN THE CAVITY OF DEFECTIVE INSULATION OF

CONDUCTOR IN METAL CLAMP

TABLE IIIVALUES OF ELECTRIC FIELD STRENGTH AROUND THE CONDUCTOR IN

THE INSULATED CLAMP

TABLE IVELECTRIC FIELD STRENGTH BETWEEN INSULATED CLAMP

AND METAL SUPPORTER

metal clamp. (With this clamp, the entire electric field is con-centrated on the insulation of the conductor). Thus, with the in-sulated clamp, the stress on the conductor insulation is reduced.In Table III, the highest values of electric field strength aroundthe conductor, achieved in conductor insulation are given. As isobvious from Table III, the difference between the values ofat the clamp for porcelain and glass insulator is much greater,with an insulated clamp, than in the previous cases.

In this case, the value of in the immediate proximity of theinsulated clamp and between the metal supporter and the con-ductor has increased by three orders of magnitude, in compar-ison with the values in the case of a metal clamp. The electricfield strength in the cement compound is twice as high as inglass.

Data from graphical presentation of the maximal magnitudeof the electric field strength in the area between the con-ductor and the metal supporter (Y direction in Fig. 8) is givenin Table IV. In the system with a glass insulator, the values arefive times as high as in the case of the porcelain insulator.

D. Covered Conductor With a Defect in the ConductorInsulation in the Insulated Clamp

Table V shows the biggest magnitudes of electric fieldstrength in the air cavity, as shown in Fig. 9.

TABLE VELECTRIC FIELD STRENGTH IN THE CAVITY OF DEFECTIVE INSULATION OF

CONDUCTOR IN THE INSULATED CLAMP

TABLE VIMAXIMUM MAGNITUDES OF E IN CONDUCTOR INSULATION AND IN AIR

CAVITY BY DEFECTIVE INSULATION OF CONDUCTOR IN THE INSULATED CLAMP

WITH PARTIALLY [FIG. 6(a)] AND ENTIRELY [FIG. 6(b)] METALLIZED EDGE

In this case (Table V), the electric field strength has amountedto only one quarter of the value for the system with the metalclamp (Table II).

The magnitudes of electric field strength between the clampand the metal supporter on one side, and the screws of the clampon the other side, are very similar to those in the case of theinsulated clamp without an air cavity.

E. Covered Conductor in Insulated Clamp With Partially orEntirely Metallized Edge

On the basis of experiences gained during the research ofcombinations of the covered conductor and the accompanyingaccessories, two new combinations have been put together. Theyhave been tested for the most difficult operational situation, i.e.,insulation defect with a glass insulator. The innovation of thesetwo combinations is that the insulated clamp has been partiallyor entirely metallized, as shown in Fig. 6. The magnitudes offor various voltage potentials are given in Table VI. In the caseof the partially metallized insulated clamp, they are the magni-tudes of almost two times lower than in the case of the entirelymetallized clamp. If the insulation of covered conductor (in thearea where the conductor is fixed in to the clamp) is defective(top part of Fig. 9), the electric field strength increases only inthe air cavity.

The calculation of the electric field strength between theconductor and the metal supporter [Fig. 5(c)] has shown values


much lower in the and direction in the case of the entirelymetallized insulated clamp, in comparison to the case of thepartially metallized clamp. But in both cases are still below3 MV/m. In the system with a defect insulation, the magnitudesof for the partially and entirely metal insulated clamp are al-most equal.

VI. PARTIAL DISCHARGES IN INSULATION ELEMENTS

“Partial discharge” is an electrical discharge that only par-tially bridges the insulation between conductors or around theconductor. It is a transient gaseous ionization, which occurswherever, and whenever the localized voltage stress in a gas(usually air) exceeds a critical value which is often related tothe electric strength properties of air. Partial discharges oftenoccur in electric fields of nonuniform strength such as at points,or edges of exposed conductors where they are referred to as“corona” due to the visible glow which they produce. Also theyoccur in gaseous cavities in or adjacent to solid insulation. Theyresult in gradual deterioration of any organic insulation materialor system upon which they impinge, often resulting in ultimatefailure from erosion, surface tracking or other mechanism.

Measurement of partial discharges in insulation elementsis one of the most frequently used methods for assessing thequality of these elements and the conditions, which they are ex-posed to during the operation. Partial discharge measurementsare only indirectly comparable to electric field calculation.With these measurements, we can arrange by quality differentcombinations of covered conductors and the accompanyingequipment.

A. Measurement of Partial Discharges

The measurement of partial discharges can be very difficult,so that they can be disturbed by numerous factors that can bedivided in the following two groups.

• Disturbances when the measurement circuit is open. Theyare caused by switching other circuits on or off, such ascommutation machines, proximity of high-voltage testing,radio broadcasting, and noises of the measurement systemitself;

• Disturbances when the measurement circuit is closed.These are disturbances that appear outside of the testeddevice and are caused by partial discharges in trans-formers, high-voltage conductors and insulators (onlythose which are not part of the tested device), or emergedue to bad grounding of devices.

These disturbances can be reduced by good radial grounding ofall conducting parts in the proximity of the measurement, and byfiltering the voltage and current used for energizing the testingcircuit. The most efficient solution has been achieved using ametal-enclosed and grounded measurement environment. Theconnections between the elements of the testing circuit have tobe made with coaxial cables. All exposed high-voltage connec-tions have to be shielded in order to prevent the influences ofexternal discharges.

This investigation employs a measurement technique forcorona and partial discharge which is in general use for mea-suring the radio noise levels in decibels produced on insulators.

Fig. 10. Partial discharges in dB for the system of porcelain (PPI) and glass(SPI) insulators and insulated conductor affixed in a metal clamp.

The measurements have been performed in accordance withthe IEC 60 270 [6].

B. Tested Devices and Measurement Combinations

The basic purpose of measurements of partial discharges isto obtain a confirmation of simulation calculations of electricfield strength in the conductor insulation and in its surroundingat operating voltage. We will check whether they are more in-tensive if the insulated conductor is in metal or plastic clamp,and if glass or porcelain insulator is used.

Some of the most frequent combinations of covered 20-kVoverhead conductors with the accompanying accessories havebeen put together for testing. The conductors have been fixed inboth the metal and insulated clamp (Fig. 1). The clamp has beenconnected to glass (Fig. 2) and porcelain (Fig. 3) pin insulators.

For these measurements, the same combinations as in com-puter simulations have been used.

VII. RESULTS OF PARTIAL DISCHARGES MEASUREMENTS

Partial discharges have been measured on various combina-tions of the system of covered conductors [5]. During the mea-surements, the different insulators have been connected to awooden pole with a metal console.

Fig. 10 shows the magnitudes of partial discharges in decibelsas a function of the actual conductor potential, both for glass andporcelain insulators.

In all cases the covered conductor has been affixed with ametal clamp. At 20 kV (the electric field calculation has beenexecuted for this peak voltage—Table I), the values of PD for aporcelain and glass insulator differ by about 20%. Similar valuescan be drawn for the calculated electric field strength .

Fig. 11 shows a comparison of partial discharges for defec-tive conductor insulation. Besides the previously used porcelaininsulator (PPI) with the metal clamp, a glass pin insulator with ametal (SPIAl) and insulated (SPIpl) upper edge of the clamp hasalso been used. In the system with a glass insulator with a metalclamp, the magnitude of partial discharges at 20 kV has beentwo times higher than in the system with the insulated clamp.These values are similar to the calculated (Table V; Table VI).

A comparison of the results of measurements for variouscombinations of conductors, clamps and insulators shows that


Fig. 11. Partial discharges in dB for porcelain and glass insulator, andinsulated conductor with defect insulation affixed in metal and insulated clamp.

the intensity of partial discharges is influenced by the mate-rial and construction of the insulator, the shape and materialof the clamp, the condition of the conductor insulation, andatmospheric conditions (air humidity). Differences in the levelof partial discharges for systems with glass and porcelaininsulators have been confirmed by measurements to be similarin proportion to the corresponding calculated electric fieldstrengths .

Glass and porcelain have similar dielectric constants; bothare in the range between 4.9 and 6. Material is therefore notthe reason for such differences. The primary reason lies in theconstruction of the SPI that is shown in Fig. 2. It consists of twoglass ribs. Between each two ribs there is insulation materialwith a different dielectric constant. Distribution of the electricfield strength and electric potential is extremely unfavorable forsuch a dielectric sequence. The metal supporter is much closerto the conductor than in the system with a porcelain insulator,which is shown in Fig. 3. The shape of the clamp that holds thecovered conductor is also important.

VIII. CONCLUSIONS

This paper presents the results of theoretical research of thebehavior of a system of medium voltage covered conductors.This research work has been studied by the use of computer-aided electric field calculations. We have checked partial dis-charge measurements to determine if they are more intensive ifthe insulated conductor is in a metal or in a plastic clamp, andif glass or porcelain insulator is used.

In the case of the calculation of the electric field in the im-mediate proximity of the conductor with a metal clamp, betterresults are obtained with the use of a porcelain insulator. In spiteof this fact, the magnitude of the electric field strength exceeds25 MV/m in the case of a testing voltage of 125 kV (peak value).If an insulated clamp replaces the metal clamp, the magnitudeof drops to about one fifth of the previous value. In the areaaround the conductor, there is a thick layer of insulation materialthat is the reason for a reduction of the electric field around theconductor. One consequence of this fact is a higher magnitude ofelectric field strength , between the clamp and the metal sup-porter. In the systems with an insulated clamp, a glass insulatorhas up to two times higher magnitudes of than the porcelainone. The reason for this lies in the nonhomogenous structure

of the material comprising the supporter, which causes nonho-mogenous distribution of the electric field. Beside this, the metalsupporter in the system with glass insulators is 2 cm closer tothe conductor. The solution to this problem has been found inthe reduction of in the area between the conductor and thesupporter without a significant change of its magnitude in thearea around the conductor. The insulated clamp has been cov-ered with metal in order to reach a low magnitude of in thearea between the supporter and the conductor, as well as in thearea around the conductor.

In the case of defective conductor insulation, reaches mag-nitudes from 2 to 4 times higher than in the case of a nonde-fective conductor insulation system with a metal and insulatedclamp. The highest magnitudes are again achieved in the systemwith a glass insulator.

The measured intensity of partial discharges has changed atnormal operating voltages in the same proportion as the calcu-lated electric field strength for the same combination of coveredoverhead conductors with associated 24-kV accessories. Thusthe simulation calculations for operating voltage have been ver-ified and a powerful theoretical tool for aid in design of systemof covered overhead conductors has been obtained. The calcu-lated magnitudes of electric field strength at 125 kV, on the otherhand, serve as a warning of the possibility of insulation break-down at the most severe operating conditions. For the highestvoltages (lightning) the calculations are questionable, since adynamic treatment of the problem should be used. This will bedone in the further steps of our research work, since the hard-ware and software have to be adequately adapted.

The experience gained thus far in the use of medium-voltageoverhead covered conductors, the results of partial dischargesmeasurements, and the calculations of confirm that the se-rious problems would be insulation damage and electromag-netic interference. For this reason, it is necessary to pay spe-cial attention to the choice of conductors and the accompanyingequipment before designing a line with covered conductors. Thereplacement of bare conductors with covered ones has to be ap-proached in the right way; it is also necessary to take into ac-count the theoretical laws. Therefore, based on theoretical find-ings and practical measurements, as well as on the standard-ization of individual components, the optimum combination ofequipment has to be determined already in the design phase.

REFERENCES

[1] R. Lee, D. Fritz, P. Stiller, and D. Shankle, “Prevention of covered con-ductor burndown on distribution circuits—Arcing protection devices,”IEEE Trans. Power App. Syst., vol. PAS-101, no. 8, pp. 2434–2438,1982.

[2] H. L. Graham, “Broken conductor and high impedance fault detectionby high frequency impedance monitoring,” in 32nd IEEE Power Engi-neering Society Winter Meeting, 1980, pA80064-6/1-5.

[3] O. Biro and K. R. Richter, CAD in Electromagnetism, Advances in Elec-tronics and Electron Physics. New York: Academic, 1991, vol. 82.

[4] J. Podvinsek, “Analysis of Electric Field of Middle Voltage Pin Insu-lator,” Diploma thesis, Univ. Maribor, Faculty of Elect. Eng. Comput.Sci., Maribor, Slovenia, 1999.

[5] Z. Kokol, “Partial Discharges in Middle Voltage Covered Overhead Con-ductors,” Diploma dissertation, Univ. Maribor, Faculty of Elect. Eng.Comput. Sci., Maribor, Slovenia, 1998.

[6] International Electrotechnical Commission IEC Standard, BureauCentral de la Commission Electrotechniqeu Internationale, PartialDischarge Measurement, IEC 60270, 2000.


Joze Pihler (M’92) was born in Ptuj, Slovenia, in1955. He received the B.S., M.S., and Ph.oxlD. de-grees in electrical engineering from the Faculty ofElectrical Engineering and Computer Science, Uni-versity of Maribor, Maribor, Slovenia, in 1978, 1991,and 1995, respectively.

Since 1988, he has been working at the Facultyof Electrical Engineering and Computer ScienceDepartment of Electrical Engineering, Institutefor Power Systems, as a researcher and AssociateProfessor. His special field of interest is switching

devices and switchgear.Dr. Pihler is a member of CIGRE and EZ.

Igor Ticar (M’00) was born in Maribor, Slovenia, in1949. He received his B.S. degree in electrical engi-neering from the University of Ljubljana, Ljubljana,Slovenia, in 1975, the M.S. and Ph.D. degrees fromthe University of Maribor, Maribor, Slovenia.

In 1977, he joined the University of Maribor asan assistant. At the moment he is Full Professor andworking on the numerical field calculation.

Dr. Ticar is a member of ICS.

design of systems of covered overhead conductors by means of electric field calculation

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