analysis of the deterioration effects of stator insulation on the its … · 2019. 2. 4. ·...

Abstract—Electrical machinery especially electric generators

which are the main parts of any electrical network, through the time and everyday functions like any other electrical devices experience a series of electrical, mechanical and thermal stresses. Continuous of this transient and stable tension over the time, cause lasting effects on the machine insulation parts, especially the insulation of stator slot which is the most important part of the insulation in machines. These events include creation of voids in the various layers of insulation; creation the gaps between layers of insulation and increasing the insulation dissipation factor due to change in capacitance of insulation. Therefore, in this paper we tried to analysis the impact of these destructive factors on the stator slot insulation electro-thermal analysis using finite element analysis and finally estimate the remaining life of the insulation in these cases with regard to the effect of the electrical and thermal stress simultaneously.

Keywords—Deterioration, estimated life, generators, void

I. INTRODUCTION HE insulating system is one of the main parts of electrical rotary machine. High operating and reliability demands

require the proper technological steps to manufacture the insulating system [1]. Much effort has been invested over several decades in studying the aging characteristics of the various electrical insulating system designs and insulating materials employed in high-voltage equipments. These studies aim mainly to allow reasonable estimates of the service life expectancies of such equipment and to assess their reliability in operating conditions after a given number of years in service. The aging of a polymeric material, or of any other material for that matter, inherently involves the alterations of the material’s physical and/or chemical structure which is expected to be related to the changes in the physical and chemical properties of the material [2],[3]. When the aging of a dielectric material is evoked, it usually implies that the alterations in the properties of the material are detrimental to its service operation and service reliability. When these properties have deteriorated to the point where the material can no longer

Diako Azizi is with the University of Science and Technology, Tehran,

Iran (corresponding author to provide phone: +9821-44491750; e-mail: [email protected]).

Dr. Ahmad Gholami was with University of Science and Technology, Tehran, Iran. He is now with the Department of electrical engineering, (e-mail: [email protected]).

Dr. Abolfazl vahedi is with the Electrical Engineering Department, University of Science and Technology, Tehran, Iran, (e-mail: [email protected]).

operate safely under normal stress conditions, it implies that it has reached the end of its useful life. The causes of aging are yet to be fully understood, but obviously, the degree of aging strongly depends on the nature of the involved material and on the nature and duration of the applied stresses. Indeed, aging can be induced by a combination of the various stresses (electrical, mechanical, thermal or environmental) to which the insulation system is subjected. The simultaneous application of these stresses leads to the interaction of aging mechanisms.

Emerge of some signs of wear and aging in the insulation, such as voids and their delaminating will accelerate its deterioration. As a result in the location of the voids, the electric field intensity will increase many times and cause partial discharge in the voids and thus will increase the void size and reduce the insulation life.

There are different ways to measure the insulation life. One of them is the laboratory method [4]. But this method is not used much, because of being time consuming. Therefore, this paper is tried to use simulation and numerical analysis of data which is obtained to estimate the remaining life of insulation using coupled Electro-thermal analysis based on strong method of finite element.

Analysis of the cosmetic results of the remaining life shows that in the adjacent Walls of the voids, the life of the insulation is reduced severely. This issue necessitates the need to repair the damaged insulators.

II. CASE OF STUDY To study a sample, it is required to determine or design the

sample first. Therefore, in order to do field analysis and to study the generator insulation Status, a generator with a given profile has been selected. The selected generator is synchronous with Regular winding, three phase, two poles that have 24 slots in stator. Rated frequency, voltage and power are respectively 50Hz, 13.8 KV and 1 MVA.

III. ELECTROMAGNETIC MODELS To derive the electromagnetic system equation, we start

with the Ampere’s law,

tDJBvE

tDJH e

∂∂

++×+=∂∂

+=×∇ σσ (1)

These quantities are: • The electric field intensity, E • The electric displacement or electric flux density, D

Analysis of the Deterioration Effects of Stator Insulation on the its Electro-Thermal Property

Diako Azizi, Ahmad Gholami, Abolfazl Vahedi

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International Journal of Electrical and Electronics Engineering 3:11 2009

697

• The magnetic field intensity, H • The magnetic flux density, B • The current density, J • Externally generated current, • Electrical conductivity, σ • Velocity, v

Now we include the effect of the time variant-harmonic fields and use these definitions for the potentials:

AB ×∇= (2)

tAVE

∂∂

−−∇= (3)

And combine them with the constitutive relationships )(0 MHB += μ and PED += 0ε to rewrite the Ampere’s law

as:

( ) ( )( ) ( ) PjJVjAv

MAAje ωωεσσ

μεωωσ

+=∇++×∇×−

−×∇×∇+− −

0

100

2

(4)

In which ω, 0ε , 0μ , M and P respectively refer to Angular

frequency, Relative permittivity, Relative permeability, magnetization vector and electric polarization vector.

In the case of 2-dimensional-plane, there are no variations in the z-direction, so the electric field is parallel to the z-axis. Therefore you can write V∇ as −ΔV/L where ΔV is the potential difference over the distance L. Now these equations are simplified to:

( )

zez

zzx

yz

PjJLV

AjAvMM

A

ωσ

εωωσσμ

++Δ

=

⎟⎟⎠

⎞⎜⎜⎝

⎛−+∇+⎥

⎦

⎤⎢⎣

⎡−−∇∇− − 0

210 ..

(5)

In the ax-symmetric case, another form of the gradient of

the electric potential has been used,r

VV loop

π2−

=∇ , because the

electric field is only present in the azimuthally direction. The above equation, in cylindrical coordinates, becomes:

[ ]

( )

PjJr

V

uVujrz

ur

uvr

MM

uz

zu

ru

rzr

eloop

r

r

z

ωπ

σ

σεωωσσ

μμ

ϕ ++=

+−+⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂∂

∂∂

+

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛⎥⎦

⎤⎢⎣

⎡−

−⎥⎦

⎤⎢⎣

⎡+

⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂∂

∂∂

∂∂

∂∂− −−

2

2.

0.

02

10

10

(6)

The dependent variable u is the nonzero component of the

magnetic potential divided by the radial coordinate r, so that:

rAu ϕ= (7)

The application mode performs this transformation to avoid

singularities on the symmetry axis.

IV. INSULATION LIFE Arrhenius model usually has been used for estimating the life of the insulation parts when thermal stress is applied alone, but with regard to faster deterioration in the sections of the insulation when placed under the both electrical and thermal tensions, Arrhenius model must be comprehensive [5], [6]. Therefore, Hatch and Endicott presented the Eyring model for taking to account the thermal and voltage stresses simultaneously. But using special methods for determining the coefficients of the equation to estimate the life of Eyring insulation is actually a challenge [7], [8]. Thus Ramu proposed [9] a model for the estimation of the insulation life which use the power law and its parameters are temperature dependent.

When the insulation is only under thermal stress, the life is obtained using the following relationship:

⎥⎦⎤

⎢⎣⎡=kTEAL exp (8)

E is the activation energy and k is the Boltzmann’s constant.

A detailed analysis of the data which is obtained in the presence of thermal stress alone or when both electric and heat are applied shows that activation energy applying both tensions simultaneously, is less than the case which one stress is applied. Taking to account this point, the model for Arrhenius can be corrected by decreasing the activation energy using the

⎥⎦⎤

⎢⎣⎡−

kTσξexp factor. New model is expressed as

follows:

⎥⎦⎤

⎢⎣⎡ −=

kTEAL σξexp (9)

The ξ is electrically field (stress) applied.

V. DETERIORATION EFFECTS

A. Void In this section we assumed that the second insulation

layer has a void with rectangular dimensions, 0.0001 meters. In Fig.1 distribution of the electric field with the air void is presented. Fig.1 shows the lines of electric field, Fig.2 compares the electric field distribution in two cases with and without holes in the second layer of insulation and Fig.3 presents the remained life of insulation.


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Fig. 1 Distribution of the electric field and respective lines if the air

void is in the second layer of insulation (V/m)

Fig. 2 Comparing the electric field distribution in two cases with and

without holes in the second layer of insulation

Fig. 3 Effect of void in the remaining life

As it is obvious from the results, the intensity of the electric

field in voids increases suddenly. These local increasing causes partial discharge in that location and thus increases the void size and the insulation will be damaged locally. Fig. 4 shows the distribution of the temperature with respect to 25 percent increasing in the dissipation factor. The results of the reduction of the insulation life in these cases have been presented in the Fig.5.

B. Increasing of the dissipation factor In this section the aging effect has been modeled by the increasing of the dissipation factor in insulation layers and its effects on the field distribution has been specific. Here we assumed that the dissipation factor of the insulation layer is increased 25 percent.

Fig. 4 The distribution of the temperature with respect to 25 percent

increasing in the dissipation factor (ºC)

Fig. 5 The impact of 25 percent increasing of the dissipation factor

on the insulation life with regards to the thermal and electrical stresses

As can be seen, 25 percent increasing of the dissipation

factor increases the insulation temperature 3 percent. It should be noticed that certain changes in the distribution of the electric field has not been detected in two cases.


699

C. Gap creation between the insulation layers Defect of good assembly and connection of appropriate

insulation layers and conductors together, makes available to time, being utilized in long-terms, the shaking and insulation exhaustion, an air gap will be created in the part of the layers. Due to the lower air permittivity coefficient in comparison to the insulation, the electric field intensity will be greatly increased in these regions and the localized tensions cause’s trivial discharges which damage the insulation wall [10], [11]. The spread of this destructive phenomenon during the time can seriously damage the insulation sections. In this section the distribution of the electric field is analyzed again while considering the above mentioned phenomenon. Fig. 6 shows the Electric field distribution assuming air layer creation and Fig.7 presents the impact of delaminating on the life of insulation layers considering the thermal and electrical stresses.

Fig. 6 Electric field distribution assuming air layer creation (V/m)

Fig. 7 Impact of delaminating on the life of insulation layers

considering the thermal and electrical stresses

Since the created air layer is directly connected the conductor, it consists the warmest part of the system. Also this

sector is located in the conductor separation curve and because of the insulation departing in this section; the electric field intensity is greatly increased as it is shown in the figure which increases the temperature of the layer. This issue can cause serious damage to the insulation section.

VI. COMPARING THE RESULTS Table I shows different impacts of the insulation aging on

the distribution of electric and thermal field, and the insulation life.

TABLE I Effects of the insulation deterioration on the distribution of electric and

thermal field, and the remained life in the deteriorated area deterioration

type effect on

electrical field effect on

thermal field effect on

remaining life with void 55% increasing 1% increasing 20% increasing

20%

increasing in dissipation

factor

without variation 3% increasing 30% decreasing

layering 180% increasing 4% increasing 70% decreasing

Delaminating is the most important effect on the intensity

of electric field and the temperature section. This problem has caused the greatest effect on the reduction of the insulation life which is reduced to 70 percent. This issue reveals the need to use insulators with high quality materials, more accurate operation and assembly, and ultimately complete seal between the insulation layers.

VII. CONCLUSION Any electrical device has a life time that with passing of

this times gradually, the need to repair and maintenance will be clearer. Generators and motors which are the basic parts of electrical networks are not exempted from this vital need. For this purpose different methods have been used. This paper was focused to the analysis of fields, such as electrical and thermal fields. These fields cause tensions in different parts of the insulation which monitor the various scenarios and determine the common equations for the insulation life estimation, and the life in different parts of the insulation is estimated while considering the aging effect. The results prove that, void creation and delaminating of the insulation will dramatically decrease the remaining life of the insulator.

REFERENCES [1] Mentlik Vaclav, Tranka Pavel, Pihera Josef, ‘‘Using of the aging

models and rotating machine insulation evaluation’’, Electrical Insulation. 2008, ISEI 2008, Conference Record of the 2008 International Symposium on, 9-12, June 2008, Pages 408-411.

[2] T. J. Lewis, ‘‘Ageing- A perspective’’, IEEE Electr. Insul. Mag., vol. 14, no. 4, pp. 6-16, 2001.

[3] V.K. Agarwal, H.M. Banford, B.S. Bernstein, E.L. Brancato, R.A. Fouracre, G.C. Montanari, J.L. Parpal, J.N. Seguin, D.M. Ryder and J. Tanaka, ‘‘The mysteries of multifactor ageing’’, IEEE Electr. Insul. Mag., vol. 11, no. 3, 1995.


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[4] James H. Dymond, K. W. Stranges and Nick Stranges, ‘‘The effect of surge testing on the voltage endurance life of stator coils’’, IEEE Transactions Industry Applications, vol.41, no.1, Jan/Feb 2005.

[5] G. C. Montanari and M. Cacciari, ‘‘A probabilistic insulation life model for combined thermal-electrical stresses’’, IEEE Transactions on Electrical Insulation vol. EI-20 no.3, June 1985.

[6] G. J. Anders, SM J. Endrenyi, F G.L. Ford, M G.C. Stone, SM, ‘‘A probabilistic model for evaluating the remaining life of electrical insulation in rotating machines’’, lEEE Transactions on Energy Conversion, vol. 5, no. 4, December 1990.

[7] S.B.Pandey, ‘‘Estimation for a life model of transformer insulation under combined electrical & thermal stress’’, IEEE Transaction on reliability. vol.41, no.3, 1992 September.

[8] H.S.Endicott, B.D.Hatch, R.G.Sohmer, ‘‘Application of eyring model to capacitor aging data’’, IEEE Trans. Component parts, vol 12, 1965 Mar, pp 34-41.

[9] T.S.Ramu, ‘‘On the estimation of life power apparatus insulation under combined electrical and thermal stress’, IEEE Trans. Electrical insulation, vol EI-20, 1985 Feb, pp 70-78.

[10] Victor V.Kuzmin, Lev L.khaimovitch, ‘‘Equigradient lines of electric field in stator slot, isodestruction lines of insulation and high voltage design of turbo- and hydro generators winding’’, Conference record of the 2000 IEEE international symposium on electrical insulation, Anaheim, CA USA, April 2-5, 2000.

[11] Grec C.Ston, Edward A.Boulter, Ian Culbert, Hussein Dhirani, ‘‘Electrical insulation for rotating machines’’, IEEE Press series on power engineering, 2004.

Diako Azizi was born in 1985. He has received B.Sc. degree in Electrical Engineering from Tabriz University, Tabriz, Iran in 2007. And he received the Master degree in Electrical Power Engineering from the University of Science and Technology, Tehran, Iran in 2009. He is presently pursuing the Ph.D. degree in Electrical Power Engineering, Iran University of Science and Technology. His research

interests are aging of insulations in electrical machines.


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analysis of the deterioration effects of stator insulation on the its … · 2019. 2. 4. ·...

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