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PERFORMANCE OF HIGH-VOLTAGE OUTDOOR INSULATORS EMBEDDED WITH METAL OXIDE VARISTORS

H.I. Mohamed Nour

In this paper, the flashover performance of dynamic solid insulators in apolluted environmentispresented. Adynamic insulator consists of a metal oxide varistor enclosed by an insulating housing. This insulator assembly is designed to achieve improved performance under bothtransient and steady state conditions. Themechanismof surface breakdown of these insulators under highvoltage stress is analyzed. A general arc propagation model is utilized for evaluating the critical flashover voltage of a simple configuration. The effects of surface contamination and its nonuniformity on the critical flashover voltage are also studied.

I"

Outdoor insulators in high-voltage power systems are exposed to environmental pollution forming, in the presence of moisture, a conductive layer along the insulator surfaces. Under these con- ditions, interfacial breakdown may occur leading to system outages. The performance of these solid insulatorsunder surface contamination conditions is therefore greatly influenced by the phenomenon of surface flashover. A method for improving the performance of these insulators under both transient and steady state conditions has previously been shown, Alonso et al. [7] and Nour [ 4 ] . The method utilizes the non-linear characteristics of metal oxide varistors to control the flashoverbehavior of an insulator assembly consisting of a varistor and a polymer concrete container.

Experimental results have shown that this configuration results in improvement of flashover performance of outdoor insulators. Under contamination conditions, the varistor generates heat necessary to dry out the surface moisture thus reducing the surface discharge activities. When overvoltages are introduced by lightning or by switching, the non-linear element acts as a switch and suppresses the excessive voltages.

H.I. Mohamed Nour is with the Department of Electeical Engineering, California State University, Long Beach, California, USA

0-7803-01 29-3/92 $3.00 0 1992 IEEE

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In this paper, a general arc propagation model is utilized for evaluating the critical flashover voltage of dynamic insulators under various contamination conditions. The effects of surface contamination nonuniformity on the flashover performance'of these insulator are also analyzed.

Metal oxide varistors have nonlinear I-V characteristics that may be used to control the breakdown process of transmission line insulators. In particular, the zinc oxide varistor has a highly non-ohmic I-V relation in a wide range of current and voltage, Lauf [!I. These varistors are composed of semiconducting ZnO grains mixed with ,insulating oxides such as Biz03 in combination with a group of oxides including MnO, MgO, NiO, Ti02, SnO2, etc. , Eda [ 2 ] . The characteristics of zinc oxide varistors are described by the following empirical relations:

I , = cva ( 1 )

I, = r o m p ( g) where 1, is the current through the zinc oxide block and V is the applied voltage. !%typically ranges from 30-100 and 5 5 0.6-0.0 eV near 300'K. C and IO are constants.

The objective of the internal varistor current is to generate the heat necessary to reduce the surface moisture. As the applied voltage is increased beyondthe operatingvoltage level, the varistor will conduct at some threshold. This conduction threshold must be below the critical surface flashover voltage of the insulator assembly.

In an earlier investigation, Cheng and Nour [l], the mechanism of surface breakdown of contaminated insulators has been analyzed in terms of a generalized arc model. The generalized model is described by the following equations, Nour [ 4 ] :

u k = v 0 - A x k 1 ; " - V , ( 3 )

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F ( k ) = f k - f r . , : k = 1.2 ,..., m .

Where A and n are the discharge constants, Xk and Ik are respectively the length and current of the kth arc. Ve is the electrode voltage drop. The fo'rm factor fk is given by

f k = b ( x . [ , > o k ( x ) ) ( 5 )

The form factor function is influenced by the arc length, the internal current and the conductivitySk(x) which is now a function of x allowing the representation of nonlinear initial contamination conditions. This internal current generates heat which dries the insulator surface and alters its conductivity thus affecting the form factor.

The current flowing along the insulator surface, Is, is

The total current through the insulator unit is the combination of the internal and external currents, i.e.,

l = l z + l s = l z + E l k k - I

( 7 )

Equations (1) -(7) have been solved numerically for the critical flashover conditions. The method of solution followed is described by Nour [4].

> FLASHOVER PERFORMANCE

Adisk-type insulatorconfiguration is shown inFig.1. This insulator model has a leakage length of 32.3 cm. The dynamic insulator concept is illustrated in Fig.2. in which 1, represents the current through the zinc oxide elementandIS is the surface leakage current including the arc currents. The flashover performance of disk type insulator is evaluated under different contamination conditions. The results are summarized in Figs. 3 and 4. In Fig.3, the critical flashover voltage (CFO) is expressed as a function of surface contamination level in terms of the equivalent salt deposit density (ESDD). It is noted from this figure that an improvement of about 20-35% may be achieved in flashover performance due to the enclosed varistors. The nonuniform contamination conditions are shown in Fig.4. A different contamination level was assigned to each of the top and bottom insulator surfaces. The top to bottom contamination level ratio was found to have an influence on the flashover performance. Higher critical flashover voltages have resulted from nonuniform Contamination compared with uniform contamination for the same average surface ESDD level. Furthermore, an improvement of 15-40% has been achieved by utilizing zinc oxide varistors.

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CONCLUSIONS

The flashover performance of contaminated outdoor insulators has been investigated. A generalized arc propagating model has been utilizedto evaluate the critical flashover voltages of a disk-type insulators with and without zinc oxide elements enclosed. It is shown that an improvement of 15-409 may be achieved in flashover performance as a result of embedding the insulators with metal oxide varistors.

1.

2. 3. 4.

5.

6. 7.

8.

Cheng, T.C. and Nour, H.I.M., 1989,

Eda, k., 1989, Elect. m u l . Ma- Muf, R., and Bond, w, 1984, - Nour, H.I.M., 1989, e*Proceedings", IEEE Electric and Electronic Insulation Conference, Chicago, USA. Nour, H.I.M., and Sarbaz, F., 1990, "Annual Report", IEEE Conference on Electrical Insulation and Dielectric Phenomena, Pennsylvania, USA.

Rodriguez, A., Nour, H., Wang, F., and Dale, S., 1985, "Pro- ceedings", IEEE Electric and Electronic Conference, Boston, Massachusetts, USA. Wang, F., Nour, H., Wang, L. and Cheng, T.C., 1987, "Procee- dings1@, IEEE Electric and Electronic Insulation Conference, Chicago, USA.

, 5 , 28-41. -, 2, 113-117.

' , 61, 278-281.

Philipp, H., and Levinson, L., 1977, J.ed P m ' ,a.

Fig. 1. Simple insulator geometry

VARISTOR

1 1

P Fig. 2. Dynamic insulator

assembly

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E 115-

g 2.3- 15- -

13-- 5

U-,

UNIFORM CGlVTAMlNAllON - *.

L.- -E>

0 NO VARIST9R ENCLCSEL~ 0 ZINC OXIRE VMISTOR LNCLGSEO

-- --d --_

0 NO VARlSlOR ENCLOSED

* ZINC OXIDE VMISIOK ENCLOSED

OC 0.005 0.055 0.105

ESOD [mr3/cm2]

Fig. 3. Flashover performance under uniform contamination

Fig. 4. Flashover performance under nonuniform contamination