ON THE DEBIGN OF METAL-OXIDE VARIBTORS FOR DYNAMIC INSULATION OF HIGH VOLTAGE SYBTEMB

H. I. M. Nour

Electrical Engineering Department California State University

Long Beach, California

ABSTRACT

This paper describes the performance of metal-oxide varistors as applied to dynamic insulation of high voltage power systems. A mathematical model for a dynamic insulator is constructed. The model takes into consideration the effects of the surface contamination conditions as well as the arc development and propaga- tion on the flashover behavior of outdoor insulators. The nonlinear I-V character- istics of metal-oxide elements are uti- lized to both suppress system overvoltages and improve the flashover performance of these insulators. A method for designing optimum configuration (i.e. which corre- sponds to best transient and flashover performance) of metal-oxide units of given characteristics is developed. Experimen- tal data of zinc-oxide varistors are used to illustrate the design procedure. It is shown that the optimum zinc-oxide configu- ration depends mainly on the system voltage level, geometry of the insulating surface and on the atmospheric contamina- tion level. The new design procedure is applied for developing a simple dynamic insulator unit resulting in an improvement of over 14% in the flashover performance under uniform contamination conditions. The concept of manufacturing solid insula- tor units with built-in dynamic insulation properties is suggested. The size and shape of these units, however, will depend on the electrical and mechanical proper- ties of this metal-oxide product.

The flashover performance of outdoor insulators has a direct effect on the reliability and availability of high voltage transmission systems. Line out- ages may occur due to insulator flashovers caused by environmental contamination, switching overvoltages or lightning surges. Under dry and clean weather conditions, outdoor insulators can with- stand voltage levels much higher than the normal operating level. As polluted insulator surfaces are exposed to atmo- spheric moisture due to fog, rain or dew, a leakage current flows through the conductive layer over the insulator surface. The flow of leakage current causes dry bands to form along the insulator surface resulting in concentrat- ed electric field intensities. Partial

discharging activities then occur and may lead to a complete flashover. When lightning strikes a power transmission line, a current surge travels along the line in both directions. As the surge reaches the nearest tower, a voltage develops across the insulator string. This voltage can be much larger than the nominal voltage and may cause insulator breakdown. The insulator breakdown may also be caused by switching overvoltages exceeding the insulator withstand voltage.

It is well recognized that the surface flashover performance of outdoqr. insula- tors can be improved by raising the insulator surface temperature. In a previous work1 , the advantages and disad- vantages of different methods of heating the insulator surface have been outlined. It was then concluded that heating the insulator body internally has a drastic effect on improving the flashover perfor- mance. Applicability of zinc oxide varis- tors for achieving the internal heat effect was latter demonstrated.2 A pre- liminary procedure for designing a dynamic insulator unit using zinc oxide varistors was also presented.

Introducing a heat source such as a metal oxide varistor inside the insulator body affects the mechanism of the surface flashover. Previous investigations3 of the discharge characteristics of contami- nated insulator surfaces have indicated that the breakdown voltage of an insulator unit is a function of both the insulator profile and the distribution of the surface contamination. The insulator pro- file and contamination conditions are expressed by a function known as the insulator form factor. The heat generated by the metal oxide varistors alters the conductivity of the surface contamination thus influencing the flashover mechanism.

In this paper, a mathematical model for evaluating the critical flashover voltage of these insulators under various surface contamination conditions is developed. The I-V characteristics of metal oxide varistors are implemented into the mathe- matical model. Using this model, an optimization technique for designing a dynamic insulator unit is outlined. The goal of the design procedure is to both improve the steady state performance of outdoor insulators under dc applied voltage and to mitigate the effects of transient overvoltages caused by lightning or switching surges. The optimization technique is based on the availability of

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castable insulating material and on the knowledge of I-V characteristics of various combinations of metal oxide blocks. An illustrative example of the design procedure is presented where a simple insulator shape is assumed.

CHARACTERISTICS OF METAL OXIDE VARISTORS Metal oxide 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, TiO2, Sn02, etc.6 These varistors exhibit a highly nonlinear current-voltage (I-V) characteristic. In Fig.l., typical I-V characteristics of two tmes of ZnO blocks are shown - a commercially available varistor having a breakdown voltage of about 1800 V/cm and a newly7 fabricated ZnO varistor giving a higher breakdown voltage of 6000 V/cm. At low voltage, the I-V characteristic of a typical ZnO block is linear and is dependent on the varistor temperature. The temperature dependence is given by6

I , = I o e x p ( g) where Io is a constant and 6 fi: 0 . 6 - 0 . 8 eV near 300°K. In the breakdown region, the highly nonlinear I-V relation is expressed by the empirical formula

I, = C V " ( 2 )

where U typically ranges from 25-50.

A SOL-GEL ZnO VARISTOR'

0 0.000 0.500 1.000 1.500 2.000

CURRENT OENSITY (@A/crn2)

Fig.1. Typical I-V characteristics of ZnO varistors

It will be shown that this nonlinear I-V characteristic of ZnO varistors can be applied for improving the performance of outdoor insulation systems under both steady state and transient conditions. The following section describes a semi-em- pirical model for a contaminated insulator unit enclosing one or more ZnO varistor discs.

CONTAMINATED INSULATOR MODEL Critical Conditions

Under steady state conditions, a contaminated insulator unit enclosing a zinc oxide element may be modeled as shown in Fig.2. A discharge is sustained along the insulator surface bridging over a length x. The unbridged portion of the surface is represented by a resistive element whose conductivity is dependent on the surface contamination conditions. The V-I characteristics of this insulator scheme are now derived from the basic circuit laws in combination with empirical arc equations.

Insula tor surface7

Varistor

Fig.2. Arc model for a dynamic insulator unit

The voltage across the insulator unit as a function of the arc length, x, is given by4

V(x) = AIT"x + F I , + V, ( 3 )

where A and n are the discharge constants, Is is the leakage current along the insulator surface and VE is the voltage drop across the insulator electrodes. F is the form factor describing both the geometry and contamination state of the unbridged portion of the insulator sur- face. The form factor can be evaluated analytically for simple insulator shapes with uniform contamination conditions. For insulators with complex configura- tions, however, the form factor may be numerically estimated. In general, the form factor is written as

F = 4( x , I =) (4)

The function @.J indicates that the form factor is influenced by both the arc length and the internal current. This internal current generates heat which dries the insulator surface and alters its conductivity thus affecting the form factor. Finally, the total current flowing through the insulator unit I(x) is the combination of the internal and external currents, i.e. ,

( 5 ) I(x) = I, + I ,

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The minimum voltage required to sustain an arc of length x is found by by taking the first derivative of Eqn. ( 3 ) and equating it to zero, i.e.,

Equation (6) gives Vmin as a function of the arc length x. The critical length, xc can be found by taking the derivative of Vmin with respect to x and equating it to zero. Further propagation of the arc beyond xc will result in a complete flashover of the insulator unit. There- fore ,

( 7 )

Equation (7) yields the critical length xc which corresponds to the critical flashover voltage Vc. solution Method By solving equations (1)-(7) , the critical flashover voltage of a dynamic insulator unit can be obtained. The solution, however, requires several iterations due to the dependence of the outside surface conditions on the current flowing inter- nally in the metal oxide varistors. Thus the form factor or the conductivity of the contamination layer is dependent on the amount of heat propagated to the insulator surface. The local temperature also influences the I-V characteristics of the metal oxide varistors as noted from Eqn.(l!. In solving Eqn.(6), it is recognized that 1, (and therefore 9) is a function of the applied voltage. A continuous system simulation program is written to numerically solve these simul- taneous equations. The outcome of the program is the critical flashover voltage of the dynamic insulator as a function of the insulator geometrical configuration and its surface contamination conditions.

In what follows, a procedure for designing a dynamic insulator unit is described. Our main objective is to improve the flashover performance of the insulator unit and utilize the same insulator unit for suppressing transient overvoltages caused by lightning or by switching surges.

DESIGN OF DYNAMIC INSULATION SYSTEM The design procedure described here is based on the following assumptions: 1.

2.

The metal oxide varistors have identi- cal geometries and have very similar I-V characteristics. The I-V charac- teristics of two, three or more varistors when connected in series (put on top of each other) are also known. The normal operating voltage VN is given as well as the contamination level in terms of the Equivalent Salt Deposit Density (ESDD) which is a measure of the severity of the insulator surface pollution.

3 .

4 .

As

A castable material such as polymer concrete is available for construction of insulating shells with desired shapes. The mechanical constraints such as tensile or compressive strengths are not taken into considera- tion at this stage. The metal oxide varistors are capable of handling the energy imposed by overvoltages which exceed the varistor breakdown voltage.

shown in Fis.3.. a dynamic insulator unit is compos&d of an insulating shell enclosing one or more of varistor blocks. Under normal operating conditions, small leakage current flows internally through the varistors. The internal current generates heat (due to 12R loss) which by thermal conduction flows out to the insulator surface. The temperature dif- ference between the insulator surface and the surrounding medium, when properly maintained-, can drastically improve the surface flashover perf0rmance.l Assume that the minimum acceptable improvement in flashover performance is pmin. During transient overvoltages exceeding the varistor breakdown voltage, the metal oxide blocks will conduct thus allowing the travelling surges to discharge into ground. In light of these objectives, the following guidelines for designing a dynamic insulator are outlined:

n U

Electrodes

Fig. 3.

Step 1.

Step 2.

Step 3.

c Dynamic insulator configuration

Based on the insulator application (suspension, post, etc.), select preliminary figures for the major geometrical parameters such as outside diameter, unit height and electrode size. Evaluate the optimum insulator profile for the overall size estimated in step 1 and determine the critical flashover voltage VC using the new mathematical model. Knowing VN, determine the minimum number of metal oxide blocks to use for each insulator unit. The knee point (breakdown voltage) of the combined varistor I-V charac- teristic must be higher than VN.

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Step 4. From the combined I-V curve of the selected number of varistors and knowing VN, calculate2 the effects of the internally generated heat in terms of the insulator with- stand voltage VW (Vw2Vc). If the surface temperature is higher than necessary, increase the number of metal oxide blocks by one and repeat step 2.

Step 5. If VpVc, the effect of internal heating is insignificant. De- crease the number of selected varistors by one and repeat step 4.

Step 6. If VWWC, calculate the percentage improvement in flashover perfor- mance due to internal heating, p =

otherwise repeat step 5 . ~OO(Vw~Vc)/Vw. If filpminr stop,

LEAKAGE LENGTH OPERATING VOLTAGE

y w CONTAMINATION LEVEL NUMBER OF VARIS-2 TORS

P

The guidelines for designing a dynamic insulator unit are summarized in the flow chart shown in Fig.4. This design proce- dure has been applied for a simple cylindrical insulator shape. Figure 5 illustrates the effects of embedding one or more metal oxide varistors on both the critical flashover voltage and the criti- cal arc length. It is noted that, under uniform contamination conditions, while the critical arc length remains essential- ly constant, the flashover voltage is considerably increased by adding the varistors. Furthermore, the effect of adding a second varistor on the flashover voltage seems insignificant. In Fig.6, the influence of the contamination distri- bution on the critical flashover condi- tions of a dynamic insulator unit is shown. The effect of contamination nonu-

15 cm 6.5 kV 8.0 kV 0.06 mg/cm2

14.2% I

8

I 1 I 1m

Fig.4. Flow chart of the design procedure

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niformity on the critical arc length is more significant that its effect on the flashover voltage. Shorter critical arc lengths indicate larger stability margins than those of longer arc lengths for the. same critical flashover voltage. In Table 1, the optimum values of the design parameters considered for this simple insulator shape are listed. For sim- plicity, uniform contamination levels were assumed. Although, using one varistor would be sufficient to achieve the same improvement in the withstand voltage as with two varistors , the f inal iteration suggests the use of two metal oxide units to reduce the leakage length to a minimum and to keep a stable margin between the operating voltage and the varistor knee point.

0 NO VARISTORS ENCLOSED

0 ONE VARISTOR ENCLOSED A TWO VARISTORS ENCLOSED

2. 1400

o o o : h o 0400 0600 ' 0800 1

NORMALIZED CRITICAL ARC LENGTH (Xc/L)

Fig.5. Critical conditions of a cylindrical insulator

2.200

1.400

ONE VARISTOR ENCLOSED IN ALL CASES

e 1.200

0.200

0 . o o o l I 0.000 0.200 0.400 0.600 0.800 1.1

NORMALIZED CRITICAL ARC LENGTH (Xc/L)

Fig.6. Effects of contamination nonuniformity

I DO

30

Table 1: Optimization results of a simple dynamic insulator unit

~

INSULATOR SHAPE /CYLINDRICAL

The concept of controlling the V-I characteristics of solid insulator units by embedding metal oxide varistors may be applied toward developing solid insulating materials with built-in semiconductive properties. The proposed material should have, in addition to the electrical characteristics described in this paper, the mechanical strength necessary for supporting the conductors. The new met- al-oxide product may be utilized for designing suspension insulators, trans- former bushings, post-type insulators, wall bushings, cable termination joints, etc. Unlike the conventional outdoor insulators , the proposed units would be designed with different profile and different I-V characteristics for each site with a given pollution severity.

1.

2.

3.

4.

CONCLUSIONS The effects of insulator internal heating using metal oxide varistors on the flashover performance of outdoor insulators have been quantitatively evaluated. Under dc applied voltage, the withstand voltage of the insulator unit considerably increases when the current flowing through the metal oxide elements is properly maintained. This metal oxide current also results in voltage linearization thus alleviating the electric filed stress along insula- tor strings. A mathematical model has been developed for evaluating the critical flashover voltage of contaminated insulators enclosing metal oxide varistors. The model takes into account the effects of current flowing through the metal oxide element on the insulator contamination conditions. The flow of current in the metal oxide varistors affects both the critical flashover voltage of the insulator unit and the critical arc length. The critical flashover voltage is increased at all times but the critical arc length may increase or decrease depend- ing on the surface contamination level. Implementation of the dynamic insulator concept using metal oxide varsitors seems very promising. Further research is necessary for developing materials with built-in semiconductive properties that can be used in producing solid insulators for high voltage applicati- ons.

ACKNOWLEDGMENTS

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REFERENCES A. Rodriguez, H.I. Nour, F. Wang and S. J. Dale, "Effect of Insulator Surface Temperature on the Flashover Perfor- mance of Outdoor Proceed- ings of the IEEE Electric and Electron- ic Insulation Conference, Boston, Mass., September, 1985. F. Wang, HI Nour, L.J. Wang and T.C. Cheng, "Application of Zinc Oxide Varistors in Dynamic Insulation of HVDC System, 1' Proceedings of the IEEE Electric and Electronic Insulation Conference, Chicago, Ill., 1987, pp. 33-37. T.C. Cheng, C.Y. Wu and H. Nour, "DC Interfacial Breakdown on Contaminated Electrolytic SurfacesIn1 IEEE Transac- tions on Electrical Insulation, Vol.

T.C. Cheng and H.I.M. Nour, "A Study on the Profile of HVDC Insulators - Mathematical Modeling and Design Con- siderations, IEEE Transactions on Electrical Insulation, Vol. 24, No. 1, pp. 113-117, Feb. 1989. M. Fazelian, C.Y. Wu, T.C. Cheng, H.I.M. Nour and L.J. Wang, "A Study on the Profile of HVDC Insulators - dc Flashover PerformanceIt1 IEEE Transac- tions on Electrical Insulation, Vol. 24, No. 1, pp. 119-125, Feb. 1989. H. Philipp and L. Levinson, "Varistors - A Clue to Conduction Mechanisms,11 Journal of Applied Physics, Vol. 48, No. 4, April 1977. R. J. Lauf and W. BondIf1 Fabrication of High-Field Zinc Oxide Varistors by SOL-GEL ProcessingIg1 Ceramic Bulletin,

R.E. Koch, J.A. Timoshinko, J.G. Anderson and C.H. Shih, IIDesign of Zinc Oxide Transmission Line Arrestors for Application on 138 kV TowersI1' IEEE Transactions on Power Apparatus and Systems, Vol. PAS-104, NO. 10, pp.

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9. P.R. Emtage, @*The Physics of Zinc Oxide Varistors, Journal of Applied Physics,

10.G.D. Mahan, L.M. Levinson and H.R. Philip, #@Theory of Conduction in ZnO Varistors, Journal of Applied Physics, vol. 50, NO. 4, pp. 2799-2810, April 1979.

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The author would like to thank the Faculty Development Center and the EE Department, CSULB, for supporting this work. Thanks are also extended to F. Wang of USC for his helpful comments.

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