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1294 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 26, NO. 2, APRIL 2011

Importance of Capacitance on Metal Oxide Arrester Block Modelfor VFTO Applications

P. Valsalal, S. Usa, and K. Udayakumar

AbstractPresently, due to several factors, metaloxide ar-resters find very limited applications against very-fast transientovervoltages (VFTOs). To enhance the utilization of these arrestersand to meet the needs of the dynamic performance of arresterunder VFTOs, an improved model for the prorated block is pro-posed and its performance is analyzed by applying current surgesof different front times (5 ns to 8 s), tail times, and peak values.There is a time lag between the residual voltage and dischargecurrent due to block capacitance. The delay thus observed in theinitial peak of residual voltage is experimentally verified.

Index TermsArrester block model, block capacitance, fronttime, residual voltage, transient behavior, very-fast transientovervoltage (VFTO).

I. INTRODUCTION

THE switching operations performed in a gas-insulatedsubstation (GIS), especially at extra high-voltage (EHV)

levels result in anomalous or inexplicable failures within GISand its connected powerplant. This is attributed to the presenceof very-fast transient overvoltage (VFTO) with the front timesin the order of 5 ns [1]. VFTO also occurs during the switchingoperations in vacuum circuit breakers (CBs); there has alsobeen increased evidence of the occurrence of a very fast rate ofrise of lightning currents at the substations. All of these attestto the importance of a well-coordinated arrester protectionagainst VFTOs. Several electrical models with acceptable

levels of accuracy have been brought out for simulating itsfrequency-dependent behavior under current surges with thefront time of 1 s or more [2] and the Micaela model [3] isfound to be more accurate for computing the peak value of theresidual voltage .

While analyzing the residual voltage of the arrester, it is es-sential to consider its peak value and rise time. So far, the per-formance of the arrester is assessed only by for the givencurrent surge. The calculation of rise time has not been givendue consideration especially for current surges with a front timeof 1 s or more because the residual voltage reaches peak be-fore the arrester discharge current reaches its peak [4]. But forcurrent surges with a front time of less than 1 s, the residual

voltage reaches peak after the arrester discharge current reachesits peak, causing a delay in response. This makes it importantto estimate the actual rise time while performing the study oftransient behavior of the surge arrester for VFTO.

Manuscript received April 08, 2010; accepted June 21, 2010. Date of pub-lication August 16, 2010; date of current version March 25, 2011. Paper no.PESL-00044-2010.

The authors are with the Division of High Voltage Engineering, Departmentof Electrical and Electronics Engineering, CEG, Anna University, Chennai 600025, India (e-mail: valsalal@annauniv.edu; s_usa@annauniv.edu; uday@an-nauniv.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRD.2010.2060093

Fig. 1. (a) Micaela model. (b) Proposed model for VFTO.

Fig. 2. (a) Showing V , t , V , and t . (b) Residual voltage waveforms.

II. PROPOSED MODEL

The perfect model for a metaloxide arrester will produce thefeatures of its current or voltage characteristics and the basic in-formation revealed by the dependence of capacitance on voltageand frequency. The capacitive effect of the arrester block de-pends on the steepness of the applied surge. The rate of rise ofthe incoming surge current plays an important role in triggeringthe operation of the surge arrester. Since the circuit capacitanceplays an important role during very fast transients, the incor-poration of an accurate value of capacitance is very much es-sential in the arrester model for VFTO studies. So the Micaelamodel [Fig. 1(a)] is modified with the inclusion of an accuratevalue of block capacitance ( 0.9-nF or 3-kV arrester),shown in Fig. 1(b). The effect of capacitance is also consideredin some existing models, but the formula prescribed to calculatethe block capacitance (approximate value) does not incorporatethe relative permittivity of the material.

Using Fig. 1, the values of residual voltage for the 3-kVarrester with different current surges are computed. There isno variation in time as to when the occurs. But a percep-tible drop in the rate of rise of residual voltage is noticed.It is mainly due to the capacitive effect. Further analysis istherefore performed based on an initial peak value and thecorresponding initial rise time . Fig. 2(a) shows , , ,and the time at which peak occurs for the 8/20- s, 10-kAsurge.

It is observed that the steepness of current surge and the ar-

rester capacitance influence the occurrence of and . The

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VALSALAL et al.: IMPORTANCE OF CAPACITANCE ON METAL OXIDE ARRESTER BLOCK MODEL FOR VFTO APPLICATIONS 1295

TABLE IINITIAL PEAK AND INITIAL RISE TIME OF RESIDUAL VOLTAGE FOR DIFFERENT

FRONT TIMES USING THE MICAELA MODEL AND PROPOSED MODEL(PEAK: 10 kA, TAIL TIME: 10 s)

dynamic behavior of the surge arrester under our study is evalu-ated for a differentfronttime , tailtime , andpeak values

of the current surge.

III. BEHAVIOR OF THE ARRESTER PRORATEDBLOCK UNDER VFTO

The performance of the proposed model is studied for currentsurge of different varying from 0.1 s (100 ns) to 0.005 s (5ns) with of 10 s and a peak value of 10 kA.

The performance of the Micaela model and the proposedmodel are compared for different of 0.1 s, 0.01 s, and0.005 s (i.e., without and with ). This helps to discoverthe influence of block capacitance on its performance [Fig. 2(b)for 5 ns]. The percentage difference between the ofthe Micaela model and the one under our proposed study, withreference to of the surge is denoted by . The values of(Table I) are computed by using

proposed Micaela(1)

It is noted, in general, that with the increase in frequency, thedelay in the increases as a result of the capacitive effect of thesystem. Thus, the is a crucial parameter for successful opera-tion of the arrester and the model proposed by Micaela does notpresent this effect on . It can be easily seen from Table I thatthe effects of block capacitance are clearly visible only in themodel now proposed. It may also be noted that increaseswith the frequency (with decreasing ). The is a function of

, and the tail times do not influence the initial response of thearrester. The values of and of residual voltages are alsocomputed by considering the current surge of , 5 ns with dif-ferent peak values (Table II). increases and decreases withan increase in the peak of current surges, showing faster arresterresponse for higher currents.

IV. EXPERIMENTAL RESULTS

To successfully turn on the metaloxide arrester, the residualvoltage should reach its peak before the occurrence of dischargecurrent, which is true for the current surges of front time of1 s or more. When the front time decreases below 1 s, the

TABLE IIRESIDUAL VOLTAGE OF 3-kV ARRESTER PRORATED BLOCK

FOR DIFFERENT PEAK VALUES

Fig. 3. (a) Residual voltage t = 2 s . (b) Residual voltage t = 420 ns.

residual voltage reaches its peak only after the occurrence ofthe discharge current peak, causing a time lag between the peaksof the residual voltage and discharge current, and the time lagincreases with steepness. This shows that there is a delay toattain the initial peak of residual voltage of the arrester and it iscaused by the capacitance of the arrester blocks. This influenceis verified with the studies performed on a metaloxide arresterof rating of 12 (Fig. 3).

V. CONCLUSION

The arrester model suggested by Micaela is more suitable andaccurate for the studies associated with the dynamic behavior ofthe metaloxide arrester prorated blocks especially for the cur-rent surges with a front time of 1 s or more. The modificationof this model facilitates the studies conducted for VFTOs. Afterincluding the arrester block capacitance, this model is used forthe estimation of residual voltages with VFTO. The finite-ele-ment method (FEM) is used to calculate the stray capacitance ofthe complete arrester assembly, which is more for higher ratedarresters; thereby, the time lag is more, causing inaction of themetaloxide surge arrester during the application of VFTOs.

REFERENCES

[1] Working Group 33/13-09, Very fast transient phenomena associatedwith gas insulated substations. CIGRE, 33-13, pp. 120, 1988.

[2] C. A. Christodoulou, L. Ekonomou, G. P. Fotis, P. Karampelas, andI. A. Stathopulos, Parameters optimisation for surge arrester circuitmodels, Inst. Eng. Technol. Sci. Meas. Technol., vol. 4, no. 2, pp.8692, 2010.

[3] M. C. Magro, M. Giannettoni, and P. Pinceti, Validation of ZnO surgearresters model for overvoltage studies, IEEE Trans. Power Del., vol.19, no. 4, pp. 16921695, Oct. 2004.

[4] F. Fernandez and R. Diaz, Metal-oxide surge arrester model for fasttransient simulations, presented at the Int. Conf. Power System Tran-sients-IPST, Rio de Janeiro, Brazil, Jun. 2024, 2001.