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composition changes continually reflecting the customers' pattern of using various appliances and devices. It depends on the cus¬ tomer's lifestyle,the weather, ...

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Evaluation of Surge Degradation of Metal Oxide Surge Arrester Article  in  IEEE Transactions on Power Apparatus and Systems · May 1982 DOI: 10.1109/TPAS.1982.317164 · Source: IEEE Xplore





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teristics have been accumulated. Some of the reported component data are reviewed and summarized in a table. However, this is only a small part of the problem. The exact composition of the load is often very difficult to estimate. Load composition changes continually reflecting the customers' pattern of using various appliances and devices. It depends on the cus¬ tomer's lifestyle, the weather, the state of economy, and many other factors. It is important to estimate the load composition at time of critical interest such as under a heavy or light load condition. Much work still seems necessary for a reliable estimation of load com¬ position. Even if the load composition were known exactly, it would be impractical to represent each individual load component, as there are usually many thousands of components. There is a compu¬ tational problem. Even if one succeeds in getting relatively simple models of each component, if they are not of similar mathematical forms, the model of the composite load will be impractically complex and will require considerable mathematical processing to obtain a reasonably manageable overall system model. Thus, in one's zeal to obtain the optimum and most elegant form of a component model, one may aggravate the difficulties of the ag¬ gregation problem. There is, in fact, a problem even in combining models of the same type in the case of dynamic loads such as induction motors. One might be inclined to give up this constructive approach and to turn directly to measurements of the actual behavior of the major bus loads. But this also has its limitations. In the first place, it is difficult to make voltage changes of much more than ± 10%. One is often interested in a much larger range, especially on the low side. Frequency changes of any appreciable magnitude are practically impossible to make except by special isolation of, possibly, un¬ typical loads. In the second place, unless the load composition is

analyzed in some detail, and unless buses having loads of fairly different compositions are measured, there will be no under¬ standing of the results so that they can be extrapolated to different conditions. Thus, one cannot escape a necessity to relate bus load behavior to component behavior. Further, there are always con¬ tinual variations in the load itself and slight transient effects that complicate the interpretation of results. In spite of these limitations, such field measurements are essential as a final, even if only partial, check on the validity of load models, however they may have been obtained. Many measurements have been reported in the literature. Some of the reported results of field tests are discussed. It is hoped that this paper will encourage further work in the area of load modeling. Acknowledgments The preparation of this paper was partly funded by a research contract DEAC01-28ET29028, "Physically Based Load Modeling Methodology", from U.S. Department of Energy. The authors wish to thank Dr. T. Trygar of DOE for his support.

81 SM 325-0

April 1982, p. 978

Evaluation of Surge Degradation of Metal Oxide Surge Arrester

Y. Fujiwara, Y. Shibuya, M. Imataki and T. Nitta, Member IEEE Mitsubishi Electric Corporation, Amagasaki, Japan

In the present paper, the surge degradation of zinc oxide elements under a constant ac stress is investigated experimentally. Two types of elements having different degradation patterns are studied concurrently.Formation I the standard material in production and Formation II a new material found to show virtually no ac de¬

terioration. The surge degradation with ac stress is formulated based on experimental data. Fig. 1 shows an example of the leakage current growth in a long term test. The full lines indicate the case, that 10 shots of 8 x 24 fis surges were superimposed to ac stress, the dotted lines indicate the case of ac stress only. Since the difference between full and dotted lines, m, is kept constant throughout the experiment, m is used as an indicator of the surge degradation. The value of m depends on the surge waveform, surge current, temperature, number of shots and ac stress level. The dependence of m on the number of surges N takes the form m a + b(N *\), where a and b are constants which depend on other factors. The experimental results are summed up in a set of figures so that the degradation of M.O.A. can be estimated for any given condition. It is shown that the overall degradation during the life of M.O.A. is much smaller in Formation II compared to Formation I. The influence of degradation on the life of M.O.A. is investigated on the basis of a dynamic thermal stability [2] considering the =

temperature rise (A7 60°C) due to switching surge absorptions at the elevated ambient temperature {T0 60°C). Fig. 2 shows the estimated permissible number of surges during the life of M.O.A. by Formations I and II, respectively. Comparing (a) and (b) of the figure, the curve of applied voltage ratio 80% for Formation II is comparable to that of 65% for Formation I assuming the life of 50 years. This shows the performance of M.O.A. is expected to be greatly improved by using the Formation II elements. =


References [1] F. S. Sakshaug, et al., IEEE Trans., vol. PAS-96, No. 2,1977. [2] S. Tominaga, et al., IEEE Trans., vol. PAS-99, No. 4,1980.








Fig. 1 Leakage current growth observed in the two types of elements, Formations I and If. (Surge: 8 x 24\is, 440 A/cm2, Applied voltage: 85%, Temperature: 81°C1

Metal oxide surge arrester (M.O.A.) utilizing highly nonlinear zinc oxide element is an innovative surge protection device in power systems [1]. However, the zinc oxide elements show a degradation under a constant ac voltage and by high current surge operations.



mix, the two-term Edgeworth expansion gives a very good ap¬ proximation down to a LOLP value of 5 x 10~4. For low forced-

outage rates (/.,) calculations, the Texas Electric power system was used with L, 0.1, 0.05, and 0.02. As the FOR's decrease, the =

Edgeworth series becomes progressively more inaccurate with negative values present when the L, 0.05 and 0.02. For very small power systems, the El Paso Electric power system was used for calculations. Here, inclusion of higher-order terms makes the Edgeworth expansion less accurate. For studying the effects of adding large units to an already present generator mix, Public Service of Oklahoma was used. Inclusion of large units makes the Edgeworth approximation less accurate. Analytic studies demonstrated explicitly origins and criteria for the above observed inaccuracies. In summary, for the case of a finite number of generators (a,) Nand low forced outage rates /.,, we have shown why adding a generator much larger than those already present makes the Edgeworth-type expansion more inaccurate than adding one of the optimal size, where: =





15 000 MW), but can be very inaccurate for small systems or those with low forced outage rates. This is because the approximating Edgeworth-type series are ap¬ propriate only for continuous probability densities, while discrete lattice-type density functions describe a typical power system's probability properties. This paper investigates these inaccuracies for small systems (

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