Article

Reliability evaluation with weibull distribution on AC withstand voltage test of substation equipment

R&D Center, Tokyo Electr. Power Co., Yokohama
IEEE Transactions on Dielectrics and Electrical Insulation (Impact Factor: 1.36). 11/2008; DOI: 10.1109/TDEI.2008.4656231
Source: IEEE Xplore

ABSTRACT For the development of a ldquoshort-duration AC withstand voltage testrdquo, an insulation specification of substation equipment, there is a precise method of reliability evaluation using a Weibull distribution function. Regarding this method, there remains a subject of handling coexistence of multiple voltage levels. This paper first defines the two reliability evaluation methods, ldquoindependence methodrdquo; and ldquoaccumulation methodrdquo, applying to Weibull evaluation for coexistence of multiple voltage levels in relation to their physical meanings. Next, the influence of the Weibull parameter values are examined on the cumulative fault probabilities and test voltages calculated using these methods. When the time shape parameter a>1, the accumulation method gives higher values than the independence method; When a=1, the two methods give the same values; When a<1, the former gives lower values than the latter. Then, appropriate reliability evaluation methods are investigated for various insulation media and insulation structures of substation equipment from the viewpoint of inception and development mechanisms of dielectric breakdown and partial discharge. According to the result of engineering evaluation of the presently available data, the independence method may be appropriate for both gas insulated switchgear and oil-immersed transformers.

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The second purpose of this chapter, closely related to the first, is to highlight the rationale behind a proper and accurate selection of a reliability model for the above devices, namely a selection which is based on phenomenological and physical models of aging, i.e., on the probabilistic laws governing the process of stress and degradation acting on the device. This “technological” approach, which is also denoted in the recent literature as an “indirect reliability assessment” (IRA), might be in practice the only feasible in the presence of a limited amount of data, as typically occurs in the field of modern power system. Although the present contribution does not address, for reasons of brevity, the topic of model or parameter statistical estimation, which is well covered in reliability literature, we believe that the development of the IRA is perfectly coherent—from a “philosophical” point of view—with the recent success and fast-growing adoption of the Bayesian estimation methodology in reliability. This success is proved by the ever-increasing number of papers devoted to such methodology, in particular, in the field of electric and electronic engineering. Indeed, the Bayesian approach makes use of prior information, which in such kind of analyses is provided by technological information available to the engineer, and—as well known—proves to be very efficient in the presence of data scarcity. Loosely speaking, IRA is a way of using prior information not (only) for random parameter assessment, but for a rational “model assessment”. In the framework of the investigation of innovations in reliability analyses regarding modern power systems, the present chapter takes its stimulus from the observation that the modern, deregulated, electrical energy market, striving toward higher system availability at lower costs, requires an accurate reliability estimation of electrical components. As witnessed by many papers appearing on the subject in literature, this is becoming an increasingly important, as well as difficult, task. Indeed, utilities have to face on one hand the progressive aging of many power system devices and on the other hand the high-reliability of such devices, for which only a small number of lifetime values are observed. This chapter gives theoretical and practical aids for the proper selection of reliability models for power system components. 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