Mapping Reliability Predictors of Low-Voltage Metal Oxide Surge Arresters Using Contour Plots

Pitshou Bokoro,Wesley Doorsamy

doi:10.1109/access.2020.3022596

Pitshou Bokoro, Wesley Doorsamy

Open Access

https://doi.org/10.1109/access.2020.3022596

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Abstract

Reliability assessment of MOSA devices requires accurate estimation of life distribution parameters or reliability predictors. Estimated reliability predictors consist of reasonable indicators of failure probability and mean life trends as a result of applied stress over a given period of time. Recent literature suggests finite-value or point-based estimation approach of reliability predictors in the context of continuously-applied distorted voltage stress to MOSA devices. However, this technique is prone to inaccuracies, and may not effectively reflect the impact of applied stress on reliability predictors. In this work, the use of contour plots is proposed as an alternative method for discrete mapping of estimated reliability predictors. Therefore, accelerated life tests - consisting of constant electro-thermal stress of: 85% of the reference voltage with and without harmonics, at the temperature of 135 °C for the time-period of 96 hours - are conducted on a set of low-voltage MOSA sourced from different manufacturers. The experimentally obtained times to failure are consistent with the two-parameter Weibull life distribution. The log-likelihood estimates on the Weibull probability density function are invoked using computational algorithms which yield the contour plots. These plots depict the reliability predictor mappings in the αβ plane. Results show that harmonic content in the applied stress causes extended peak amplitude which represents the point of highest probability of reduced reliability, and thus the highest accuracy point for the estimated shape parameter.

Highlights

The continued ac or dc field-induced conduction is reported to be one of the major causes of electrical degradation or reduced reliability of metal oxide varistor (MOV)-based surge arresters [1]–[4]
The results indicate at 90 % confidence level that: voltage harmonics cause the peak amplitude of the predictor maps to extend upward and its area to shrink and/or shift leftside
Contour plots approach provide an alternative technique to effectively determine the mappings of discretely estimated predictors

Summary

Introduction

The continued ac or dc field-induced conduction is reported to be one of the major causes of electrical degradation or reduced reliability of metal oxide varistor (MOV)-based surge arresters [1]–[4]. In the context of reliability assessment of MOSA devices, the knowledge of the most suitable life distribution − which best describes the behaviour or performance of MOSA devices under defined operating conditions (electrical and non-electrical stresses) − consists of an essential adopted step. Estimation techniques of reliability predictors such as currently proposed in the literature mostly suggest a finite value-based approach to determine these predictors [13] This approach is prone to innacuracies and incorrect reliability assessment. This work is aimed at mapping discretely estimated reliability predictors using contour plots This approach improves the existing estimation techniques and eliminates the possibility of inaccuracies associated with finite value-based estimation of predictors. The responses of discretely estimated reliability predictors to variables or additional factors in the applied stress are described in terms of shape mutation of the obtained maps

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