EVALUATION UNAVAILABILITY DUE
TO AGING FAILURES


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Abstract

The paper presents a method to evaluate unavailability due to aging failures and incorporate that in power system reliability evaluation. it use of normal and weibull distribution for modeling aging failures probability distribution functions. The defined unavailability of aging failures has a consistent form which is the same as that for repairable failures.

INTRODUCTION

There are two failure modes for a power system component: 1) repairable and 2) nonrepairable.
Almost all of the methods and tools presented so far have only considered the repairable failure mode. These failures characterized with average failure frequency and repair time. These characteristics are constant with varying time. Nonrepairable failures including aging failures have not repair time and if system fails due to aging failure, it will be died forever. the aging failures in the young age systems are less important and can be neglected. But in old age systems they have major role in availability assessment. Therefore neglecting them in old systems leads to underestimation of system risk.

CASE STUDY

The three representative transformers were used in this example. They are of the same type at the voltage level 230/12 kV. Based on 15 years' historical records in the BC Hydro reliability database, this type of transformer has the following average repairable failure data: the failure frequency is 0.0834 failures/year and the repair time 59.06 hours. A posteriori normal distribution was used to model aging failures.

CONCLUSIONS

The paper presents a method to incorporate aging failures in power system reliability evaluation. It includes development of the calculation approach with the two possible probability distribution models for unavailability of aging failures. The defined unavailability of aging failures has a consistent form as that for repairable failures and therefore it is simple and straightforward to include it in system reliability assessment. This modeling is easily added in existing reliability evaluation techniques and tools. The differences between the two models for unavailability of aging failures using normal and Weibull distributions have been discussed.