Component Reliability

Introduction

In electrical power transmission system, protection devices are provided to isolate the equipment from the system whenever there is a possibility of exceeding the limiting forces of currents and voltages. However, whenever fault develops in any or combination of equipment of transmission system, there is sudden disturbance of these forces resulting in an inrush of current and voltage build up in micro seconds which may result in fire in equipment and subsequent damage. In this chapter, practical cases that are taken from Andhra Pradesh state electrical utility are presented in support of the validity of the bathtub curve for analyzing the component reliability.

The forces may be so high that circuit breakers may implode, insulators may shatter, the equipment like current transformers (CTs) and capacitor voltage transformers (CVTs)/potential transformers (PTs) may explode, conductors may snap and entire sub-station may be on fire. Every equipment connected with power supply is susceptible for damage depending on the nature of fault and location of work. These forces show effects on other running equipment whenever a load is suddenly added or thrown off in the system.

Lack of performance by an item of its required function or functions is defined as failure. Failure can be a major one or minor. Major failure is defined as a failure of a switchgear or control gear which causes the cessation of one or more of its fundamental functions. A major failure will result in an immediate change in the system operating conditions, e.g., the backup protective equipment being required to remove the fault, or will result in mandatory removal from service within 30 minutes for unscheduled maintenance. Minor failure is defined as a failure of equipment other than a major failure or any failure, even complete, of a constructional element or a sub-assembly which does not cause a major failure of the equipment.

Failures of high voltage equipment may be considered either from a system, or from an equipment point of view. With a system approach, the important issue is whether a fault caused a system outage or not, and related consequences such as transfer of power to other paths, or even loss of supplied power. With an equipment approach, the focus is on failure mechanisms, properties of the equipment, etc. The definitions of “major failure” and “minor failure” relate to the performance of the equipment.

The analysis of engineering components connected with power industry requires consideration of the uncertainties associated with all the variables intervening in the performance criteria. Because of these uncertainties there is always a possibility that the component may not perform as intended. Statistical failure data collection and analysis offers a methodology to systematically study the probability of non-performance. The calculation of empirical reliability requires data collection of failed units.

Customized performance based design approach is one method that can be adopted for better quality with stringent pre-defined standards. Another approach will be the development of a codified procedure. Components fail randomly in service life either from sudden increase in applied load or the unpredictable environmental variations. Alternatively, components fail due to continuous applied stress.

A few products, i.e., power components, especially electrical equipment such as current transformers (CTs), capacitor voltage transformers (CVTs), lightning arresters (LAs), power transformers, show a decreasing failure rate in the early life and an increasing failure rate in the later stage. Despite the fact that the above said electrical equipment is expected to serve for a period of 25 years, they fail in the first year itself from the date of commissioning, due to defects in product design and manufacturing process.

The bathtub curve represents the hzf as the sum (superposition) of a decreasing hzf, a constant hzf, and an increasing hzf.

For component Reliability assessment the following four critical components of the power systems (APTRANSCO, 2005, 2007) are considered for analysis:

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  Power transformers.

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  Circuit breakers.

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  Instrument transformers such as CTs and CVTs, etc.

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  Surge arrestors.