Introduction
Circuit breakers are some of the most important components in modern electric power systems. The circuit breaker has to operate within extremely tight tolerances when a disturbance is detected in the network to protect sensitive and costly components such as transformers. They have to operate after months or in some cases years of inactivity. To ensure proper function and optimize network
reliability, reliable and efficient test instruments and methods are needed. New developments have made it possible to improve and re-evaluate conventional methods that sometimes involves time consuming and cumbersome process steps. The aim of this publication is to increase the understanding of circuit breaker testing.
Why test circuit breakers
Some of the most important of the many reasons for testing circuit breakers are to ensure they:
■ Provide protection for expensive equipment
■ Prevent outages that lead to loss of income
■ Ensure reliability of the electricity supply
■ Prevent downtime and darkness
■ Verify breaker performance
Substation breaker testing is an important task for any power utility. The breakers are there to facilitate the flow of current during normal operation and to interrupt current flow in the event of a fault. However, all electrically operated devices are, sooner or later, likely to experience some kind of failure. That
failure can be caused by many factors, including ageing and external faults. The power utility operator has to be prepared and
have a plan in place to handle every situation.
This document will help readers to understand what is involved with keeping circuit breakers operating at peak performance.
Breakers are mechanically sophisticated devices requiring periodic adjustments. The need for some of these adjustments can be
determined visually and they can be given the attention needed without testing.
However, in most cases, it will be necessary to carry out electrical testing to find out what is the cause of out-of-tolerance conditions. This guide primarily deals with electrical testing.
HV Circuit Breakers in a transmission scheme can be viewed as forming a tree starting with the generating station, fanning out to the transmission grid, to the distribution grid, and finally to the point of consumption.
The task for the utility is to generate power, transmit it and distribute it with maximum availability. While doing this, it is imperative that losses are kept to a minimum, and acceptable levels of power quality and safety are maintained. All of this must be done in an environmentally friendly manner. Breakers play an important part in making this happen. High voltage circuit breakers are extremely important for the function of modern electric power supply systems. The breaker is the active link that ultimately has the role of quickly opening the primary circuit
when a fault occurs. Often, the breaker has to perform its duty within some tens of milliseconds, after months, perhaps years of
idly being in circuit. Since RCM (reliability centred maintenance) and condition based maintenance have become the established
strategies for most owners and operators of electric power delivery systems, the need for reliable and accurate test instruments for
field use is clear.
Protection systems are put in place to detect all electrical faults or other abnormal operating conditions and they are coordinated to disconnect the smallest possible part of a power network in the event of a fault. With good system design, it should be possible to quickly restore normal operation. When a fault is detected by a protective relay and a trip impulse is sent to the breaker operating mechanism, the breaker has to function as specified and interrupt the current as soon as possible or severe damage may occur. The cost of damage caused by a malfunctioning circuit breaker can sometimes reach large sums.
Proper functioning of a breaker is reliant on a number of individual components that have
to be calibrated and tested at regular intervals. The trigger for maintenance intervals differs greatly between power utilities but the intervals are often based on time since last test, number of operations, or severity of fault current operations. Environmental considerations such as humidity and temperature, whether the breaker is located in a desert or coastal region, also play into the maintenance scheme.
Mechanical wear and lubrication often affects the performance of breakers, so being
able to trend mission critical parameters and compare these with factory thresholds
helps to verify proper breaker function.
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