Breaking the mould in circuit breaker testing
Verifying the performance of the high-voltage circuit breakers used in transmission systems is no easy task. Since the contact mechanisms are invariably fully enclosed, visual inspection is impossible without costly and time-consuming dismantling. And even electrical testing is problematic, not least because of the dangerous voltages that may be induced in the breaker under test from adjacent lines.
To address these safety issues, dual-ground testing, where both sides of the circuit breaker are grounded during testing, is now increasingly widely used. Indeed, some utilities have already made dual ground testing obligatory for work carried out on their plant.
Not all test sets are however, suitable for use under dual ground condition and some that are have shortcomings in terms of performance, portability and convenience. New technologies, some of which are the subject of patents held by Megger, have been developed to solve these problems.
Making accurate resistance measurements on circuit breaker contacts under dynamic conditions as the contacts operate involves injecting large currents, especially under dual-ground conditions where a substantial proportion of the test current may be shunted via the path provided by the two ground connections. Large currents are also needed to differentiate between the point at which the main contacts operate and the point at which the arcing contacts operate.
In the past, the current source for these tests has typically been a heavy power supply or a cumbersome secondary battery. The most recent development, however, makes use of one of the new generation of super capacitors as a current source. Available with capacities up to hundreds of farads, these capacitors are far lighter and more compact than secondary batteries, yet they can easily generate large currents for the time needed to carry out a circuit breaker resistance test.
For example, Megger’s new SDRM202 static and dynamic resistance module, which uses this technology and is designed for use in conjunction with the Circuit Breaker Analyser product line, weighs just 1.8 kg, yet it is capable of generating an output current of 200 A when used with the test leads supplied. The capacitor is continuously charged and it takes less than one minute to recharge it to full capacity after a test with the standard duration of 1.6 seconds, however, several tests can be carried out without waiting for full recharge.
In addition to resistance testing, a common requirement with high voltage circuit breakers is to carry out timing tests to ensure that all of the contact sets operate as nearly as possible simultaneously. While the timing can be inferred from resistance tests, accurate results can often be obtained more easily by using dynamic capacitance measurement (DCM), a technique that is fully compatible with dual-ground testing.
With the DCM technique, the circuit breaker contacts are treated as a variable capacitor that, during the test, is used to form part of a series resonant circuit. Under pre-test conditions, the circuit is arranged to resonate at the frequency applied by the test set, which with most instruments is typically in the 4 MHz range.
As the contacts move, the change in capacitance between them shifts the resonant frequency, resulting in a large change in the test current. The test set automatically translates this current change into contact status.
While very satisfactory in performance and widely used, the DCM technique as described is not applicable to circuit breakers that include grading capacitors. These capacitors shunt the circuit breaker contacts and can have values of 500 pF or more, which at frequencies in the 4 MHz range, behave for practical purposes as a short circuit. No meaningful results can, therefore, be obtained about the change in capacitance between the contacts as they move.
A novel solution developed and patented by Megger adapts the DCM technique to make use of a variable frequency test signal, and to configure the circuit for parallel rather than series resonance. With this arrangement, resonance is achieved at a much lower frequency, which means that the grading capacitor is no longer seen as a short circuit. As a result, timing tests can readily be carried out on all types of breaker, irrespective of whether or not grading capacitors are fitted.
With both sided grounded the induced current will not pass through the test instrument.
In all substations there is capacitive coupling from live high voltage conductors inducing currents in all parallel conductors. Through a disconnected circuit breaker with one side connected to earth and both disconnectors open this current can reach double digit values in mA. The induced current is sometimes called 'hum' current. This current can be large enough to hurt or be lethal to humans.
The first instruments to use this novel measuring technique for circuit breaker timing will be those in Megger.
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