Even if the potential for dangerous mistakes is excluded, testing high voltage circuit breakers can be a hazardous task, not least because of the risk of induced voltages from adjacent circuits. However these risks are eliminated by a novel test technique that allows both sides of the breaker to remain grounded throughout the testing process.
The most important parameter in the operation of large circuit breakers is the timing of the main contacts. If this departs markedly from the values recommended by the manufacturer, it is a clear indication that the breaker needs servicing. In addition, errors in timing have an adverse effect on synchronization between phases in three-phase systems, and between multiple contacts in single-phase systems.
For these reasons, all circuit breaker test sets provide facilities for measuring contact timing. The conventional test method, which is widely used in the power industry, is simply to apply a DC test voltage across the contacts and monitor the resulting current flow, so that the instant of opening and closing can be determined.
For this method to work, however, only one side of the breaker can be grounded. If both sides are grounded, the test current flows in the earth circuit, and an accurate determination of the contact timing becomes impossible.
At first sight, this may not appear to be a major problem. Unfortunately, leaving one side of the circuit breaker ungrounded creates a major safety hazard. An error in setting up the test conditions may, for example, allow one side of the breaker to remain live when the contacts are open. Even more likely, the ungrounded side of the breaker may be energized by induction from nearby circuits.
Clearly, testing with both sides of the breaker grounded – dual-ground testing – is highly recommended. Indeed, it could be argued that dual ground testing is essential if the employers of those who carry out the tests are to demonstrate that they have carried out their duty of care toward their employees.
A number of methods have, therefore, been devised to permit dual-ground testing. One is the indirect measurement of contact timing, by performing the tests not on the circuit breaker main contacts, but on auxiliaries. Unfortunately, the values obtained in this way are not necessarily a reliable indicator of circuit breaker performance.
Another approach is dynamic resistance measurement, which involves the injection of large currents into the circuit breaker under test. While this method is, in some cases, compatible with dual-ground testing, it involves the use of test equipment that is inconveniently heavy and bulky, because it needs to be able to supply the large currents involved.
To address the shortcomings of these existing techniques, Programma, a part of the Megger organization, has developed a radically new method of circuit breaker testing. This is based on viewing the circuit breaker contacts as two plates of a capacitor that, in conjunction with the stray inductances in the contact assembly, form a resonant circuit.
The key to the operation of this new test method is that, as the contacts move, the capacitance between them – and hence the resonant frequency of the circuit – changes. The innovative Programma TM1800 circuit breaker test set operates by first applying a sweep frequency in the megahertz range to the contact assembly. This enables the baseline resonant frequency to be determined.
The instrument then tracks the changes in this frequency throughout the breaker operating cycle, and the information obtained allows the exact contact timing to be readily evaluated.
The key benefit of this method is that at the high test frequencies used, the external circuits connected to the breaker appear as high impedances. Grounding these external circuits therefore has no effect on the accuracy of the test results, or the ease with which they are obtained. In other words, dual-ground testing, with its inherently enhanced safety, is fully supported.
Additionally, because the impedance of the external circuits effectively isolates them from the contacts under test, it is never necessary to disconnect the breaker’s main circuits. This is a big advantage where bolted busbar connections are used, as these are difficult to disassemble, and can cause major problems if precisely the right torque is not used for each bolt during reassembly.
Results from the high-frequency circuit breaker test set are presented in exactly the same way as those from a traditional circuit breaker tester, in the form of a thick or thin line to show the status of the contacts. The tester is, therefore, easy to use and its results are easy to interpret. A further benefit of capacitive evaluation of circuit breaker timing is that, as no large currents are involved, the test set can be made compact, light and easily portable.
There’s no doubt that dual-ground testing of circuit breakers not only cuts costs, it also has the potential to save lives. Until now, it has been difficult and inconvenient to implement but, with the advent of the patented high-frequency testing method used in the Programma TM1800 circuit breaker test sets from Megger, these shortcomings have been completely overcome.
Given, in particular, the safety benefits, what possible reason can there be, therefore, for testing circuit breaker contact timing in any other way?
For further information contact:
Trusted Service – Powering Solutions
Protec Equipment Resources
1517 W.N. Carrier Parkway
Grand Prairie, TX 75050