RE: Digital Testing of High Voltage Circuit Breaker
The functionality of high-voltage circuit breakers is tested in high-power laboratories. Due to the necessary power and the physical size of the equipment, testing is rather expensive and time consuming. The steps followed so far by the authors in order to enable the digital testing of HV circuit breakers are described in this paper. At the end of the article, examples of digital testing are also presented.
Electrical Energy, Transients and Circuit Breakers.
With the introduction of alternating current (AC) electrical energy as a versatile power source for every conceivable application by the end of the 19th Century, the problem of transporting and distributing this energy arose /a/. In the case of changing the topology of a power system and protecting against (total) failure, the Circuit Breaker (CB) is an irreplaceable element. Although it is commonly said that: "the circuit breaker opens the circuit", it is in fact the electric-arc (arc for short) formed inside the circuit breaker, which interrupts the circuit current. How the arc is able to interrupt a (short-circuit) current is known through many years of practical experience and from the science of plasma physics e.g. /b,c,d,e,f,g,h,i,j,k/. However, since many energy exchange processes play a role in the extinguishing process of the arc, we are still unable to predict, with a near to 100% probability, whether a (newly built) circuit breaker will interrupt a certain current in a specific circuit. It is for this reason that circuit breakers are put to the test during the design and proving stages. These tests are carried out in so called 'High Power Laboratories' /l/. High Power Laboratories test especially High-Voltage Circuit-Breakers in a separate test circuit, since in-grid-testing would jeopardize normal operations of the power system. Test circuits try to simulate the most conceivable network or circuit conditions /m/. This is quite difficult since the electrical phenomena which occur when a circuit is interrupted are rather complicated and depend on numerous network (and arc) conditions. The most harsh conditions occur when a circuit breaker has to interrupt a short circuit current. At first the breaker is subjected to a high current which heats it up considerably (or the arc for that matter) and by means of the accompanying Lorentz forces parts are put under great mechanical stress. When the breaker interrupts the current (at a natural current zero) the subsequent Transient Recovery Voltage (TRV) subjects the breaker (or former arc channel) to a high dielectric stress. Both phenomena, i.e. the high current and the high (transient) voltage, have to be withstood by the breaker and cannot be avoided in any way.
With the advancement of power system, the lines and other equipment operate at very high voltages and carry large currents. High-voltage circuit breakers play an important role in transmission and distribution systems. A circuit breaker can make or break a circuit, either manually or automatically under all conditions viz. no-load, full-load and short-circuit conditions. The American National Standard Institute (ANSI) defines circuit breaker as: "A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal circuit conditions such as those of short circuit". A circuit breaker is usually intended to operate infrequently, although some types are suitable for frequent operation.
THE NEED FOR TESTING
Almost all people have experienced the effects of protective devices operating properly. When an overload or a short circuit occurs in the home, the usual result is a blown fuse or a tripped circuit breaker. Fortunately few have the misfortune to see the results of a defective device, which may include burned wiring, fires, explosions, and electrical shock.
It is often assumed that the fuses and circuit breakers in the home or industry are infallible, and will operate safely when called upon to do so ten, twenty, or more years after their installation. In the case of fuses, this may be a safe assumption, because a defective fuse usually blows too quickly, causing premature opening of the circuit, and forcing replacement of the faulty component. Circuit breakers, however, are mechanical devices, which are subject to deterioration due to wear, corrosion and environmental contamination, any of which could cause the device to remain closed during a fault condition. At the very least, the specified time delay may have shifted so much that proper protection is no longer afforded to devices on the circuit, or improper coordination causes a main circuit breaker or fuse to open in an inconvenient location.