W. F. Skeats

Circuit Breaker Recovery Voltages Magnitudes and Rates of Rise

This paper shows the conditions affecting the magnitude and rate of rise of recovery voltage at the terminals of an oil circuit breaker upon interruption of a short circuit, and explains their effect in quantitative terms. Factors are presented whose numerical value takes into account the number of phases involved in the short circuit and the ground connections of short circuit and generator, the decrement of short-circuit current previous to interruption, and the effect of unequal reactances in the direct and quadrature axes of synchronous machines. It is also indicated how to take into account the effect of displacement and of initial load current, and the effect of saturation is discussed very briefly. Magnetic oscillograms are presented which verify the most important of the points brought out. The most common locations and a rough idea of the magnitude of the capacitances affecting the rate of rise of recovery voltage for various types of short circuit are indicated. Calculations of the recovery voltage curve for several representative cases, at both low and high voltages, are presented, and in some cases cathode ray oscillograms are presented for comparison with the calculated curves. While no systematic data appear to be available at present regarding the effect of the rate of recovery voltage rise upon circuit breaker operation, several tests showing a very pronounced effect, some of them made by the authors and some by others, are reported briefly.

Effect of Restriking on Recovery Voltage

IT IS well known that when any portion of an electrical circuit is either opened or closed a transient condition will usually exist for some period, depending on the damping, before the circuit assumes its new steady state. In power circuits, such switching operations are the application and removal of faults or loads and the connecting or separating of various parts of a system. The transient conditions resulting from these switching operations give rise in some cases to overvoltages, which can be calculated by straightforward and more or less well-known methods, if the transient circuit parameters are known. However, in some instances voltages much higher than those predicted by such calculations have been obtained, and various explanations1–9 have been offered for these occurrences.

System Short-Circuit Currents Proposed New Calculating Procedure for Application of Interrupting Devices and Relays

THE current flowing in a short circuit in a simple a-c system can be expressed as a function of time in the form of three exponentially decaying components and a term which is substantially constant. These components are termed the subtransient, the transient, the d-c components, and the steady-state component. In actual systems all these terms are affected to some extent by the number of machines and the phase angles between them and also by their excitations. The phase relations between machines may change during the period of short circuit and thus introduce further complications. Automatic voltage regulators also introduce another factor which tends to increase the short-circuit current, since their action causes increase in machine excitation. Consequently, the accurate determination of short circuit current at any instant may be and usually is quite difficult.

The Oil-Blast Circuit Breaker

A further development of the explosion-chamber principle avoids the mixing of arc and oil in the main gap, utilizing the high oil pressure together with smooth and controlled flow of oil from the chamber to sweep away the arc, replacing it with a stream of clean oil. With this design tests show that a voltage of 44 kv. per inch across the working gap actually is reached.

Special Tests on Impulse Circuit Breakers

A novel circuit arrangement that supplies a large current to a circuit breaker under test and then subjects the open terminals to a high recovery voltage has been used in testing the 2,500,000-kva 287.5-kv impulse oil circuit breakers built for the Boulder Dam-Los Angeles transmission line. A description of this arrangement, which makes available several times the apparent power of the testing apparatus, is given in this paper, together with oscillograms which show details of the arc extinction process not previously known, and the results of a study of voltage distribution.

Practical Calculation of Circuit Transient Recovery Voltages

WHILE there have been a number of papers on recovery voltage and its effect on the operation of circuit-interrupting devices, and although the transient recovery voltage and its characteristic are assuming a more important place in the design and application of circuit breakers with the growth and increasing capacity of power systems, there has not been available to the average power-company engineer a convenient method for determining this characteristic. The present paper offers such a method and also tabulates capacitance data for the more important circuit elements.

Field Tests on High-Capacity Air-Blast Station-Type Circuit Breakers

IN January 1940 there was presented before the Institute a paper1 describing a new high-capacity air-blast circuit breaker. Since then, breakers of higher interrupting rating have been built, following out the general principles of design and construction disclosed at that time. As part of an organized development program, such breakers have been subjected to extensive interrupting tests under factory laboratory conditions. However, it is recognized that the final proof of the interrupting performance of high-capacity circuit breakers comes as a result of tests made on actual operating systems.

Dielectric Strength of Oil for High- Voltage Testing of Oil Circuit Breakers

THE purpose of this paper is to report on work undertaken in response to a request, made to the Institute, to determine the minimum dielectric strength of oil for high potential testing of oil circuit breakers. Many tests were made with 60-cycle and impulse voltages. The tests were made with a rod to plane gap, cone to disc gap, and on oil circuit breakers of commercial types and sizes. The results of these tests have been collected, analyzed, and are included in tabular form in the paper. The impulse strength of oil circuit breakers was found to be practically unaffected by the 60-cycle breakdown strength of the oil within the range of 16 to 30 kv, as measured in a standard test cup. The 60-cycle dielectric strength of oil circuit breakers is shown not to be changed in direct proportion to the differences in the dielectric strength of oil as measured in a test cup with 60-cycle voltage. The effect of low dielectric strength oil on voltage breakdown is discussed and conclusions are drawn which show that it is desirable to test all oil circuit breakers with oil having dielectric strength of 22 kv or higher.

Bushing-Type Current Transformers for Metering

This paper describes a new development in connection with bushing type current-transformer metering circuits. While this development utilizes the two-stage principle and has the same order of accuracy as that of two-stage current transformers, it is different from the conventional two-stage current transformers in that it does not require two-stage wattmeters and watthour meters, but may be used with any wattmeter or watthour meter. The principle and connections of the transformer are explained, and performance curves of a typical unit are given.

Design and Test of High-Speed High-Interrupting-Capacity Railway Circuit Breaker

THIS PAPER describes an oil circuit breaker built to the following specifications