Abstract
Abstract The frequencies of natural oscillations formed in three-phase windings of electric power circuits vary over a wide range, depending on the internal and external connections of the circuits. Star and delta connection of the winding, insulated and grounded condition of the system, and the influence of the first and the last interrupting pole in three-phase circuit breakers are analysed in their influence on the natural frequencies excited in such multiple-coupled systems. The results of many tests on actual machines show excellent agreement with the calculated results. Since the natural frequencies of insulated circuits are found always to be higher than those of grounded systems and, in addition, are independent of the random character of the sequence of interruption of the three poles of a circuit breaker, it is expedient to base tests of the interrupting capacityof three-phase circuit breakers exclusively on ungrounded systems.
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