Comparison between peak values of induction motor currents’ waveforms during nearby three‐phase short‐circuits and results from standardized calculations

Abstract

AbstractThis article shows a comparison of peak values of measured and computed waveforms of induction motor currents during nearby three‐phase short circuits with the results from the standardized calculations of those currents. The methods for computing short‐circuit currents according to IEC and ANSI standards are based on phasors at grid frequency, and the correspondent transient result is approximated to a simple sine wave plus a decaying DC component. However, the measured and simulated waveforms are far away from those simple waveforms. Thus, the peak values computed by standardized methods must be obviously different than the peak values of measured and simulated waveforms. It is herein shown that these differences are larger in smaller motors, and it is also shown that standardized methods to compute these motor currents are accurate enough although they are simply based on phasors at grid frequency. On the other hand, the maximum value of the first peak of motor contribution does not necessarily occur at the same time of the maximum first peak of grid contribution. This fact had been rarely mentioned, and an analysis about its influence was not found in the literature. Herein, it is shown that the influence of this point is not very important in the analyzed cases.