Digital Processing of Electromagnetic Transient Current in an Electric Machine

Specific features of electromagnetic transient processes in electric machines place raised demands on the algorithm of digital processing of short-circuit current in a small time window. The fast bus transfer system needs a procedure for rapid determination of the orthogonal components of a short-circuit current. This problem is solved here by directed suppression of all significant components of the current, except for one of the two elementary components of the imaginary fundamental frequency, the second component being suppressed as well. It is shown that decaying components with unknown attenuation coefficient are better suppressed using two-stage stop filters. Such a filter of orthogonal components has multiple sections, shifts the frequency along the imaginary axis, suppresses non-zero frequency components, and smoothes the output signal. The suppression sections are modified depending on the complexity of the transient process. The smoothing window, which also additionally suppresses the harmonics that are not suppressed individually, is selected in a similar way. The multi-section filter was tested against three-phase and phase-to-phase short-circuit currents in a synchronous electric machine.