High current low ratio

This article is one of the few references describing high current testing of protective relays using low ratio current transformers (CTs). Maximum fault currents on power plant auxiliary buses are routinely 40 kA and can be as high as 80 kA. Accurate measurement of high-current magnitudes requires high-ratio CTs. The rests show the limitations of Fourier and cosine filters used in microprocessor relays that extract the fundamental phasors and eliminate harmonics. The tests validate the operation of a cosine-peak adaptive filter designed to cope with the highly distorted saturated waveforms produced by the low-ratio CTs subjected to high current. The details of relay operation are shown in unfiltered event records of the test cases. This article reports on the results of primary high-current tests of overcurrent, motor, and distance relays using low-ratio CTs. The test currents ranging from 6-50 kA were used with CTs with ratios of 50:5, 300:5, and 600:5. This article compares the internal unfiltered event records with MATLAB simulations.