Primary high-current testing of relays with low ratio current transformers

This paper will serve as one of few references describing primary high-current testing of protective relays using low ratio current transformers. The tests 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. A report is given on the results of primary high-current tests of overcurrent, motor, and distance relays using low ratio CTs. The test currents ranging from 6 kA to 50 kA were used with current transformers with ratios of 50:5, 300:5, and 600:5. The paper compares the internal unfiltered event records with MATLAB simulations of the same cases.