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

This paper serves as one of few references describing primary high-current testing of protective relays using low ratio current transformers. Accurate measurement of high-current magnitudes requires high ratio current transformers. However, it is the practice to apply low ratio CTs based on small motor loads. The protection engineer then relies on the motor relay to trip for the enormous current of a phase-to-phase fault in the motor cable. The assumption is that fault current exceeding 300 times the CT rating can produce a secondary current large enough for a fast instantaneous trip. This leap of faith is based on the lack of knowledge of the highly distorted secondary waveform and how it is processed in the relay. 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. This paper also reports on the results of primary high-current tests of overcurrent, motor, and distance relays using low ratio CTs. Lastly, this paper compares the internal unfiltered event records with MATLAB/spl reg/ simulations of the same cases.