This paper reviews the C and K bushing relay accuracy ratings for current transformers (cts) and their implications in relay application. The paper relates the concept of knee-point and the 10 ampere excitation voltage of a conventional excitation curve to the actual secondary waveforms produced at and above the ANSI voltage rating. The paper uses the volt-time area concept to introduce ct saturation and the voltage rating using an idealized B-H curve. Computer simulation
using the actual shape of the B-H curve is then introduced and used to analyze specific ct applications in transformer and generator differential relays.
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