Performance of CTs and relays at low frequencies

Abstract

When generators or motors are started via variable frequency drive (VFD), the initial frequency is very low, on the order of few hertz, and can be low for a considerable amount of time or even permanently. It is not well recognized in the industry about consequences of the low frequency for the currents transformers (CTs) performance and how this affects the protection performance as well. First of all, CTs are designed to operate at the nominal system frequency, at lower frequency CTs will saturate at much lower currents, this can happen even at CT nominal current. Secondly, there are limits of the system frequency measurements in the protective relays, which will impact the correct current magnitude estimation and therefore accuracy of the protection. And lastly, protective relays need full power cycle to measure current and power cycle will be considerably longer at lower frequency, therefore affecting fault clearance time. This paper will educate engineers on how to take into account CTs and protective relay's performance at lower frequencies to ensure adequate protection. It will also explore other solutions to address impact of the low frequency on the CTs and protection.