Considerations for Dependability of the Motor Protection on Current Transformers Performance in VFD Applications

When generators or motors are started via variable frequency drive (VFD), 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 the consequences of low frequency on current transformers (CTs) performance and how this affects the protection performance as well. First, CTs are designed to operate at the nominal system frequency; however, when operating at low frequency, CTs could saturate at much lower currents; this can happen even at CT nominal current. Second, there are limits of the system frequency measurements in the protective relays, which will impact the correct current magnitude estimation, and therefore, the accuracy of the protection. And finally, protective relays need full power-cycle to measure current; power-cycle will be considerably longer at lower frequency, therefore affecting fault clearance time. This paper will educate engineers on how to consider CTs and protective relay's performance at lower frequencies to ensure adequate protection. It will also explore other solutions to address impact of low frequency on the CTs and protection.