CCVT failures and their effects on distance relays

Distance relays rely on accurate voltage and current signals to correctly determine if a fault is within their zone of protection, as determined by the impedance reach setting. The current signal provided to the relay comes from a current transformer (CT), which is a simple device consisting of a steel core and wire wrapped around that core. At high voltage levels, the voltage signal typically comes from a coupling-capacitor voltage transformer (CCVT), which is more complicated and less reliable than a CT. Although a CCVT is more complex than a CT, very little additional attention is typically given to monitor the performance of a CCVT. Typically, loss-of-potential (LOP) logic is used to detect a loss of the voltage signal to the relay, but the LOP logic does not effectively monitor and alarm for transient CCVT errors, errors small in scale, or errors that develop gradually. In this paper, we review three unique CCVT failure events and discuss relay performance during these failures. We analyze the performance of two different distance relays during a CCVT failure in which the voltage applied to the relays was erratic and influenced the frequency tracking in one relay but not the other. Next, we discuss an event in which a CCVT transient led to a relay overreach but also revealed that the CCVT was failing. Last, we analyze an event in which a CCVT failure caused a standing measurement error prior to a phase-to-ground fault and evaluate the influence of a standing unbalance of the directional elements of the relay. From these events, we offer solutions for monitoring CCVT performance that include steady-state monitoring as well as transient characteristics to look for while analyzing events.