Capacitive voltage transformer: transient overreach concerns and solutions for distance relaying

Abstract

Capacitive voltage transformers (CVTs) are common in high-voltage transmission line applications. These same applications require fast, yet secure protection. However, as the requirement for faster protective relays grows, so does the concern over the poor transient response of some CVTs for certain system conditions. Solid-state and microprocessor relays can respond to a CVT transient due to their high operating speed and increased sensitivity. This paper discusses CVT models whose purpose is to identify which major CVT components contribute to the CVT transient. Some surprises include a recommendation for CVT burden and the type of ferroresonant-suppression circuit that gives the least CVT transient. This paper also reviews how the source impedance ratio (SIR) affects the CVT transient response. The higher the SIR, the worse the CVT transient for a given CVT. Finally, this paper discusses improvements in relaying logic. The new method of detecting CVT transients is more precise than past detection methods and does not penalize distance protection speed for close-in faults.