CCVT transient fundamentals

Potential transformers and coupling capacitor voltage transformers (CCVT´s) have been used successfully for providing voltage to the inputs of meters and relays since the 1960´s. At voltage levels above 138kV economics dictate that CCVT´s be used to provide secondary voltages to relays. As we´ll see later CCVT´s consist of a capacitive voltage divider along with a step down transformer and other associated equipment. Placing capacitance in series and shunt to the natural inductance of the power system leads to resonant circuits and RC time constants that affect the faithful reproduction of the primary voltage on the output terminals of the CCVT. These transients weren´t a problem for electromechanical relays, but with the advent of faster solid state relays and modern microprocessor relays CCVT transients became a problem that needed attention. This paper will discuss coupling capacitor voltage transformer design. What factors in the design influence the transient behavior of the CCVT. What factors in the power system contribute to the transient behavior of the CCVT. How CCVT transient behavior affects relay performance. Finally, how relay manufacturers take this transient behavior into account in relay design. A Capacitive Voltage Transformer (CVT) is a CCVT without carrier accessories. The paper will use these terms interchangeably. Even though there is a slight difference between the two, the portion of the device in that pertains to the transient response is the same.