A case study on Capacitive Voltage Transformer (CVT) behaviour on multiple earthing of CVT secondary circuit in a 400 kV Transmission line.

Abstract

The neutral point of secondary circuits of Instrument transformers e.g., Capacitive Voltage Transformer and Current Transformer (CVT & CT) must be earthed, IEEE emphasises that the secondary of instrument transformer must be grounded to protect the connected equipment and to ensure safety of working personnel. It recommends grounding of all in-service secondary circuits at the first point of its application (for CVT, typically a relay or meter) to counter the rise in electrostatic potential in secondary circuit due to stray capacitance. On the contrary, if the secondary neutral point of CVT has multiple earthing, the output voltage is not a replication of primary voltage during fault. This leads to mal-operation or non-operation of protection relays. As no discrepancy in secondary voltages is sensed by relays during normal operation, hence multiple earthing remains unnoticed and thereby compromising the integrity of relay operation. In this case study the behaviour of CVT having multiple neutral earthing has been illustrated, based on the site experience along with disturbance records and relay data. This paper, based on field data and vectorial mathematical analytics establishes the causes of distortions in the CVT secondary voltages due to multiple earthing on its neutral, causing maloperation of protection/relay. Study conclusively brings out correlation between CVT secondary magnitude, phase angle and harmonic generation and suggests formulation of a suitable algorithm/logic in Intelligent electronics device (IEDs)/relay for automatic identification of problem. There is a global trend of automation in the Power system to improve reliability and security. Automatic identification of multiple earthing as illustrated in this paper is a step toward the assimilation of automation in the power system that helps in timely rectification of faults that otherwise cause maloperations or outage of important elements.