Impact of Instrument Transformer Secondary Connections on Performance of Protection System—Analysis of Field Events from Indian Power Sector

Protective relays are commonly connected to the secondary windings of instrument transformers i.e., current transformers (CTs), and/or capacitive voltage transformers (CVTs). The purpose of CT and CVT is to provide galvanic isolation from high voltages and reduce primary currents and voltages to a nominal quantity recognized by the protective relays. Selecting the correct instrument transformers for an application is imperative: failing to do so may compromise the relay’s performance, as the output of the instrument transformer may no longer be an accurately scaled representation of the primary quantity. Having accurately rated instrument transformer is of no use if these devices are not properly connected. The performance of the protective relay is reliant on its programmed settings and on the current and voltage inputs from the instrument transformers secondary. This paper will help in understanding the fundamental concepts of the connections of instrument transformers to the protection relays and the effect of incorrect connection on the performance of protective relays. Multiple real-world case studies of protection system mis-operations due to incorrect connections of instrument transformers are discussed in detail in this paper. All the case studies presented in this paper are the actual field events of high-voltage AC transmission networks of Indian power transmission sectors. Apart from the connection issue of instrument transformers to protective relays, this paper also discusses about the effect of multiple grounding of CTs and CVTs secondary on the performance of the protection system. We hope that the case studies presented in this paper will help the readers to analyze the problem through real-world challenges in the complex power system networks.