Effect of High Fault Currents on Protection and Metering

This chapter reviews the literature on effects of high fault currents on protection and metering equipment. It discusses the capabilities and limitations of existing short-circuit protection devices. High fault currents are well known to cause saturation of iron core current transformers (CTs). This can adversely affect the performance of system protection devices. CTs are intended to deliver a secondary current that is directly proportional to the primary current with as little distortion as possible. During normal operation, the CT secondary winding induces a magnetic flux that opposes and nearly cancels the primary induced flux. CT saturation may cause overcurrent relays to misoperate or fail to operate, resulting in a failure of the protection system. Protective relaying is an integral part of any electrical power system. The fundamental objective of system protection is to quickly isolate a problem so that the unaffected portions of a system can continue to function.