Lessons learned in static var compensator protection

Static VAR Compensators (SVCs) are an increasingly common solution for power system voltage stability problems by providing rapid var support following system disturbances. From a protection perspective, an extensive protection system is typically necessary to ensure the SVC's operating range is available to provide rapid and reliable var support to the power system. SVC protection often involves novel applications of traditional protective relay schemes, and coordination of protective control functions with SVC controls. One example of this is the use of multifunction current differential relays to protect Thyristor Controlled Reactors (TCR) or Thyristor Switched Capacitors (TSC). In these applications some features of modern relay systems, such as harmonic blocking, may serve no purpose but can adversely affect the performance of the protection system if ignored or applied incorrectly. This paper provides an overview of common approaches to SVC protection focusing on TSC, TCR, and Harmonic filter branches. An analysis of event reports for TSC and TCR faults are presented to demonstrate the strengths and weaknesses of typical protection methods, and to highlight common misconceptions in the application of differential relays to delta connected reactive elements.