Investigation and Analysis of Protection Misoperations Due To Switching Transient In Dc Control Circuits

Since 1990s, Hydro One has widely applied microprocessor-based protection relays across its transmission and distribution systems. In addition to advanced protection capabilities, these digital relays have also provided valuable features such as fault records and event sequence records with timestamps, which are greatly helpful and convenient for event analysis.

With large scale applications, occasional misoperations were expected. These misoperations can be caused by various factors such as incorrect wirings, configuration errors, or hardware/firmware defects.

Around 2010, Hydro One began replacing old electromechanical relays with digital protections at major stations. After these digital relays were in services, several circuit breakers were tripped unexpectedly without operations of their primary protection schemes. Therefore, the operations were classified as misoperations.

Detailed investigations revealed a common root cause: protection relay contact inputs were falsely triggered by short duration (2–4 ms) trip signals. These signals were identified from switching transients in DC control circuits during normal operations of unrelated breakers or switches.

To verify the proposed solutions, EMTP simulations were conducted. Based on the simulation results, mitigation methods were implemented. Over the past 10 years, these solutions have been proven effective in preventing further misoperations.