Influence of Auxiliary Trip Relays and Battery Ground Faults on Power System Protection Security

The auxiliary DC power supply stands as a critical cornerstone within protective relaying and control systems. The reliability of this auxiliary DC system is paramount, as its failure can result in delayed fault clearance during power system faults. Beyond this, a malfunctioning auxiliary DC system can significantly impact the operation and reliability of the broader electrical grid, causing switchgear non-operation and inaccurate or suspicious status reporting to remote control centers. The auxiliary DC control power system comprises essential components, including batteries, battery chargers, distribution systems, switching and protective devices, as well as monitoring equipment. It necessitates proper sizing, design, and maintenance to function effectively. Despite advancements in relay technologies and the adoption of modern numerical protection relays, auxiliary trip relays continue to play a crucial role in circuit breaker tripping during fault events, aligning with specific scheme requirements within substations. The inadvertent operation of auxiliary trip relays, stemming from issues within the auxiliary DC power system, can lead to mis-operation of the protection system. This paper delves into real-world case studies from the Indian power sector, illustrating instances where unintended operation of auxiliary trip relays resulted in transmission element outages. It further explores the coordination of auxiliary trip relays with DC ground fault relays, presents techniques to mitigate the unintended operation of auxiliary trip relays, and discusses methods for monitoring ungrounded DC systems to detect transient ground faults.