GSU transformer - Current transformer connections and differential relay applications

The application and connection of current transformers for electromechanical differential circuits creates vulnerability for mis-operation under rare but identified scenarios. As a lesson learned from a real-world incident, discovery of the vulnerability occurred during the investigation of a generating unit trip from a nearby switchyard fault immediately outside a generator step-up transformer (GSU) differential zone of protection. Problem resolution is correctable or minimized with the available features of microprocessor differential relays, with sufficient winding inputs, and the proper application of current transformer connections. However, shortcuts in the design and installation of new circuits and microprocessor relays can introduce similar vulnerabilities.