Input Source Error Concerns for Protective Relays

Capacitive coupled voltage transformers (CCVTs) and bushing potential devices (BPDs) have supplied high voltage relay potential circuits for many years. The transient performance of a CCVT has been analyzed, and solutions for their transient performance have been incorporated in relaying systems since the 1970s. Little analysis has been performed on BPD transient performance. A BPD has been tested and characterized to determine the parameters that affect the output voltage during system fault conditions. Current transformers (CTs) that saturate during system faults produce a nonsinusoid output that, when filtered, presents a reduced magnitude and phase shifted current to protective relays. The response of distance relays for various levels of CT saturation are analyzed and presented. Finally, utility experience with relays sourced by CCVTs and BPDs with the intent of validating the models and relay setting adjustments for input source errors are presented