Transient Induced Loss of Protection During Fault

Eversource Energy experienced an event where System 1 and System 2 protection failed to operate due to a significant DC system surge due to a high energy lightning strike. Analysis of the lightning-related transient event indicated electrical relays and other Protection & Control (P&C) equipment were subjected to surge conditions that exceeded the industry standard design for surge withstand ratings. Root-cause analysis revealed severe protection system equipment failures, unintended operation and/or mis-operation at a critical 345kV substation facility. System 1 and System 2 protection groups shut down because of the lightning-strike-induced surge at one terminal of a 345 kV transmission line. Neither System 1 nor System 2 local relay protection cleared the SLG fault resulting from the lightning strike. The fault continued for over 1.5 seconds until protection at the remote terminals tripped as designed via time-delayed elements. Eversource conducted extensive on site testing and collaborated with relay vendors to conduct forensic analysis. Both manufacturers were asked to conduct similar factory acceptance tests with an emphasis placed on the surge withstand specifications of each relay. Additionally, Eversource partnered with an industry expert to perform detailed grounding system modeling and transient analysis. The analysis simulated the lightning event to determine the resulting Ground Potential Rise/Ground Potential Difference (GPR/GPD) levels across the substation yard and to identify the mechanism upon which the surge was transferred to the relays. It became evident to Eversource that protective relay systems, when exposed to surges beyond their specified criteria, may not operate as intended or fail. This paper documents the event analysis and design practice modifications made by Eversource to limit coupling of transients from the ground system to secondary AC and DC control cables via implementation of improved cable shielding and shield grounding methods.