Study of overhead ground-wire shunting rates during grounding failure, for 500 kV substation grounding system design

Ensuring the prescribed grounding resistance for 500 kV substations has become difficult due to various factors including increased short-circuit capacity, reduction of substation area, and construction of substations in mountainous locations, making it necessary to drive electrodes deep into the ground at high cost. To find a solution to this problem, we reviewed overhead ground-wire shunting rates during grounding failure, using fault current distribution analysis as well as actual measurements using the newest in-service 500 kV substation system and a 1/100 scale model of this system. We found that the rate reaches 70%-80% under some conditions; this is greater than the 50% that has been the fixed standard value used to date. We have clarified that this result can be explained by taking into consideration relatively high grounding resistance and the number of circuits connected to a substation, and have established new guidelines for designing economical grounding systems.