Setting suggestions of inverse-time zero-sequence overcurrent backup protection for EHV and UHV AC lines

The setting principle of inverse-time zero-sequence overcurrent protection for extra high voltage （EHV） and ultra high voltage（UHV） alternating current （AC） lines is put forward according to the“six unified”line protection devices in State Grid Corporation of China. There is no difficulty in ensuring sensitivity for inverse-time zero-sequence overcurrent protection，and the key problem of setting principle is to solve the contradiction between selectivity and quickness. According to the action logic and characteristics of the inverse-time zero-sequence overcurrent protection，the concept of turning point current and the cooperation principle of the grounding distance protection and the inverse-time zero-sequence overcurrent protection are proposed. Fully considering the influence of matching time setting and minimum time setting on the protection action characteristics，combining the actual short circuit current level of EHV and UHV AC power grids and the transition resistance tolerance of the grounding distance protection，the selectivity of inverse-time zero-sequence overcurrent protection in low current range is combined with the selectivity of grounding distance protection in high current zone，which improves the ability of selective excision of the grounding fault in the whole current range. The principle of using directional element is put forward. The effectiveness of the proposed setting principle is verified by an example of a provincial power grid. The proposed setting principle has been applied in the actual power grid. © 2024 Electric Power Automation Equipment Press. All rights reserved.