Research on Two Stage Zero Sequence Inverse Time Current Protection and Its Setting Method

The principle and composition of traditional stage based fixed time zero sequence overcurrent protection are simple, but as the structure of ultra-high voltage power grids becomes more complex, its setting coordination becomes increasingly difficult. The zero sequence inverse time current protection has the characteristic of “larger current, faster action”. When there is a difference in the short-circuit current of adjacent lines, it can naturally achieve selective cooperation between upper and lower lines, and is gradually widely used in backup protection. In existing applications of zero sequence inverse time current protection, a single inverse time curve is used to form zero sequence inverse time current protection. This paper analyzes the widely adopted setting principle of “adopting unified inverse time characteristics and setting value for the entire network” in China, and points out that using a single inverse time curve to form zero sequence inverse time current protection may lead to the problem of long protection action time when high resistance grounding occurs. In response to this issue, this paper proposes a solution to form a zero sequence inverse time current protection using two inverse time curves, and proposes setting methods for each of the two inverse time curves. Theoretical analysis and numerical example verification show that this scheme can effectively improve the speediness of zero sequence inverse time current protection when high resistance grounding occurs, and has certain practical application value.