High Sensitive Zero-sequence Stage Current Protection for Low-resistance Grounding System

Abstract

Due to the over-rated value in a low-resistance grounding system, the high-resistance earth fault cannot be reliably protected by traditional definite-time zero-sequence overcurrent protection. Within this context, this work evaluated the distribution characteristics of zero-sequence currents in the case of single-phase-to-ground fault in a low-resistance grounding system. The results showed that regardless of the transition resistance of the fault point, the magnitude of zero-sequence current in the fault feeder was always close to the zero-sequence current of the neutral point, which was more than ten times higher than the zero-sequence current in the healthy line. Moreover, a high-sensitivity zero-sequence overcurrent protection method was proposed to decompose the existing zero-sequence overcurrent protection into multi-stage definite-time zero-sequence overcurrent protection. During this novel protection method, the algorithm decreased the minimum start current setting value, increased the protection sensitivity at high-resistance ground fault, and improved the protection selectivity at low-resistance earth fault with the use of horizontal coordination between each outlet protection. Therefore, the specific setting principles and calculation method of the specific value of the protection current setting and the operation time of each stage were given according to engineering reality. And the simulation proof of maturity of the protection tolerant transition resistance was almost up to 1.5kΩ. The method did not need to re-develop and install a new device, and the original protection device of the system could be realized through reasonable setting, which was convenient for field application.