Neutral-point Grounding Mode in Medium Voltage Network and Technique to Eliminate Single-phase Grounding Fault

摘要

It is comprehensive technology to choose an appropriate neutral-point grounding mode for a medium voltage power grid. The technology features a tight combination of theoretical research and industrial practice. It is the technical foundation and system engineering to ensure a power grid system to be operated economically and safely. This paper first introduces a series of neutral-point grounding methods in 6-66KV medium voltage power grid of China along with their advantages and disadvantages. Then, it presents an innovative technique that precisely diagnoses and find out single-phase ground fault and ferro-resonance in a neutral-point non-ground system and automatically eliminates them. Finally, the paper describes a comprehensive intelligence device that implements above technique. The most significant parts of the innovation include: 1) developing a new voltage method that eliminates electric arc by a reactor that limits overvoltage between faulty phase and the ground. This reactor can effectually prevent electric arc from reigniting and hence eliminate arc-grounding overvoltage and accomplish the reliability of power supply. 2) inducing the rule of the single-phase grounding fault in a non-grounded power grid system and establishing a mathematical model that characterizes the relationship of earthing resistance, phase voltage, and neutral-point displacement voltage. As a result, we got a criterion that can judge faulty phase. According to this criterion we have designed and implemented a diagnostic and protective program that can destroy the single-phase fault at the early stage of its forming and hence can effectually protect power network from high-resistance ground fault (HRGF) no matter how the earthing resistance varies. 3) adopting a new method to suppress resonance: when resonance happens, a reactor is intelligently added in parallel between the problematic phase bus and the ground to apart circuit from resonance point so that the resonance overvoltage is limited. The power grid protective device described above has passed a series of in situ tests of 10KV substations conducted by relevant government inspection agencies. These tests include an extensive and rigorous electrical performance test, an environmental type test, and an electromagnetic compatibility test This new technique and protective device are proven to be reliable and operational. A national-level new product appraisal was completed and a certification was granted. It is stated in the certificate that: “The device design is innovative. The device has a high protective performance. Its method of eliminating electric arc and resonance and its diagnose and protective techniques for HRGF are at the leading class level in China.”