Research Of The Transformaer Neutral Grounding Method Impact On The Reliability Of Power System

The power system neutral grouding is a very wide range covers comprehensive technical problem which has a great relationship with voltage, power structure, insulation levels, reliability, etc. The part transformer neutral grounding commonly used by 110kV system in China had advantages:circuit breaker interrupting capacity was not limited by the current limit single phase; zero-sequence protection was simple and reliable; interference on the communications was little. However, the neutral point protection, gap in parallel with the arrester, can not be achieved reliable over-voltage protection. The main problem of the protection is the gap difficult to match with the arrester, ground fault would cause the gap breakdown error and extended power outages, otherwise no action on gap lead the arrester explosion or equipment insulation damage. It is need that study better mode of neutral grouding.110kV transformer neutral point grounding though reactor can not only prevents effective ground system the occurrence of "lost groud" and reduces the overvoltage threat to the neutral insulation, but also obviously limits the neutral short-circuit current that flows through the main transformer and improve the transformer ability of anti-short shocks. Neutral point grounding though reactor not only has all the advantages of part transformer neutral grounding, but also to overcome the shortcomings. There is rich running experience of grounding through coil and high-impedance on the 63kV network abroad,500kV autotransformer neutral point grounding by small reactor method has also been a long run of success and achieved good results in China. About the choice of reactance on neutral point there are different views in the literature. The key technology is also far from comprehensive coverage, is still not perfect, is necessary to do further research.In this paper, the zero-sequence-impedence changes of transformer and power system are discussed in the condition of grouding by reactor, and the effects of reactance on the short circuit, zero-sequence protection and the insulation level of the neutral point of transformer are also analyzed. By analyzing the simulation results of EMTP, it can be proved that grouding by appropriate reactance can reduce the magnitude of system short circuit current, degrade the insulation level of neutral point of transformer; meanwhile the insulation level of the neutral point of transformer can maintain in 35kV, and MOA can be used to protect the neutral point against the over-voltage. At last, the range of the reactance is recommended as about one third of zero-sequence impedance of transformer.