A New Neutral Operation Mode Of 35kV Grids In Mountainous Area

35kV grid is an important power supply system in suburb and rural area of China, but ever with high trip-out rate from lightning strike. As the permission for no over-head earthing wire along total line,non-transposition,3-4 insulators only, and difficulty to reduce earthing impedance of line supporter,the line is prone to lightning strike and its lightning resisting level is not high. Besides, it's also difficult to improve lightning resisting level in both engineering and economical considerations for poor geographical conditions of line corridor in mountainous area of West China. Therefore, to reduce trip-out rate of 35kV grid, it's reasonable to minimize arc building rate of line, which is closely related to operation mode of neutral point.According to theoretical analysis and practical operation experience, most of trip-out from lightning strike is caused by inter-phase flash-over which is developed from single phase flash-over, and grounding via arc suppressing coil could effectively eliminate single-phase grounding arc and reduce arc building rate of line. So, it's definitely specified by DL/T 620—1997 that insulation mode should be applied for neutral point of 35kV grids when single-phase grounding current is less than 10A, and grounding via arc suppressing coil when greater than 10A.However, through a lot of cite measurements,calculations and analysis of three operation patterns of QilinBa substation and BeiGu substation, which is subsidiary to ChongQing QiNan Supply Bureau, it is discovered in this paper that, because of the unsymmetrical phase-to-ground capacity values of 35kV grids, severe voltage unbalance of three phases would occur even under routine condition when arc suppressing coil is regulated normally, while improving compensation of suppressing coil to rectify voltage imbalance would bring up new problem that arc suppressing coil can not extinguish arc from flashover effectively , which lead to high trip-out rate. Also it is found that when insulation operation mode is applied on neutral point, the symmetry of phase voltage meets routine operation requirement; while without consideration on voltage symmetry or deviation under normal state, usually regulating scale of suppressing coil can meet requirements to extinguish grounding arc effectively and reduce recovery velocity of in-fault phase voltage for a specified operation pattern. Based on situation that power supply won't be broken off temporarily under single phase-to-ground fault in grid with neutral point insulated or grounded via suppressing coil, a new operation mode for neutral point, which is suitable for 35kV grids in mountainous area, is proposed in this paper that neutral point is separated in routine operation, and grounded quickly via suppressing coil regulated beforehand when arc fault is confirmed after judgement on single phase-to-ground fault type, and once fault is eliminated then quit the coil to restore the insulation mode of neutral point. This new neutral operation mode can ensure both voltage balance and the effective extinguishment effect of arc suppressing coil .In this paper a lot of calculations have been done at transient course of the new operation mode of neutral point with outcomes that possible maximum over-voltage of system is obtained as well as possible over-voltage and impulsive current exerted on arc suppressing coil and switching device during the complete transient course of new operation mode. And the result indicates that the said over-voltage is within the limit of insulation coordination (4 p.u.), also the afore-mentioned impulsive current maximum is only 32.79A, which makes feasible the switching of arc suppressing coil in engineering according to researches. And besides, elementary research has been done at judgement on single phase-to-ground fault type with judging theory and corresponding circuit proposed, which lay a foundation for implementation of new mode. In short words, the new operation mode of neutral point is feasible and bears great academic and engineering significance in both enhancing operation level of grids and reducing trip out rate of line.