Research Into The Neutral Point Grounding Mode Of Distribution Power Grid And Its Decision Algorithm

The decision of neutral point mode in distribution network is a widely concerned technical and economic problem, which should be considered from following three main fields, overvoltage deciding system insulation coordination, the effect in system reliability and in power continuity. Peterson coil and resistor are largely used as neutral point grounding devices. Factors including overvoltage, fault current, cooperation with relay, electromegnetic interference and safety of human have to be considered when the value of resistor is calculated. Tuning TCR Peterson coil, there are harmony currents because of thyristor, which can be limited in permitted level by a new design method: multiple shunt circuit method. Change TCR Peterson coil into three-winding construction and connect proper inductor controlled by switch in the middle winding, then appears adjustable inductor neutral point grounding device, which can extinguish fault arc and avoid unnecessary trip when instantaneity single phase fault occurs, and it can coordinate with relay to trip fault line when permanent fault occurs. Adjustable inductor grounding device is a promising neutral point grounding device.Neutral point mode is important to distribution reliability, it effects the trip ratio directly. The effect of neutral point mode to distribution reliability has not been calculated quantitatively. Power system is a grey system whose reliability parameters and operational parameters are statistic, so interval theory should be introduced to analysis the model of effect of neutral point mode to distribution reliability by interval function compare method based on relative superiority extended from interval number compare method, the model is proved by IEEE RBTS example. Further more the effect of neutral point mode to different connection modes in middle voltage distribution system typical reliability indices and customer withstand fault index which is a parameter to scale power supply continuity is analysized. Weighting multi-attribute decision arithmetic is given to decide distribution system neutral point mode, which is based on the analytic hierarchy process theory. The arithmetic includes overvoltage, the effect in system reliability and in power supply continuity. The result of analysis indicates that the adjustable inductor grounding mode is the best for distribution system consist of cable. According to the linear relation and shift relation of interval numbers, the rules of interval calculation are revised to degree of grayness.