Research And Decision-making About The Neutral Point Grounding Mode For Medium-voltage Distribution Network

Neutral point grounding mode for distribution network relates to aspects ofdistribution system, and is closely related to the reliability of power supply, relayprotection, overvoltage protection, communication interference and electrical safety, toensure human safety, equipment safety and reliability of the system and stability, theabove factors should considered for the selection. With the rapid development ofChina’s medium voltage distribution networks, cable coverage has big promotion,network structure is strengthened, the degree of automation is increasing continuouslyand new energy widely access medium voltage distribution network, neutral pointgrounding mode has been diversified, how to suit one’s measures to local conditions forthe selection of neutral grounding mode has important meaning for the safe and stableoperation of distribution network.By comparing the domestic and international present application of neutral pointgrounding mode for medium voltage distribution network, analyzed the single-phasegrounding fault characteristics of different neutral grounding mode, calculated faultcurrent, power frequency overvoltage and intermittent arc grounding overvoltage underdifferent conditions, and established corresponding models, got a conclusion that thefault current and abnormal voltage differences of the neutral point grounding mode ismainly because the contribution of the zero sequence impedance is different when thesingle-phase grounding fault happened. In addition, studied the different neutralgrounding mode of relay protection, safety protection measures and design principles,compared advantages and disadvantages of various line selection methods and alsoanalyzed the influence factors of line selection accuracy for small current groundingsystem; put forward a resistance value selection method for neutral point groundedthrough resistor system, that can guarantee sensitive and reliable action of zerosequence current protection, over voltage does not exceed a certain range and themaximum the fault current in safety requirements as constraints.Built a fault trip probability line model for distribution network with differentneutral grounding modes, analyzed the line selection accuracy for the grounding modes,used the radiation network as an example, evaluated the reliability of different neutralpoint grounding mode in different cable percent. Based on the reliability evaluationresults, put forward a kind of neutral point grounding mode decision method which made the reliability and economic coordination. Combined the reliability evaluation,safety protection and construction and operation cost of distribution network undervarious neutral point grounding modes, through the whole life cycle cost and reliabilityindexes coordinated to achieve neutral grounding mode decision, verified the feasibilityand effectiveness of the decision making method for distribution network, provided atheory basis for neutral point grounding mode decision making.For the new type distribution network which has distributed generation access,established equivalent model for a DG access and multiple DG access, for different DGaccess capacity, location and connected transformer grounding mode, throughtheoretical analysis and simulation research of DG access, studied the influence whenDG access on the distribution network with different neutral grounding mode, gave thematching principle of neutral grounding mode for DG connected transformer and maintransformer of distribution network.