| With the development of our national economy, the contradiction between power supply and demand is growing. The existing lOkV distribution network has many limitations in the capacity of power supply and other aspects. According to actual conditions in China, it has been demonstrated that 20kV voltage is technically feasible and economically advantageous. Neutral grounding methods is one of the integrated technologies in the process of 20kV medium voltage distribution network boost transformation facing, which has great relation with the relay protection during the process of the boosting transformation, relevant equipment selection, investment budget, reliability in power supply, etc. When the distribution network voltage level increased from lOkV to 20kV, neutral grounding methods and the corresponding relay protection measures should be reselected reasonably based on industry standards and the operating experience of existing lOkV medium voltage distribution network, combined with the actual state of 20kV distribution system. This thesis mainly studies the subject as following:1.In this thesis, five kinds of 20kV distribution system are established, including both urban power grids with higher load density majored with cable outlet and rural power grids with lower load density majored with overhead line outlet. According to the outlet type of 20kV side in substation, different neutral grounding options for different structure of 20kV power distribution system are given.2.The selection of the resistance of the resistor and the relay rehabilitation of 20kV distribution system with neutral grounding by middle resistor are discussed. Considering overvoltage, relaying protection, heat capacity of middle-resistance, communication interference and personal safety etc, the principle about determining the magnitude of middle-resistance is made; With simulating computations using PSCAD/EMTDC, the magnitude of the middle resistor is decided for diferent 20kV distribution systems within a certain range of ground capacitive current, also the qualifications of resistor are presented in the thesis. The relay protection characteristics under the conditions of 20kV distribution system with neutral grounding by middle resistor are studied and reconstruction methods are proposed.3.For the mixed-outlet distribution system being reconstructed from lOkV to 20kV, a neutral flexible ground method is proposed in this thesis. For such method, the neutral equipment consists of a auto-tuned Petersen coil and a paralleled automatically switched resistor. The Petersen coil is mainly used to compensate the capacitive current and then to increase the self-restoration probability after single phase fault, while the resistor is used to restrain the over voltage due to arc grounding and resonance. Further more, the paralleled ground resistor can be adjusted to produce enough active component in the fault current that can help to identify the faulty feeder from other feeders in a substation, and even the fault location on the feeder.Through numerical calculations and analysis of the single phase to ground fault of a flexible ground system by the PSCAD/EMTDC, automatically switched paralleled resistor of 200～1000Ωis suitable, furthermore, With the capacitive current of the system increasing, suitable high resistance paralleled of neutral point is also increased.A new simple detection method for the faulty feeder is put forward based on both the value and direction of the active component of zero-sequence current. The method ensures correct selection of faulty feeder in a network for a faulty resistance lower than about 3000Ω. This paper also proposed a new fault locating method based on the active component increment of zero-sequence current for the 20kV distribution system.
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