Asymmetric Fault Analysis And Relay Protection Of Distribution Network With Large Scale Charging Stations

With the development of electric vehicles,more and more charging piles and charging stations have been enabled to be connected to the distribution network.Due to their advantages,such as compact structure,less transformation,fast response and high power,DC charging stations are widely used to regulate the power bidirectional flow then for promoting the V2G.However,large-scale charging stations connect to the grid with losts of power electronic equipments accessed,complex the fault characteristics of the power system and highlight the nonlinear feature.Traditional relay protection faces the problems of adaptation to the complex fault characteristics.It is very important to study the fault analysis and relay protection of the distribution network with large-scale charging stations connected.Therefore,considering the grid-connected inverter control strategy,this paper studies the AC side fault characteristics under the asymmetric faults and the protection scheme of the distribution network with large charging stations connected charging station.Considering the control ways of grid-connected inverter,this paper studis the fault characteristics on both the DC side and AC side under asymmetric fault conditions,analyzes the influence of the negative sequence component on the control link during the fault propagation.Besides,the generation mechanism of the harmonics on both sides and the quantitative analysis of the magnitude of each harmonic component are studied,and the analytical formula of the fault voltage and current are derived theoretically.Secondly,this paper studies the influence of the control link on the fault characteristics in aspects of power electronic components,and analyzes how the parameters of the PI regulation controller and the limit value influence the voltage on the DC and AC sides under the double closed-loop control strategy of the grid-connected inverter.Finally,this paper obtains the waveform of the voltage,current and harmonic components,verifies the correctness of theoretical derivation for the fault voltage under asymmetric fault conditions and analyzes the influence of the control parameters on fault characteristics qualitatively by the simulation model.By analyzing the fault characteristics of large-scale charging stations connected to the grid,this paper studies the influence of fault characteristics on the directional components and the positive sequence components,and proposed a principle of positive sequence integrated impedance longitudinal protection based on the amplitude difference of positive sequence differential impedance when the fault occurs inside and outside the feeder section of the distribution network with the large scale charging stations connected.For the dead zone problem that may occur in the case of three-phase short circuit,the fault characteristics of positive sequence impedance and positive sequence current are used to construct the auxiliary criterion,and the applicability of the protection principle is verified in the simulation.