Power Flow And Short-circuit Current Calculation Method Of Distribution Network With Inverter Based Distributed Generation

DG(Distributed Generation)is widely used in distribution network due to decreasing fossil energy and bad environmental p roblems,where most DGs belong to IBDG(Inverter Based DG).The large access of IBDG not only changes the power flow and voltage profile of distribution network,but also increases the short-circuit current of distribution network,which endangers the reliability of relay protection and the safety of power equipment.Power flow and short-circuit current calculation are the basis of analyzing and operating of distribution network.Their key issues and solutions in distribution network with IBDG are focused in this thesis.The power flow and short-circuit current calculation equivalent model of IBDG in distribution network are established firstly.The equivalent models and their parameters calculation method are analyzed based on the analysis of output characteristic of IBDG under the PQ control strategy.In power flow calculation,the IBDG is modeled as the three-phase symmetric positive sequence I? bus and the three-phase symmetric positive sequence PQ bus respectively depending on wheth er IBDG is in current limiting states or not.In short-circuit current calculation,in consideration of current limiting characteristics and low voltage ride through requirements of IBDG,the IBDG in upstream of short-circuit point is modeled as a voltage controlled current source controlled by local voltage and the IBDG in downstream of metallic sh ort-circuit point is modeled as a constant current source.The three-phase power flow and continuous power flow calculation methods of distribution network considering current limiting status of IBDG are proposed secondly.Due to the different power flow calculation equivalent model of IBDG before and after the current limiting states,the judgment link is added in the process of power flow calculation to find the IBDG in current limiting state.According to the principle of Newton iterative method for solving nonlinear equations,the elements related to IBDG in current limiting states in Jacobian matrix(?)?P/(?)U and(?)?Q/(?)U are changed after the IBDG goes into current limiting states and changes to 10 bus.The short-circuit current calculation method of distribution network with IBDG is proposed thirdly.In this method,all power sources including IBDG are equivalent to current source and according to the position of IBDG relative to short-circuit point,IBDG in upstream of short-circuit point is modeled as a voltage control current source and in downstream of short-circuit point is modeled as a constant current source.Short-circuit current parameters are divided into normal and fault component by fault component method and normal component must be calculated according to the normal component network equation of distribution network.The output current from IBDG in upstream of short-circuit point is modified according to the voltage at point of common coupling of IBDG during each iteration.Finally,an integrated power flow and short-circuit current calculation method for distribution network with IBDG is proposed.The equivalent model of IBDG in power flow and short-circuit current calculation adopts modified PQ and I? bus with local LVRT characteristics of IBDG uniformly.And the power flow and short-circuit current is calculated by the power flow calculation method of distribution network with 10 bus.In order to take into account the current limiting state of IBDG and the difference of node admittance matrix between power flow and short-circuit calculation,the judgment for current limiting IBDG and modification for node admittance matrix are added in the process of integrated calculation.On the basis of theoretical analysis,the calculation programs are written in Matlab.The power flow and short-circuit current for standard distribution network examples with IBDG are calculated by this program and the corresponding simulation models are built in Matlab/Simulink.The comparison between program calculation results and simulation test data shows the both calculation results are consistent,which verifies the correctness and validity of the proposed IBDG model and power flow and short-circuit current calculation methods.