Short-Circuit Current Calculation In Distribution Networks With Distributed Generation

The distribution network with Distributed Generation(DG)has become the development trend of future distribution networks.When a large number of DGs are connected to the distribution network,the electrical characteristics of the network change from a single power source structure to a multi-terminal power source structure.At the same time,the traditional practical short-circuit current calculation method for distribution networks is based on the transient linear model of AC synchronous motors and linear networks,and cannot be used for distribution networks with inverters in DGs.Therefore,researching practical short-circuit current calculation methods suitable for distribution networks with multiple types of DGs has important theoretical and practical significance.The key to the practical short-circuit current calculation method for distribution networks with Distributed Generation(DG)is the establishment of equivalent models for DG,particularly for those with inverters.In this regard,a simulation model was developed to study the electrical characteristics of DG at the port,based on which the equivalent models for practical short-circuit current calculation were investigated.To this end,the identification of asymmetric electrical quantities under asymmetric faults and the discussion and simulation of phase-locked loop techniques were carried out to obtain a method and technology with good dynamic response.Secondly,based on the grid connection requirements and the control objectives of DG during network faults,control strategies for inverters and doubly-fed asynchronous wind turbines under large and small disturbances were respectively proposed.Then,according to the control strategies of DG during network faults,an equivalent model for short-circuit current calculation was proposed,which considered the participation of the GridSide Converter(GSC)in the low voltage ride-through operation of Doubly Fed Induction Generator(DFIG)under large disturbances.This was done to solve the problem of the incompleteness and inaccuracy of the equivalent model,which is usually caused by the neglect of the GSC-side output current in existing large disturbance models.Finally,two types of distributed power models connected to the distribution network were simulated and verified using Matlab/Simulink 2021 b.The simulation results showed that the established equivalent models effectively improved the model accuracy and had a high accuracy level.Based on the proposed DG short-circuit current calculation equivalent model,a general short-circuit current calculation method for distribution networks with DG is improved.By introducing DFIG disturbance equivalent models,this method solves the problem of inaccurate processing of DFIG equivalent models in the existing method,and obtains a shortcircuit current calculation method that can be applied to multiple types of DG.The method uses the compensation method to treat the distribution network fault as a change in the network structure.According to the different characteristics of the DG’s output current,it is divided into two types: the unit direct solution type and the iterative solution type with inverters.Using the superposition theorem,the node voltage components of the two types of distributed generators acting on the distribution network after the fault are calculated separately,and then added to obtain the total node voltage of the distribution network after the fault.Finally,using the node voltage equations,the branch currents are calculated.Based on the proposed DG short-circuit current calculation equivalent model,a general short-circuit current calculation method for distribution networks with DG was improved.By introducing DFIG disturbance equivalent models,the problem of inaccurate processing of DFIG equivalent models in the existing method was solved by this method,and obtained a short-circuit current calculation method that can be applied to multiple types of DG.The compensation method to treat the distribution network fault as a change in the network structure was used by this method.According to the different characteristics of the DG’s output current,it was divided into two types: the unit direct solution type and the iterative solution type with inverters.Using the superposition theorem,the node voltage components of the two types of distributed generators acting on the distribution network after the fault were calculated separately,and then added to obtain the total node voltage of the distribution network after the fault.Finally,the branch currents were calculated,using the node voltage equations.Finally,an algorithm program was developed and a local 35 k V distribution network in a certain city was taken as an example to verify the correctness and convergence of the shortcircuit current calculation method for distribution networks containing DG.