Research On Calculation Method Of Short-circuit Current Of High Proportion Distributed Generation Connected To Power Grid

With the proposal and implementation of the energy transformation strategy,the proportion of distributed new energy sources such as wind power and photovoltaic in the power grid is increasing,bringing new challenges to the calculation and analysis of the power grid.In the calculation of short circuit in the power grid,distributed power sources rely on power electronic devices to connect to the power grid.The output current characteristics during the short circuit are affected by the low breakdown control strategy,which is completely different from the fault characteristics of traditional AC synchronous generators.With the increasing penetration rate of distributed power sources,the calculation of short-circuit current when considering a large number of distributed power sources with complex control strategies connected to the power grid is an urgent problem that needs to be solved.Therefore,it is necessary to study short-circuit calculation methods that take into account distributed power control strategies for this scenario.Firstly,two main distributed generation control strategies are studied:negative sequence current active Inhibitory control control strategy and negative sequence current optimization control strategy.Based on the two-phase rotating coordinate system,the short-circuit current output characteristics were analyzed,and the short-circuit calculation analytical expressions for different control strategies were solved.A short-circuit calculation positive and negative sequence model considering the distributed power supply control strategy was established.Subsequently,based on the current distribution of distributed power sources in the power grid,which exhibits the characteristics of multiple points,wide areas,and high local density,a calculation method for power grid partition short-circuit taking into account the control strategy of distributed power sources was studied.In this calculation method,based on the principle of node tearing,the large power grid is divided into a main power grid and several distributed power partition subnets.Each distributed power partition subnet iterates locally and exchanges interface information with the main power grid,The collaborative calculation between the main power grid and various distributed power generation sub networks greatly improves the short-circuit calculation efficiency of large-scale power grids under the connection of high proportion distributed power sources.By using the node admittance matrix of block diagonal and the method of self calculation of sub networks,the dimensionality of the calculation equation is reduced while ensuring the same calculation accuracy.Finally,the current short-circuit calculation method:simple Iterative method is analyzed.Through the basic principle of simple Iterative method convergence,the current short-circuit calculation method taking into account the distributed generation control strategy has the problem of non convergence when the external power grid becomes weak and the penetration rate of distributed generation increases.This conclusion is verified through theoretical derivation.In order to solve the problem of short-circuit calculation under the scenario of high proportion of weak power grid,A quasi Newton iterative calculation method was proposed,and based on the convergence theorem of the quasi Newton iterative method,the feasibility of the quasi Newton iterative method in this scenario was derived and verified.This method adapts to the short-circuit calculation of power grids under various fault conditions by adding an additional equation set based on the fault boundary conditions and solving the node voltage equation simultaneously.Through PSCAD/EMTDC platform simulation and MATLAB comparative calculation,it is verified that the quasi Newton iterative calculation method has good convergence characteristics and stable Rate of convergence.