Research On Protection Scheme For Distribution System With Distributed Generations

Due to the exhaustion of traditional fossil energy and the environmental problems caused by its use,the development of renewable energy has been strongly supported,and its access and application in the distribution network is more and more extensive.With the access of distributed generation(DG),the distribution network is transformed from a single power system to a multi-power system.When short-circuit fault occurs in the distribution network,the level and distribution of fault current are significantly changed compared with the passive distribution network,thus seriously affecting the performance of traditional three-stage current protection.For this reason,this thesis focuses on the protection scheme of distribution system with DGs,in order to eliminate the impact on distribution network protection caused by the access of DGs.Accurate analysis of the impact of DG on distribution network protection has important guiding significance for the proposed protection scheme.Therefore,the fault output characteristic model of inverter-type DG under P/Q decoupling control strategy and V/f control strategy is given.Based on the fault output characteristic model of DG,a fault current calculation method of distribution network with DG based on Gauss iteration method is introduced.Based on the fault current calculation method,the fault current characteristics of distribution network with DG are analyzed qualitatively,and the influence of DG on distribution network protection is analyzed quantitatively,which lays a theoretical foundation for Proposal of protection scheme of distribution network with DGs.In the initial stage of DG development,reasonable planning of DG access location and capacity can effectively eliminate the impact of DG on distribution network protection without changing the original protection configuration and settings.Therefore,taking the maximum DG capacity of distribution network as the objective function and the coordination of current protection as the constraint,a calculation method of the allowable DG access capacity and location in distribution network is proposed based on differential evolution algorithm.The validity and rationality of the proposed method is verified by the analysis of the results of simulation.In the future,the grid will be a Smart Grid dominated by clean energy.Simply restricting the DG access capacity to eliminate its impact on protection will seriously restrict the development of the grid to the green and intelligent direction,optimizing and upgrading the protection of distribution network will be an unavoidable problem.Therefore,considering the high penetration of distributed generation,the randomness of operation of DG,the active management scheme,the islanded operation mode and multiple loads,etc.,combined with distributed intelligence and centralized intelligence,a divided-period adaptive overcurrent protection scheme is proposed in this paper based on time-varying characteristics of various influence factors.Based on MATLAB software,the proposed protection scheme is simulated and analyzed,which verifies the effectiveness and rationality of the proposed scheme.