Research On Optimal Coordination Of Directional Overcurrent Relays In Microgrids Considering Different Network Topologies

With the improvement of the permeability of Distributed Generation(DG)and the development of the Microgrid,the relay protection of microgrid is of great significance to ensure the safe,stable and reliable operation of power system.Distribution network connected by microgrid has the structural particularity of active and non-unidirectional radiation power flow,which puts forward a great challenge to the relay protection technology.In recent years,there are some problems in the current microgrid reverse-time overcurrent protection strategies:(1)different setting parameters are adopted in grid-connected operation mode and island operation mode,while the instability caused by frequent switching of relay setting parameters has a negative impact on relay protection in microgrid;(2)due to unexpected line interruption,power grid recondition of the actual power system,the operation mode can't be maintained uniquely,that is,the network topology is constantly changing,which results in changes in fault current characteristics.And the original relay protection setting scheme will lead to relay coordination mismatch.Therefore,the research on the above problems has important theoretical and practical value.The biological background of Grey Wolf Optimizer(GWO)is introduced,and its basic mathematical model and optimization mechanism are described in detail.The results of singlepeak and multi-peak test functions show that GWO algorithm has some advantages in exploiting the optimal solution,exploring the global solution and convergence when dealing with high-dimensional non-convex optimization problems with many inequality constraints,which lays a foundation for the application of GWO algorithm in the setting parameters optimization of microgrid reverse-time overcurrent protection.The GWO is applied to solve problem of the relay parameters optimization,and two different optimization schemes are used to simulate and compare the results.Unique-setting scheme(USS)and multi-setting scheme(MSS)are defined.The results of the case study show that MSS has better speed performance and USS scheme is suitable for two operation modes with lower cost and higher stability.GWO is an effective method to solve the setting value optimization of reverse-time overcurrent protection relay in microgrid.Under CTI constraints,high quality setting parameter groups are obtained.For the number of GWO search population,when the selected number is 30,GWO has a high level in solving performance and convergence speed.This paper presents an optimization scheme for setting parameters of reverse-time overcurrent protection considering N-1 safety principle of relay protection.In order to solve the problem of relay coordination mismatch caused by the changes of topological structure,the network topology caused by accidental disconnection of the single transmission line and the single distributed generation is incorporated into the relay setting scheme.The results show that,compared with the original setting scheme,considering the N-1 principle,the optimal scheme can improve that the main protection/backup protection relay can satisfy CTI constraints without coordination mismatch,and effectively enhance the robustness of the reverse-time overcurrent protection under the premise of fast-moving relay protection.