Study On The Influence Of Interconnecting Distributed Resources With Electric Power Systems And The Protection Of Active Distribution Network

In order to achieve the goal of healthy and sustainable development,due to energy crisis,environment pollution and power system construction issues,the distributed resources technology has developed rapidly in recent years.Distributed Resources(DR)generally connects to distribution network,interconnecting DR with electric power systems changes the structure of traditional distribution network.The traditional protection is likely to be affected.And it can no longer meet the requirement of safe operation of the power grid.Therefore,it is necessary to study the protection of active distribution network on the basis of establishing reasonable active distribution network and understanding the influence of interconnecting DR with electric power systems.This paper firstly describes the control principle and fault characteristics of the DR.The DR based on the DIgSILENT software is established and its correctness is verified.The transformer connections that are widely used to interconnect DR to the utility system is studied.The characteristics of various transformer connections are analyzed,and the advantages and disadvantages of various transformer connections are summarized.According to the typical design of Chinese medium-voltage distribution network,the principle of neutral point grounding selection and the actual parameters of grid physical system,a practical model of medium-voltage distribution network is established,and the specific model parameters are given.A reasonable active distribution network model based on DIgSILENT software is built by interconnecting DR with medium-voltage distribution network directly or using appropriate interconnection transformer,so that a foundation is established to study the the influence of interconnecting DR with electric power systems and the protection of active distribution network.Then,the protection setting value for traditional distribution network is given according to the principle of common current protection and setting method.By using the method of hierarchical modeling,the models of protection equipments corresponding to the common current protection were established based on the DIgSILENT software,the correctness of the protection model is verified.Through the theoretical analysis and simulation results,the influence of DR access to distribution network on traditional current protection is discussed.Finally,the influence of inverter DR access to distribution network on traditional current protection and distance protection is analyzed,which shows that distance protection is more suitable for active distribution network protection.The influence of the inverter DR access to distribution network on the measuring impedance is analyzed according to the different connection location,and the relationship between the measuring impedance and the DR fault current is obtained.An improved distance protection setting method is introduced for the case that the DR fault current can be measured and there is a feasible communication between interconnection device and protection device.For the case that there is no feasible communication between interconnection device and protection device,since the fault current of the DR is related to the positive sequence voltage of the Point of Common Coupling(PCC)after the fault,DR reactive current is controlled by voltage deviation of PCC,under the fact that the reactance is bigger than resistance in distribution network and reactive current of DR fault current is under priority control during ride-through,a new distance protection setting method that substituting reactive current which comply with Chinese standards for grid-integration of distributed resources for DR fault current is studied for three phase and two phase fault.The simulation results show the superiority and effectiveness of the method.