Research On Adaptive Protection Strategy For Active Distribution Networ

In recent years,due to the rapid development of Distributed Generation(DG)such as photovoltaic and wind power,the network structure and tide of distribution network has changed greatly due to the high penetration ratio of distributed power,and the active distribution network has changed from single-ended power supply to multi-ended simultaneous power supply.The research on the relay protection strategy in active distribution network has good engineering practical significance.The main object of this paper is to study the active distribution network of DG with low voltage ridethrough capability.When the voltage at grid connection point of DG decreases,DG can run continuously for a period of time according to the standard requirement,so that DG will not leave the network when the protection equipment on the line operates and avoid the impact on the main network caused by frequent grid connection of DG.Firstly,the changing characteristics of system currents when DG is connected to the grid at different locations in the distribution network are analyzed and simulated for verification.Based on the analysis of the basic principles and characteristics of traditional current protection,the changes of electrical quantities at the relay protection device after DG is connected to the grid are analyzed,and the adaptive positive sequence current protection scheme based on the positive sequence current and positive sequence voltage at the location of the protection device and the adaptive current differential protection scheme based on the electric flow on both sides of the line are designed respectively,and the effectiveness of these two protection schemes is verified by simulation.Secondly,in order to improve the reliability of protection to isolate faults,this paper proposes a new active distribution network protection strategy combining adaptive positive sequence current protection and adaptive current differential protection.The active distribution network is divided into feeder line area(area 1)and DG grid-connected line area(area 2),and the lines in area 1 and area 2 are equipped with adaptive positive sequence current protection and adaptive current differential protection,respectively.When one section of the adaptive positive sequence current protection rejects because of fault,the other two sections of protection can disconnect the fault area and protect the non-fault area,and set the action time limit between the three sections of protection to effectively avoid the phenomenon of protection robbery.The adaptive time-limited positive sequence current quick-break protection and adaptive positive sequence fixed time overcurrent protection can not only protect the whole length of this level line,but also protect the adjacent lines.The simulation verifies that the use of the three sections of protection can ensure the stability of the main network operation.Finally,the measured currents at both ends of the adaptive current differential protection line are fitted with the Sigmoid function to obtain the action criteria of the new protection strategy,which can be operated when the current flow on both sides of the line meets the action criteria,so as to better determine the fault occurrence area.The simulation verifies the effectiveness of the new active distribution network protection strategy combining adaptive positive sequence current protection and adaptive current differential protection,and the new strategy can adaptively adjust the protection settings according to the DG grid connection and fault conditions,which can improve the reliability of the grid operation.