Research On Grid-connected Strategy Of Distributed Generator And Protection Scheme For Distribution Network

As an important utilization way of clean energy,Inverter Interfaced Distributed Generator(IIDG)is more and more widely used in modern medium and low voltage distribution network.But the output characteristics of IIDG are significantly different than that of the traditional synchronous generator,which has brought great difficulties to the distribution network fault analysis and the relay protection.It is of great significance to study the control strategy of IIDG and establish a comprehensive fault output model,and to study the fault analysis method and adaptive protection setting principle for IIDG multi access distribution network,which can ensure the safe and reliable operation of modern distribution network.In this paper,the basic control principle of IIDG is analyzed.According to the three-phase four-wire low-voltage distribution network,the control strategy of three-phase four-leg grid-connected inverter is proposed.The control strategy realizes the grid connected power output by the positive sequence power control,and realizes the three-phase voltage balance through the negative sequence and zero sequence voltage control.According to the functional goal of the control strategy,a comprehensive current limiting scheme suitable for three-phase four leg inverter is proposed.The priority of each sequence current can be determined according to the control target,and the maximum limit of the sequence current can be adaptively adjusted.For the three-phase grid-connected inverters with constant power control strategy,the influence of the control strategy in normal operation,the negative sequence current eliminating strategy in asymmetric operation,the maximum fault current limiting and the low voltage ride through(LVRT)control strategy in fault operation conditions to the fault output characteristic of IIDG has been analyzed.the comprehensive fault output model is established.According to the location and type of the fault,the node voltage equations of distribution network which takes the IIDG output into consider is established.The node voltage equations and the nonlinear output equations of IIDG are solved by alternating iterative method.Then,the fault analysis of distribution network after multiple IIDG access is realized.In view of the situation that the existing adaptive protection relies on the communication to obtain the upstream IIDG operation parameters,the principle and method of using the local information to obtain the current active command of the IIDG are studied and put forward,which can obtain the output characteristics of the remote distributed power supply without communication.Based on this method,the fault current is analyzed when two-phase short circuit and three-phase short circuit fault occurs at different positions in the distribution network with distributed generator.The solution method of equivalent impedance for different fault types under different fault conditions is given,and the adaptive setting scheme of each protection is put forward.The research contents of this paper can provide a certain basis for the application of distributed generation technology in distribution network.