Research On Control Strategy Of Synchronous Constant Frequency AC Microgrid With Nonlinear And Unbalanced Loads

The reserves of non renewable energy are decreasing year by year and the environmental pollution caused by them is becoming more and more serious.At the same time,as an important medium of the transition from large power grid combined with distributed generation technology to smart grid,microgrid is a scheme that takes into account both economy and environmental protection,which not only reduces environmental pollution,but also improves the utilization rate of renewable energy and power supply reliability.Therefore,microgrid has attracted wide attention.The most common unbalanced load and nonlinear load in microgrid will affect power quality.In this paper,the control strategy of synchronous constant frequency microgrid is analyzed under balanced load,unbalanced load and nonlinear load.Firstly,aiming at the problems of frequency offset and voltage sag in traditional droop control of microgrid,a synchronous constant frequency microgrid control method based on GPS is proposed.In this method,GPS 1PPS is used to calculate the synchronous phase angle of dq rotating coordinate system,so that the control process of each microgrid inverter is carried out in the same coordinate system,and then a fixed frequency(50 Hz)output voltage is generated.By analyzing the power distribution characteristics of microgrid,the factors affecting power distribution and circulating current are determined.Virtual resistance method is used to reduce the influence of line impedance difference on power distribution.Then,according to the characteristics of positive and negative sequence components in the output voltage of microgrid inverter under unbalanced load,a synchronous constant frequency microgrid sequence control strategy based on DSOGI is adopted to reduce the imbalance of output voltage.The positive and negative sequence separation module based on dsogi is used to separate the positive and negative sequence components,and the output voltage of microgrid inverter is controlled by positive sequence control loop and negative sequence control loop.At the same time,aiming at the problem of unequal line impedance between microgrid inverters,the virtual resistance method is used to realize the reasonable current sharing of microgrid.Secondly,according to the characteristics of positive sequence component and odd harmonic component in the output voltage of microgrid inverter under nonlinear load,the synchronous constant frequency microgrid sequence control strategy based on harmonic decoupling network is adopted to reduce the harmonic distortion rate of output voltage.In this control strategy,CGI based harmonic decoupling network is used to separate positive sequence and harmonic components,and the output voltage of microgrid inverter is controlled by positive sequence control loop and harmonic control loop.At the same time,aiming at the problem of unequal line impedance between microgrid inverters,the virtual resistance method is used to realize the reasonable current sharing of microgrid.Finally,time domain simulation is built in MATLAB simulation software to verify the feasibility of the proposed control strategy.