Research On Control Strategy Of Off-Grid Parallel Inverters Under Unbalanced Load

The off-grid microgrid combined with renewable energy generations can flexibly supply power to users in special occasions such as remote areas and islands.Multi-inverter parallel system based on droop control,which can realize stable operation without communication lines,has been widely applied in off-grid microgrid in virtue of its high power supply reliability.However,the practical running effect of parallel inverter system based on droop control is affected by factors such as line impedance,which leads to the problem of low power sharing accuracy.In addition,the access of unbalanced loads causes voltage imbalance,which results in low power quality of the system.This papers takes the off-grid parallel inverters as the object to study the control strategy of power sharing and unbalanced voltage suppression.Firstly,the basic theory of off-grid parallel inverters is analyzed.The mathematical models of the inverter in different coordinate systems are established,and the droop control principle under the condition of inductive line impedance is analyzed.Furthermore,the power sharing characteristics of parallel inverters based on droop control and the characteristics of inverter output voltage,current and power under unbalanced load conditions are analyzed.Secondly,the ccntrol strategy of off-grid parallel inverters based on droop control is studied.In the traditional inverter control,the virtual impedance method cannot effectively adjust the negative sequence impedance under the condition of unbalanced loads,which makes it difficult to achieve accurate negative sequence power sharing of the parallel inverters.Therefore,a control strategy of double sequence virtual impedance based on delay signal cancellation-complex coefficient filter(DSC-CCF)is proposed.The dsc-ccf method is used to separate and filter the current,and the obtained positive and negative sequence currents are applied to double sequence virtual impedance.With the double sequence impedance controlled independently,high accuracy of the positive and negative sequence power sharing is achieved.In addition,the principle of inverter unbalanced voltage control is analyzed,and it is found that the unbalanced voltage at the point of common coupling(PCC)can be suppressed by improving the control performance of inverter for negative sequence voltage and reducing the negative sequence equivalent line impedance.On this basis,an unbalanced voltage control strategy of PI-quasi proportional resonant combined with virtual impedance is designed.Considering the many parameters of the PI-quasi proportional resonant controller and the difficulty of tuning,a voltage controller offline parameter tuning method based on the particle swarm optimization(PSO)algorithm is adopted.Finally,a simulation model for off-grid parallel inverters control system was built on the Matlab/Simulink platform,and an experimental prototype of the parallel inverters was built with the core of TMS320F28335 controller.The effectiveness of the proposed control strategy is verified by simulation and experiment.