Research On Active Power Decoupling And Parallel Technology Of Three-phase Inverter Under Unbalanced Loads

The three-phase inverter is widely used in distributed power generation,auxiliary power supply for locomotive,uninterruptible power supply and other application scenarios with its excellent performance.However,when the three-phase inverter is operated with unbalanced loads,the secondary pulsating power and unbalanced current generated in the system will have adverse effects on the DC side of three-phase inverter and the normal operation of parallel system.Therefore,it has great theoretical and practical value to study the power decoupling and parallel control strategy of three-phase inverter under the condition of unbalanced loads.This thesis takes the widely used isolated three-phase three-leg inverter and non-isolated three-phase four-leg inverter under the condition of unbalanced loads as the research objects.The main research works are introduced as follows.(1)When the three-phase inverter is operated with unbalanced loads,the secondary pulsating power generated on AC side will be coupled to DC side,resulting in power and current pulsations on DC side.To address this issue,the power coupling existing in the traditional isolated three-phase three-leg inverter with unbalanced loads is analyzed firstly.Based on this,the control strategy of the three-phase three-leg active power decoupling inverter topology is proposed,where the secondary pulsating power is provided by the AC side filter capacitors.Thus the secondary pulsating component of the DC side source current could be suppressed to achieve active power decoupling.Subsequently the minimum voltage on DC side and current stress of the three-phase three-leg active power decoupling inverter are analyzed.Finally,the steady and dynamic response performances of the proposed control are verified by simulation and experimental results.(2)Three-phase three-leg active power decoupling inverters are applied to the parallel system.The positive and negative sequence equivalent circuits of three-phase three-leg active power decoupling inverters in parallel are established firstly.On this basis,the distribution principles of positive sequence average power,positive sequence current,secondary pulsating power and negative sequence current are analyzed.Furthermore,active power decoupling parallel control strategy based on virtual impedance is proposed,which can achieve proper distribution of power and current while suppress the secondary pulsation component of the DC side source current of each inverter.Finally,the theoretical analysis is verified by simulation and experiment.(3)The active power decoupling and parallel control are generalized to the three-phase four-leg inverters.The improved three-phase four-leg inverter is proposed firstly,which can simultaneously ensure the output voltage balance and suppress the pulsation on the DC side source current.The control principle and characteristics of the voltage on DC side and current stress of the proposed topology are analyzed.Then the theoretical analysis is verified by simulation and experiment.Secondly,based on the parallel control of the three-phase three-leg inverters,the zero-sequence equivalent circuit of the improved three-phase four-leg inverters in parallel is analyzed.The distribution principle of the zero-sequence current is derived.Furthermore,the parallel control is presented.The simulation results show that the proposed control could achieve the proper distribution of power and current as well as active power decoupling.Finally,with regard to the problem of common mode leakage current existing in such non-isolated system as the three-phase four-leg inverter,the common mode equivalent models of both traditional and improved three-phase four-leg inverters are built.The common mode current paths are analyzed.It is concluded that the impedance of the new current branch of the improved inverter at high frequencies is far less than that of the parasitic capacitance to ground.The simulation results show that the common-mode leakage current can be effectively suppressed by the improved topology.