Active Power Decoupling Methods For Three-Phase Grid-Connected Converters Under Unbalanced Conditions

Three-phase Voltage-Source Converters(VSC)have been widely adopted as the key interfaces between power sources and loads in distribution systems.However,different types of grid disturbances and faults exist in practice,which pose great challenges to the normal operation of the VSC.Grid unbalance is one of such abnormal conditions,which may lead to negative current and large double-frequency current/voltage ripples on the DC link.In light of the operation of VSC under unbalanced conditions,there is still not a control method in the literature that can achieve sinusoidal symmetrical AC grid current and ripple-free DC current/voltage at the same time so far.To tackle these issues,this thesis proposed three improved three VSC topologies with high-performance operation under unbalanced conditions.In this thesis,the main contents include:(1)To tackle the problems caused by three-phase unbalanced grid,three three-phase power-decoupled converter topologies are proposed,including Modified Four-leg Converter(MFC),Three-phase Differential Buck Converter(DBC)and Three-phase Split Capacitor Converter(SCC).Circuit analysis and power flow analysis of three topologies are shown,and the magnitude and phase of the decoupling capacitor voltage are calculated when the oscillation power of the topology is decoupled.Besides,the control strategies of power-decoupled topologies under unbalanced grid condition are proposed,including the analysis of SRF-PLL and DSOGI-PLL,the control strategy of the circuit with shared leg and the control strategy of decoupling circuit.Finally,simulation results are given to prove the effectiveness of the power-decoupled topologies and its control strategies,which can achieve the balanced sinusoidal AC grid-connected currents and constant DC voltage/current simultaneously under unbalanced grid conditions.(2)The AC equivalent circuits of power-decoupled converters are studied and the expressions of the minimum DC voltage and AC current stress are calculated.In addition,the vector diagram and expression of the three-phase unbalanced grid voltages caused by grid faults are given.The minimum DC-link voltages and current stresses of the proposed topologies have been quantified and analyzed for all types of faulted voltages.By comparing and analyzing the topologies,the optimal power-decoupled topology is obtained,which is also compared with the traditional converter.At the same time,other functions and applications of the optimal topology are described.(3)A hardware-in-the-loop simulation platform is built by using dSPACE and power-decoupled converters.The experimental results verify the effectiveness of the proposed topologies and their control strategies,as well as the correctness of the theoretical analysis of the thesis.