Control of Quasi-Z Source Inverter with Super Capacitor under Unbalanced Grid Fault Conditions

The quasi-Z-source inverter (qZSI) has some unique advantages and is suitable for renewable energy sources such as solar and wind generation system. In order to make the inverter have the ride-through ability and the current into the grid symmetrical when unbalanced grid faults occur, the topology of qZSI with super capacitor substituting the low voltage capacitor and the unbalance control strategy were proposed. With a super capacitor the system can balance and smooth the power between the generation unit and grid, also the current injected into the grid can be symmetrical through the unbalance control method. The circuit of the new topology and the mathematical model of the three-phase converter under grid voltage unbalance conditions have been analyzed. And then, closed-loop control scheme for the proposed system is used to control the power flow of generation unit, grid and super capacitor in this system, also unbalance control strategies in positive (dq) + and negative (dq) – reference frames have been implemented in the grid-connected qZSI system to make the current into the grid symmetrical during the grid fault. The operating mode during unbalanced grid fault is simulated in MATLAB/Simulink, and the simulation results are presented to verify the theoretical analysis and the control scheme.