Four-leg parallel Z-source inverter based DG systems to enhance the grid performance under unbalanced conditions

This paper presents a DG system based on four-leg parallel Z-source inverters in integrating a renewable generation system into the grid. Particularly, four-leg distribution schemes give flexibility into the DG system by supporting other functions of power distribution like control of zero sequence components and unbalance mitigation. To increase the capacity and to have redundancy, a parallel structure for the Z-source inverter is proposed. The emphasis is given to component count and the modular structure, thereby reducing the cost while achieving the system reliability. A modulation method is proposed based on interleaved carriers to reduce the output current ripple. Separate controllers are designed in stationary reference frame for the AC side of each inverter. The AC side controller is designed using a combination of p+resonance and negative feedback time delay. The combined controller would deliver good reference tracking and harmonic rejection properties. Another controller is designed for the DC side Z-source impedance network to mitigate the fluctuations in the renewable source. The whole system is driven from a higher level controller that would generate current references to operate the total system in two operating modes, to deliver specified power and to control the unbalances and zero sequence. Proposed control method is tested with simulation results obtained using Matlab/Simulink and PLECS. Subsequently, it is experimentally validated using a laboratory prototype.