Decoupling control for transformerless grid-connected inverters in parallel operation

The modular non-isolated inverter structure enhances the efficiency and redundancy of the PV generation system. Current coupling between the transformerless grid-connected inverter modules for PV power system is researched. The operation modes of the non-isolated inverters with different topologies and modulations are analyzed and the current decoupling conditions are obtained. Inverters which are capable of eliminating the common-mode current, such as H5 topology, can be operated in parallel with no extra control. And a current decoupling control for the unipolar double frequency PWM full-bridge inverters in parallel operation is proposed with regulating the currents of two split filter inductors respectively. Prototype systems are built and experimental results verify the validity of the proposed theory and control strategy.