Operation and control of a new grid-connected PV systems based on common DC bus

This paper proposes a new three-phase grid-connected PV system based on common dc bus, this system is connected to the grid by cascaded multi-level DC/AC converters (CMCs). PV array strings will be connected to boost converters which are connected to a common DC bus through separate interfacing circuits. Dual-active-bridge (DAB) converters are employed to this system, which can provide galvanic isolation. The low voltage side of each DAB converter is connected to the common DC bus, and the high voltage side is connected to the CMC. In this system, the output power of the PV arrays will be unbalanced once the operating states of the PV arrays are not identical. The unbalanced output power will lead to the asymmetric of the grid-side currents of the CMC. In order to achieve the symmetric of grid-side currents of the CMC, a zero-sequence injection method is presented to balance the grid-side currents. A 10kV/4.5MW PV system with the proposed control strategy is built in PSCAD/EMTDC. Time-domain simulations are conducted to verify the effectiveness of the proposed system topology and control strategies.