Modular Multilevel MMI(HB) Topology for Single-Stage Grid Connected PV Plant

A new topology for the single-stage grid connection of Photovoltaic (PV) system is proposed in this paper. PV inverters of various types have been proposed previously. Modular multilevel inverter, MMI(HB) using half-bridge submodules is recent and a potential candidate for PV application. Conventionally, the DC link of MMI has a centralized large DC source e.g. PV system connected across it. In the proposed topology, distributed PV system e.g. PV strings/ arrays are connected directly across dc link of MMI(HB) submodules (SMs). This allows easy expansion, independent monitoring of each PV string voltage and maximum power point optimization. The dynamics of PV generator is different from the conventional power generator, hence, a new combination of MPPT and decoupled dq controller is proposed to maximize the real power output from the PV strings and limit reactive power exchange with the grid as per the system requirements. In order to validate the proposed concept of new single-stage grid connected PV topology and its controller, simulations were conducted. A detailed model of the proposed 1800W, PV system consisting of 36 PV modules connected across a 13 level three phase MMI (HB) was established. The simulation results of new PV system during fast changes in solar irradiation and reactive power exchange has confirmed the feasibility of proposed MMI(HB) interface for single-stage grid connected high voltage, high power PV system and has verified the validity of the control strategy adopted.