Modular multi-level power converter for large scale grid-connected photovoltaic system
Rapid improvement in photovoltaic (PV) power generation technology and plummeting price of higher efficiency solar panels and accessories are offering the power companies a competitive deal to invest in PV power generation. In order to meet a good share of the ever-increasing demand for electric power, high efficiency and high power density grid-connected large-scale (in Megawatt power scale) PV systems are required. Research has been going on to find innovative power converter configurations in terms of better efficiency, power density, reliability and cost-effectiveness. A modular multilevel power converter configuration for grid connected photovoltaic (PV) system is proposed in this research. The proposed converter configuration replaces the conventional bulky line frequency transformer with several high frequency transformers, potentially reducing the power electronics and balance of systems cost of PV systems. The front-end converter for each port is a neutral-point diode clamped (NPC) multi-level dc-dc dual-active bridge (ML-DAB) which allows maximum power point tracking (MPPT). The integrated medium/high frequency transformer provides the galvanic isolation between the PV and grid side and also steps up the low dc voltage from PV source. A simple symmetric phase-shift modulation scheme, based on least number of angular parameters (i.e. α, β, &phis;) rather than duty-ratio, has been proposed for the dc-dc ML-DAB converter. Parameters α, β can be used independently to control any voltage mismatch between two clamping capacitors at the dc-link. Mathematical equations have been derived for power flow at different operational cases. Soft-switching analysis under various constraint has been performed. Following the ML-DAB stage, in each port, is a NPC inverter. N number of NPC inverters' outputs are cascaded to attain the per-phase line-to-neutral voltage to connect directly to the distribution grid (e.g. 11 kV, 12.46 kV, 13.2 kV, 13.8 kV or more). The cascaded NPC (CNPC) inverters have the inherent advantages of using lower rated devices, smaller filters and low total harmonic distortion required for PV grid interconnection. A novel voltage balance control at each module based on power mismatch among N-ports, have been presented and verified in simulation. The proposed configuration has advantages of high power density due to requirement of smaller sized medium/high frequency transformer, medium voltage operation with lower rated switching devices because of multi-level operation and reduced Balance of Systems (BoS) cost due to higher internal voltage. The proposed converter system is modular, scalable, serviceable with zero downtime with lower foot print and lower overall cost. The overall converter performance is verified in simulation using PSIM®, Matlab ®, Simulilnk® which is also supported by the experimental results obtained from two 3.34 kW hardware prototypes.