Abstract
A modular multilevel power converter configuration for grid connected photovoltaic (PV) systems is proposed. The converter configuration replaces the conventional bulky line frequency transformer with several high frequency transformers, potentially reducing the 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 high frequency transformer provides the galvanic isolation between the PV and grid side and also steps up the low dc voltage from PV source. 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 (i.e., 13.8 kV). The cascaded NPC (CNPC) inverters have the inherent advantage of using lower rated devices, smaller filters and low total harmonic distortion required for PV grid interconnection. The proposed converter system is modular, scalable, and serviceable with zero downtime with lower foot print and lower overall cost. A novel voltage balance control at each module based on power mismatch among N-ports, have been presented and verified in simulation. Analysis and simulation results are presented for the N-port converter. The converter performance has also been verified on a hardware prototype.
Highlights
Grid-connected large-scale photovoltaic (PV) systems have to consider challenges such as energy yield, power conversion efficiency, power density, reliability and cost which includes solar panels, power electronics and balance of systems (BOS) cost
A 3 MW 12 kV modular cascaded H-bridge (CHB) for grid-connected PV system is proposed in [17] where the dc-dc conversion stage is a two-level dual active bridge (DAB) and multiple of such dc outputs are added to feed an inverter module in the cascade
The above analysis of soft-switching operation in multi-level dc-dc dual-active bridge (ML-DAB) becomes important if the IGBTs are replaced by silicon carbide (SiC) Mosfets operating at higher switching frequencies
Summary
Grid-connected large-scale photovoltaic (PV) systems have to consider challenges such as energy yield, power conversion efficiency, power density, reliability and cost which includes solar panels, power electronics and balance of systems (BOS) cost. For PV applications modularity at the input side with two stage power conversion is advantageous as shown in a study on a commercially available large-scale PV inverter that replaces the PV side dc combiners with dc-dc converters at string-level along with maximum power point tracking (MPPT) which increases the energy yield by 34–46% in comparison to a single central inverter for certain shading conditions [15]. A 3 MW 12 kV modular CHB for grid-connected PV system is proposed in [17] where the dc-dc conversion stage is a two-level dual active bridge (DAB) and multiple of such dc outputs are added to feed an inverter module in the cascade.
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