A Line-Frequency Transformer-Less High Frequency Medium Voltage PV Grid Connected Inverter With Extended High Voltage Gain Range

In this paper, a new high voltage gain PV medium voltage (MV) grid-connected inverter system that eliminates the line frequency step-up transformer is proposed. The proposed MV PV step-up inverter configuration consists of (1) modular high voltage gain resonant dc/dc converter modules with extended high efficiency from full load to reduced load conditions, and (2) a high voltage neutral point clamped (NPC) 5-level grid-connected inverter system. The presented step-up dc/dc converter module is able to regulate the dc-link voltage by the phase shift control and PWM control in the auxiliary switch of the output voltage quadrupler (VQ). In addition, maximum power point tracking for each PV array is achieved via variable frequency (VF) control in each dc/dc converter module. A 5-level NPC grid-connected inverter with integrated dc-link voltage balancer was also presented. The steady-state and dynamic performance of the proposed modular system are validated through simulation results on a silicon carbide (SiC) based 27kVrms (L-L) 60Hz output, 410kW system and preliminary experimental results on a proof-of-concept 6.6kV dc-link, 10kW SiC laboratory prototype.