Front-end isolated quasi-Z-source DC-DC converter modules in series for photovoltaic high-voltage DC applications

A quasi-Z-source modular cascaded converter (qZS-MCC) is proposed for high-voltage (HV) dc integration of photovoltaic (PV) power. The qZS-MCC comprises front-end isolated qZS half-bridge (HB) dc-dc converter submodules (SMs) in series. By the qZS-HB handling PV voltage and power variations, a unified duty cycle is applicable for the front-end isolation converter of all SMs. Resultantly, the proposed system improves the quasi-Z-source cascaded multilevel inverter and the modular multilevel converter based PV counterparts in terms of no double-line-frequency pulsating power so as to low qZS impedance, HV dc collection of PV power thus to reduce conversion stages for dc transmission, and overcoming the limit of series-output voltage with simple galvanic isolation. Operating principle and power loss evaluation of the qZS-MCC are presented. Parameter design guidelines and simulation are addressed based on a 60-kW SM; experimental results are carried out on a downscaled prototype as a proof-of-concept, demonstrating the validity of the proposed system.