An improved voltage balancing technique for a soft-switched high-gain converter with low voltage stress using duty ratio control for wind energy application

This paper presents a simple passive voltage balancing circuit for a modular DC/DC step-up converter for medium voltage (MV) DC grid in wind energy systems. The proposed auxiliary circuit is able to balance the capacitor voltages over a wide range of operating points. Regarding the converter, high frequency step-up resonant circuits with high gain rectifier modules are combined to form a modular step-up DC/DC converter. This converter is able to achieve high voltage gain efficiently without using high frequency step-up transformers with large turns ratio. Soft switching operation is also achieved by using LCL resonant circuit modules. The voltage stress of each power switch is also reduced to half of the DC-link voltage by using a series connection of two strings of switch pairs. In addition, the proposed converter is capable to regulate the output MV grid voltage by utilizing a duty cycle control. Simulation results are given on a 3kV/30kV, 1.5MW converter for a wide range of load conditions. Experimental results are also provided on a laboratory-scale 70V/300V, 28kHz proof-of-concept prototype to highlight the merits of this work.