A Step-Up Transformerless, ZV–ZCS High-Gain DC/DC Converter With Output Voltage Regulation Using Modular Step-Up Resonant Cells for DC Grid in Wind Systems

The use of medium voltage dc (MVDC) grid in offshore wind farms has been presented as an alternative solution to eliminate the conventional bulky low frequency step-up transformers used in the medium voltage ac grid. In order to reduce the transmission losses, the trend is to increase the wind turbine output voltage to at least thousands of volts. In this paper, a variable frequency controlled MVDC converter with modular approach of combining multiple step-up resonant circuits and multistring of switches configuration is proposed for MVDC grid. The proposed converter has the following features: 1) the multistring arrangement of the switches allows much lower voltage stress across each transistor; 2) the modular step-up resonant circuits are able to achieve high voltage gain so that transformer with large turns ratio is not required; 3) zero voltage switching turn-on and zero current switching turn-off are achieved in all the switches; 4) the output MVDC level is controlled by variable frequency control of the step-up converter; and 5) circulating energy in all the resonant circuit modules is minimized through close-to-resonant operation for different load conditions. Simulation results are provided on a 4 kVac/50 kV, 2 MW converter for a wide range of load conditions. Experimental results are provided on a laboratory-scale 200 V/1.6 kV proof-of-concept prototype to highlight the merits of this paper.