This paper is introducing a new method of operation for a series resonant converter, with intended application in megawatt high-voltage DC wind turbines. Compared to a frequency controlled series resonant converter operated in sub resonant mode, the method (entitled pulse removal technique) allows the design of the medium frequency transformer for highest switching frequency, while being operated at lower frequency without saturation. The main focus of this paper is to identify and analyse the operating modes of the converter with pulse removal technique. With the use of variable frequency and variable phase displacement in sub resonant mode, the new method of operation promises transformer size reduction and facilitates soft-switching transition of the IGBTs and line frequency diodes on rectifier side. Four modes of operation are identified, while equations for output power, voltage and current stress are identified. Experimental results are concluded on a 1 kW, 250V / 500V prototype.

A new modulation scheme is introduced for a single-phase series-resonant converter, which permits continuous regulation of power from nominal level to zero, in presence of variable input and output dc voltage levels. Rearranging the circuit to locate the resonant LC tank on the rectifier side of the high turns-ratio transformer combined with frequency control and phase-shifted inverter modulation keep transformer flux constant from nominal frequency down to DC, always in sub-resonant continuous or discontinuous conduction mode. This overcomes the principal deficit of series-resonant converters, and the resulting compact and efficient transformer, and softcommutated inverter, present particular advantages in highpower, high-voltage applications, like DC offshore wind turbines. With transformer excitation frequency in hundreds of Hz range, line-frequency diodes can be employed in the high-voltage rectifier valve. Circuit operation and conduction modes, governing equations and sample waveforms are presented, together with experiments from a scaled demonstrator.

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