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This paper presents a design procedure and loss estimation for a high-power, medium-voltage series resonant converter (entitled SRC#), intended for application in megawatt medium-voltage dc wind turbines. The converter is operated with a novel method of operation, entitled pulse removal technique, characterized by variable frequency and phase-shift modulation, below the resonant point of the LC tank. A step-by-step design procedure is proposed and circuit component ratings and efficiency are compared for two variants. The new method of operation reduces transformer size to below 50%, whereas losses are kept below 1.5% from zero to nominal power, due to soft-switching characteristics. An experimental setup, rated for 10 kW and 5 kV output was assembled to extract losses and validate the semiconductor loss model. [ABSTRACT FROM AUTHOR]

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 subresonant 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 analyze the operating modes of the converter with pulse removal technique. With the use of variable frequency and variable phase displacement in subresonant mode, the new method of operation promises transformer size reduction and facilitates soft-switching transition of the insulated gate bipolar transistors (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, 250 V/500 V prototype. [ABSTRACT FROM AUTHOR]

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 subresonant continuous or discontinuous conduction mode. This overcomes the principal deficit of series-resonant converters, and the resulting compact and efficient transformer, and soft-commutated inverter, present particular advantages in high-power, high-voltage applications, such as 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. [ABSTRACT FROM AUTHOR]

The equivalent model of offshore DC power collection network for the harmonic susceptibility study is proposed based on the discrete time-domain modelling technique and frequency scan approach in the frequency domain. The proposed methodology for modelling a power converter and a DC collection system in the frequency domain can satisfy harmonic studies of any configuration of wind farm network and thereby find suitable design of power components and array network. The methodology is intended to allow studies on any configuration of the wind power collection, regardless of choice of converter topology, array cable configuration, and control design. To facilitate harmonic susceptibility study, modelling DC collection network includes creating the harmonic model of the DC turbine converter and modelling the array network. The current harmonics within the DC collection network are obtained in the frequency domain to identify the resonance frequency of the array network and potential voltage amplification issues, where the harmonic model of the turbine converter is verified by the comparison of the converter switching model in the PLECS™ circuit simulation tool and laboratory test bench, and show a good agreement. [ABSTRACT FROM AUTHOR]