High power medium voltage DC/DC converter technology for DC wind turbines

High voltage direct current (HVDC) connected wind farms promise reduced electrical losses, lower bill-of material cost and undiminished functionality with the condition the wind plant medium voltage (MV) collection network becomes DC, rather than MVAC. One dearly missed building block that would enable the transition to a DC voltage collection, is the DC/DC converter for high power & high voltage. The main hypothesis of this thesis is that transition of MVAC connected wind turbines to MVDC (medium voltage direct current) output is conceivable with present and commercial technologies. Therefore, the main objective was investigation and development of proof of concept demonstrator for a high efficient, medium voltage (MV) converter topology, across relevant power (5 to 15 MW) and voltage range (±35 kV to ±50 kV). Research and concept demonstrators in DC/DC converters for high powers and voltages have emerged over the last decade, but none have become products yet, due to high cost, complexity or market volume. With priority on availability, losses and power density, an analysis of a large catalogue led to favor a variant of the series resonant converter ( entitled SRC#). As the DC turbine application does not require bidirectional power flow, forced commuated semiconductors on the ±50 kVdc side are not required. A monolithic rectifier and transformer configuration was favoured for further robustness. With amorphous steel the transformer excitation frequency of 1 kHz was deemed achievable. Even more, at this frequency, linefrequency rectifier diodes are still feasible to use in series connected design, respecting their individual characteristics in recovery charge and recombination time, to balance losses and ratings. During analysis, it was discovered that the transformer core size and losses could be significantly reduced by shifting the resonant inductor and capacitor to the transformer secondary, and operate the inverter (on the primary) with a modified pulse-pattern, entitled pulse removal technique. The new modulation scheme permits requlation of power from nominal level to zero, in presence of variable input and output DC voltage levels. Combination of variable switching frequency and phase-shift modulation, below v the resonant point of the LC tank, allows soft-switching commutation on all semiconductors and design of the medium frequency transformer for highest operating frequency. Further on, substantial effort was invested to develop the analysis and design guide line tool, including a 10 kW, 5 kV laboratory test circuit. From this scaled power circuit and simulation & design tools, the full-scale circuit performance was estimated to yield impressive 1-1.5% losses and transformer weight of less than 4 tonnes. This should be comparable to the present solution of some 2-2.5% losses and weight of 20 tones. On a different medium voltage experimental setup, target 6.5 kV igbts and press-pack diodes have been exposed to similar voltage and current waveforms as in the target converter. The proposed soft-switching characterization setup allows generation of kilovolt and kiloamps pulses for a short time, without the need of bulky and expencive dc sources and loads. The semiconductor loss model was further validated and calibrated, while further works needs to address the validation of the monolithic transformer loss model.