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This thesis presents the concept of a "distributed HVDC converter" for offshore wind farms. The proposed converter topology allows series interconnection of wind turbines obviating the necessity of transformers and an offshore platform. Each wind turbine is equipped with a 5MW permanent-magnet synchronous generator and an ac-dc-dc converter. The converter topology is a diode rectifier (ac-dc) cascaded with a single-switch step-down converter (dc-dc). The dc-dc stage allows the current to flow at all times in the dc link while regulating generator torque. The receiving end is equipped with a conventional thyristor-based HVDC converter. The inverter station is located onshore and it regulates the dc link current to be constant. Stability of the configuration and independent operation of the wind turbines are validated through simulations using the PSCAD/EMTDC software package. Protection for some key dc fault scenarios are discussed and a possible protection strategy is proposed.ii

This study presents a dc-dc converter based on the Marx generator principle of charging capacitors in parallel followed by reconnection in series for discharging and for creating higher voltage. The topology uses solid-state switches like IGBTs as well as diodes. The concept and the operation of converter are described as well as design steps. The validity of the design is confirmed using PSCAD/EMTDC software package. The topology is compared with the conventional boost where the Marx dc-dc converter is shown to be competitive and even advantageous for higher dc gain. A cascade configuration is also presented and a 5 kW prototype with two stages made of two capacitors per stage is presented for experimental validation of the concept.

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