INVESTIGATION OF THE MATRIX CONVERTER IN A DC SERIES-CONNECTED WIND FARM: Modulation, Control and Efficiency
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- Institutt for elkraftteknikk 
In this thesis a wind energy conversion system for series-connection of DC wind turbines is considered. The wind park topology offers the advantage of a high voltage transmission without the need of an offshore platform to support a step-up transformer and an AC-DC converter. The wind conversion system which is proposed is composed of a permanent magnet synchronous generator, a three phase AC-AC converter, a three phase high frequency transformer and a diode bridge rectifier. Designing the transformer for high frequency operation reduces its volume and weight, making it possible to place it in the nacelle of the turbine. Three different converters are considered for the AC-AC conversion: the matrix converter, the indirect matrix converter and the back-to-back converter. The ACAC converter feeds the high frequency voltage waveform to the transformer which brings about a high switching frequency and switching losses. An important effort is therefore been spent on finding appropriate modulation schemes which can limit the losses. "High Frequency" modulation schemes suitable for the conversion system are developed in the thesis. To make a proof of feasibility by simulation, a simulation model of the conversion system is build. The model is based on mathematical models of the different components and includes a detailed model of the converters with the modulation schemes. A control strategy for the rotor speed is developed and implemented in the simulation model. The design of a typical 750kW unit is carried out to complete the simulation model. Simulation results show that the proposed conversion system and the solutions for the modulation, control and design are valid. Semiconductor loss calculations are also build into the simulation models to evaluate the efficiency of the different AC-AC converters. The results of the simulations show that the developed modulations keep the efficiency at an acceptable level while enabling an increase of the transformer frequency. The matrix converter and indirect matrix converter have very similar efficiencies. The B2B efficiency is largely dependent on the desired stator current waveform quality. If a performance equal to the matrix type of converters are desired, the efficiency of the back-to-back is lower for all modulations while it is higher if other non-comparative criteria are used for selecting filter and switching frequency. An initial experimental verification of the proposed concept is presented in the thesis. The experimental set-up includes a matrix converter, a high frequency transformer, a diode bridge rectifier. The matrix converter is connected to a voltage source instead of a permanent magnet synchronous machine. Designing and building the matrix converter is part of the thesis and presented here, as well as the implementation of the modulation with a real-time simulation system.