Sequentially controlled two level inverters of multi-modular permanent magnet machines for wind energy systems

Lower harmonic content is sometimes the attractive feature of some control systems. High power Axial flux Permanent Magnet Synchronous Generators for wind applications are often multi modular. Such system are often driven by a multilevel inverter due to its lower harmonic distortion compared to two level inverter. However a two level inverter has the advantage of a much simpler control system and lower cost. This thesis presents a particular system, each module of the machine is connected to a two level inverter; the inverters are paralleled on the dc side. The carrier signal of each inverter is shifted from its neighbouring inverter with an angle. The angles necessary are selected such to cancel out the least harmonic components in the dc side current. There are two types of modulation considered: square wave and pulse width modulation. We analysed the dc side current mathematically for both types of modulation; finding the full expression of the dc current ripple and finding the necessary angles to reduce the current ripple. We have proven the effectiveness of the sequential control of the two level inverters in decreasing the dc current ripple through checking the mathematical calculations with some simulations. Decreasing the current ripple subsequently decrease the torque ripple of the shaft. In chapter two gives a brief about wind energy systems and the illustration of the axial flux permanent magnet machine. In chapter three focuses on the most common topologies for multilevel inverters and a comparison between the multilevel inverter and the two level inverter. In chapter four presents the mathematical model for the dc current ripple and the verification of the equation on Matlab/Simulink. Finally chapter five includes the implementation of the control on the machine model on Simulink and a comparison between Simulink results and the mathematical results. Also includes the effect of changing some control parameters on the ripple components of both current and torque.