The Modular Multilevel Converter (MMC), originally proposed by Professor Marquardt, has made it practical to realise converters with ratings up to 1000MW using standard components developed for variable speed drives. With the power electronics packed into individual "sub-modules", the ability to realise a converter where the AC and DC voltages are under direct control and have very little distortion appears to be an ideal; however, while there have been several papers published covering the design considerations required to ensure these converters operate correctly from an academic perspective, the method of specifying the design of a fully rated MMC, where several hundred sub-modules may be required for each valve to meet all conditions in service have not been discussed. The paper outlines a procedure for calculating the values of the transformer turns ratio, the transformer reactance and the valve reactance before considering how the ripple that appears on the submodule voltage affects the operating performance of the converter. This ripple limits the sub-modules from being able to fully utilise their capability so the principal operators that affect the ripple voltage are considered. The main independent variables available are the sub-module capacitor value, the number of sub-modules per valve and the average capacitor voltage setting, which is set as a servo demand within the control. All these inputs have an effect on all aspects of the converter performance and so must be set together while considering other system variables such as the variation in the AC voltage.

This paper presents modeling and analysis of electrical oscillations in a wind farm system. The detailed modeling and modal analysis of a wind farm system are presented in this paper. The approach to modeling uses detailed representation of a wind turbine generator and collection system including highvoltage direct-current (HVDC) power converter system control, facilitating a comprehensive analysis of the wind farm system. Various modes are classified according to the frequency of oscillation. The detailed modal analysis is used to characterize the critical modes. Time-domain simulation also confirms the presence of these modes. The effect of wind farm operating conditions and voltage source converter control tuning on critical oscillatory modes are also assessed and discussed in detail.Index Terms-Doubly fed induction generator (DFIG), eigenvalue, oscillations, stability, wind farm, wind turbine generator (WTG).

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The Controlled Transition Bridge (CTB) is a converter topology that combines series connected semiconductor "director switches" with chains of switched capacitor modules, chainlink circuits, in such a way that the director switches carry the main current for a significant portion of the period and the chainlink elements provide a controlled traverse of voltage between different director switches conducting. The simplest example of this is where the director switches form a six pulse bridge and the chainlink elements traverse at a constant rate between the upper director switch conducting and the lower director switch conduction etc., so that the output AC waveform is a trapezoid. The use of a trapezoid waveform reduces the level of super harmonics significantly and with a star delta transformer to remove the "triple N" harmonics, the total harmonic distortion is reduced, but not sufficiently for use in HVDC application. The use of filtering is undesirable because of the VARs they introduce and while active filtering can be used there are control difficulties that need to be overcome, so a two slope trapezoid waveform is proposed in which the slope characteristics are chosen specifically to minimise a wide range of harmonics for a given fundamental magnitude. For this a cost function is derived that includes the functions of the harmonics being considered and a search is carried out using standard algorithms such as Newton-Raphson, to minimise its value within a given region. Modelling is used to demonstrate that the resulting primary THD would meet the requirements for VSC HVDC operation.