Abstract-The alternate arm converter (AAC) was one of the first modular converter topologies to feature dc-side fault ridethrough capability with only a small penalty in power efficiency. However, the simple alternation of its arm conduction periods (with an additional short overlap period) resulted in 1) substantial sixpulse ripples in the dc current waveform, 2) large dc-side filter requirements, and 3) limited operating area close to an energy sweet spot. This paper presents a new mode of operation called extended overlap (EO) based on the extension of the overlap period to 60• , which facilitates a fundamental redefinition of the working principles of the AAC. The EO-AAC has its dc current path decoupled from the ac current paths, a fact allowing 1) smooth dc current waveforms, 2) elimination of dc filters, and 3) restriction lifting on the feasible operating point. Analysis of this new mode and EO-AAC design criteria are presented and subsequently verified with tests on an experimental prototype. Finally, a comparison with other modular converters demonstrates that the EO-AAC is at least as power efficient as a hybrid modular multilevel converter (MMC) (i.e., a dc fault ride-through-capable MMC), while offering a smaller converter footprint because of a reduced requirement for energy storage in the submodules and a reduced inductor volume.Index Terms-AC-DC power conversion, active filters, capacitive energy storage, HVDC transmission, power system faults, power transmission protection.

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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This paper presents a method to calculate the minimal size of cell capacitors in multilevel VSCs which meets a maximum voltage deviation criterion under ideal conditions. This method is applied to the Modular Multilevel Converter (MMC), the Alternate Arm Converter (AAC) and the hybrid multilevel converter with ac-side cascaded H-bridge cells (AC-CHB). The results show that the newer VSC topologies exhibits smaller energy deviation in their stacks, leading to an overall smaller volume of cell capacitors for the converter station but often accompanied by some compromises such as higher power losses or degraded DC current waveform quality.

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.