Abstract-The switching frequency of medium-voltage high power converters is limited to about 1 kHz due to semiconductor junction temperature constraint. The frequency band between the fundamental and carrier frequency is limited to a little more than one decade and the LCL filter design is usually a challenge to meet grid codes for grid connected applications. Traditional designs focus on the optimization of the filter parameters and different damping circuits. However, this design is very influenced by the modulation technique and produced low order harmonics. Widely used pulse width modulations (PWM), such as phase disposition PWM (PDPWM), produce low order harmonics that constraint the design of the filter. Selective harmonic elimination PWM (SHEPWM) can eliminate theses low order harmonics, enabling a more efficient design of the LCL filter. In this paper, the LCL grid filter of a Multi-Megawatt Medium-Voltage neutral point clamped (NPC) converter for a wind turbine is redesigned using SHEPWM modulation. Experimental results demonstrate that the efficiency of the converter, filter and overall efficiency are increased compared to that obtained with PDPWM.Index Terms-LCL filter, medium-voltage converter, selective harmonic elimination, SHE.

The paper develops a version of the synthetic loading method, suitable for testing of multiphase machines with multiple three-phase distributed windings. The method is at first discussed in general terms for a structure with k three-phase stator windings (i.e. total number of phases is n = 3k). Subsequent detailed development is described for a dual three-phase (sixphase) stator winding configuration. With a control architecture that allows the use of half of the three-phase windings as a motor and the other half as a generator, the machine (and/or the converter) can be tested under full rated power without the need for any mechanical load. Moreover, the power consumed from the grid is in essence equal only to the total losses of the system. Modelling, based on the double d-q approach, and the control layout that includes full cross-coupling decoupling are described for a permanent magnet (PM) synchronous machine. An experimental test rig with a double three-phase PM machine of 150 kW rating is detailed and the samples of experimental results are provided to verity the theoretical considerations.

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− Standard modelling approaches for a triple three-phase (i.e. nine-phase) machine include vector space decomposition (VSD) and triple d-q modelling procedure. Each is characterised with certain benefits and shortcomings. This paper introduces a novel transformation, applicable to multiple three-phase winding machines in general, especially aimed at facilitating the power (current) sharing between the three-phase systems. The easiness of the current sharing with the novel transformation, introduced here, is demonstrated via simulations in MATLAB.Index Terms − Nine-phase machine, multiphase machine transformation, multiphase machine modelling, triple three-phase machine.

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