Multi-level AC-DC power electronic converter for applications in PMG-based WECSs

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University of New Brunswick
Due to their structural and operational features, permanent magnet generators (PMGs) have gained popularity in wind energy conversion systems (WECSs). The general structure of a PMG-based WECS is composed from a voltage source (VS) ac-dc power electronic converter (PEC)(generator-side PEC), which feeds a VS dc-ac PEC (grid-side PEC) that delivers both active and reactive powers to a grid and/or a load. Such a structure (usually called a back-to-back PEC WECS) offers an independent control of both PECs to accommodate variable wind speed operation. Despite the flexible control, back-to-back PEC PMG-based WECSs suffer a disadvantage due to the current harmonics generated on inputs of the generator-side PEC. These current harmonics create distortions in the stator magnetic field of PMG, and cause pulsations in the electromagnetic torque. The pulsations in electromagnetic torque of a PMG can lead to several undesired operating conditions, including sustained mechanical vibrations in the wind turbine tower, damages to the turbine shaft and rotor mechanical assembly, wear-outs of mechanical fittings of the PMG and turbine couplings, and difficulties in realizing the maximum power point tracking (MPPT) operation. This research aims to investigate the possibility of reducing the pulsations in the electromagnetic torque of a PMG, when used in back-to-back PEC WECSs. The proposed approach is based on employing a 3ɸ, multi-level, VS, ac-dc PEC as the generator-side PEC. The ability of multi-level ac-dc PECs to reduce current harmonics on their inputs will be employed for achieving the objectives of this research.