Wind Turbine Current-Source Converter Providing Reactive Power Control and Reduced Harmonics

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Abstract
This paper presents a current-source inverter topology that is suitable for multi-megawatt wind turbines. The proposed scheme utilizes two series-connected three-phase inverters that employ fully controllable switches and a proper interconnection transformer with the mains. In order to improve the efficiency and to allow the use of high-power devices, the inverters are switched at the mains frequency. The axial-flux permanent-magnet (PM) generator is directly coupled to the turbine (gearless solution), and its design reduces the dependence of the output voltage on the load current. The overall control technique allows to independently impose two desired quantities that can be selected out of the set of three composed of: 1) the total average voltage at the dc side of the inverters, which is directly related to the turbine speed; 2) the fundamental power factor at the mains interconnection point, which can be chosen unitary, leading, or lagging; and 3) the amplitude of one desired component of the spectrum of the mains line currents. The two chosen quantities univocally determine the third one. At specific operating points of the turbine, a significant reduction of the fifth and seventh harmonics can already be achieved without additional filters and/or active harmonic compensation. Nevertheless, the introduction of an active harmonic compensator is necessary to provide the required harmonic reduction (also up to higher orders) more independently and on a wider range of operating conditions. The almost independent regulation of the dc-link current allows further control of the average generator torque. Experimental results that are obtained from a 10-kW prototype with an axial-flux PM generator are presented.

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