Evaluation of Pole-Changing Wound Rotor Synchronous Wind Generator Topologies

Abstract

With the rapid penetration levels of inverter-based grid-connected renewable energy sources, the stability of the grid comes into question. To rectify this, direct synchronous grid-connected renewable energy plants are being considered to ensure that the necessary grid services are provided, with a special focus on grid strength. Information on directly connecting wind turbine generators (WTGs) to the grid is not widespread in the literature. The limiting factor remains that direct grid-connected WTGs are only operable at a single synchronous speed. This paper proposes the use of alternative rotor designs and forms part of a broader study investigating the possibility of increasing the amount of energy that direct grid-connected WTGs can harvest. In this paper, various round-rotor field-winding topologies are evaluated to determine their suitability for ensuring that a synchronous generator (SG) is capable of operating at two distinct synchronous speeds. The considered round-rotor field windings range from various phase spreads of trapezium to triangular-formed magnetomotive force (MMF) waveforms. The proposed rotor and stator topologies are optimized according to a design process whereby torque ripple, copper losses and total harmonic distortion are considered, amongst other things. It is shown that the majority of the rotor slot and winding configurations satisfy the design process requirements, where certain phase-spread-forming trapezium MMF waves are found to be superior.