Abstract
This study studies, using experimentally verified finite element (FE) analysis, various performance measures of the brushless doubly fed induction generator (BDFIG) when magnetic wedges are used for closing stator open slots. The BDFIG is an attractive generator solution for offshore wind power and can replace doubly fed slip-ring induction generators. It is shown in this study that the use of magnetic wedges, commonly used in large induction machines, reduces the stator windings magnetising currents in a BDFIG, reflected in the values of magnetising inductances. They also increase the leakage flux in the stator slots, leading to a larger series inductance in the equivalent circuit. The series inductance significantly affects the BDFIG performance as well as the rating of its converter. The effects of magnetic wedges on BDFIG air-gap flux, synchronous torque, and stator tooth top saturation is also investigated. Two-dimensional FE analysis of an experimental 250 kW BDFIG is used in this study, verified by experimental measurements.
Highlights
The Brushless Doubly-Fed Induction Generator (BDFIG) known as Brushless Doubly Fed Machine (BDFM) is a variable speed generator or motor, which has in recent years been investigated as a potential replacement for the Doubly-Fed Induction Generator (DFIG) [1], currently used in the majority of large wind turbines
This paper has shown that the use of stator magnetic wedges in the BDFIG can significantly increase the magnetising inductances whilst at the same time significantly increase the stator slot leakage and the series inductance in the equivalent circuit
It has been shown that the use of magnetic wedges reduces undue saturation in the stator tooth top, the harmonics in the air gap flux and ripple in the machine’s torque
Summary
The Brushless Doubly-Fed Induction Generator (BDFIG) known as Brushless Doubly Fed Machine (BDFM) is a variable speed generator or motor, which has in recent years been investigated as a potential replacement for the Doubly-Fed Induction Generator (DFIG) [1], currently used in the majority of large wind turbines. Using magnetic wedges to close slot openings can reduce the effective air gap length resulting in lower magnetising currents and an improved power factor [11]. It will increase the stator leakage inductance by providing a low reluctance path for leakage fluxes. The authors showed in [10] that stator magnetic wedges have a considerable effect on inductance values in the BDFIG equivalent circuit and proposed analytical methods to derive the stator magnetising and leakage inductances. The effects of magnetic wedges on BDFIG air gap flux, synchronous torque and stator tooth top saturation is investigated using nonlinear FE models
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