This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show the system's steady-state and dynamic performance and grid low-voltage ride-through capability. The medium-speed brushless DFIG in combination with a simplified two-stage gearbox promises a low-cost low-maintenance and reliable drivetrain for wind turbine applications.

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5A design procedure for the Brushless Doubly Fed machine is based on equations derived from a simplified equivalent circuit. 6The method allows the many variables in the design of this machine to be handled in straightforward way. Relationships are 7 given for the division of slot area between the two stator windings and for the design of the magnetic circuit. The design method 8 is applied to a frame size 180 machine. In particular, calculated values for flux densities in the machine have been verified by 9 time stepping finite element analysis for actual operating conditions. The approach outlined can also be used as part of a design 10 optimization routine.

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This paper contributes to the characterisation of the brushless doubly fed induction machine (BDFIM), which is attractive as a variable speed generator in applications (offshore wind turbine) with minimum maintenance requirements. The BDFIM has two three-phase stator windings of different pole numbers housed within the same stator slots and a shortcircuited rotor winding capable of coupling fields of different pole numbers. The stator windings and rotor winding create a magnetic field distribution with a range of characteristics different to those of conventional induction machines. This paper presents an analysis to identify the field characteristics and discusses their impact on the analysis and design of the BDFIM. The characteristics are determined from an analysis of the sum of two rotating sinusoidal field waveforms and confirmed by comparison with time-stepping finite element results and measured magnetic flux density data.Index Terms-Brushless doubly fed machine (BDFM), flux density, field distribution, rotor design, core loss, standing wave.

Time-stepping finite element analysis of the BDFM for a specific load condition is shown to be a challenging problem because the excitation required cannot be predetermined and the BDFM is not open loops stable for all operating conditions. A simulation approach using feedback control to set the torque and stabilise the BDFM is presented together with implementation details. The performance of the simulation approach is demonstrated with an example and computed results are compared with measurements.

Details of a lumped parameter thermal model for studying thermal aspects of the frame size 180 nested loop rotor BDFM at the University of Cambridge are presented. Predictions of the model are verified against measured end winding and rotor bar temperatures that were measured with the machine excited from a DC source. The model is used to assess the thermal coupling between the stator windings and rotor heating. The thermal coupling between the stator windings is assessed by studying the difference of the steady state temperatures of the two stator end windings for different excitations. The rotor heating is assessed by studying the temperatures of regions of interest for different excitations.