Study Of BDFM With A Wound Rotor And Its Application In Stand-Alone Generating System

The Brushless Doubly-Fed Machine (BDFM), which removes the brush and slip ring, has the advantages of simple structure but high reliability, and compared with variable frequency motor, it could reduce the capacity and voltage class of frequency converter. Although the BDFM is proposed to have a bright prospect as an adjustable speed drive (ASD) or variable speed generator (VSG), it has not industry application due to its poor performance. The structure of rotor is considered to be the main reason that limits the performance of BDFM. The special-cage rotor is most likely used in large-capacity machine, but it has some disadvantages such as abundant harmonic contents and low conductor ratio, which decrease the efficiency and lead to the enlargement of the volume. Compared with special-cage rotor, the wound rotor has a flexible connection, which would increase the winding factor of both fundamental waves and reduce or eliminate other harmonic components by elaborate design. Those merits make the wound rotor be considered to replace the special-cage rotor to promote the industry application of BDFM. This work does some researches on the design of wound rotor, the modeling and simulation of BDFM with a wound rotor and its performance analysis using as a stand-alone generator.Firstly, the characteristic and design requirement of wound rotor are investigated through harmonic analysis. Three design methods of wound rotor are generalized as "Conjugate Method", "Pole-Changing Method" and "Tooth Harmonic Method", a new "Pole-Amplitude Modulation Method" is also presented for three-phase rotor design.Secondly, the d-q model of BDFM with a wound rotor is derived detailedly, based on which a complex analysis model is deduced on the F-B-0 coordinates. The rotor winding is regarded as two sets of sinusoid-distributed windings according to the characteristic of MMF excited by its current. A three-phase analysis model of BDFM with a wound rotor is presented and parameters calculation method is also given. The relationship between choice of reference frame and normalization of mutual inductance matrix is investigated, and an independent constant transformation matrix for rotor winding is deduced by choosing rotor rotating reference frame. Furthermore, a complex analysis model based on the F-B-0 coordinates is also deduced.Subsequently, a steady-state coupled equivalent circuit of the BDFM in the frequency of power winding is evolved from the F-B-0 complex analysis model, which then could be transformed to an equivalent circuit of BDFM by winding conversion. The parameter estimation of the equivalent circuit is also investigated, and a method combing experiments and Genetic Algorithm is presented.As a stand-alone generator, some steady-state performance characteristics of the BDFM are studied, including adjustable characteristic and external characteristic. A modified d-q model of the BDFM used as a stand-alone generator is derived to simulate the dynamic behaviors, on load and no load respectively.Some issues about the machine design are investigated considering its usage as a stand-alone generator, including the combination of pole-pairs, the determination of rated power, the allocation of magnetic and electric loading, the optimized ratio of winding factor of power winding to control winding.Finally, the experimental system of BDFM with a wound rotor is introduced. The parameters of the prototype are estimated to simulate the dynamic response of BDFM using as a stand-alone generator under closed-loop control of frequency and voltage. The simulation results are in good agreement with the experimental results, which verify the correctness of the modeling and parameters estimation of this work.