Research On Rotor Configuration Of Brushless Doubly Fed Machine And Its Applications On Stand-alone Generation System

The brushless doubly-fed machine (BDFM), as a novel type machine, has gained attractive attention in recent years along with variable speed constant frequency generation systems and variable frequency adjustable speed motor system. The presence of slip-rings and brushes in doubly-fed induction machine (DFIM) necessitates regular maintenance due to brush wear and carbon accumulation on the internal components, correspondingly, BDFM with no need for brushes, leading to a higher level reliability and an easier maintenance. Thus, the BDFM has been promised to be an alternative of DFIM in some apllications such as wind power generation system and some embedded generation systems like ship shaft generator where maintenance is becoming more difficult and expensive. However, commercialization of the BDFM has at least in part been inhibited due to its not yet perfect performance, difficulties in controlling the machine and complex manufacturing process. This dissertation summarized the operation principle of BDFM and proposed two novel rotor configurations, in which a novel wound rotor type BDFM is suitable for generators in wind turbines, a novel reluctance rotor configuration is suitable for mass production, and they both have good performance with low space harmonic content.This dissertation also proposed a method of generated voltage control of the BDFM in a stand-alone operation system. The control scheme was implemented without the parameters of BDFIG and was able to maintain constant voltage and frequency at the output of the BDFIG, irrespective of the prime mover speed and load condition variation. Meanwhile, the mathematical description, power flow and performance evaluation of the stand-alone BDFIG was comprehensively developed and analyzed. This dissertation also analyzed and solved some problems during the test of BDFM based stand-alone ship shaft power generation system, which made a certain contribution for future industrial applications of the machine.The main content of this dissertation is summarized as follows:Firstly, based on the review of the researches and developments of the BDFM done by the domestic and overseas scholars, this dissertation summarized the operation principle, the equivalent circuit and operation modes of the BDFM. According to the derived steady-state equivalent circuit and mathematical model, the simulation system can be used as a tool for analyzing the steady-state characteristic and dynamic characteristic of BDFM;Secondly, on the basis of the original equivalent circuit of BDFM, the simplified equivalent circuit of the machine was derived, and then the relationships of the air-gap flux densities and resultant magnetomotive forces (MMFs) with different pole pairs were analyzed. The standing wave theory of the BDFM was presented, which reveals the air-gap magnetic field characteristics of the machine and the rules of the pole-pairs combination;Thirdly, in accordance with the principles of slot MMF harmonics, the design method of the wound rotor type with unequal element coils for the BDFM was proposed. The design principle, design steps and design rules of this structure were analyzed in detailed with an example. The validity of novel rotor structure and design method is verified by the harmonic analytical results of different design simulations;Finally, according to the characteristics of BDFM in the application of stand-alone power generation system, its control method was propoesd. The control scheme was implemented without the parameters of BDFM and was able to maintain constant voltage and frequency at the output of the BDFM, irrespective of the prime mover speed and load condition variation. To verify the proposed method, the response of the generated voltage control under load and speed variation has been investigated by simulations and experiments. The amplitude and frequency of the generated voltage were kept almost constant regardless of load and speed variation, it shows BDFM has its promise for commercial application.This dissertation summarized the research background, the operation rules and characteristics of BDFM, and then two novel kinds of rotor configurations were proposed. Whereafter, the practical application of BDFM-based stand-alone power generation system was studied in depth. It provided some theoretical basis in the development of converting BDFM’s research from conceptual exploration to industrial application.