Steady-State Performance Analysis Of Cascade Brushless Doubly-Fed Generator

Facing the challenge from energy resource and environment, windenergy nowadays has large mercantile potential and environmentalbenefits as a green clean and recycled energy. The wind resource in ourcountry is plenty with great potential for development. It has been along-term energy resource policy of our government to develop windelectricity. Recently, with the development of wind power technology athigh rate, the type of AC exited generators (doubly-fed generators) forvariable-speed constant-frequency (VSCF) wind power becomes thedominant among the international products. The VSCF technique maymake good use of wind energy and may improve the stability of electricpower system. Development of Brushless Doubly-Fed Generation(BDFG) system is one of the promising subjects in the research area.As a sort of BDFG, the Cascade Brushless Doubly-Fed Machine(CBDFM) has the advantage of decoupling in magnetic circuits betweenthe two machines so that it may be much easier to be designed and to be manufactured. Basically, CBDFM was studied as a motor applicable tospeed regulation. This thesis, paying specific attention to its futureapplication for wind power generation, investigates the steady-stateperformance of CBDFM in detail. The main points of this thesis aresummarized as follows:(1) The structure and working principle of CBDFM used in VSCFwind power system are introduced in detail. Using matrix transformation,a steady-state model for rotor mechanical reference frame (d-q axis) isderived. Then, based on the model, the various frequencies of currents inall the windings of the cascaded machines are converted to an unitaryvalue so that the steady-state equivalent circuit may be finally obtained.(2) Based on the frequency restriction relationship of CBDFM, thesimplified equivalent circuit is used to analyze the steady-statecharacteristics of the machine. The derived conclusions are more concisein the expression and clearer in explanation of physical behavior.Combining the main parameter characteristics varied with the motorspeed, some steady-state performances of CBDFM controlled byconstant voltage/frequency ratio such as open-circuit voltagecharacteristic, power flow in the machine, nature power characteristicand the control characteristic as generator operation are analyzed to better understanding of the inherent relations between some steady-stateperformances and the main parameters.(3) The thesis puts emphasis on the investigation of thecharacteristic of nature power and behaviors of the generator undercontrol of constant voltage/frequency ratio. It shows that the power-anglecharacteristic of CBDFM is similar to that of synchronous generator andthe characteristic is also affected by the nature characteristic varied withspeed. Generator operation of CBDFM may be achieved at differentspeed (sub-synchronous speed, super-synchronous speed) except thedead areas by adjusting the excitation voltage/frequency ratio and/or thephase angle of the voltage.(4) The computer simulation program is edited by MATLABsoftware on the basis of equivalent circuit. The results of simulationexamples, including open-circuit operation, sub-synchronous operationand super-synchronous operation as a generator connected to the grid,show that the conclusions obtained from simplified equivalent circuit arefairly good. It validates the theory analyses presented.The proposed investigation is hopefully applicable for the furtherresearch of VSCF wind-turbine generator with voltage-frequency ratiocontrol.