Research On The New Type High Voltage Wind Power Generator Of Internally Fed Rotor Excitation And Its Integration Control

With the development of power electronics technology,the performance of wind turbine generator(WTG)and its matched energy conversion equipment is becoming more and more advancement.The power conversion level of wind turbine generator has grown from the original kilowatt level to MW level,especially with the rapid development of offshore wind power generator.WTG of high voltage and large capacity has become inevitable trend of the world wind power development.At present,the high voltage wind turbine generation system adopts two types:one type is a high voltage wind turbine generator and a high voltage converter,the other one is composed of low voltage wind power generator,low voltage converter and transformer.Since high voltage converter is expensive,complicated and difficult to control,large capacity generators generally adopt the second type.But with the increase of generator capacity,the lower output voltage of the generator stator end would lead terminal output current increases,which requires the cable and copper have a greater carrying capacity,and bring considerable difficulties to the production process of wind turbine generator.In addition,the stator terminal voltage is low,it is necessary to improve the voltage through two or more transformers before connecting to the grid;the grid side converter also needs to add a transformer,which is used to match the voltage level.These would undoubtedly increase the manufacturing and operating costs of the system.In order to solve the problems of the high voltage wind turbine,the main research work and innovation are as follows:(1)Based on the structure and working principle of the common doubly fed induction generator,a new type high voltage wind power generator of internally fed rotor excitation(IFREG)is proposed in this paper.The stator of the utility model is provided with two sets of stator windings which are in the same slot.A winding is the main winding of the high voltage stator,and the output terminal can be connected directly to the grid,which can output constant frequency AC high voltage;the other winding is a low voltage stator auxiliary winding,which is connected with the PWM converter on the side of the grid,which can provide the excitation current continuously to the generator through the DC bus and the rotor side PWM converter.It solves the problem of excitation current regulation when the rotor speed and load change.Through the corresponding excitation starting control strategy,control strategy and flexible no-load steady state operation control strategy,IFREG can effectively control the output of high voltage and large capacity by the small capacity excitation converter.The frequency amplitude of the output voltage can keep constant,the number of the system transformer is reduced,and the cost,volume and weight of the system are effectively reduced.(2)Based on the new generator structure and working principle analysis,a 6kV,440kW prototype was designed.Finite element simulation has been carried on the sub synchronous,synchronous and super synchronous in no-load and load conditions by using the finite element method.The feasibility of the new type of IFREG in the field of variable speed constant frequency is verified in the theory.(3)The mathematical model and equivalent circuit of the IFREG in the abc and dq coordinate systems were derived,and the electromagnetic torque equation and motion equation are given.This paper put forward the excitation starting mode and control strategy of IFREG,and analyzed the physical process of the generator.According to the mathematical model,the Matlab model of IFREG was built,and the simulation analysis was carried out under the different rotational speed.The simulation results show that the high voltage output can be obtained at the stator end in the condition of off grid wind turbine with different wind speed.(4)In this paper,the mathematical model and the double closed loop vector control strategy of the grid side PWM converter under the condition of the grid voltage stability were studied in the IFREG.On the basis of the double closed loop vector control strategy,the power compensation control strategy of grid side converter was proposed.Compared with the common doubly fed wind generator,the stator of IFREG has two set of windings,which makes the generator system model more complex.In this paper,the operating mechanism of the rotor side PWM converter under the condition of voltage stability was discussed.The stator flux oriented vector control strategy and the maximum wind energy tracking control strategy which can decouple the active and reactive power of the system were deduced.(5)This paper presented the control step and the control strategy of the no-load grid which matchs the IFREG.The simulation model was established in the power grid voltage stability under the condition of high voltage in the IFREG,and the simulation analysis was carried out on the excitation performance of the double fed wind power generation system,the performance of the no-load flexible integration,the steady-state operation of the load,the decoupling control of active and reactive power,and the maximum wind energy tracking capability.The simulation results show that the doubly fed wind power generation system can be started smoothly,and can carry out no-load and flexible grid connection.The high voltage energy which frequency and amplitude are stability can be input to the grid.(6)In order to verify the operating characteristics of variable speed constant frequency(VSCF)generation in high voltage induction generator,a 440kW,6kV IFREG prototype system is developed.A set of high voltage isolated power supply system which can be used for the power supply of power electronic switching devices in the IFREG is completed.The performance of the IFREG prototype system is studied,including the excitation starting performance,the performance of the no-load flexible grid connection,the steady state operation of the load,the decoupling control of active and reactive power,and the maximum wind energy tracking capability.Experimental results were in agreement with simulation results and theoretical results,and the feasibility of the design and the correctness of the matched control strategy were verified.