Study On Speed Sensorless Control Of Brushless Doubly-fed Wind Power Generator

Wind power generation,which is a potential form of renewable energy, has been more and more extensively researched in recent years, Double fed Variable speed constant frequency (VSCF) wind power generation technology especially obtained rapid development with its unique advantages. Currently, the AC excitation double fed generator is widely applied in wind power generation system. It is composed of metal and graphite composite brush structure; it is difficult to keep moderate lubrication between the brush and slip ring in a long-term, the chip from the brush operation needs regular cleaning. In addition, the composite material is very sensitive to moisture, it’s easy to cause uneven wear when the relative humidity is below15%or above85%.The slip ring and brush of doubly-fed wind power generator need to be replaced once a year. The humidity and the maintenance time and the maintenance fee in the offshore wind farm brought great challenges to the doubly-fed wind power generation. The brushless doubly-fed generation is particularly applicable to the offshore wind farm, because it eliminates the brush and slip ring structure with easier maintenance and more reliable performance. Now the brushless doubly-fed generator has been research widely, but the study about the control without the speed sensor is relatively less. For BDFG, the main studies of this paper can be summarized as follows:1. The rotor of BDFG has realized lower harmonic magneto motive force, higher efficiency and better practicability. The rotor structure decides the performance of the BDFG.The research abject is the BDFG with changing pole winding rotor structures. The operational principle of the BDFG and the energy conversion principle are analyzed under different speed, form sub-synchronous speed to super-synchronous speed.In order to reduce the solving variables and exploring the BDFG control law, based on the mathematical model of three-phase static coordinate system, the rotor speed coordinate model is established, where the power side and control side is still relative to the rotor speed rotating two-phase coordinate system. because the control winding is direct connected to the converter, in order to realize vector control, the double synchronous speed two-axis rotating coordinate mathematical model is established, in which the power side and control side is respectively synchronous with the rotating magnetic field itself. 2, Installation of the speed encoder will increase the hardware maintenance andenlarge the motor shaft size and cause the concentricity problems. Meanwhile, the control subsystem of wind power system is in the bottom of tower, the photoelectric encoder is susceptible to the effects of high temperature and high humidity environment from more than100m transmission line. In order to make the system more suitable for extreme wind field condition and overcome the effect of the installation speed encoder, Considering the complex electromagnetic relation between two stator windings and a rotor winding in the BDFG, this paper proposes a model reference adaptive system speed estimation method,in which the coordinate system is based on the two-axis rotating coordinate mathematical model with double synchronous speed. It realized the speed estimation by observing the stator flux linkage of the BDFG power winding in the two phase rotating coordinate system, which overcomes the defects of the initial value and DC offset which is exist in the traditional static coordinate system by observing the reference model flux linkage.3, The BDFG is a nonlinear complex system, the proportion parameter and integral parameter is fixed in the traditional MRAS speed estimation, which cannot keep the control system’s performance in the best range, so a fuzzy PI MRAS was put forward in this paper. The influence factor such as the power winding side active current and proportional, integral parameter is analyzed in the small signal model. Fuzzy PI MRAS improved the estimation speed of following performance under changing speed and noise immunity by adjusting the proportional, integral parameter in different working conditions when the torque current is changing. The feasibility and effectiveness of the control scheme is verified by relative analyzing the control performance of fuzzy PI the control without speed sensor and conventional PI control without speed sensor with simulation in the Matlab/Simulink.4, The life span of the capacitor in DC side is shorter thanthe rest of the back-to-back converter system. In order to reduce the capacitance size and stable the voltage in DC side, the internal model-feed forward compound control scheme is designed in the bidirectional converter. First, the control principle of the line-side converter of the back-to-back converter in the BDFG without speed sensor is shown and the DC condenser capacity is designed. Then in order to further reduce the capacity of DC side under the same DC side voltage fluctuation conditions, a feed forward control which bringing the generation-side converter power change to the line-side converter is performed. The control delay is reduced and the given value following is speed up by internal model control. The feasibility of the proposed compound control scheme is verified by the analyze that the voltage fluctuation can be decreased under the same capacitance.5,Based on the theoretical research, the experimental platform of a5KW BDFG variable speed constant frequency vector control system is formed, which contains the back-to-back converter withtwo DSP.Using a common ac asynchronous motor to drive the BDFQ variable speed constant frequency control is realized, the decoupling control of active and reactive power is validated. The experiment of speed tracking and the inhibition of disturbance-rejection are relatively tested between the proposed fuzzy PI MRAS and conventional PI MRAS, the result shown that the proposed control scheme is correctness and feasibility. Finally, the advantages suppressing the voltage fluctuation of the internal model-feedforward compound control scheme under the same capacitance of the DC side is verified.