Research On Control Strategy And Application Of Brushless Doubly-fed Variable-frequency Speed Regulation System

The use of converter to improve the efficiency of fan and pump loads is great significance for energy conservation,emission reduction and realizing carbon neutralization.The brushless doubly-fed machine(BDFM)eliminates the slip ring and brush through the two indirect coupling magnetic fields with different pole pairs,which improves the system reliability.In BDFM driving system,the converter only needs to provide a part of power of the system.Besides,the two sets of stator windings can be designed as high-voltage winding and low-voltage winding respectively in the high-voltage system.In this way,a low-voltage converter can be applied to speed adjustment,which has obvious advantages.At present,most of the research on the magnetic field theory and control strategy of BDFM is still at the stage of theoretical analysis and laboratory testing,which can hardly meet the system requirements in applications.This paper takes the wound-rotor BDFM as the object,and studies the starting methods,switching of the operation modes and control strategies and the control methods under non-ideal grid,which provides a relatively complete solution for the application of BDFM.The main work contents and innovations are shown as follows:1)The space-time vector diagram of the power side subsystem and the control side subsystem of the wound rotor BDFM is established,and the electromagnetic coupling relationship between the stator and rotor is studied.It is point that the magnetic fields corresponding to the two subsystems are independent of each other,and there is a cross in the rotor circuit of the two subsystems.In this way,the indirect coupling of the two stator magnetic fields through the rotor can be explained.Based on the stator and rotor magnetomotive force balance equation and the magnetic field energy storage theory,the torque expression of BDFM is obtained.At the same time,the torque expression of BDFM is derived from the perspective of the equivalent circuit based on the power balance.Two different expressions of the torque angle are presented,indicating that the torque characteristic of BDFM is similar to the synchronous machine.2)The torque characteristic of BDFM in asynchronous state is studied through the steady-state model,and the relationship between the maximum asynchronous torque and the machine parameters is analyzed.It is point that the principle of the asynchronous soft-start control of BDFM is selecting a proper resistance so that an optimal speed-torque curve obtained in total asynchronous starting process.The relationship between the turn-on ratio of low bridge arm and the virtual resistance is deduced through the simplified equivalent circuit.Then,the torque adaptive control strategy based on linear increasing turn-on ratio and the constant torque starting method based on current hysteresis control are proposed.According to the dual electrical ports structure of BDFM,a doubly-fed starting control strategy based on the control winding side open circuit voltage tracking is proposed.Based on the characteristics of the control side open circuit voltage and load torque,a variety of starting schemes are proposed to provide references for the control system designing,which are verified by simulation and experiments.3)A reduced order model is derived,and the equivalent angel between the power side and control side current vector is proposed.The stability of the speed open loop I/F control method is analysized through the equivalent angel,which is analogous to the torque angle in Chapter 2.The variation of the torque and excitation component of the control side current before and after the operation mode switching is studied.The difference between the I/F control method and the vector control method in operation mode switching are compared,and the calculation method of the initial set angle for I/F control method is proposed.The I/F control method is selected to connect the asynchronous operation mode and doubly-fed operation mode based on the speed sensorless vector control method.Based on the switching principle of voltage vector equivalent,the initial values calculation method when the control strategy switching is presented,and the above control methods are verified through the simulations and experiments.4)The harmonic equivalent circuits of BDFM system in the weak grid and the distorted grid are proposed.The harmonic distribution characteristics of the system are studied through the harmonic equivalent circuits.It is point that the harmonic distribution of the system is determined by the grid harmonic impedance and the equivalent harmonic impedance of the BDFM system,the calculation results are compared with the experiment results.The equivalent harmonic impedance of the machine can be regulated by controlling the converter output.Two harmonic control loops are designed based on the resonance control method,which can realize bidirectional harmonic current control of the system under the weak grid and reducing the stator harmonic current under the distorted grid.The parameters of the harmonic control loops are designed through the time domain analysis method,and the margin of dc bus votage for different control targets are analysized.The harmonic current control strategies are verified by simulations and experiments.