Research On Control Strategy For Staggered Dual Cage Rotor Brushless Doubly-fed Induction Generator

Under the background of China’s comprehensive promotion of "double carbon" strategy,new energy generation has become the indispensable way for the development of energy technology.Among numerous renewable energy technologies,wind power has been rapidly developed and applied due to its wide distribution of resources,large reserves and relatively mature technology.In variable speed constant frequency-wind energy conversion systems(VSCF-WECS),brushless doubly-fed induction generator(BDFIG),as a dual-electric port machine with high reliability and low system maintenance cost,is expected to become the main force of future offshore wind power systems.Similar to other types of machines,brushless doubly-fed induction machine(BDFIM)also has different topology structure,so as to obtain the expected advantages.Although the two sets of stator and rotor of the staggered dual-cage rotor brushless doubly-fed induction generator(SDCR-BDFIG)are placed in single machine frame,it still belongs to cascaded brushless doubly-fed induction machine(CBDFIM)in terms of the operational principle.Therefore,the two air-gap magnetic fields can still be separated in space,which has the advantage of low spatial harmonics.In addition,the SDCR-BDFIG achieves a more compact cup-shaped spatial structure by means of the staggered connection of the two squirrel-cages,and thus the power density and efficiency of the machine are improved,which makes it very suitable for wind power generation applications as a generator.However,the simplicity of machine structure does not mean that control strategy design is also simple.Similar to other BDFIMs,its complex mathematical model brings great challenges to the design of control system,which is also the main barrier to the commercial application of the machines at present.There are many coupled variables of the SDCR-BDFIG.Besides its operational state is also easily affected by the grid voltage conditions.This is because in order to reduce the capacity of the power converter and thus reduce the cost,the power winding(PW)of the brushless doubly-fed type WECSs are usually directly connected to grid to output the main power,whereas the control winding(CW)is responsible for transmitting the slip power through the partial-rated power converter.For the SDCR-BDFIG,since its PW is directly connected to grid,any change in grid voltage will directly affect the operation of the machine.Therefore,the design of gridconnected control strategy with high reliability and high performance is a key problem to be solved urgently in the control technology of SDCR-BDFIG.To address the control problems of the SDCR-BDFIG based WECS,this dissertation proposes different control strategies to achieve different control objectives with the dynamic mathematical model analyzed under different operation conditions,which can achieve stable and reliable operation in the whole operation domain.The main research contents of this dissertation are as follows:1.The state-of-the-art of the WECSs and its future development trend have been illustrated.Besides,the different technical routes for WECSs have been compared and discussed.The development history of BDFIM has been systematically introduced and the control strategies for BDFIM have been discussed and categorized in terms of the working principle and the control target.2.The dynamic and steady-state mathematical models as well as state space equations of the SDCRBDFIG are derived.The power flow problem of the SDCR-BDFIG as a kind of dual-electric ports machine is analyzed.and the experimental parameter measurement method based on the steady-state equivalent circuit is introduced.The experimental platform of SDCR-BDFIG-based wind energy conversion system is designed.Based on the experimental platform,not only parameters of SDCR-BDFIG are measured,but also gridconnected and standalone generation experiments are implemented.3.A grid synchronization strategy is proposed to provide premise guarantee for the generation operation of the grid-connected SDCR-BDFIG.Based on the concept of virtual flux and power decoupling model,the calculation methods of the CW currents for synchronization and PW power decoupled control are investigated,respectively.Then with the PI parameter appropriately selected,the stability of inner loop CW current controller is analyzed.Finally,a dual-objective control method for synchronization and PW power decoupled control is proposed.4.A robust nonlinear control strategy for the SDCR-BDFIG is proposed.Based on the super-twisting sliding mode control theory,an inner-loop CW current controller is designed with the stability analyzed by selecting the appropriate Lyapunov candidate.In addition,a PI based outer-loop PW power controller is also proposed with the stability analyzed.Finally,combinng the inner and otter controllers,a robust nonlinear control system is designed for SDCR-BDFIG to achieve the independent decoupled control of the PW active power and reactive power.5.A speed sensorless control strategy for the SDCR-BDFIG is proposed.The adaptive models based on the PW active power and reactive power are derived,and thus a PW power model reference adaptive system(MRAS)based speed observer is designed.Then,the stability of the observer is proved with the small signal model.Finally,combining the power decoupled control strategy,a speed sensorless power decoupled control system for the SDCR-BDFIG is designed.6.A Coordinated harmonic suppression control strategy is proposed for the SDCR-BDFIG under unbalanced grid conditions.Firstly,a coordinated control system is designed for the SDCR-BDFIG under balanced grid conditions.Then the dynamic mathematical models of the SDCR-BDFIG and the grid side converter(GSC)withstanding the grid voltage imbalance are analyzed.According to derived models,the negative sequence current references for machine side converter(MSC)and GSC are obtained to suppress current harmonics and power ripples.Finally,a negative sequence current compensation based coordinated harmonic suppression control system for the SDCR-BDFIG under unbalanced grid conditions is proposed.7.A low voltage ride through control method for the SDCR-BDFIG is proposed.Firstly,the dynamic mathematical model of the SDCR-BDFIG during voltage sag is analyzed,and it is found that the PW transient flux has a negative effect on the operation of the generator.In order to accelerate the decaying of the transient flux and to reduce the electromagnetic torque oscillation,an electromagnetic torque suppression method based on the "demagnetization" of the transient flux for the SDCR-BDFIG during voltage drops is proposed.This method can not only provide reactive power to grid to support the voltage recovery,but also accelerate the demagnetization of transient flux,and thus reduce the electromagnetic torque oscillation.