Research On Direct Torque Control And Fault-tolerant Control Of T-Type Three-level Dual Three-phase Pmsm Drives

This thesis is presented at the background of the application for low-voltage and large-power traction and build T-type three-level dual three-level permanent magnetic synchronous motor(PMSM)driving system,which embraces both the advantages of mutilevel inverters and mutiphase PMSMs.The control schemes under normal and fault states for the system are analyzed and reasoned deeply.For ease of theoretical analysis on the controlled object,the mathematical model under Natural stationary coordinate system and vector space decomposition(VSD)transformation are built in this thesis.Contrary to three-phase motor,the harmonic impedance of dual three-phase motor is very small.Therefore,the suppression of harmonic currents should be be taken into account,while controlling the torque.By combining of vector space decomposition(VSD)method and space vector modulation based direct torque control(SVM-DTC),This thesis proposed double SVMs based direct torque control and VSD-SVM based direct torque control for T-type three-level dual three-level PMSM drives.Both the two control schemes includes the optimization target of suppressing the harmonic current caused by dead-time,motor asymmetry and EMF harmonic.The proposed double SVM approach designs two identical three-level three-phase space vector modulation module.Different from conventional three-level three-phase space vector modulation,A simplified pulse width modulation method,which directly obtains the dwelling time of each level in each phase is proposed.This modulation method simplifies complex modulation process and strengthens the control over DC link mid-point voltage.The proposed VSD-SVM method is designed on the basis of vector space decomposition,which achieves the synthesis of voltage reference in torque subspace and guarantees zero average voltage in harmonic subspace.In order to optimize the process of vector synthesis,a two-step vector synthesis approach is proposed,which decreases the complexity of space vector modulation.The double SVMs based direct torque control is simple and effective,which mainly applies to multiple of three-phase motors.The VSD-SVM based direct torque control adopts relatively complicated modulation method,but applies to all multiphase motors.In order to enhance the reliability of T-type three-level dual three-level PMSM driving system further,this thesis proposes a complete set of fault-tolerant control strategies for different open-switch faults under different space vector modulation methods.In addition,voltage compensation based fault-tolerant control and hybrid fault-tolerant control is proposed for single open-phase faults of dual three-phase PMSM.The proposed voltage compensation based fault-tolerant control only compensates the voltage reference under faults and the structure of control system is not changed.The original dynamic response character and utilization of DC link voltage are not affected.The proposed hybrid fault-tolerant control adopts current closed-loop control for the remaining two phase of fault three-phase winding and maintain SVM based direct torque control,under the currents constraint conditions of minimum copper loss.Therefore,the adjustment of original system structure is small,while achieve fault-tolerant control under minimum copper loss.The experiments are given to verify the correctnes of theoretical analysis and proposed schemes.