Research On Dead Time Compensation Strat-egy Of Cascaded H-Bridge Inverter In Medi-um-Voltage Large Capacity Motor Drive

With the expansion of the scale of modern industrial production,the capacity of the corresponding production equipment is constantly increasing,which prompting the development of electric motors that drive large mechanical equipment toward high voltage,large capacity,and high performance.Therefore,how to improve the performance of medium voltage and large capacity motor drive system has been widely concerned and studied.This thesis mainly focuses on the dead time compensation of the cascaded H-bridge inverter based on motor speed regulation system,which is caused by the nonlinear factor of dead time,such as the high harmonic content of rotor current and inverter output voltage,and the increase of motor torque ripple.Firstly,the working principle of cascaded H-bridge inverter is analyzed in detail,and the principle and characteristics of bipolar and unipolar CPS-PWM modulation algorithm are introduced.On this basis,the influence of dead time under the low speed working condition of motor drive is modeled in detail,the mathematical model of rotor flux oriented motor control system is established and the influence of different factors on the harmonic content of inverter output voltage under open-loop state is analyzed.Secondly,the principle and shortcomings of three dead time compensation methods applied in the structure of this thesis are analyzed in detail.The zero-crossing region detection method was studied,the threshold value of the zero-crossing region was determined,and the simulation systems of PBDTC direct adjustment method,average voltage feedback method and invalid switch locking method based on zero-crossing current region detection were established,and their practical application value was analyzed according to the simulation results.Thirdly,the thesis expounds the control scheme of double closed loop speed regulation system of asynchronous motor,and designs the parameters in detail.The inner ring is the current ring,and the current value in the d-q coordinate system is corrected.The outer ring is a rotating speed ring and a magnetic link.The principle of repetitive control method is analyzed in detail,and the characteristics of embedded and parallel repetitive control are analyzed by using frequency characteristics.Aiming at the harmonic problem caused by dead time,a compensation method of adding repetitive control in the current inner loop is proposed,and the filtering and compensating links of repetitive controller are designed to effectively suppress the system harmonic at low speed of motor.Finally,a seven-level cascade H-bridge inverter system with DSP+FPGA as the control core is built,and the effectiveness of the composite control method proposed in this paper is verified on this platform.