Improved Synthetic Vector Direct Torque Control For Permanent Magnet Synchronous Motor Based On Three-level Fault-tolerant Inverter

The three-level inverter has many advantages such as low power switching voltage stress,less serious electromagnetic interference,low harmonic content of output voltage waveform,and high work efficiency,which makes it have great application potential.Direct torque control uses hysteresis comparators to control the torque and speed,which has the advantages of fast response speed and simple structure.This paper studies the application of new equipment and control methods in the field of ship electric propulsion.Despite the advantages of three-level inverters,the doubling of the number of power switching devices also reduces their reliability.Especially during the departure of the ship,the maintenance conditions are simple and it is difficult to repair complex faults,so it is of great significance for ocean-going ships to study the fault-tolerant control technology of three-level inverter and improve the reliability of the system during operation.In addition,the three-level inverter has inherent midpoint potential fluctuations,which will cause obvious distortion of the waveform,and also produce low-order harmonics,resulting in large torque and flux ripple of the ship’s electric propulsion system,reducing its speed and torque output performance.Therefore,this paper designs a direct torque control based on three-level fault-tolerant inverter,focusing on the fault tolerance control and midpoint potential fluctuation suppression of three-level inverter.Firstly,aiming at the problem of reducing the reliability of the power switching devices caused by the doubling of the number of power switching devices in the three-level structure,the topology of the improved three-level fault-tolerant inverter is researched and designed,its basic working principle is analyzed,its mathematical model is summarized,its fault types are classified,summarized and analyzed,and its protection method is designed in the event of single-phase short circuit fault.Secondly,aiming at the problem of midpoint potential fluctuation on the DC side of the improved synthetic vector DTC algorithm of three-level fault-tolerant inverter,the fluctuation mechanism of midpoint potential is deeply studied and the midpoint current formula is derived,and the influence of each basic voltage vector of three-level fault-tolerant inverter on midpoint potential is expounded.A new and improved synthetic vector method is designed,a small vector opposite to the effect of the medium vector is introduced in the synthetic vector sequence,the influence of the midpoint potential of the medium vector is eliminated,the control ability of the small vector alignment potential is improved,and the midpoint potential coefficient is introduced in the action time of the positive and negative small vectors,and the capacitor voltage hysteresis controller is added on the basis of the traditional flux and torque hysteresis controller,and a new voltage vector switch table determined by the three hysteresis controller signals of capacitor voltage,torque and flux link is designed.According to the different midpoint potential transformation coefficients,the value is better to suppress the midpoint potential fluctuation.Finally,the relevant simulation and experimental system was built.The result data show that the fault-tolerant three-level inverter designed in this paper and its corresponding midpoint potential fluctuation suppression direct torque control method can better deal with the single-phase fault and midpoint potential fluctuation of the inverter bridge arm,and can achieve a balance between system stability,robustness and control accuracy.