Research On Direct Torque Control System Of Three-level Inverter

The three-level neutral-point-clamped(NPC)inverter has significant advantages in energy-saving and emission-reduction.At the same time,it also has the advantage of low cost in high voltage large capacity motor.It has become the hot research topic in the world.Direct torque control(DTC)which applied to the high voltage large capacity motor has important research significance with the NPC three-level inverter technology.In this paper,the three-level neutral-point-clamped DTC system is studied on the basis of asynchronous motor.First of all,this paper introduces the mathematical model of NPC three-level and asynchronous motor.For the following theoretical groundwork,the influence of voltage space vector on flux and torque of the motor is analyzed.Secondly,because the traditional stator flux observer is an open loop system,it can not correct the stator flux linkage.To solve this problem,a closed loop flux observer based on stator flux and stator current is proposed.Then the closed-loop observer is combined with the speed identification to cancel the traditional speed sensor;a simplified DTC system method is introduced.The method does not require complex mathematical operations,and can obtain the voltage space vector of the NPC three level inverter directly in the coordinate system.Finally,aiming at the complexity of the traditional NPC three level control strategy and the unbalance of the DC side capacitor voltage of the inverter.A simplified NPC three level control strategy is proposed to improve the DC capacitor voltage imbalance.The algorithm includes the establishment of the g-h coordinate system,the transformation of the reference voltage vector,the re division of the small sector,and the control method of the neutral point potential.The system is verified by simulation experiment.