Research On Ship Electric Propulsion DTC System In Low Speed Environment

With the development of power electronic technology, automatic controltechnology and computer technology, AC electric propulsion has replaced the DCelectric propulsion and becomes the mainstream. Direct torque control (DTC)technique which is a new type of AC speed regulating system can meet the needs ofdynamic performance for main propulsion motor in the ship electric propulsionsystem. The main characteristics of ship electric propulsion are low speed of largecapacity, high output of torque and large inertia. However, the vast majority of shipoperating environment are at low speed region. Thus, the research in ship electricpropulsion DTC system of low environment is of considerable practical value.In this thesis, induction motor is chosen as the main promoting motor. Theasynchronous motor and inverter mathematical model and the basic principle of DTCare analyzed at first, then, establishes simulation models of traditional direct torquecontrol system and ship electric propulsion traditional direct torque control system inthe platform of MATLAB/Simulink, specifically analysis the performance of thesesystem at low speed environment, and point out the main factors that affect theperformance. In view of the low accuracy of flux observation and the problem of fluxdistortion caused by the variation of stator resistance at low speed environment, asimple stator resistance compensator is studied in this paper, and the fuzzy control isintroduced to construct fuzzy stator resistance estimator, it can track the variation ofstator resistance on real-time. The simulation results show that fuzzy stator resistanceestimator-DTC system has the highest precision of flux observation, stator resistancecompensator-DTC system is the second, traditional U-I model is the worst, and thetorque ripple is also better than traditional DTC system. However, the torque ripple offuzzy identification-DTC system is still large. So this thesis firstly adopts SVMcombined with fuzzy identification-DTC system, and the simulation results show thatFUZZY-SVM-DTC system performance is overall improved. Secondly, this system isapplied to ship electric propulsion system, compare to ship electric propulsiontraditional DTC system, the results proved that the modified ship electric propulsionsystem is valid.In order to meet the needs of ship electric propulsion for high voltage large capacity requirements, the DTC system based on three-level neutral-point-clamped(NPC) inverter is studied in this paper. There are two important problems in thissystem, include the selection of voltage vectors and neutral-point potential control.Then, three level SVPWM-DTC system based on double PI regulator is studied in thispaper, this structure is characterized in: not only can the voltage vector selection andcontrol of neutral point potential is distinguished respectively to design the controller,and can well suppress torque ripple, and has good dynamic performance. Theconclusion of this system is proved by the simulation results.