Research On Ship Electric Propulsion Simulation Based On Three - Level Converter

With the development of ship propulsion technology, ac electric propulsion has replaced the dc electric propulsion and become the mainstream. Multilevel inverter has advantages of low pressure requirements for switching devices, low output harmonic wave and high voltage utilization. It meets the increasing demand for high voltage and high power of ship electric propulsion. Therefore, it has become one of the key research objects in the field of high voltage and high power frequency control. In the multilevel topology, the diode-clamped three-level inverter is one of the most studied topology. Direct torque control(DTC) technology, as a new type of ac speed regulation technology, has been more and more widely investigated and used with its simplicity of control, strong robustness and good dynamic performance. Therefore, the ship electric propulsion system based on the diode-clamped type three-level inverter has been simulated and researched in this paper.This thesis analyses the principle and structure of two-level inverter and direct torque control based on the traditional direct torque control, and establishes simulation models of each component of the system in the platform of MATLAB /Simulink, then analyses the curves in which AC motor runs in constant speed state and constant torque state. In order to meet the needs of ship electric propulsion for high voltage large capacity requirements, the DTC system based on the diode-clamped type three-level inverter is studied in this paper. At first, the working principles of three-level inverter based on the diode-clamped type, neutral-point voltage imbalance and the theory of SVPWM have been elaborated. Then in order to solve the problem of voltage vectors selection and neutral voltage control in the three-level DTC system, double PI adjuster has been used to adjust the stator flux linkage and torque. This structure can separate voltage vector selection and neutral voltage control and has a good dynamic performance. At last, since the estimation accuracy of stator flux linkage can affect the system performance directly and the observation model of the commonly used U-I flux linkage has low accuracy, a new type stator flux observer based on PI phase-locked loop principle has been used to establish a three level SVPWM ship electric propulsion DTC system. The simulation reveals that the observation accuracy of flux linkage and the system performance has been improved.In the end this thesis builds the ship-motor-propeller simulation model, analyzing the speed, thrust and torque characteristics of ships, and applies the traditional DTC, three-level inverter SVPWM-DTC to ship electric propulsion system respectively, establishes ship electric propulsion DTC comprehensive simulation system. What’s more, it is comparatively analyzed on the motor characteristics of the DTC system when the propellers are loaded on and the effect on the DTC system when the propellers are put load disturbance. According to verification, in the ship electric propulsion system three-level inverter SVPWM-DTC has good steady state and dynamics performance, dynamics performance of traditional DTC is superior to its steady.