Application Study Of An Improved Direct Torque Control Strategy Applied In The Ship Electrical Propulsion

In the near decades, with the development of power electronics, computer science and modern techniques of variable speed for AC motors, electric propulsion has gained a significant evolution and has been applied in many kinds of vessels and gradually becomes the main steam of future ships. Direct Torque Control (DTC) technology, emerging after the vector control technology, is a new type of AC drive strategy with high performance and developing rapidly. This new control method will have a broad prospect of application in the ship electric propulsion system because of its innovative ideas, simple control configurations, direct control methods, and good dynamic and steady performance.In this paper, an asynchronous motor was chosen as the main propulsion motor of the electric propulsion system, and an improved DTC strategy was applied in the system. At first, the basic principle of DTC was analyzed. After that, the system simulation model was established using MATLAB/SIMULINK and the simulation results were discussed. An improved method based on Space Vector Modulation (SVM) was presented to deal with the problem of high torque ripple that traditional DTC system generally encountered, especially in the low speed. Both dynamic and steady performance had been improved. Considering the fact that electric ship's propellers are operating in a rough environment, installation and maintenance of speed sensors would be a big challenge. Two speed sensorless technologies, Model Reference Adaptive System (MRAS) and Extended Kalman Filter (EKF), were introduced and studied respectively. Simulation results showed that the identified speeds could follow the actual speeds quite well.Finally, some simulation interfaces, such as the concentrative control room and the maneuvering board, were developed in the VC++ 6.0 environment according to the practical configuration in some domestic electric propulsion ship. The interface program which is used as a client runs in the front of the entire simulation system while the MATLAB model which is treated as a server runs in the background. They are linked by the MATLAB engine, transferring instructions and data. As a result, the distributed simulation system was then realized with a multipoint communication model which was based on DCOM. And since the model and the interface system were independent of each other, each subsystem of ship electric propulsion, especially the control system, could be evaluated, tested and improved individually.The presented simulation system could be beneficial in several applications such as offering an experimental system for the study of electric propulsion, providing a testing platform of R&D issues of ship electric propulsion systems that are necessary before the real electric ships are tested. As a result, the number of tests at sea could be decreased, the experimental cost could be reduced and the development cycle could be shortened.