Research And Design On Variable Frequency Transformer Of New Structure

Cross-regional power supply and regional power grid interconnection is the trendof power grid development in the future. Compared with synchronous andasynchronous interconnection, which are the interconnection ways fully developed atthe present stage, grid interconnection with Variable Frequency Transformer (VFT) hasthe same advantages as the two methods have. As mechanical and electrical equipment,the structure of VFT is solid and durable. Besides its good stability, it can isolate faultwhen it occurson one side of electrical networks. Compared with the existing products,VFT has a promising future. Firstly, brief platform is built and the experiments ofgrid-connection in steady state and static characteristic are performed. In this way, theVFT’s interconnection ability and its static characteristic are verified. However, thestructure of the traditional VFT is complex that a separated drive motor need to beconnected to the wound-rotor asynchronous motor with the same shaft. So theamplitude value of shaft and bushing vibration is relatively high. In order to solve theseproblems, a new structure of VFT with integrated drive motor is proposed, and itsoperating principles and structure are briefly introduced in the thesis.Secondly,mathematical models of the new structure VFT in different coordinates are given.Equivalent circuit ofVFT is derived throughthe theoryanalysis of its internal magneticfield.Then, Combining the design of water turbine and high-power permanent magnetmotor, the designing ideas and methods of the new structure VFT are introduced, andhave the design of a new100MW VFT completed. Using the advanced frozenpermeability method, the d and q axis inductance characteristics and power anglecharacteristic of the designed VFT are worked out, by which the rationality of thedesign preliminarily are verified.Finally, the designed VFT of the new structure is modeled and analyzed based onfinite element method. Through the analysis of magnetic lines, air gap flux density andinduced electromotive force, relationships between the subsystem of VFT andpermanent magnet synchronous control system are obtained either they are workingtogether or separately. Decoupling is realized by designing the pole pairs reasonably.Grid-connection on both sides of power winding of stator and rotor are simulated.When there is a sudden change on the frequency of one side, rotor speed is controlled by control winding to ensure the frequency of the other side can keep relatively stableand power can be transferred and controlled smoothly.