Research On Three-phase Back To Back Electrical Converter With Bidirectional Power Flow And Its Control Strategy

With the increasingly severe energy problems, flywheel energy storage technology is becoming a hot spot of research. In flywheel energy storage system, back to back electrical converter with bidirectional power flow is the most widely used electric power converter. It has advantages of stable DC bus voltage, sinusoidal grid current, unit power factor, and bidirectional flow of energy, etc. Therefore, this thesis takes three-phase back to back electrical converter with bidirectional power flow as research object to carry out deep research.Based on the analysis of the working principle of the grid-side three-phase PWM converter, the paper sets up its general mathematical models and power mathematical models, and presents the mathematical models of three-phase induction motor under different coordinate system. According to the principle of instantaneous power, a detailed analysis of the energy flow of the system is presented. And on this basis, the paper introduces the working principle of back-to-back converter.Aiming at the shortcomings of the grid-side three-phase PWM converter traditional control strategy, this paper presents a constant frequency direct power control strategy. It solves the problems of traditional control strategy that switching frequency is not stable. On this basis, combining with the concept of virtual flux, this paper proposes a power error discrete control strategy based on virtual flux, which uses virtual flux and power error to calculate the rectifier voltage directly. This control strategy simplifies the structure of control system and improves the stability of control system. To verify the feasibility of control strategy, this paper introduces the simulation analysis of the control strategy.In order to solve the problems that the decline of the efficiency in flywheel energy storage system caused by independent control strategy of back to back converter, this paper presents a load power feed-forward control strategy based on rotor field-orientation vector control of three-phase inductor motor. The control strategy can effectively restrain the fluctuations of DC bus voltage when the load changes, and improves the dynamic response speed of the system. The simulation results demonstrate the superiority of the control strategy.In the actual cases, the unbalanced power grid phenomena occurs frequently. Therefore, on the basis of the above proposed control strategy, combined with the mathematical model of three-phase PWM converter in the unbalanced case, a power error discrete control strategy based on virtual flux under unbalanced grid voltage condition is proposed, and the corresponding simulation is put forward. This control strategy can effectively suppress the second harmonic of the dc bus voltage and active power, and ensures the stable operation of the system.On the basis of the theory and simulation analysis, the paper sets up the experimental platform to test. The results of experiment verify the features of back to back converter of stable DC voltage, bidirectional energy flow and unit power factor. The whole system has good operating performance.