Research On Control Methods Of Modular AC-DC-AC High-Frequency Power Conversion System

Power conversion technology, which combines power electronics and control theory, can change electric power from one form to another by controlling power conversion processes, and therefore power conversion technology is the foundation of utilizing new energy sources and improving the quality of power supply. In power conversion technology, the control of power conversion processes is the key of improving the performance of power conversions, so the control of power conversion processes is a hot research topic. Based on analyzing the topological structure of power conversions, we focus on studying the rules of the system operation in power conversion processes and discuss control methods for AC-DC-AC power conversion process.The main research work is as follows:1. Referring to the idea that the energy decides system behavior, a power control method based on power balance theory is proposed for AC-DC power conversion process in this dissertation. According to the mathematical model of three-phase rectifiers in the synchronous dq coordinates system and the power relation of AC and DC sides in power conversion process, the proposed control scheme controls the instantaneous active power on the track of referenced dynamical active power and the instantaneous reactive power to zero, and therefore steady output voltage and high power factor is realized. In addition, the compensator for the load current is designed and the output error which is introduced from constructed referenced active power in direct power control strategy is avoided in the proposed scheme.2. Concerning with the uncertain load condition in AC-DC power conversion process, an adaptive control scheme is proposed for three-phase PWM rectifiers in this dissertation. According to the adaptive estimate of the load parameter and the relation of the system energy, the proposed scheme constructs dynamic reference curve for three-phase current. Then the proposed scheme makes three-phase current and dynamic reference current convergent together, and therefore zero output error and high power factor are realized. The restriction that loads should be positive is broken in the adaptive parameter estimator, so the proposed scheme extends the extent of application. 3. A control method based on compensating unbalanced effect completely by the fourth leg is proposed for DC-AC power conversion process using four-leg topology. The proposed control method follows the rule that the sum of four legs current is zero and discovers the essence of the control law for four-leg inverters by analyzing the functions of four legs. In addition, the proposed control method exerts the function of the fourth leg to compensating unbalanced effect according to four-leg topology.4. To compensate voltages of three-phase inductors under uncertain load conditions, a unique integrated control strategy is proposed in this dissertation. Based on compensating unbalanced effect by the fourth leg, the proposed control strategy compensates symmetrically three-phase output voltages. In addition, the proposed control method uses the integral current instead of the differential current as the control variable in order to make controller practical.5. A closed loop control scheme based on SVM is proposed for DC-AC power conversion process using single-phase full-bridge topology. In the proposed scheme, traditional control mode on 1-DOF space is breaken and a 2-DOF vectors plane is structured. Based on the geometrical relation of leg voltage, output voltage and the inductance current on the 2-DOF vectors plane, a closed loop controller is designed. The proposed scheme makes full use of the ability of SVM to lessen switching losses and mitigate harmonic.6. Concerning with the uncertain load condition in power inversion process using single-phase full-bridge topology, the control schemes based on state-feedback and feedforward compensation is proposed. The proposed schemes eliminate bad effect on output waveform under uncertain load conditions. In addition, the control parameters are designed based on the stabilization and the control capability of the system.7. Some unsolved problems in the research and the application are indicated, and the prospects are presented.