Research On Control Technology Of Four-Quadrant Multilevel Converters

With the development of power electronic devices and related control technologies, a variety of medium-voltage high-power multilevel topology are widely applied in industrial frequency control system. Four-quadrant converters have achieved significant energy saving and reduced energy consumption of unit output value. Although the topologies of a variety of multilevel converters are different, there are still some common problems to be solved, such as the power network quality control, circulating-current restraining method for parallel modules, coordinated control instantaneous power for back-to-back modules, switch frequency limit and other issues. This paper, which focuses on a number of key control technologies of parallel converters with common DC-bus and cascaded H-bridge converters with independent DC-bus, is organized as follows:Compared with two-quadrant converters, four-quadrant converters need to increase input reactors in rectifiers. The split multi-winding transformer can be designed to replace input reactors in high voltage access applications. It can not only reduce the voltage and isolation, but also play the role of energy storage and filter. To analyze the impact of the multi-winding transformer short-circuit impedance on the system stability, the equivalent circuit model associated with short-circuit impedances is presented. Rectifiers are modulated with phase-shifted carriers. It can not only significantly reduce the input current distortion and improve the power network quality, but also reduce the transformer harmonic losses and improve work efficiency.When MMP converters are modulated with phase-shifted carriers, there are fundamental circulating-current, harmonic circulating-current and DC bias current between the transformer secondary windings. Therefore, the current sharing control is related to AC side circulating-current restraining, DC side circulating-current restraining and transformer DC bias restraining. Firstly, active power and reactive power are respectively adjusted by q-axis voltage and d-axis voltage. The active and reactive power circulating-current are effectively restrained by the power droop control strategy. Secondly, the power module connections and the main stray inductance are rationally designed, and the appropriate resonant frequency is selected to restrain DC side circulating-current. Finally, the proportion integration resonant regulator eliminates the influence of the bias voltage on system stability and restrains transformer DC bias.Based on the mathematical model of power cells in four-quadrant CHB converters, a current control strategy for PWM rectifier is proposed with the control target being the balance of the input and output instantaneous power. The complex vector PI (CVPI) regulators are introduced to track various frequency currents and decoupled control completely. CVPI regulators are proved to have better robustness than traditional regulators with the resonant frequency change. Moreover, the load current model based on Kalman filter is established to identify the current magnitude and frequency in real time. The proposed instantaneous active power feed-forward control strategy with CVPI regulators, which significantly reduce the twice synchronous frequency oscillation component of DC bus voltage, restrains the motor torque ripple.Medium-voltage high-power multilevel converters strictly limit operating frequency of the switching devices. Synchronous Optimal PWM technology can effectively solve the problem about power quality with low switch frequency and instantaneous power balance. Based on harmonic current minimum evaluation function, multilevel SOPWM can be applied to CHB converters for induction motor vector control. Firstly, the specific function optimization goals of harmonic current minimum and the nonlinear constraints of H-bridge power equilibrium are designed. Using genetic algorithm and constrained optimization, switching angles are calculated offline. SOPWM with continuous switch angles ensure the smooth handoff in the entire modulation range. Secondly, the smooth transition research of different PWM modulations for induction motor vector control is carried out. Finally, the feasibility of multilevel SOPWM is confirmed by stator flux closed-loop control simulation. A new idea using multilevel SOPWM for high-power AC motor control with low switch frequency is presented.