Research On Control Strategies And Application Techniques Of Current Source PWM Rectifiers

According to the characteristics of dc-link supplies, PWM rectifiers can be classified into two types: voltage-source rectifiers(VSR) and current-source rectifiers(CSR). CSR has the following particular advantages: sinusoidal input current, adjustable power factor; direct current control, etc. These features make the CSR an attractive solution in many industrial applications, such as SMES; battery charger for EV; active front-end converter; motor drives, etc. Furthermore, the recent advances in semiconductor devices and magnetic components technology may help to overcome the obvious fundamental disadvantage of conduction losses and dc series inductor losses. Hence, the CSR is expected to have great application potentials.In this dissertation, operation characteristics, parameters design and control strategies of a three-phase CSR have developed, the main contents include the following aspects. The relatively higher power dissipation is major obstacle for the application of three-phase CSR. This dissertation calculates and compares the efficiency and loss distribution in the two non-isolated converter topologies of electric vehicle charging systems: CSR and VSR+Buck converter. To start with, under space vector PWM modulation(SVM) techniques, the commutation process and swithing characteristics of the two types of converters are analyzed. According to the power device datasheet provided by the manufacturers, the switching losses can be evaluated based on the curve fitting theory. Through the calculation of each loss factor such as switching losses, conduction losses and filter losses, a systematic method for calculating all the losses of PWM rectifiers is presented, and this method is able to provide reference for practical engineering. The results show that the CSR system gets higher efficiency than VSR+Buck converter under the rated load of 10.5k W.The principle of space vector PWM technique is introduced, and simplified algorithm of SVPWM is also provided, which can not only avoid trigonometric functions and nonlinear computing, but increase the computing speed.A novel fast-charging strategy considering the inner characterization of batteries is developed based on a three-phase CSR, including a variable constant current(VCC) control scheme and a constant voltage(VC) control scheme. Aim to the VCC mode, a second-order Thevenin equivalent circuit is used to model the dynamics of battery pack. Subsequently, according to the experimental data, the polarization voltage curves versus state of charge(SOC) are established at different charging current. On this basis, the polarization voltage and its change rate are considered as the input variables of the fuzzy control algorithm, so reference charging current in the next instant can be deduced. Then,the VCC mode is switched into the CV mode once the cut-off voltage is reached. So, the proposed charging mode can balance charging speed and charging polarization, and extend battery life. For the two modes above,a voltage-oriented CSR indirect current control strategy is proposed,in which a delay-angle of power factor is introduced into modulation signal. In the frequency domain, the parameters of current inner-loop and voltage outer-loop controllers are optimized. The proposed strategy can provide unit power factor and nearly sinusoidal input current in two kinds of control modes.This paper presents a novel power control strategy for PWM CSR in stationary frame based on instantaneous power theory. In the proposed control strategy, the virtual resistance based on the capacitor voltage feedback is used to realize active damping, and the proportional resonant(PR) controller under two-phase stationary coordinate is designed to track the ac reference current as well as to avoid the strong coupling brought about by the coordinate transformation. The limitations on improving steady-state performance of PR controller is investigated and mitigated using the cascaded lead-lag compensator. In the z-domain, a straightforward procedure is developed to analyze and design the control-loop with the help of MATLAB/SISO software tools. In addition, the control algorithms is realized adopting model-based modern software design approach, which can speed up the development process from algorithm scheme to system implementation. Finally, the simulation and experimental results verify the proposed control scheme and design method.By analyzing the system of three-phase CSR, and with feed-forward decoupling method in d-q reference frame, a linear large-signal model based on the multi-loop current controller is established in this paper. Meanwhile, as an alternative approach for active damping, capacitor-voltage feedback is used to suppress the inductor-capacitor filter resonance. Therefore, the transfer function of the inner current loop is analyzed using the frequency-response method, with considering the effect of system control delay and active damping compensation. By using the frequency domain method, the zero position and loop gain of the inner loop controller are optimized with a compromise between dynamic response and stability of the controller. Due to the gain of the outer loop transfer function is always dependent on the operating point of the CSR, an outer current selector switch controller is proposed, and the controller is selected according to the load voltage value and corresponding to mapping rules. Finally, simulation and experimental results demonstrate that the proposed control strategy and parameter design not only have excellent dynamic and stability performances, but also can achieve unity input displacement power factor and can reduce the input-current distortion in a wide range of output voltages or currents.Finally, the dissertation makes a summary of full paper and an expectation of future study direction of current source PWM converter.