Study On Three-Phase Current-Source PWM Rectifiers And Their Control Strategies

According to the dc-link energy storage elements, PWM rectifiers are classified into voltage-source PWM rectifiers and current-source PWM rectifiers. The current-source PWM rectifiers will be very attractive in medium power applications due to their advantages such as controlling the output current more directly and conveniently, faster dynamic response, and implicit short-circuit protection. The development of the super-conducting magnetic energy storage (SMES) technology, the efficiency problem of the current-source PWM rectifiers will be revolved, which supply advantages for the current-source PWM rectifiers in high power applications. This dissertation studied the current-source PWM rectifiers and their control strategies, mainly including following aspects.Low frequency and high frequency time space mathematical model of 3-phase current-source PWM rectifiers have been founded in abc stationary coordinate, αβ coordinate and dq coordinate. These Mathematical models of current-source PWM rectifiers establish theoretical foundation for the successive research on control system analysis and design in the dissertation. The models in frequency space of the three-phase voltage-source and current-source PWM rectifies have been studied, frequency spectra components of SPWM output in two rectifiers were analysis and dc-link outputs of the two rectifiers were compared. The transfers band of the two rectifiers applied to APF were studied and compared.The dissertation presents tri-logical SPWM digital implement technique of three-phase current-source PWM rectifiers. The proposed method fully uses the inner resources of DSP and distributes driver signals of six switches in principle of the switch time minimum. Consequently, the method reduces switch losses and system cost. Studies show the signal distribution method is in essential accordant with traditional SVM of the current-source PWM rectifiers. The proposed digital implement method changed little can be applied to SVM of the current-source PWM rectifiers. The dissertation primarily discussed the effect of overlap-current time on output in SPWM modulation. The proposed tri-logic SPWM digital implement method is confirmed by experiment, which lay experimental foundation for successive study of control strategies in the dissertation.Direct current control plays most important role in the control of the three-phase current-source PWM rectifiers. After PI control and pole-placement state-feedback control were studied, the dissertation presents linear quadratic (LQ) optimal control and improved model predictive control (MPC) for the current-source PWM rectifiers. LQ control method finds optimal poles of the system according to the minimum steady-state error, consequently, LQ control can reality steady-state characteristic optimal. Nevertheless, LQ control is based on small signal model, there are inherent drawbacks, so the dissertation proposed MPC for the current PWM rectifiers. MPC has merits of forecast and real-time optimization, however, in traditional MPC, non-parameter model based on pulse response is usually adopted as predictive control model, which results in complex computation, so this method is difficult to be applied to real-time control system directly. In this dissertation, the traditional MPC was improved, the first-order differential equations were derived from the transfer functions of controlling variables and controlled variables in the current-source PWM rectifiers, and the equations were used as predictive control model. Meanwhile, the merits of feedback correction and dynamic optimization in traditional MPC were reserved in the new MPC, which makes MPC applicable to fast-control system current-source PWM rectifiers. The simulation and experimental results verified the feasibility and correctness of the proposed method. Compare with traditional PI control and improved MPC when used in the unity power factor current-source PWM rectifiers system, the proposed MPC gets better robustness and faster dynamics characteristics than the traditional PI control.Direct current control is relatively complex compared with indirect current control, and when current-source PWM rectifiers feed a load demanding power factor regulation and energy regeneration, such as a dc motor drive, its realization is difficult. Indirect current control is deep researched in three-phase voltage-source PWM rectifiers. The dissertation studied indirect current control of current-source PWM rectifiers referring the idea that of voltage-source PWM rectifiers, unity power factor of current-source PWM rectifiers was obtained in abc stationary coordinate. This dissertation proposed an indirect current control technique based on dq coordinate. In the proposed technique, dc output of the converter is used as active current reference value of the network current, reactive current of the network is regulated by the produce of the active current reference value and tgcp, then the ac-link current steady-state equations of current-source PWM rectifiers are used to determine instant control variable that control network current of the rectifiers indirectly. Consequently, active power and reactive power of the rectifiers are controlled, and adjustable power factor and reversible energy are obtained. Experimental results of the proposedmethod were given, and the steady-state operation range of the rectifiers is analyzed in quantification.Under unbalanced source voltage supply, if control methods of balanced source voltage are employed directly in current-source PWM rectifiers, the input and output performance of the rectifiers will deteriorate, even work failure. The dissertation analyzes in detail the effect of unbalanced source voltage on ac-side and dc-side of current-source PWM rectifiers. Derived from complex power of system, a compensation control method eliminating the second harmonics of the dc output current is presented under unbalanced source voltage. In the proposed method, the second harmonics active power and average reactive power are set to zero. These conditions are used to calculate the reference values of the network currents, and then control variables are determined to real-time control rectifiers. Accordingly, the second harmonic component of the output dc current can be eliminated and low-order harmonics of the source current are reduced effectively. The validity of the proposed method is proved by simulation and experimental results. If the control condition of the method above depicted is altered, a compensation control method restraining negative sequence component of source current is achieved, simulation results confirmed the method.Finally, the dissertation makes a summary of full paper and an expectation of future study direction and application prospect about current-source PWM rectifiers.