Research Of Modulation And Control Strategies For AC-DC Matrix Converter

The AC-DC matrix converter (AC-DC MC) is a novel step-down AC-DC power conversion topology possessing many unique advantages over the existing rectifiers. Due to the absence of bulky energy storage components, the AC-DC MC shows a highly compact structure, high power density, high reliability and good maintainability, which make it very suitable for the application areas where weight, volume, efficiency and reliability are of importance. Besides, it can generate DC output with arbitrary polarity and wide-range controllable magnitude while drawing sinusoidal input current from the power source at unity or any specified input power factor, and very easy to extend to regenerative operation. All these highly attractive characteristics make the AC-DC MC an ideal future solution for AC-DC power conversion. Systematic and deeply studies have been conducted in this dissertation focused on the modulation algorithms and the control strategies of the AC-DC MC, as well as its input filter resonance mitigation methods by means of theoretical analysis, computer simulation and prototype experiment. These researches provide theoretical and technological foundation for the practical implementation of the AC-DC MC.Based on the in-depth analysis of the basic space vector modulation (SVM) strategy, an improved SVM method is developed for practical implemented AC-DC MC. The objectives of reducing the common-mode voltage (CMV) and eliminating the narrow pulses are accomplished by replacing the zero space vectors with suitable couple of active ones. Further considering the commutation time, the relationship between the modulation index, the sector angle and the probability of narrow pulse in the conventional and the proposed SVM method are derived. The proposed scheme not only can reduce the magnitude, the root mean square value and the harmonic components of the CMV, but also guarantees a better input and output performance in wider modulation ratio range. Simulation and experimental results verified the conclusions and the validity of the optimized strategy.In order to improve the poor performance of the AC-DC MC when the power source is distorted, the input and output characteristics of the AC-DC MC are theoretically analyzed in the conditions that the source voltages are unbalanced and non-sinusoidal respectively. Then, a universal implementation method in the αβ stationary coordinates for the SVM strategy is proposed, which allows the change of the performance of the AC-DC MC by flexibly modifying the demanded input current vector. At last, an input current reference construction method is proposed with the purpose of generating steady output DC voltage. The proposed compensation strategy not only eliminates the low-frequency ripples at the output side of the AC-DC MC caused by the magnitude and the angular frequency variation of the source voltage vector, but also simplifies the calculation process by reducing the requirements for complex real-time calculation operations including square root, trigonometric functions and anti-trigonometric functions. The correctness of the theoretical investigation is verified and the effectiveness of the proposed compensation method is testified by computer simulations.A novel direct control method applying the sliding-mode variable structure control technique and the space vector approach, aiming at both regulating the input currents and the output voltage, is presented. The proposed method is developed based on the modeling of the variable structure system considering the switching matrix and the dynamics of its associated input and output filters. Afterwards, the sliding surfaces and the switching laws are determined, the sliding surface reaching conditions are checked and the entire switching table is established. With the purposes of maximizing the sliding mode extension domain and minimizing the switching losses, the influence of the controller parameters to the converter performance is investigated in detail, and a practical parameter tuning approach is given. The designed sliding mode controller is tested by simulations and experiments. The obtained results show that the proposed control scheme guarantees fast dynamic response and precise control actions, especially ensuring a unity input power factor regardless of the system parameters and providing better output behavior with distorted source voltages.The input LC filter is essential in the AC-DC MC system, but its inherent resonance figure makes it easily to be excited by the harmonic pollution in the power source or by the harmonic components in the input currents drawn by the converter itself. This phenomenon may cause instability during operation, even result in severe system failure. With the purpose of enhancing the performance of the AC-DC matrix converter, a double closed-loop control method is proposed under the dq rotating reference frame. The inner-loop input current controller and the outer-loop load voltage controller are designed based on the one-cycle control (OCC) algorithm and the classical control theory respectively. And the consistency of the proposed OCC control method and the SVM strategy is mathematically proofed. A frequency selective active damping (AD) method by employing the grid-side input currents as feedback signals is developed so as to mitigate the input filter resonance actively. The proposed AD method is introduced into the inner-loop OCC controller to avoid the requirement of normalization and complex modulation index control in the existing strategies. A model including the proposed method and the input filter is established with considering delay caused by the actual controller, based on which, the active damping parameter design principle is investigated. A realization approach of the proposed OCC controller is proposed, which is suitable for the implemented digital control system utilizing DSP and CPLD chips. The experimental results are given to demonstrate the correctness of the theoretical analysis and the practicability of the proposed digital OCC control approach.This dissertation mainly pays attention on the modulation and control strategies of the AC-DC matrix converter. Some valuable experience and ideal results are given, which found the technological foundation for the AC-DC matrix converter’s further industry applications.