Research On Key Technical Problems Of VIENNA Rectifier

With the power electronics technology is widely used in the field of power conversion, it brought lots of harmonic pollution to the grid, and rectifiers are the major factor. Therefore, high power factor, low harmonic content rectifier device and their control algorithm are currently research hotspot. Among them, VIENNA rectifier and its control algorithm attracted more and more scholars in recent years, because it greatly reduced the number of switches, high power factor and low current harmonic content, the bridge arm switch without dead time voltage, control circuits is simple, and so on. VIENNA rectifier is an excellent three-level topology, and it is an important technical method to improve the power factor.To carry out indepth study of its control strategy can be further improved VIENNA rectifier performance.In this paper, the key issues of VIENNA rectifier, such as space vector modulation algorithm, current control strategies and control strategies under grid unbalanced, were studied. And the new ideas and strategies to solve the problem were proposed. The contributions of the thesis are shown as follows.1. The work process and the mathematical model of VIENNA rectifier are analysis in brief. All this work, provides a theoretical basis for the control strategy of the research paper.2. The distribution of VIENNA rectifier space vector and its influence to the neutral-point potential, and the VIENNA rectifier neutral-point voltage unbalance principle are analyzed by theoretically. It proved that there are three times fundamental frequency low frequency oscillation when VIENNA rectifier using traditional SVPWM modulation algorithm. And if the modulation M is relatively large and low power factor, the neutral-point voltage caused by middle vector can not be completely balanced. Aim at the shortcomings of the traditional SVPWM modulation, virtual space vector modulation strategy is proposed and used in VIENNA rectifier. The algorithm can be adjusted in the whole modulation degree and load power factor range of the capacitor voltage balance control, but the algorithm increases the switching frequency, resulting in more loss. Comprehensive the shortcoming of these two algorithms, a hybrid space vector modulation method is proposed, which full consider the advantages and disadvantages of traditional space vecto modulation algorithm and virtual space vector modulation algorithm. At the same time, neutral-point potentialoptimization algorithm of VIENNA rectifier, which based on a combination of accurate compensation control and hysteresis control, is proposed.3. VIENNA rectifier current control strategy based on PI algorithm and its limitation are analyzed. Since VIENNA rectifier is a strong nonlinear systems, PI algorithm is more difficult to achieve the desired control effect, especially in the dynamic characteristics. To solve this problem, passive control strategy for VIENNA rectifier is proposed, and VIENNA rectifier control strategy model are set up respectively using EL model and PCHD model. For EL model, passive sliding model variable structure control algorithm is used; and for PCHD model, IDAPB algorithm is used. Three d-axis reference current generating methods of VIENNA rectifier, i.e., according to the energy balance principle, using voltage outer loop based on PI algorithm, using voltage outer loop based on variable structure control algorithm, are analysed and discussed. The dynamic characteristics of voltage outer loop using PI algorithm is analyzed in theoretically. When VIENNA rectifier startup or load mutations, the parameters of the PI controller are related to DC-side output voltage and the rate of change for the d-axis current. Therefore, the dynamic characteristics of PI controller is relatively poor. In order to meet the system dynamic characteristics and robustness, sliding model variable structure control strategy used in VIENNA rectifier voltage outer loop is proposed. Simulation and experiment for all kinds of control strategy are done. The result show that the control strategy, that is, inner current loop using passive and sliding mode control, voltage outer loop using sliding mode control, achieve a good control performance, strong anti-interference ability and robustness, especially in system startup and load mutation.4. The impact of the grid voltage unbalance to VIENNA rectifier are analysed by theoretical, and a conventional power control are analysed too. In order to suppress VIENNA rectifier’s AC negative sequence current and DC side voltage second harmonic, related control strategy are proposed, and positive and negative-sequence current inner loop independently controlled method of VIENNA rectifier under power grid voltage imbalance are proposed. And then, a new instantaneous power analysis methods of VIENNA rectifier under power grid voltage imbalance is proposed too. A novel reference current calculation method for the gridside power control, rectifier input side power control under αβ coordinate system are proposed. Final, VIENNA rectifier controller based on proportion resonant control and sliding mode control is design.