| The multilevel converters have many advantages, such as the active switch with low voltage stress, the output currents with low harmonic currents and the voltage with low rate(dv/dt) in the ac side, which are suitable for medium and high power applications. Among them, the diode clamped three level converter is one of the most popular topology. This article selects the diode clamped three-level Vienna rectifier as the research object, and the nonlinear control algorithm, space vector modulation technology, carrier wave modulation and the neutral point potential balancing control method are studied in this thesis.In this paper, the modulation technology of the diode clamp three level converter is researched deeply. Presently, three-level space vector modulation of diversifications are many, such as the different partitions technique, the SVPWM technology in the different coordinates, but they do not have the unified theory. So this paper will derive the variety of three-level SVPWM modulation of the three-level system in detail, then the new unified theory will be produced, it is easy to apply the new theory in multilevel modulation technology. This paper discusses the unity relations between SPWM and SVPWM with more than 8 segments in detail; and the unified equivalent theory also provides a new idea for the three-level space vector modulation and the carrier modulation technology convert into each other. The simulation and experimental results effectively evolves the unity theory of SPWM and SVPWM successfully.The neutral-point potential unbalance is the inherent problem of the diode clamped three-level converter. The neutral-point potential unbalance that the two capacitances’ voltages of the dc side are not same, it will bring a series of problems. Firstly, the mechanism of the neutral-point potential unbalance is analyzed, then the average neutral point current model is established and the specific expression of the neutral-point potential is given. To overcome the problem that these methods cannot enough control the neutral-point balance and simple the complexity of the three-level SVPWM technique. Take the Vienna rectifier as an example, the proposed equivalent SVPWM can precise control midpoint voltage balance for zero-sequence components adding the potential balance factor, and the static and dynamic characteristics of the neutral-point voltage balance are improved.In order to improve the performance of Vienna rectifier, this paper proposes a series of control algorithms. Firstly, this paper proposes a single closed loop nonlinear control strategy without current sensor, which can simplify the design process of the control system and save the cost of production. Additionally, the method can shorten the design and debugging period of the controller. In order to enrich the mathematical model of Vienna rectifier, the mathematical model of the rectifier is established in the polar coordinate system. The traditional PI controller parameters are tuned process complexly, and the performances depend on the parameters of the PI controller in large extent. In order to overcome the above shortcomings, this paper proposes two hybrid nonlinear control strategies. The first one is sliding mode PI direct power control strategy; the inner loop adopts PI regulation, the outer loop adopts slide mode controller to improve the dynamic performance. The other one is a hybrid nonlinear control strategy, which combines the passive control and the sliding mode variable structure control. The inner loop adopts the passive controller, and the outer loop adopts slide mode controller. Finally, the results show that the nonlinear hybrid control strategies have the advantages of good dynamic and anti-disturbance performances.Sliding mode variable structure control is one of the important methods to solve the problem in the nonlinear system, which has been successfully applied to the three-phase rectifier in many literatures. Based on the previous studied, this paper presents a novel direct power control based on sliding mode control strategy for the Vienna rectifier, which adopted direct power control based on sliding mode control(SMC-DPC) for inner loop without transforming to d-p rotating coordinate system and used sliding mode control with the square of voltage as the feedback for the outer loop; the principle and design process of the strategy are discussed in detail, including the establishment of the system variable structure model, the selection of sliding mode surface, the condition of the sliding mode and the control rate. Both the simulation and experimental results are utilized to prove the effectiveness and feasibility. |
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