Study On Three-level NPC Rectifier With A Low Switching Frequency

Green, energy saving, environmental protection have become the theme of our times. Medium voltage high-power multi-level rectifiers are widely used because of its sinusoidal output current, lower harmonic content and the energy flowing in both directions. It’s essential to reduce switching frequency in order to improve device output power as well as the switching losses. However, the reduction of switching frequency would cause serious inertial characteristics of PWM and take an adverse impact on the dynamic performance of the system. For this reason, this thesis carries out deep research on mathematic model, current regulator and its application in three-level neutral-point-clamped rectifier.The research objective of this thesis is diode-clamped three-level PWM rectifier. The mathematic models in three-phase static a-b-c coordinate and two-phase synchronous d-q coordinate are derived based on switch function. A PWM rectifier mathematical model is built according to the complex vector modeling method at low switching frequency. The deficiencies of conventional PI regulator and PI regulator with feedforward decoupling are analyzed with the low switching frequency. A novel current regulator with complex state variables at low switching frequency is proposed on the basis of low coupling. The comparisons in decoupling, robustness, steady and dynamic performance are analysed between conventional PI regulator and complex regulator. Simulation and experimental results show that the novel current regulator can improve the performance of the system at low switching frequency.Model predictive control(MPC) is studied to improve the dynamic performance at low switching frequency. With current, neutral point potential balancing and switching frequency as cost function, three-level PWM rectifier can achieve optimal control by rolling optimization. The method of order reduction is adopted from three-level to two-level using the reference vector decomposition to simplify the control algorithm. Meanwhile, the control strategy combining MPC and sliding discrete fourier transform(s DFT) is adopted to achieve the grid standards. The fundamental and other harmonics extracted from current will be the cost function as well as the other control objectives. The effectiveness of the control method is verified by simulations and experiments.The mathematical model and three kinds of positive/negative sequence component extraction(PNSCE) method are established for three-level rectifier under the non-ideal grid. An unbalanced control strategy based on MPC is studied, which can meet the control objectives of current symmetry, neutral point potential balancing and lower switching frequency. Finally, the simulation results show the effectiveness and feasibility of the strategy under the non-ideal grid.