Research On The Control Strategy Of PWM Rectifier Under Unbalanced Grid Voltage

When compared with thyristor or diode rectifiers, the muti-level pulsewidth modulation(PWM) AC/DC converter has some advantages, such as sinusoidal input current, smaller output voltage ripple, bidirectional power flow and controllable power factor. A large number of studies on PWM rectifier are under balanced grid voltage in recent years, but the PWM rectifier is sensitive to voltage dips and has many problems when under the unbalanced conditions, because there are negative sequence components existing in the PWM rectifier system, making the converter operating performance decline. For this reason, this paper carries out deep research and design on mathematic model and new current-reference generator under unbalanced voltage dips conditions in three-level neutral-point-clamped rectifier.The research objective of this paper 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. When using the traditional control strategy, dc-link voltage harmonics and odd ac input currents harmonics will appear under unbalanced grid condition. A new stationary frame control scheme is proposed in this paper, No phase-locked loop(PLL) strategies and coordinate transformations are needed for the new current-reference generator. This allows using PR controllers for simultaneous compensation of both positive and negative current sequence components. Also, the new control strategy can control the power factor.The traditional PWM recitifier control strategy has many defects, such as the slow dynamic response, the poor parameter robustness. Model predictive control(MPC) is to predict the future behavior of the variables over a time frame(integer multiple of the sample time). These predictions are evaluated based on a cost function and then the sequence that minimizes the cost function is chosen. MPC is the nonlinear control theory with excellent robustness, fast dynamic response, and insensitivity to parameter variations. However, the traditional MPC method has several disadvantages like the considerable computation and inconvenient realization, to reduce the number of rolling cycles, simplify the control algorithm, The method of order reduction is adopted from three-level to two-level using the reference vector decomposition to simplify the control algorithm. An unbalanced control strategy based on MPC is studied under the non-ideal grid, which can meet the control objectives of current symmetry, neutral point potential balancing. Finally, the simulation results show the effectiveness and feasibility of the strategy under the non-ideal grid.Finally, based on the three-level neutral-point-clamped rectifier hardware experiment platform, the DSP and FPGA algorithms are designed. The effectiveness of the control scheme are analysed under the non-ideal grid.