The Research Of Model Predictive Control For Three-level Converters

Three level PWM converter is widely used in high voltage and high power applications by virtue of lower switching frequency, small current THD, energy bidirectional flow and so on. The research object of this paper is three-level PWM converter, and its control strategy mainly including:voltage oriented control, direct power control, model predictive control. The problem of the three level topology (such as neutral point potential balance control, voltage amplitude high jump suppression) are presented in this paper, and the corresponding solutions are given. For the direct power control, the use of positive and negative redundant small vectors to control the neutral point potential balance, and the high voltage jump problem is smooth transition from a common vector or an intermediate vector. For the model predictive control, the voltage difference of the capacitor on the DC bus bar is used as an error signal, which can be considered as an cost function to solve the control problem of neutral point potential balance, and the problem of high voltage amplitude jump is solved by using a predefined vector table that does not generate voltage amplitude jump.In the practical application, the system operating environment and the temperature will change, that may cause the system parameters change which will affect the performance of the control system. In view of the variation of the parameters of the system in the actual operation process, this paper studies the online parameter identification technology, mainly using the recursive lest squares method and the model reference adaptive method to identify the system parameters online. In addition, in order to actually reduce the hardware cost and increase system reliability, this paper also studied the technology without grid voltage sensor, using four cascaded low pass filters to replace the traditional integrator, to eliminate the problems brought by the traditional integral integrator saturation and drift.In the high power field, the switching frequency of the three level PWM converter can not be too high. Therefore, a multi step predictive control is adopted in this paper. Compared to single predictive control, multi step predictive control has lower switching frequency and good control performance. According to the range of prediction step, the optimal voltage vector is selected to achieve the overall optimal control. There are two methods used in this paper, namely, hysteresis control and sphere decoding.Grid voltage will be unbalanced or contain harmonics and other special circumstances in actual operation. At this time as the traditional control strategy will cause the distortion of the grid current, and it will cause local collapse of the power grid. Therefore, the research applied to grid voltage unbalance and containing the harmonic condition will have important practical significance. Therefore, a control strategy is proposed under grid voltage unbalance and containing the harmonic condition. The control strategy can be divided into two types, the traditional reactive power definition and the new reactive power definition. Finally, the effectiveness of the proposed method is proved by simulation and experiment results.