Abstract
In order to concentrate the frequency spectrum of the output voltage and improve the quality of grid currents for the three-level neutral point clamped inverter with the model predictive control, this paper proposes an optimal switching sequence model predictive control algorithm. Based on the increments of grid currents and the neutral-point voltage, the predictive model of the inverter is established in αβ frame. Moreover, including grid currents and the neutral point voltage tracking, the cost function is designed and simplified by arranging the voltage vector sequence in a sampling cycle appropriately. Meanwhile, the calculation of the optimal dwell time for each voltage vector sequence is derived by Lagrange multiplier method, and its solving process is simplified to reduce online computations. Furthermore, according to different voltage vector sectors, a voltage vector sequence preselection principle is introduced in this paper, besides, considering redundant small vectors in a pair have opposite affections on the neutral point voltage, the amount of voltage vector sequence that needs to be verified is further cut down, for obtaining the optimal voltage vector sequence. Finally, the experimental results show that the proposed method can concentrate the output frequency spectrums, which maintains a good quality of grid currents with a small neutral-point voltage fluctuation.
Highlights
With the rapid development of digital signal processors (DSP), some complex control algorithms have been researched extensively as effective approaches for controlling power converters and electrical drives, such as sliding model control, fuzzy logic control, model predictive control (MPC) and the like [1][2]
In order to concentrate the frequency spectrum of the output voltage and improve the quality of grid currents for the 3−level neutral point clamped inverter with the model predictive control (MPC), this paper proposes an optimal switching sequence MPC algorithm
When conventional finite control set MPC (FCS-MPC) method is applied to power converters, only an output voltage vector that minimizes the cost function is applied during the whole sampling cycle, which leads to the unfixed switching frequency of the inverter output voltage [6]-[8]
Summary
With the rapid development of digital signal processors (DSP), some complex control algorithms have been researched extensively as effective approaches for controlling power converters and electrical drives, such as sliding model control, fuzzy logic control, model predictive control (MPC) and the like [1][2]. When conventional FCS-MPC method is applied to power converters, only an output voltage vector that minimizes the cost function is applied during the whole sampling cycle, which leads to the unfixed switching frequency of the inverter output voltage [6]-[8]. The conventional FCS-MPC uses a high sampling frequency to achieve good performances, which leads to larger inductors These issues have been researched in some literatures. In [14]-[18], a modulated MPC algorithm is proposed for the indirect matrix converter to obtain the fixed switching frequency by using a similar output mode with SVM method to control the input instantaneous reactive power and output current. A tuning parameter is introduced to trade the grid current tracking error versus control input effort, which affects the grid current control
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