Abstract
Parallel three-level simplified neutral point clamped inverters (3L-SNPCIs) are applied in this paper because of fewer switching devices, low harmonic, larger power capacity and more flexibility. However, the challenge of zero-sequence circulating current (ZSCC) is inevitable, which causes the output current distortion and system instability. The conventional control and modulation methods introduced by far cannot solve them properly due to the limitations of fewer voltage vectors selection. To overcome these limitations, a modified model predictive control (MMPC) strategy is proposed based on the discrete mathematical model of parallel 3L-SNPCIs. The MMPC strategy realizes circulating current suppression and neutral-point voltage balancing by selecting P-type or N-type small voltage vector. In addition, the control objectives are optimized and the implementation is simplified, which can reduce the burden of calculation and reserve the advantages of the conventional model predictive control (MPC) algorithm. The proposed control strategy can be flexibly extended to parallel multilevel inverters. The efficiency of the proposed method is proved by simulation and experimental results under various conditions.
Highlights
Three-phase inverters have been extensively employed in different kinds of industrial applications, for example, photovoltaic grid-connected power generation, superconducting energy storage, and so on [1]–[4]
For the purpose of solving this problem, the multilevel inverter was proposed owing to low harmonics of the output currents [5]–[7]
In contrast to parallel two-level inverters, the neutral point (NP) voltage balance and the circulating current suppression should be considered at the same time
Summary
Three-phase inverters have been extensively employed in different kinds of industrial applications, for example, photovoltaic grid-connected power generation, superconducting energy storage, and so on [1]–[4]. A nonlinear control method was given to suppress the circulating current in [19] This algorithm [19] was too complex to implement in industrial applications. In contrast to parallel two-level inverters, the neutral point (NP) voltage balance and the circulating current suppression should be considered at the same time. In parallel 3L-SNPCIs, it is necessary to note that, because of the novel topology, the existing MPC method for conventional T-type three-level inverter is not applicable. Inspired by the newly MPC method [23]–[25], to balance the NP voltage and reduce the ZSCC at the same time, a modified model predictive control (MMPC) strategy is developed. A one-step prediction is employed to compensate the delay time In this case, the voltage vector of the 3L-SNPCI is given as.
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