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Model Predictive Control for Modular Multilevel Converter with Reduced Switching Frequency Based on Improved Insertion Priority of the Sub-modules
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Model predictive control (MPC) has been widely investigated in modular multilevel converters (MMCs) due to its superiority in achieving multiple control objectives. The three-phase model-based MPC, which contains the common-mode voltage in the output current dynamic model and considers interaction among phases, shows better performance than conventional per-phase model-based predictive control in a three-phase MMC system. However, it suffers from a heavy computational burden as the number of submodules (SMs) increases. To address this issue, this paper first analyzes the relationship among the numbers of inserted SMs, the controllability of DC-link current and circulating currents. Then, according to this analysis, two simplified MPC methods based on the three-phase model with reduced computational burden are proposed. Specifically, fewer insertion index combinations are selected in advance to ensure good output currents, controllable DC-link and circulating currents. The effectiveness of the proposed methods is verified through experimental results.
Modular multilevel converters (MMCs) have been a promising topology in medium-voltage motor drive applications over the past decades. The main challenge that hinders the widespread use of the MMC in medium-voltage motor drives is the large voltage fluctuation under low-speed conditions. In this paper, we propose a model predictive control (MPC) for the MMC operating in a wide frequency range. Unlike conventional MPC methods, a novel cost function has been designed to realize the output current control and the voltage balancing control for both low- and high-frequency operations. Compared with the high-frequency circulating current injection method, the proposed method can easily achieve a trade-off between the modulation range and the amplitude of injected common-mode voltage under different operating conditions. In addition, due to the exclusion of the switching process between the low- and high-frequency operation, the control structure of this method is more straightforward. At last, the experimental results verify the effectiveness and superiority of the proposed method.
Abstract In recent years, the modular multi‐level converter (MMC) has been widely used in high and medium voltage DC transmission systems because of its topological advantages. However, for an MMC with a two‐stage model predictive control (TSMPC) method, it is difficult to precisely and reasonably design the weighting factor in the cost function. Here, an improved TSMPC method is proposed which not only can avoid choosing the weighting factor for both first and second stage control but also can raise the output voltage level (OVL) to 2N + 1 without increasing computation burden. The discrete‐time mathematical model of the MMC is first derived. Two circulating current factors are introduced to calculate the optimal number of submodules (SMs) of the upper and lower arms in the first stage control. Secondly, the second stage control calculates the optimal number of SMs through the superior control and forms the optimisation array by adding or subtracting one SM. Then the objective function is developed, and the SMs with the minimum value of the objective function are selected for the final input. Finally, the algorithm of reducing switching frequency (RSF) is applied to achieve the balance of the SM capacitor voltage. The simulation and experimental results verify the effectiveness of the proposed method.
Modular multilevel converters (MMCs) have attracted a great deal of research attention in medium-voltage motor drive applications. However, the excessive fundamental-frequency submodule (SM) capacitor voltage ripple at low speeds or during start-up poses a major technological challenge. This paper develops a new control strategy for SM capacitor voltage ripple suppression based on model predictive control (MPC). The system mathematical model with common-mode voltage (CMV) and the SM capacitor voltage ripple suppression models are established. Then, a cost function of the system is established, and the control system is designed. When compared to existing strategies, the modulation process, PI regulation process, and high frequency injection control are avoided due to the MPC strategy. The performance of the proposed approach is verified through MATLAB simulations and experiments on a MMC system.
Modular multilevel converters (MMC) have revolutionized the voltage-sourced converter-based high-voltage direct current transmission, but not yet got widespread application in medium-voltage variable-speed motor drives, because of the large capacitor voltage ripples at lowmotor speeds. In this paper, a novel hybrid MMC topology is introduced, which significantly reduces the voltage ripple of capacitors, particularly at low motor speeds. Moreover, this topology does not introduce any motor common-mode voltage; as a result, there are no insulation and bearing current problems. Additionally, the current stress can remain at rated value throughout the whole speed range; thus, no device needs to be oversized and converter efficiency can be ensured. Operating principle of this hybrid topology is explained, and control schemes are also developed. Validity and performance of the proposed topology are verified by simulation and experimental results.
Submodule (SM) capacitor voltage ripple is one of the major concerns in modular multilevel converters (MMCs). Capacitor voltage ripple leads to the double-frequency circulating current (CC) in legs, thereby resulting in a cascading effect of increased peak value of the arm current, semiconductor device stress, and power losses in MMCs. In this study, a model predictive control (MPC) with common-mode voltage (CMV) injection is proposed to minimize capacitor voltage ripple and the magnitude of CC. A discrete-time mathematical model of the MMC with CMV is presented to predict the future behavior of the control variables. The injection of CMV guarantees arm voltage balancing without CC control and long-term stability of MMC without large capacitors. The dynamic and steady-state performances of MPC with CMV injection are verified on an MMC with three-level flying capacitor SMs. A performance comparison between the proposed approach and the conventional MPCis also presented. Simulation and experimental studies show that CMV injection significantly reduces the capacitor voltage ripple and theCCin legs. The proposed approach also improves output voltage and current waveform quality.
Modular multilevel converters (MMC) represent an interesting and emerging topology in medium-voltage motor drive applications. The main challenge of using such a topology in variable-speed drives is the large voltage ripple of submodule capacitors at low speed with constant torque. In this paper, an improved circulating current injection method is proposed, which does not completely eliminate the capacitor voltage ripple, but maintains it bounded within reasonable values. As a result, magnitude of injected circulating current is reduced, leading to converter efficiency improvement and reduction of semiconductor current ratings. Dimensioning of submodule capacitance is also discussed, which is an important consideration when designing the MMCs in variable-speed drives. The proposed method has been successfully validated by simulation and experimental results.
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