Abstract
The Modular Multilevel Converter (MMC) is one of the most attractive converter topologies in the High Voltage Direct Current (HVDC) transmission field. The latest widely used sorting method has a low algorithm complexity. It can effectively balance the sub-module (SM) capacitor voltages, but it would cause relatively high switching frequency and power loss. Aiming at the problem that the sub-module (SM) capacitor voltage sorting algorithm has a large switching loss due to the high switching frequency of the device, this paper proposes an auto-optimized capacitor voltage balancing control strategy. Firstly, the topology and operation principle of MMC are analyzed. Secondly, a SM capacitor voltage control method based on the dynamic deviation threshold is proposed. Considering the switch switching state of the SM and the difference between the voltages of each SM, the algorithm can obtain the dynamic deviation valve using the closed-loop control. The method can avoid the unnecessary repeated switching of the Insulated Gate Bipolar Transistor (IGBT) under the premise of ensuring that the capacitance voltages of the SMs are basically the same, which effectively result in reducing the switching frequency of the MMC SM and reducing the switching loss, thereby improving the operating efficiency of the system. Finally aiming at the proposed control strategy, the simulation and experimental verification are carried out which shows that the proposed algorithm can better control the system voltage deviation, reduce the switching loss of the system and improve the stability of the system.
Highlights
The modular multilevel converter (MMC), first proposed by Marquardt and Lesnicar in 2003 [1], has become the most attractive topology for high voltage applications
Extensive research has been focused on the technical challenges associated with the designs, operation and control of the MMC to improve its performance for various applications [2,3,4,5,6,7,8,9,10,11,12,13]: voltage source converter-based high voltage direct current transmission (VSC-High Voltage Direct Current (HVDC)) [5,6,7,8]
When the capacitance target deviation is 0.5 V, the SM capacitor voltage volatility is reduced by 35.2% and the total switching frequency is reduced by 6.7%
Summary
The modular multilevel converter (MMC), first proposed by Marquardt and Lesnicar in 2003 [1], has become the most attractive topology for high voltage applications. In References [20], the SM capacitor voltage is balanced by a carrier-based pulse width modulation method This method can better reduce the capacitance voltage deviation between SMs, but it is necessary to add a controller to each SM for control, which increases economic cost and system complexity. The algorithm based on a sorting technique is given in Reference [21] In this method, the computational complexity is reduced and the capacitance voltage deviation between SMs is well suppressed compared to the previous methods, but the switching frequency of the system is high. Based on the analysis of the working principles of MMC topology and sorting algorithm, an auto-optimized capacitor voltage balancing method based on a maximum voltage deviation automatic threshold for a modular multilevel converter is proposed in this paper, which can reduce the switching frequency of the system and the deviation of the capacitor voltage.
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