| Featured by better quality waveform, lower switching frequency and less loss compared to two-level or three-level inverter, modular multilevel converter(MMC) is becoming a new generation topology for flexible HVDC transmission system. MMC-based flexible HVDC system has broad application prospects in fields of offshore wind power network and power grid capacity increase of urban transformation. Reducing the volume and mass of MMC submodules which individually accounts for 50% and 80% of the whole submodule, is the urgent need of industries in order to reduce installation and maintenance costs. However, too few current research in reducing submodule capacitor volume and mass makes this project of great significance.This paper presents a submodule topology which is capable of reducing capacitor volunm and mass of the novel MMC submodule, and relevant control strategies are studied thoroughly. The detailed contents are as follows:(1)The energy density as well as ripple current withstand capability of the metallized film capacitors are statistically analyzed and calculated. The principles of reducing capacity volume are elaborated. Topology and operating principle of stacked switching capacitor which is capable to reduce the volume of film capacitor is researched and analyzed. The energy efficiency of different stacked switched capacitor is studied, to provide theoretical guidance for stacked switched capacitor design.(2)A novel submodule which is able to reduce capacitor size based on stacked switched capacitor is proposed and its mass and volunm are theoretically analyzed. This paper studies the working principle of the new MMC submodule, proposes the new MMC submodule controll strategies at rated power, and deadband control strategies is proposed for the new MMC submodule safe and stable operation. Futhermore,A different control strategy of the new MMC submodule is proposed when the operating power below the rated power so as to reduce losses.(3)An MMC AC-DC decoupling mathematical model is established based on the new MMC submodule, and the MMC control system is divided into AC circuit controll and circulating current suppressing controll based on this model. A double-loop AC circuit controll is designed. The circulating current and output harmonic mechanism of the new MMC submodule based system are studied and analyzed, circulating current and harmonic suppression controllers are designed corresponding for second?fourth circulating current and fifth harmonic. Capacitor voltage balancing control strategy is designed for the new MMC submodule.(4)A three-phase 11-level MMC system is built in Matlab/simulink based on the new MMC submodule. Capacitor value and volume of the new MMC submodule and the halfbridge submodule are comparatively studied. When voltage ripple coefficient is 10%, the DC voltage of the new MMC submodule is 2 kV, the toal volume of the new MMC submodule capacitor occupies only 58.73% of half bridge submodule capacitor.The steady and dynamic characteristics of the new MMC submodule controller and system-level controller under rated power and partial power is verified by simulation. Converter loss constitution of the MMC is analyzed, IGBT module parameters is extracted based on the data identify method, loss of the new MMC submodule and halfbridge submodule is calculated and comparatively analyzed under different working conditions. |
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