Abstract
DC-link voltage control is of great importance for modular multilevel converter (MMC)-based high-voltage direct-current (HVDC) systems. Existing methods need to measure the actual dc-link voltage, which are referred to as direct control methods in this article and require a reliable dc voltage measuring device. The measurement system is complicated. Also, once malfunction occurs, dc-link voltage control may fail. This article aims to develop a dc-link voltage control method without actual voltage measurement. The idea is based on the coupling effect between dc-link voltage and submodule (SM) capacitor voltage. Based on the analysis, an indirect dc-link voltage control method is proposed. In this method, the existing dc-link voltage control loop is first replaced by the total capacitor voltage regulation control loop. Then, a compensation signal is introduced to the switching functions to mitigate undesired voltage deviation. Two approaches are proposed to generate a proper compensation signal. The proposed method can be integrated into the existing MMC control system. Also, in measurement failure cases, it can come into service and continue maintaining MMC dc-link voltage, improving system reliability. Both simulation and experimental results confirm the validity and effectiveness of the proposed method.
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