Master–slave structure‐based capacitor voltage measuring technique for hybrid modular multilevel converters

Abstract

The measurement of the capacitor voltage of each sub-module (SM) is a crucial part of achieving robust control of modular multilevel converters (MMCs). This study proposes a master-slave structure-based capacitor voltage measuring technique (MS-MT) for hybrid MMCs, in which the SM capacitor voltages are calculated using nine processes at the corresponding sampling instants according to the changing rule of the arm AC-side voltage and SM operation states. In MS-MT, the master sensor is used to measure the overall arm voltage while the slave sensor, which measures the full bridge, is used additionally to improve accuracy. Furthermore, MS-MT performs well even under pre-charging and DC short-circuit fault conditions, for the reason that a single SM short-circuit fault can be detected during above two conditions by comparing the voltages measured by the two sensors. Compared with the direct measuring technique, which has low measurement errors, and the overall measuring technique, which requires less hardware cost, MS-MT inherits both advantages. Finally, extensive simulation and experimental results validate the effectiveness of the proposed MS-MT. Nomenclature N SM number in every arm f c carrier frequency u c SM capacitor voltage u Hout output voltage of HBSM u Fout output voltage of FBSM U dc DC bus voltage of MMC u a , u b and u c MMC AC-side phase voltages u m_au measured upper arm AC-side voltage by the master sensor u m_auFs measured voltage of the slave SM in the upper arm S aui switching state of the ith SM in the upper arm of phase A i arm_au , i arm_al upper and lower arm currents T c sampling period S j switching state of the jth SM Δu ecj estimated voltage variation of the jth SM u cs measured voltage by the slave sensor ΔU ceb voltage drop caused by auxiliary circuit