Voltage Prediction Based Capacitor Voltage Balancing Control for MMCs With Lower Switching Frequency

The capacitor voltage balancing (CVB) is one of the most important issues for safe and reliable operation of the modular multilevel converters (MMCs). This article proposes a voltage prediction (VP)-based CVB control to realize the capacitor voltage balancing with low switching frequency. First, the virtual operation states of Submodules (SMs) are generated. And then, based on the virtual operation states, the SM capacitor voltages and average capacitor voltage at the end of control cycle are predicted. Finally, according to prediction voltages, the SM virtual operation states are adjusted in advance before the capacitor voltage deviation exceeding its limit. The proposed VP-CVB control can effectively balance the capacitor voltages and reduce the switching frequency of MMCs, and therefore reduce the switching losses and improve the system efficiency. Electromagnetic transient (EMT) simulation studies with professional tool power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of proposed control.