Interpreting the Individual Capacitor Voltage Regulation Control of PSC-PWM MMC via Consensus Theory

In computer science, consensus theory has been studied for decades. “Consensus” in a multi-agent system means reaching an agreement regarding a certain quantity of interest. The modular multilevel converter (MMC) is a promising topology for medium- and high-power applications owing to its salient features. In MMC operation, one key point is to regulate the capacitor voltage of each submodule (SM). The SMs in MMCs are interconnected both physically and via a control system, and these SMs can be regarded as a multi-agent system, with the control objective of regulating their capacitor voltages, which could formulate a consensus problem. In this paper, through mathematical derivation, it is found that individual capacitor voltage regulation control of phase-shifted-carrier pulse-width modulation (PSC-PWM) MMC is one special case of consensus theory. Furthermore, based on the consensus theory, the influence of different communication structures on the MMC voltage regulation performance is discussed. The analysis proves that the existing one has the highest rate of convergence. The impact of current-tracking dynamics on consensus-based interpretation is also analyzed. In addition, the proportional relationship between the rate of convergence and the P controller parameter is also justified. The experiments on a seven-level down-scaled prototype are conducted to confirm the analysis.