Small-signal modelling considering internal perturbations for modular multilevel converters

Small-signal modelling of modular multilevel converters (MMC) has drawn lots of attention recently. Existing research mainly studies MMC terminal impedances by injecting corresponding AC-side and DC-side external small-signal perturbations. This paper focuses on the MMC internal small-signal perturbations, that is, perturbations in control signal and their effect. There are two kinds, that is, differential-mode and common-mode, of internal small-signal perturbations, which influence MMC external and internal state variables, respectively. Then, the existing MMC small-signal model is improved. With its help, accurate MMC control-signal to state-variables transfer functions are established, which can aid precise MMC controller analysis and performance optimization. In this paper, an analysis of MMC phase capacitor voltage regulation control is demonstrated as an example. The stability can be precisely ascertained based on the obtained transfer function. The effect of parameter variation of the proportional-integral controller is also studied. Moreover, an additional compensator is introduced in the existing control loop to attenuate resonance peaks in the transfer function, which mitigates the large voltage overshoot and makes the transient state much smoother. Simulation and experimental results support and well confirm the analysis of this paper.