Small-Signal Stability Analysis of MMC-HVDC System Based on Hybrid Passivity

The admittance model of the modular multilevel converter (MMC) has been well developed in small-signal stability research. Based on the admittance model, the frequency-domain passivity theory can guide the system design. That is, the system will not exhibit oscillatory instability events in the passive frequency band. This paper proposes the hybrid passivity property based on the 3
^rd
-order hybrid admittance model of the MMC. Hybrid passivity criterion is a comprehensive criterion for assessing the stability of the entire MMC high voltage direct current (MMC-HVDC) system containing the AC- and DC-side systems. Furthermore, the effects of the control system, including the DC voltage loop, AC current loop, circulating current (CC) loop, and phase-locked loop (PLL), on hybrid passivity property are analyzed in detail. In addition, virtual damping control as a means to improve the hybrid passivity of the MMC is also investigated. Importantly, a set of guidelines for parameter design based on hybrid passivity criterion is proposed. In Matlab/Simulink, two simulation cases validate the hybrid passivity analysis.