Small-signal Stability Analysis of MMC-HVDC System Based on the Admittance Model under Unbalanced Condition

The admittance model under unbalanced condition varies greatly, which deserves in-depth study. In this paper, an MMC hybrid AC/DC admittance model with a complete control system under unbalanced condition is constructed based on the harmonic transfer function (HTF) method. The complete control system includes the decoupled double synchronous reference frame PLL (DDSRF-PLL), outer loop, circulating current suppressing controller (CCSC), positive- and negative-sequence current controller. The ac- and deside SISO admittance model is developed and used to comparatively analyze the ac- and deside admittance characteristics under both balanced and unbalanced condition. The results show that the unbalanced condition not only deteriorates the ac-side admittance characteristic, i.e. increase in amplitude, but also introduces a resonance in the deside admittance, both of which are detrimental to the stability of the system. Finally, the simulation verifies the analysis and points out that the improper controller parameters may cause oscillations on the ac- and deside of the system under unbalanced condition.