Harmonic State-Space Based Hybrid AC/DC D-Q Admittance Model of MMC

Modular multilevel converters (MMCs) play a vital role in the formation of hybrid AC/DC power system. Despite their numerous advantages, however, impacted by the unique admittance characteristics, the wide-band oscillations occur more common recently. To address this issue and properly investigate the admittance characteristics of MMCs, this manuscript establishes the hybrid AC/DC admittance model in the dq synchronous reference frame with the harmonic state-space (HSS) method. Compared with the conventional AC and DC admittance models, the hybrid AC/DC admittance model offers salient features as follows: 1) it stays immune to the variation of the AC and DC grid admittances; 2) it simultaneously portrays the AC and DC side admittance characteristics and describes the self-admittances with its main-diagonal elements; and 3) it thoroughly captures the coupling interaction between the AC and DC sides and expresses the coupling interaction with its off-diagonal elements. Based on the aforementioned advantages, the hybrid AC/DC admittance model can analyze the stability of system more comprehensively. Finally, the consistence of the theoretical results and simulation results proves the accuracy of the proposed hybrid AC/DC admittance model.