Impedance Modeling of MMC-HVDC System Connected to Passive Network

MMC-HVDC systems have suffered resonance problems frequently in late years, which made the operation of power grids are vulnerable to be affected. The impedance model is an effective tool for analyzing system stability. Impedance modeling of the interconnected system between MMC-HVDC and weak grids or wind farms has been deeply studied. However, MMC-HVDC system connected to passive network is ignored, which has great impedance coupling between MMC-HVDC and passive network. The control strategy of passive network is different from other systems, which should be considered into impedance model. In this paper, the small signal impedance model on the basis of harmonic state space (HSS) of MMC-HVDC connected to passive network has been established. The proposed model can reflect the harmonic dynamics inside the MMC, which can reveal the impedance coupling between the converter and the passive network. Furthermore, time delay, voltage outer loop, current loop controller, circulating current suppressor, and phase-locked loop (PLL) is taken into account. The precision of established model is validated by impedance sweep through electromagnetic transient model. The dominant influence factors of MMC impedance and system stability also has been analyzed and verified.