Interaction Analysis Among Multiple Series-Parallel Connected LCC/MMC in Hybrid Cascaded HVDC System

The series-parallel structure of multiple LCC/MMCs in hybrid cascaded HVDC (HC-HVDC) system facilitates flexible power transmission to different load areas, however, could introduce potential risk of instability arising from intricate interactions among multiple converters under weak AC system. To investigate the interaction paths among multiple converters and quantitatively evaluate their contributions to the system stability, this article establishes the state-space model and the motion equation model integrating multiple converters for HC-HVDC system. Then, based on the path decomposition method, the open-loop transfer function is decomposed into five damping paths viewed from the equivalent inductor of LCC inverter. This offers a perspective on system damping characteristics to reflect the key paths and contributing converters leading to the weak damping of dominant oscillatory mode. The impact of LCC inverter's self-stabilizing path, as well as the en-stabilizing paths involving the components related to interactions between LCC inverter and each MMC, and interactions among series-parallel connected multiple converters, on system stability is clearly quantified. Moreover, the outcome of controller bandwidth variations on the damping characteristics of different paths is studied, providing insights into adjusting control parameters to enhance the system damping under weak AC system. It is indicated that the interaction between DC-voltage-controlled <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MMC}_{1}$</tex-math></inline-formula> and LCC inverter, as well as the interactions among multiple converters, provide negative damping on system stability. The critical interaction paths contributing to negative damping and the identified sensitivity parameters can offer valuable insights for further enhancement of system stability in HC-HVDC system.