Analysis of interactive oscillation modes for LCC-HVDC system in single-infeed mode and split-connection mode

There exist single-infeed mode and split-connection mode for Yongfu HVDC project in China. Differences of system stability characteristic between the two connection modes are unclear. In this paper, a small signal dynamic model of LCC-HVDC system is established. By means of eigenvalue analysis and participation factor analysis, weakly damped interactive oscillation modes reflecting the interaction between the inverter-side control system and receiving AC girds are identified, which may be the key modes dominating the system stability. On this basis, variation characteristics of interactive oscillation modes in two connection modes are investigated by the analysis of eigenvalue loci with varying DC voltage control parameter under different SCR conditions. The theoretical analysis and simulation results manifest that, in the single-infeed mode, the system stability is dominated by the common-mode. In the split-connection mode, when two receiving AC grid strengths are identical/ similar, the stability characteristic of the two poles are similar, and there is an extra anti-mode to further weaken the system stability, compared to the corresponding single-infeed mode. When two receiving AC grid strengths differ greatly, the stability characteristics of the two poles are relatively independent.