Impedance Based Stability Analysis of the Multi-terminal Cascaded Hybrid HVDC System

The cascaded hybrid high voltage direct current (HVDC) combines the strengths of the line commutated converter (LCC) and the modular multilevel converter (MMC) in the long distance large capacity power transmission. However, its distributed muti-terminal grid structure raises the complexity of the whole system drastically. To clarify the oscillation mechanism of the multi-terminal cascaded hybrid HVDC system, utilizing the merits of the impedance model, this paper proposes an impedance based stability analysis method, which decomposes stability analysis problem into a hierarchical structure. In addition, an equivalent single-input single-output (SISO) impedance based method is proposed together for oscillation propagation analysis, which could depict how the oscillation spread from the perspective of the physical impedance network. Oscillation suppressing methods including impedance reshaping and parameter retuning could be tailored with the instruction of the impedance based stability analysis and thereby efficiency is improved considerably. The analytical results and the control method are verified through the electromagnetic transient simulation with practical data.