The Subsynchronous Oscillations Caused by an LCC HVDC Line in a Power System Under the Condition of Near Strong Modal Resonance

This paper derives a multi-input multi-output closed-loop model of a power system with an LCC HVDC line. The derived model is comprised of the HVDC subsystem and the subsystem of rest of power system (ROPS). Analysis in the paper indicates that the closeness of open-loop subsynchronous oscillations (SSO) modes of two subsystems on the complex plane is a special condition of near strong modal resonance (NESMOR). Under the condition of NESMOR, strong dynamic interactions are introduced by the LCC HVDC line to degrade the SSO stability of the power system. Theoretical derivation is presented in the paper to estimate the degree of SSO stability degradation caused by the NESMOR. Based on the derivation, a method of open-loop modal analysis is proposed to detect the SSO instability risk brought about by the multivariable dynamic interactions introduced by the LCC HVDC line. An example of multimachine power system with an LCC HVDC line is presented to demonstrate and evaluate analysis and conclusions made in the paper. Dynamic interactions of the LCC HVDC line with a VSC-based HVDC line or a PMSG wind farm occurred in the example power system under the condition of NESMOR, which caused growing SSOs.