Strong sub-synchronous control interactions caused by open-loop modal resonance between the DFIG and series-compensated power system

This paper examines sub-synchronous control interactions (SSCIs) caused by open-loop modal resonance in a series-compensated power system with a doubly-fed induction generator (DFIG). The examination employs a closed-loop interconnected model consisting of an open-loop DFIG subsystem and an open-loop electrical subsystem with series compensation. The open-loop modal resonance is created when two open-loop sub-synchronous oscillation (SSO) modes outside of the DFIG and electrical subsystems are close to each other on the complex plane. The analysis results indicate that the open-loop modal resonance may cause strong SSCIs between the DFIG and electrical subsystem, and thereby degrade the ability to dampen SSOs in the power system. The degree of damping degradation can be estimated by computing the open-loop resonance residue. The principle of open-loop modal resonance introduced in this paper is based on improvements to a previous modal resonance analysis involving the concept of near strong modal resonance. Here, the relationship between the open-loop modal resonance and the near strong modal resonance is evaluated to determine the mechanism by which DFIGs result in strong SSCIs from the perspective of open-loop modal resonance.