Impact Analysis of Power Outer-Loop on Sub-Synchronous Oscillation for VSC Systems

The integration of renewable energy sources with the voltage source converters (VSC) as grid-connected interfaces is increasing in power systems, however, the system stability is deteriorated due to acute wide frequency band oscillations. At present, frequency domain impedance-based research about the sub-synchronous oscillations (SSO) induced by the interactions of the VSC and AC grid mainly focuses on the loop reshaping of the phase-locked loop (PLL) and the current inner loop (CIL), which ignores the impact of the active power outer loop (AOL) on SSOs. In this paper, the impact of AOL on SSOs is investigated, with which the fact that a large integral coefficient of the AOL can enhance the system stability is revealed. Finally, frequency-domain and time-domain simulations in MATLAB are performed to verify the effectiveness of the AOL in suppressing oscillations.