Damping Degradation of Power System Low-Frequency Electromechanical Oscillations Caused by Open-Loop Modal Resonance

This paper investigates the damping degradation of power system low-frequency electromechanical oscillations caused by the open-loop modal resonance. The investigation is based on a closed-loop interconnected model of a multimachine power system, wherein, the electromechanical dynamics of a synchronous generator being examined and the remainder of power system are modeled as two separate open-loop subsystems. The open-loop modal resonance is the closeness of an open-loop electromechanical oscillation mode of the synchronous generator to an open-loop electromechanical oscillation mode of the power system on the complex plane. Theoretical analysis indicates that when the open-loop modal resonance happens, the dynamic interactions between the synchronous generator and the remainder of power system are strong and degrade the damping of the low-frequency electromechanical oscillations. Test results in two example power systems show that the synchronous generator geographically far away took part in the local low-frequency electromechanical oscillations when the open-loop modal resonance occurred. The macroscopic effect of the open-loop modal resonance was the expansion of area on the complex plane, where the electromechanical oscillation modes were located, when the scale of the power systems increased.