UHVDC Islanded Operation System Ultra-low- frequency Oscillation and Its Countermeasures

There exists serious governor control instability in the islanded operation system of large capacity UHVDC with its matching hydraulic power generators. Ultra-low-frequency oscillations (ULFO) will happen because of the inconsistent control features between UHVDC and frequency regulation of generators. Using frequency domain analysis, there exists a frequency band of negative damping in the hydraulic turbine control system. The larger value of water starting time constant, the more serious negative damping exists in the frequency control system. And if the forward gain of governor becomes larger, the negative damping will be more remarkable. Effective frequency control strategies for UHVDC islanded system are proposed in this work by withdrawing a part of primary frequency control functions of generators or coordinating the hydraulic generator governors and UHVDC frequency limitation controller (FLC). Using the proposed strategies, islanded system frequency is mainly controlled by UHVDC FLC, which can successfully suppress the ULFO. Verification simulations are carried out and the simulation results prove the effectiveness of the proposed strategies.