There exists serious governor control instability in the islanded operation system of large capacity UHVDC with its matching hydraulic power stations. Yunnan-Guangdong UHVDC islanded system also has ultra-low-frequency oscillation(ULFO)phenomenon because of the inconsistent control features between UHVDC and frequency regulation of generators. The ULFO analysis model is put forward based on the complete coherence characteristics of all generator rotors. There exists a frequency band of negative damping in the hydraulic turbine control system by frequency domain analysis. The larger value of turbine 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. An effective frequency control strategy for UHVDC islanded system frequency control is proposed by withdrawing a part of primary frequency control functions of generators or coordinating the dead zone values of governors and UHVDC frequency limitation controller(FLC). By the proposed strategy

islanded system frequency is mainly controlled by UHVDC FLC

which can successfully suppress the ULFO problem. Verification simulations are carried out and the simulation results prove the effectiveness of the proposed frequency control strategy.