| Presently, the proportions of large units of hydropower or thermal power is increasing. In some specified operating conditions, such as isolated island operation with load, DC isolated island operation and asynchronous interconnection operation, the character controllers and operation modes of those large capacity units may effect the stability and security of power system, The low frequency oscillation problem is one of important aspects, so it is necessary to implement a research in term of this phenomenon.This paper researches a problem of damping very low frequencies oscillations(VLFO) by installing damping control in governor side, which relates to the speed governing system. Firstly, this text analyzes the mechanism of the Power System Stabilizer(PSS) in the field side, and its disadvantages, in view of the water hammer problem and the incapability of Governor Power System Stabilizer(GPSS), a compound control consisting of a GPSS on the governor side and a feed-forward, the variation of angular velocity of feed-forward is △ω so this control can damp VLFO problem related to the speed governing system better. Secondly a generator infinite system model, a hydropower isolated grid operation model and a four-machine two-area system model are simulated to analyze, Results of simulations shows that the strategy of compound damping control in the governor side can restrain the VLFO more significantly compared with PSS in the field side.
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