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The asynchronous interconnection between the Southwest China power grid (SCPG) and Central China power grid (CCPG) reduces the inertia of rotation of SCPG to 1/5, which leads to a prominent risk of frequency stability. Moreover, in order to suppress the ultra-low frequency oscillations, the parameters of hydropower governors were optimized, which further worsen the frequency regulation capability of the system. Therefore, the frequency control (FC) of high voltage direct current (HVDC) line along with automatic generation control (AGC) are designed to improve the frequency restoration after a large power disturbance. However, the characteristic of FC regulation is contradictory to the subsequent AGC regulation. In order to improve the post-contingency frequency response of power system, an improved AGC strategy is developed. The proposed strategy involves the FC regulation power and distribute the control power according to the characteristics of different generator units. Case study on the actual model of SCPG proves the effectiveness of the strategy.

Southwest China power grid (SCPG) and central China power grid are planned to be unsynchronized by using back-to-back voltage source converter based high voltage direct current transmission between Chongqing and Hubei power grids. After asynchronous interconnection, the proportion of hydro in installed capacity is about 70%. In other words, the SCPG is a typical hydrodominant power system. Due to the negative damping effect of governors of hydrogenerators, there is a serious risk of ultralow-frequency oscillation (ULFO) in the range of 0.05 Hz. In this paper, optimization strategies of governor for hydropower unit are presented to eliminate ULFOs. First, the control mode is selected based on comparing damping characteristic of different control modes of governor. Then, a relevant coefficient index is proposed to select sensitive generators of ULFO. Afterward, PID parameters of governor are optimized based on a new performance criteria considering the damping characteristic of governor. Finally, optimization strategies are presented and applied to SCPG. Simulation results show effectiveness and practicability of the proposed measures, which have been adopted and will be implemented before the asynchronous interconnection.