According to two units with hydraulic coupling in hydropower plants, the effect of expanded unit connection to low frequency oscillation’s damping was studied in detail. With the mathematical model of hydraulic system which accurately reflectes each vibration mode of water flow, and the established mathematical model of expanded unit connection deduced from the five order integrative stability model of generator and the interface model between generator and electric power network, the whole state equations to describe the hydraulic、 mechanical and electrical system were derved, furthermore, the effect of expanded unit connection to low frequency oscillation’s damping and stability was analyzed with a given case. The results indicated that, for a system with two units having hydraulic coupling, low frequency oscillation’s damping was increased by the effect of expanded unit connection, and the effect to low frequency oscillation suppression was identical for larger increased damping on one unit and smaller increased damping on two units, while the hydraulic coupling had less effect to the distribution of damping.
