Investigating Oscillations in Bulb Turbine Plant During Islanded Black Start Operation

This paper investigates the frequency oscillations displayed by a bulb turbine-based run-of-river (ROR) hydropower system under islanded operation. From a field test it was observed that these oscillations grow as the generator loading increases. These oscillations prevent higher loading of these generators (under islanded conditions) and hence reduce the grid restoration capacity for these individual generators. To investigate the problem, first, a transfer function model of the bulb-turbine system, sensitive to the effects of head variation, blade angle, and other non-linearities pertaining to turbine characteristics is derived. Next, by analyzing this transfer function model and by simulations, it is found that system inertia, hydrogovernor derivative filter time constant, and hydroturbine's unobserved damping and friction are among factors that are responsible for these oscillations. Also, through simulations it is shown that the same ROR system with same controls is non-oscillatory under grid connected condition but oscillates when islanded.