Governing the Turbine-Generator Unit of a Small-Scale Hydropower Plant with a Long Penstock

where GD is the flywheel effect; n is the rated speed; N is the capacity. Such a combination of parameters makes it a challenge to meet the head and speed rise limits (the waterhammer pressure no higher than 65 m and the maximum speed no higher than 160% of the rating) required by the manufacturer to guarantee stable governing (Fig. 1). In the 1950s, a surge tank was usually placed in the penstock if T W > 2 sec. Load rejection analyses show that when the wicket-gate closing time is 10 sec, the turbine rapidly overspeeds (Fig. 1a) reaching a speed of 2.03, which is higher than the runaway speed (1.99) because of a severe water hammer. Usually, the longer the closing time, the higher the maximum speed after load rejection, because the flow spins the turbine when the wicket gates close. In the case being considered, the situation is opposite because of a severe water hammer (up to 230%): the shorter the closing time, the higher the maximum speed. With increase in the closing time and decrease in the waterhammer pressure, the maximum speed decreases, tending to the runaway speed. It remains at a level of (200 ± 2)% for any closing time. As part of the general analysis of possibilities of ensuring stable governing, we will consider alternatives of variation in the parameters of the turbine-generator unit and the penstock, some of which are abstract. Calculations show that Power Technology and Engineering Vol. 49, No. 4, November, 2015