Application of variable speed operation on Francis turbines

Francis turbines that are directly coupled to a synchronous generator operate at constant rotational speed around a design point characterized by a given water head, flow and guide vane aperture. When important changes occur in headwater level in power stations with large reservoirs, the turbines suffer a significant loss of efficiency. By applying variable speed technology it may be possible to adapt the runner speed and to operate with a higher efficiency over a wide range of water heads. This investigation is intended to reveal the possible benefits of using variable speed operation in regard to gains in efficiency and power output. Based on model test data it is possible to determine the characteristic curves of unitary speed and unitary flow of the respective prototype turbine for varying guide vane apertures. By varying rotor speed it is possible to maintain values that correspond to maximum efficiency. An analysis is made keeping guide vane aperture constant and introducing a proportionality factor of water flow to corresponding power output. The results show that for guide vane apertures and heads different from the design point, best efficiencies can be kept by adjusting rotor speed. At heads lower than the design head, significant efficiency gains can be achieved. Consequently, a significant proportion of the flow can be saved while generating the same amount of power.