Abstract
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.
Highlights
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
The examined Francis turbine is used in a high-head hydropower plant, where distinctive changes of water head are less common
The results allow several conclusions. It is possible for most heads and most guide vane apertures to improve the Francis turbine’s efficiency by applying variable speed technology
Summary
In order to change the runner speed, the rotor velocity of the generator has to be variable. As only a limited change of the rotor speed would be necessary, a less expensive doubly fed induction generator (DFIG) and a power electronic converter with a lower converter rate in comparison to total machine rating is the most promising alternative. The rotor in contrast gets its voltage and frequency from an electronic power converter. It introduces a slip frequency AC field current, thereby allowing the rotor to spin faster or slower than the grid frequency. This change accounts for up to ±30% of the original frequency. As the DFIG rotor is directly connected by the shaft to the turbine runner, it is possible to vary the turbine speed
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