Introduction. Instability of pump-turbines during start-up in turbine mode

Instability of pump-turbines during start-up in turbine mode

References

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[References 4 – 28 are omitted]

Thomas Staubli, Florian Senn, Manfred Sallaberger

During the last decade the deregulation in the European electricity market has resulted in rapidly changing conditions on the market. Due to the growing demand for balancing power and frequency control an investment in increased pumped storage capacity became economically feasible. Reversible pump-turbines seem to be in many cases the most cost-effective solution. Occasionally torque fluctuations of reversible pump-turbines are encountered in power plants during start-up in turbine mode operation. Such fluctuations can slow down the process of synchronization what is highly undesirable when fast peak power production is required. During start up there is practically no load on the turbine shaft and the turbine operates close to the runaway characteristic. The guide vanes are opened only a few degrees during this phase.

A first case study of such oscillations on a model pump-turbine was presented by Yamabe [1] and [2]. He observed oscillations with pronounced hysteretic behavior which interacted with unsteady cavitation patterns. A case study and a simple cure of the problem by detuning some guide vanes are given by Klemm [3]. A linear stability analysis to predict the occurrence of the oscillations was successfully introduced by Martin [4] and [5]. Also Doerfler [6] presented a case study on how stable operation could be achieved in spite of the instability at no load.

Recent experiences with single stage reversible pump turbines are published by Billdal and Wedmark [7]. They propagandize multiflow guide vanes (MGV) to overcome difficulties with synchronization and to obtain stable speed after load rejection. All authors agree that the so-called S-shape of the four quadrant characteristic of the pump turbines is responsible for the oscillations at no load operation.

[Some details are omitted]

In the following a numerical study will be presented which focuses on the prediction of the characteristic near runaway and on the flow phenomena leading to the instability. To do so, tools were developed to analyze local and time-dependent flow, momentum and energy exchange in each of the runner and guide vane channels and in the vaneless spaces. For validation of a model of a reversible pump-turbine with a known unstable behavior and well documented model test data was chosen.

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