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Influence of large oscillations on stability of simple surge tank Wei, Jiin-Piau

Abstract

This study investigates the stability of a simple surge tank operating in conjunction with a turbine. The turbine is assumed to be governed for constant power output or for constant gate opening, depending on whether the instantaneous head acting on the turbine is greater or smaller than the rated head. Early investigations on the stability of oscillations assumed that the governor on a turbine ensured constant power output. In practice, the governor on a turbine can maintain constant power output only when the oscillations remain above the rated head and the full-gate position is not reached. For this case, a computer study showed that results of the investigation are in close agreement with the results of several investigators. A revised form of the Thoma's formula for surge tank stability with oscillations of large amplitude is proposed. When the head during the maximum downsurge falls below the rated head of the turbine the constant gate opening condition must be considered. A computer program was developed to study the stability of oscillations of large amplitude under constant gate opening condition. The transient conditions for three cases of load demand with initial steady state discharge of zero, one quarter and one half of the final steady state discharge were studied. The graphical form of the results allows the prediction of surge tank stability for oscillations of large amplitude for a simple surge tank. The graphs for all three conditions show that an increase in the rated head has a stabilizing influence on surge tank oscillations.

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