Effect of water inertia in penstocks on regulating characteristics of hydraulic units

Abstract

In recent years, in the design of divided-fall hydroelectric plants with long penstocks there has been a tendency toward elimination of surge tanks, in order to reduce construction costs. In this connection, to reduce the magnitude of the water hammer to an allowable value when the loads are dropped, the closure time of the turbine control is increased; this leads in turn to an increase in the maximum value of the rotative speed of the unit. Since, in general under present-day conditions, cutting-off of a unit from a power system is not attended by operation of a hydroelectric plant with an internal load, the allowable increase in the rotational speed is determined only by the strength of the generator rotor. The current norms GOST 5616-63 stipulate that the generator rotor should be sufficiently strong for a maximum speed not less than 1.75 times the rated speed. From this condition, the closure time of the turbine control can be increased.The elimination of surge tanks leads to an increase in the travel distance of the impact wave in the penstocks, which entails an increase in the inertia of the water mass enclosed in the high-pressure stretch. This inertia is characterized by the rime constant of the inertia for the penstockin which L i and V i are, respectively, the length and velocity of the flow at different sections of the high-pressure stretch.The first experience in our country with construction of hydroelectric plants having increased values of T w and of the closure time of the turbine controls was provided by the plants in the Kuban' system. The results of tests at the first two hydroelectric plants in this system indicated that increasing the length of the penstocks does not cause an increase in the pressure pulsations in the internal passage of the turbine or a deterioration of the vibration characteristics of the unit; from this standpoint, it is entirely admissible to increase the length of the penstocks. This circumstance provided grounds for revising the norms stipulating the values of T w for design of hydroelectric plants. The current norms permit using T w = 5 sec and over [1] for hydroelectric plants whose capacity does not exceed 2~o of the capacity of the power systems; formerly, values of over 2 sec were seldom used for T w.Of great practical importance, in the light of the foregoing considerations, is an examination of the question of the effect of the value ofT w on the regulating characteristics of hydraulic units under different operating regimens. A well-known approach is Professor G. I. Krivchenko's method [2] of quantitative evaluation of the time response of variations in the output unit, according to which the time response is evaluated from the equivalent time lag At e of the output (Fig. 1). This criterion was the basis for full-scale evaluation of the regulating characteNs'tics of the units at the Kuban' hydroelectric plants.The rate ofvatiationof unit output is of great significance under emergency conditions. Let us try to establish the applicability of the above-mentioned ...

Modelling of a Hydro Power Station in an Island Operation

“…The objective of the surge tank is to absorb the pressure and therefore take care of the sudden pressure rise in the pressure shaft, like has been stated. Omitting the surge tank leads to an increase in the travel distance of the impact waves in the conduit which causes increase in inertia of the water mass (Kiselev 1974).…”