Sensitivity analysis of transient flow of two parallel pump-turbines operating at runaway

Transient flow analysis in hydroelectric power plants is one of the most important issues for the prevention of undesirable pressure fluctuations in waterways. In this paper sensitivity analysis of hydraulic transient for two parallel pump-turbine units operating at runaway (unstable S-shaped zone of pump-turbine hill-chart) have been numerically investigated. The characteristics of Siah Bishe power plant located at the north of Iran, has been considered. Firstly, simultaneous operation of two units at runaway is simulated. For this purpose, the governing equations of transient flow are solved along the waterways using the method of characteristics and modified Suter transformations. Then, the sensitivity of pressure rise at the spiral case, water level oscillating at surge tank, rotational speed and flow rate and torque oscillation at unstable zone of hill-chart to moment of inertia, head loss coefficient of connection tunnel of surge tank, connection tunnel diameter and acoustic wave velocity has been performed. The results show that the moment of inertia has considerable effect on the maximum torque at unstable condition of runaway. Also, more moment of inertia can well postpone the peak of pressure and rotational speed. The maximum water level of surge tank and the maximum torque at unstable condition of runaway are strongly affected by head loss coefficient of connection tunnel of surge tank and connection tunnel diameter. By using different values for acoustic wave velocity, output conditions don't show any clear trend.