Modelling the Interaction of Air Cushioning & Unstable Surge in Francis Turbine during Load Rejection

During the full load rejection test of francis turbine in hydroelectric power plant the negative pressure in draught tube lower than the permissible limit can be appeared. Also in the hydroelectric power plants with long tailrace tunnel the unstable surge can be appeared in draught tube. The physics of this Phenomenon is explained in this paper and it is showed that the equations of classic theories of water hammer are not true for analysis of the said phenomenon and based on fundamentals of applied thermofluids, a new model is constructed and introduced. The full load rejection test to be performed according to the recommendation and instruction based on test simulation report. Subject to to inefficiency of the instruction of the turbine’s manufacturer, the optimized method for closing of the guide vanes during load rejection test to be specified at site by adjustment of governor. There are few softwares for simulation of load rejection test. One of these programms is computer program of SIPROHS which has been written by the university of stuttgart in Germany based on the order from Voith Company. If the load rejection tests performed after simulation and adjustments based on new closing laws and the measurement results showed that the unstable surge problem is still remaining, the air injection to draught tube to be done for reducing the surge intensity. The reason of the difference between the classic simulation values and the test values is incapability of the classic Softwares(e.g. SIPROHS) in modeling of the occurred unstable surge phenomena in draught tube connected to long tailrace tunnel. The interaction of admitted air cushioning and unstable surge is modeled based on physics of phenomenon.

Modelling the Interaction of Air Cushioning & Unstable Surge in Francis Turbine during Load Rejection Keywords:

Load Rejection , Francis Turbine , Unstable Surge , Tailrace Tunnel , Draught Tube