The situation: Von Mises stress calculations on Hydro unit forced plant operators to run well below rated capacity of 57.5MW. Running at 55MW, the rotor was calculated to be exceeding the material limits of 8000psi shear and 13,333.33psi von Mises combined stress when running at capacity.
The solution: live torque monitoring of the shaft (as opposed to calculated values based on generator output) has proven itself a valuable tool in determining whether the unit is operating within specified shaft stress limits.
Measurements from the TDMS were analyzed and true shear stress on the unit was determined to be 7,186psi, well below the 8000psi limit. This gave the operator the option to waive the 55MW limitation and operate at capacity.