Investigation of rotor hotspot temperature using Fiber Bragg Gratings

Outward migration of the turn insulations of several poles was observed on every generators in one of Hydro-Quebec's power plant and was attributed to thermomechanical stresses. The concern for these rotors is more related to consequences of such degradation and to decide what should be the next course of action considering that the units have been operating for just over thirty years out of 50 years of expected life. If shorted turns start to occur and cause magnetic, thermal or mechanical problems, this may lead to unplanned outage. A prudent alternative would be to re-insulate the poles from class-B to class-F insulation. However, since the actual hotspot temperature is not known and could be above the 155°C of maximum for class-F insulation, the problem could manifest itself again. Therefore, determination of the hotspot temperature was the primary goal of this investigation. Fiber Bragg Gratings (FBG) were used in conjunction with an optical rotating joint to carried out measurements of four of the poles of one unit in the plant. The results of this investigation are presented, highlighting the benefits of carrying localized on-line rotor temperature measurements.