Development of measuring techniques used in evaluating the insulation condition of large turbogenerator rotor windings

The authors briefly illustrate the main measuring methods currently developed and standardized by ENEL to search for insulation faults between turns. Such a category of faults is more insidious for the continuity of service because it is not directly detectable from the protection system during operation. The information obtained through impedance and reflectometric measurements supplies sufficient data to ascertain the existence and location of the fault. However these are not reliable enough to measure the actual state of degradation of the fault. All this has shown the need for further testing methods whose results could have a direct correlation to the actual energy lost by the fault during operation. This is the aim of recent experimentation on the spectral content of the shaft voltage