Influence of Moisture on Fault Diagnosis for Rotor Poles of Large Synchronous Machines Using Frequency Response Analysis

Frequency Response Analysis (FRA) is a well-established technique for the condition monitoring of power transformers. The use of FRA for rotating electrical machines is nowadays a prominent research area. In previous works, the use of FRA for synchronous machine field winding diagnostics has been studied, but some key factors remain uncertain in this application. The moisture content is one of the uncertainty factors that could be envisaged as potentially influential. Therefore, the influence of moisture content on the FRA response of poles of synchronous machines is addressed in this paper. For this purpose, numerous experimental tests have been carried out on an extracted pole of a 40-MVA hydro-generator, with different moisture contents, considering healthy state, ground faults, and interturn faults. The main contribution of this work is to prove that, in contrast to what occurs in ordinary insulation testing, the moisture content does not have a significant influence on the FRA response of the pole, discarding this method for moisture diagnostics. However, this fact makes FRA very interesting for in-situ inspection, as it is independent of the moisture content. This way, it is not required to dry the machine before testing.