Influence of Rotational Speed on the Frequency Response Analysis of the Field Winding of Large Hydrogenerators

Frequency Response Analysis (FRA) is a well-known technique used for power transformer condition monitoring for more than 40 years. Some research lines use this technique in other machines such as synchronous machines. This paper presents onsite tests on the field winding of a 125 MVA synchronous machine installed in a pumped storage power plant. The tests have been accomplished in standstill and rated speed conditions. The main contribution of this article is to demonstrate that the speed of the rotor has a significant influence on the field winding FRA test results. A comparison between standstill condition and rated speed FRA tests is presented. The frequency range where the differences take place indicates that centrifugal forces at rated speed change the capacitance of the field windings.