Rotor Pole Inter-Turn Insulation Deterioration Assessment in Hydrogenerator: Methodes Comparison Including SFRA

The deterioration of rotor pole interturn insulation is a significant failure mode in large salient pole generators. The failure mode in this case can lead to the short circuit between turns that can be considered as a stray current. This causes a reduction of magnetic flux, resulting in magnetic imbalances and vibrations, which can lead to costly unplanned maintenance. To address this, modern techniques have been developed to detect insulation failures by monitoring the rotor pole flux during operation either by stray or air gap flux. Additionally, several tests can be performed during maintenance outages. These include the voltage pole drop test, the Impulse Frequency Response Analysis (IFRA/Surge), and more recently, the Sweep Frequency Response Analysis (SFRA) has been getting interest. Other tests can also be conducted with the rotor poles removed from the generator. However, the results from these measurements often contradict each other, and the causes are not always clear. Factors such as the sensitivity of the tests, environmental conditions, and the specific nature of the inter-turn short-circuit (ITSC) in the rotor poles can all influence the outcomes. An investigation is conducted on a 310 MVA hydrogenerator that exhibits significant anomalies in the pole winding inter-turn insulation upon visual inspection were a lot of poles has shown migration of this insulation. In service, this generator also shows signs of vibrations. The study presents comparative various tests, including flux measurement, Surge, SFRA, pole voltage drops tests at different frequencies all with the rotor poles in situ including experimental measurements.