Influence of Starts and Stops on the Aging of Hydroelectric Generator Stators by Thermal Cycling: Empirical Study and Accelerated Lifetime Model
Résumé
Due to the deregulation of the energy market and the integration of renewable energies, hydropower plant operators are faced with an increasing number of start and stops and load changes that can reduce the life of equipment. This paper proposes to assess the influence of the start and stop cycles on the ageing of stators in hydroelectric generators. In a first step, different modes of generator degradation are identified. The most affected component by start and stop cycles is the stator insulation,because of the thermal stress induced by these cycles, and the insulation default is considered to be the first cause of a premature end-of-life. The stator lifetime is first estimated using a Weibull analysis on the winding replacement dates recorded on a large number of units subject to a variable number of start and stop cycles per day. The results show that there is a significant difference in lifetime between installations subject respectively to a high or a lower number of start and stop cycles. As the degradation of the generators' insulation is mainly due to thermal stress, a model using Coffin Manson's law is then used to explicitly take into account this stress and to determine the acceleration factor that allows predicting the reduction of the stator's lifetime due to thermal cycling. The proposed accelerated model is used on actual temperature monitoring data and the results show that the value of the acceleration factor is greater than one and increases with the cycles frequency which means that the life of the generator stator decreases as the number of starts and stops per day increases.
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