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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.

-Power plants are subject to introduce disturbances in the power grid, resulting from interactions with the dynamical behavior of the energy source subsystem. In the case of hydropower plants when used to compensate for variations of power generation and consumption, instabilities or undesirable disturbances may arise. They may be caused by phenomena such as part load vortex rope pulsations in the draft tube of Francis turbines. This may affect the dynamical behavior of the power plant and lead to troublesome interactions with the grid. This paper presents a case study of an existing hydropower plant that illustrates the effects of pressure pulsations due to vortex rope precession on the draft tube of Francis turbines. It also showcases possible solutions to the mitigation of the effects of this disturbing hydraulic phenomenon over the operation of the generators and electrical system. The investigated system is a 1 GW hydropower plant (4 × 250 MW units). The assessment of the power swings is performed through modal analysis combined with frequencydomain and time-domain simulations, which are then compared with on-site measurements.

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