Impact of thermal cycling on high voltage coils used in marine generators using FEA methods

Premature failures of stator insulation account for a large percentage of repairs of marine generator systems. The failure mechanisms of such faults have been presented in many parts of the literature. Partial discharge activity, thermal degradation, thermal cycling, harmonics and transients are some examples of such failure mechanisms. Whilst there has been an insight into the failure mechanisms, there is still no definite answer to how these defects manifest in the first place. Most of the failures that have been identified within literature are on end windings, especially slot ends. Some failure mechanisms have also been linked with thermal cycling. Frequent and rigorous stop/start cycles stress coils by inducing mechanical forces between elements of the coil and housing owing to differential thermal expansion. This differential expansion is dependent on the rate of rise of temperature and also the different coefficients of thermal expansion of the materials. The present paper will evaluate the thermal degradation of insulation systems used on marine generators using Finite Element Analysis (FEA) methods. On board temperature measurements of stator coils during a high speed run are used as one of the parameters within the FEA simulations, to investigate if there is any risk of differential thermal expansions during such an operational cycle. Different ramp rates are also analyzed within the FEA simulations to understand the effect of uneven thermal expansions and the risk of material degradation of the insulation in coils on marine systems. A brief review of the standards available for thermal cycling and testing are also presented within the paper.