Publications
Finite element analysis of filled epoxy encapsulated high voltage transformer
Encapsulation of high voltage transformers can be a difficult undertaking. Stresses arise due to the coefficient of thermal expansion (CTE) mismatch of the components. Due to the viscoelastic nature of the encapsulation, these stresses can change over time. Excessive tensile stress in the ceramic cores results in cracks which can affect the performance of the transformer. The transformer that is the subject of this paper performed well after manufacturing and an initial thermal cycle; four years later however, the same transformer failed during the heat-up portion of a similar thermal cycle. X-rays revealed a large crack in the ceramic core. This paper summarizes the elastic and nonlinear viscoelastic finite element modeling that was done in support of the failure investigation and redesign of the transformer. In both the elastic and viscoelastic finite element models, the maximum principal tensile stresses at the low temperature condition of the thermal cycle exceeded the estimated ultimate tensile strength of the core material. At room temperature, the models predicted that the maximum principal tensile stresses were sufficiently high to produce subcritical crack growth in the core material. The viscoelastic model indicated that the core could experience a significant increase in stress due to physical aging of the encapsulation. Modeling stresses compared well to the cracks found in the failed transformer. The final design utilized a silicone coating applied to the interior surfaces of the cores. The coating acts as a stress relief layer that decouples the high CTE encapsulation from the ceramic core. The addition of the silicone coating resulted in a significant stress reduction. X-rays of transformers made with the silicone coating reveal no cracks in the cores.