FEM-study on converter transformer failures in the Celilo HVDC converter station

Two converter transformer failures occurring at the Celilo terminal of the Pacific Intertie HVDC transmission system, (one during commissioning in 1989 and one in operation in 1995), caused considerable concern regarding the reliability of the remaining converter transformers. These failures and discoloration of bushing porcelains observed during maintenance demanded a review of electric field stresses. This was done using a FEM field simulator and building a model which most accurately reflects the bushing/transformer arrangement with regard to the shape and data of the various conducting and insulating materials. Study results show that factory tests do not necessarily cover actual service stresses. Also the magnitude of the stress within critical areas might exceed withstand capabilities.