The need for this procedure was created when a transformer failed and high levels of PCBs were found. The PCBs were traced back to the transformer vacuum braces. The utility was faced with the monumental task of determining which braces in the more than 800 transformers on their system contained liquid. The original plan required drilling holes in each brace. This would have been effective but would have been extremely slow and very expensive. A quick, non-destructive, field-hardened procedure was needed.
SI fabricated a mock up and developed an inspection procedure utilizing an L-wave technique for detecting liquid inside the brace from the outside. The procedure was simplified to a pass-fail method. After successful laboratory demonstration at a utility facility, field tests were initiated. Liquid was detected in approximately 30% of the vacuum braces inspected. When liquid was detected, holes were drilled in the braces and samples were drawn and tested. Several braces were determined to contain higher than desired PCB levels.
Oil is used in transformers primarily as an insulating and damping medium. The transformer that failed was not known to contain high levels of PCB's. Its oil had been tested several times and found to be within acceptable levels. Subsequent to the failure the utility determined that during original factory construction the manufacturer weighted the hollow "box beam" stiffeners with PCB's to provide sound deadening. This was done after the unit failed to meet the guaranteed sound level. The utility had not been informed the beams had been PCB filled.
During manufacture, a vacuum is pulled on the transformer tank for oil filling. In order to keep the tank from imploding during this process, channel braces are welded to the outside of the tank. It was recently discovered that at least one transformer manufacturer occasionally filled the vacuum braces with dense PCB's in order to meet specified noise levels. This was a highly unusual practice and was not indicated on any documentation provided to the utility. As this vacuum brace volume is isolated from the main body, the PCB level in the main tank oil did not reveal the potential hazard in the braces.
For more information contact Structural Integrity Associates, Inc., 3315 Almaden Expressway, Suite 24, San Jose, CA 95118; Tel: 408-978-8200; Fax: 408-978-8964.