When evaluating the PCB status of a transformer, the oil in the main transformer tank is typically the primary fluid tested. However, older oil-filled transformer bushings can contain insulating fluid with PCB concentrations that differ from the oil in the main tank.
Under normal operating conditions, routinely sampling the oil inside a sealed bushing is generally not practical or recommended simply to determine its PCB concentration. Accessing the internal fluid may require opening or otherwise disturbing the bushing’s sealed insulating system. For this reason, PCB analysis of bushing fluid is more commonly relevant after a bushing has failed, leaked, been damaged, or reached the end of its service life.
The more important concern for transformer owners is what happens if fluid from a PCB-containing bushing enters the transformer.
Can a Failed Bushing Contaminate Transformer Oil with PCBs?
Yes. If an oil-filled bushing contains PCBs and leaks or fails in a way that allows its internal insulating fluid to mix with the transformer’s main tank oil, the PCB concentration of the transformer oil can change.
This can be particularly important for transformers that previously tested at a low or non-detectable PCB concentration. A previous transformer oil test reflects the PCB concentration of the oil at the time the sample was collected. It does not account for PCB-containing fluid that may later enter the tank from another component.
A bushing failure, leak, or other event involving the release of bushing fluid can therefore create a reason to reevaluate the PCB status of the transformer.
Why Older Transformer Bushings Can Be a PCB Concern
Bushings are separate transformer components with their own insulating systems. Some older oil-filled bushings may contain insulating fluid that has a different PCB concentration than the main transformer tank.
Over the transformer’s service life, the main tank oil may have been replaced, processed, reclaimed, or retrofilled while the original bushings remained in service. As a result, a transformer with low PCB concentrations in its main oil can potentially have older bushings containing higher concentrations.
This difference may not become significant until a bushing leaks or fails and its fluid has the opportunity to mix with the main transformer oil.
When Should Bushing Fluid Be Tested for PCBs?
PCB testing of bushing fluid is generally most relevant after the bushing has been removed from service, damaged, or failed and the fluid can be safely accessed.
Testing at this stage can help determine whether the bushing contained PCBs and provide useful information when evaluating a leak, failure, cleanup, component disposal, or possible contamination of the transformer.
PCB concentrations in electrical insulating liquids can be determined using ASTM D4059 laboratory analysis.
Retest Transformer Oil After a Bushing Leak or Failure
If an oil-filled bushing leaks or fails and there is reason to believe its fluid may have entered the main transformer tank, the transformer oil should be retested for PCBs.
Even if the transformer oil has previously been tested, a new sample provides an updated PCB concentration following the event.
Retesting can help transformer owners determine whether the bushing failure changed the PCB concentration of the main insulating oil and provide the information needed for subsequent maintenance, fluid handling, repair, or disposal decisions.
Previous PCB Results May No Longer Tell the Whole Story
PCB testing provides information about the fluid that was sampled at a specific point in time. A transformer that previously tested below a particular PCB concentration may not necessarily remain at that concentration if fluid from another component enters the tank.
For older transformers with oil-filled bushings, a bushing leak or failure is therefore an important event to consider when reviewing historical PCB results.
If contamination is possible, collecting a new transformer oil sample is the most direct way to establish the transformer oil’s current PCB concentration.
PCB Analysis for Transformer Oil
MVA Diagnostics provides PCB analysis of electrical insulating liquids using ASTM D4059 methodology.
If your transformer has experienced a bushing leak or failure, or you need to confirm the PCB concentration of the transformer oil following maintenance or equipment damage, request a customized transformer oil testing quote to discuss your PCB testing needs.