Transformers are electrical devices used to transfer power from one circuit to another through electromagnetic induction. Since their creation in the 1880s, transformers are now found everywhere, from compact devices used to charge our cell phones and laptops to industrial usage seen in manufacturing plants and power grids.
Along with their prolific use to electrify the world, transformer failures can occur even when the best industrial practices are used.
In 2012, the Institute of Electrical and Electronics Engineers (IEEE) published A Case Study of the Failure of Two 13.8 kV Primary Transformers. In this report, the IEEE uncovered that the resulting failures were destructive to the transformers and related electrical equipment line-up.
The first failure studied in the report occurred on 27 August 2007 involving a cast-epoxy 15kVA 13.8kV primary transformer. This control power transformer (CPT), fed one side of the line-up, failed on its primary winding side insulation. Turn-to-turn and layer-to-layer electrical shorting happened.
Interestingly, the protective “E” type primary fuse was not activated to clear the fault even though the transformer ultimately failed. The transformer had cast-epoxy inside. Investigation of this failure found the CPT failure progressed over time until the epoxy cast primary was overheated, boiling off its resin creating significant smoke, thus alerting the fire department.
The most likely failure scenario was a shorted turn in the winding due to insulation failure. As the temperature increased, the remaining insulation would fail and this would not require a substantial amount of electrical current to cause the failure. The extreme heat caused electrical arcing, progressively destroying the insulation and welding the wire together.
The IEEE study of this failure quoted from the manufacturer’s installation instructions for replacement CPTs, stating: “If a transformer develops a winding fault, in many instances the primary fuses do not clear the fault until the transformer has split open from internal pressure.”
In the second case study, a Metering Potential Transformer (MPT) failure that occurred on 17 December 2010 was also analysed. That failure involved a cast-epoxy 13.8kV primary transformer and was destructive to it and the related switchgear line-up.
As in the first case study, the “E” type primary fuse did not clear the fault. The failure of the MPT caused a short-circuit and ground fault at the MPT’s input contacts, melting and vaporising the contacts. That damage caused an arc flash and arc blast sufficient to mechanically destroy facility equipment, blowing the lower compartment front door off the enclosure holding the MPT and damaging the rear panels to that compartment.
Originally, in the instance of the first case study, when the failure happened, temperature rise was the likely initiator and proactive replacement of CPTs was begun. However, after examining the second case study failure it became apparent that an MPT is not a powerhandling transformer that would have a problem with temperature rising during usage. What became apparent is that degradation of the epoxy can cause catastrophic failures. The normal life span of transformers is 20 years and many have continued to work long after that up to 80 years of age.
Civil case involving a control power transformer
The concerns with cast epoxy breakdown were explored in a recent civil case. An electrical facilities building had a catastrophic failure of a CPT transformer that caused a fire and power outage. That failure caused fire and smoke damage to equipment at the facility. Initial examination revealed the fire originated in an electronic equipment cubicle in the building.
Closer examination revealed it was a CPT. That CPT showed substantial internal electrical arcing and ground faulting. An experienced engineering consultant specialising in transformer failures was retained to investigate.
The investigation found no evidence of long-term overloading or any system problems that would have caused external electrical activity to damage the CPT. The epoxy insulation in the failed CPT did not prevent electrical arcing as it should have. The expert’s enquiry further showed that inadequate epoxy insulation allowed arcing, heating the copper windings and transferring additional heat to the epoxy insulation. Long-term arcing inside the high voltage windings caused further breakdown of the epoxy insulation, creating a void that permitted ground faulting and ultimate failure of the transformer.
The intense, localised arcing that damaged and deformed the high voltage windings had not caused a long-term overload or an external fire. The plaintiff contended in the civil action that the CPT with inadequate insulation permitting electrical arcing was a manufacturing defect as well as a design defect under strict liability tort law. After extensive investigation, filing suit and discovery, the subrogation matter was confidentially resolved. In common law jurisdictions, tort is a civil wrong that unfairly causes someone else to suffer loss or harm resulting in legal liability for the person who commits the tortious act.
Observations and recommendations
As noted in the IEEE report, transformer failures put the entire connected electrical supply system at risk of failure. Improper epoxy insulation in transformers can cause fires. If you are faced with a transformer loss, engaging an experienced engineering expert is essential along with preserving related physical evidence. Failure to assess epoxy insulation as part of your expert’s fault tree transformer failure analysis will result in a costly burden for the utility to cover along with customers’ furore over being left in the dark.