Why Electric Utility Transformers Fail

As a key part of a utility’s infrastructure, transformers are the electrical devices that take electricity of one voltage and change it into another voltage. This change in voltage levels ensures that safer, lower voltage power is delivered from generating plants and transmission lines to commercial, industrial, and residential customers. Because transformers are installed on top of utility poles, on concrete pads in neighborhoods and shopping centers, under city streets, and in enclosed substations, they often go unnoticed by the public.

Utility risk managers and insurers, however, do take notice. They see transformers as the utilities’ most critical equipment because of the large amount of oil they contain, in contact with high voltage elements. To understand the risks posed, it helps to know something about how transformers are constructed.

Potential Danger Within

The metal tank of a transformer must be insulated from the internal live electrical assembly. This is almost always accomplished through a system of insulating paper and fluid, known as dielectric coolant. The fluid cools the transformer and provides part of the electrical insulation between internal live parts. Occasionally the insulation system breaks down, with increasing probability the older the transformer, and high temperature arcing occurs. Conventional petroleum-based mineral oils have a lower fire and flash point, which means that they can catch fire at standard operating temperatures. On the other hand, biodegradable bio-based transformer fluid, which is rated “less flammable,” has flash and fire points much higher than the standard operating temperatures. There have been no sustained fires due to arcing with bio-based fluid, even under “worst-case scenario” arc ignition testing.

To ensure that the insulating capability of the transformer fluid does not deteriorate, the transformer casing is completely sealed against moisture. Like any operating device, however, transformers can develop weaknesses over time. They can leak. They can be compromised by intrusion from animals and pests. They can be hit by lightning or by cars. Their insulation system weakens with heat and time. These conditions can all cause internal arcing, leading to transformer fires and spills. These cannot be dismissed as freak occurrences. In fact, it’s been estimated that in the United States, petroleum-based mineral oil transformer fires occur daily.

Add the fact that today’s competitive energy markets demand longer life and greater production from transformers, and the result is aging equipment and overloaded devices. That means higher susceptibility to leaks and faults.

And that can have a disastrous impact.