One evening quite a few years ago, as I was driving through my hometown I saw the telltale flashing lights of the local volunteer fire department ahead. I passed by a side road where all the activity was: a utility pole on fire. I could see smoke and flames shooting from the transformer and I could hear the loud, angry 60 Hz buzzing that sounded like a million hornet nests. As I passed, the transformer exploded and released a cloud of flaming liquid that rained down on the road and lawns underneath. It seemed like a good time to quit rubbernecking and beat it as fast as I could.
I knew at the time that the flaming liquid was transformer oil, but I never really knew what it was for or why it was in there. Oil is just one of many liquid dielectrics that are found in a lot of power distribution equipment, from those transformers on the pole to the big capacitors and switchgear in the local substation. Liquid dielectrics are interesting materials that are worth taking a look at.
Same Thing, Different State
If you know what a dielectric is, you’ve got a pretty good idea already of why a dielectric in a liquid state would be a useful thing. A dielectric is just a material that doesn’t conduct electricity but can be polarized by an electric field. So dielectrics are just special cases of insulators, where the properties of the material make it easy for charges to separate but not flow. Therefore, all dielectrics are insulators, but not all insulators are dielectrics.
Liquid dielectrics are just dielectric materials in the liquid state, and maintain all the properties of the solid dielectrics commonly found in the capacitors we’re all familiar with. But for practical purposes, the more useful property of liquid dielectrics is their insulating ability as opposed to their charge separation properties. In fact, electrical insulation is probably the most important property of a liquid dielectric, at least as far as their use in power distribution gear.
That transformer I watched explode was likely filled with mineral oil, which is probably the most common liquid dielectric in use today. Mineral oil is just highly refined petroleum – baby oil is perfumed mineral oil – and it’s used in transformers because of its high flash point and excellent insulating properties. Its primary job is to conduct heat away from the windings by convection; some large transformers even have radiator fins to increase heat transfer to the atmosphere. The transformer oil also serves to exclude oxygen and moisture that would attack the interior components, especially the paper used to insulate the windings.
Another reason that electrical gear is often filled with oil is to limit corona discharge and quench arcing. Coronas occur when an electric field ionizes the fluid surrounding it. You can hear corona discharge under most high-tension lines, where the fluid being ionized is the air around the insulators. That’s not such a bad thing high in the air, but inside an enclosure it could lead to disaster. Liquid dielectric, with a lower relative permittivity (the preferred term over “dielectric constant”) than air, solves that problem. Liquid dielectrics are also used in the massive switches that control high-voltage systems, to damp the long, powerful arcs that can occur when circuits are broken in free air.
One interesting device that monitors the health of a transformer through its cooling oil is the Buchholz relay. This is an electromechanical device that sits in an oil-filled chamber plumbed to between the transformer and its radiator. It has two mercury tilt switches: one attached to a ball float at the top of the chamber, the other attached to a flap near the bottom where the pipes enter and exit. Minor transformer faults will decompose the oil and produce gas bubbles; if enough gas accumulates in the chamber, the float will drop enough for the mercury switch to make contact and trigger an alarm. Similarly, a major fault will produce a large slug of gas that will move the flap to trigger a circuit breaker to isolate the transformer and prevent further damage.
Mineral oil isn’t perfect, of course. As I saw dramatically demonstrated, mineral oil is flammable. The search for a better transformer oil in decades past led to the use of polychlorinated biphenyls, or PCBs, organic chlorine compounds that have excellent dielectric properties and are practically non-flammable. Unfortunately they’re also highly toxic; think Love Canal and Agent Orange toxic. Their production was banned in the 1970s, but a not insignificant number of transformers filled with or contaminated by PCBs are still hanging on poles.
Other substitutes for mineral oil in transformers and switchgear include expensive silicone and fluorocarbon oils, used where flammability is a safety issue, and even oils derived from plants, like castor oil and plain vegetable oil. The natural oils are more easily biodegradable than the mineral-derived dielectrics and work well where there’s a possibility for leaks. Considerable effort has been put into optimizing these natural oils with additives and to using them as the basis for synthetic oils with the desired properties.
[Featured image: Transformer oil under test, KEP Power Testing Blog]