Breathing new life into old transformers

Technology from ABB can extend a transformer’s lifetime by up to 25 years, using a drying process that can finish the job in four or five days without moving the unit.

Transformers are among the simplest electrical devices, and they come in all sizes – from a tiny coupling transformer inside a microphone, to an 800-ton phase-shifting unit that regulates power flow in national power grids.

The problem is that the paper and mineral oil insulating the interior of many transformers degrades over time. This leaves a residue of water and acid that begins to accumulate inside the transformer, and eventually accelerates its breakdown.

Without proper insulation, a transformer gradually loses its ability to withstand the rigor of daily operations and stressful events like short circuits.

But ABB has developed a new drying method that combines low frequency heating and oil filtration to save the insulation, and ultimately, the transformer.

When this method was used to dry two, 69-tonne, 200-MVA, 300/125/17 kV Nett transformers for the Norse utility Lyse Energi, each transformer’s lifecycle was extended by 10 to 15 years, a significant savings for the utility.

Working on site ABB extracted 70 litres of water from one of the transformers. Before processing, moisture content of the insulation paper was 2.5 percent. After drying it was 1.0 percent, comparable to the specs of a new transformer straight out of the factory. Transformer oil was reclaimed and the tank and cover, refurbished due to rust. Everything took place in the field with minimal disruption.

In the past, removing water from a transformer involved filtering the transformer oil repeatedly. But since most of the water is not in the transformer oil but in the active part of the transformer, removing water in this way could take several weeks.

The ABB low-frequency method heats the transformer core with a low-frequency electrical pulse that effectively pushes moisture from the insulation into the oil, where it can be quickly and safely filtered out. The high- and low-voltage windings can be set to different temperatures according to their varying heat tolerances.

ABB service engineer Paul Koestinger stands next to a 200-MVA transformer which was dried for utility Lyse Energie. Transformer oil is reclaimed and the transformer appears almost new, with a much longer life expectancy.