Back to the future

20 October 1999

Going back to the basic principles of Maxwell's equations, ABB has developed Dryformer, a dry power transformer based on cylindrical windings.

Following on from the development of the revolutionary Powerformer, ABB has developed a new transformer, Dryformer. The Dryformer contains no oil, with the electrical system being made entirely out of solid dielectric, cross linked polyethylene (XLPE). The first Dryformer unit will be installed later this year at the Lottefors hydropower plant, 200 km north of Stockholm.

The windings in Dryformer are made from dry polymer cables with cylindrical conductors. A cylindrical conductor gives an even electric field distribution, as can be derived from Maxwell's equations. Powerformer had initiated the use of cylindrical conductors, even though square conductors were easier to produce.

Without oil in the transformer, and with controlled exposure to electric fields in the surrounding area, many of the limitations that reduce the availability of site location disappear. Personnel safety is notably higher for Dryformer than it is for a conventional transformer. The only exposure to voltage is at the cable terminations, which can be kilometres away from the transformer. With no oil, the risks of fire or explosion are minimized.

Fire risks

The amount of inflammable material in the Dryformer is significantly less compared with a conventional transformer, as shown in Table 1 below. Dry cable terminations, with outer insulation of silicone rubber, replaces traditional porcelain bushings. There are, therefore, no dangers from cracked porcelain or oil vapour.

It has also been demonstrated that the flammable material used in Dryformer has less tendency to develop a fire than the material used in a conventional transformer. This was demonstrated experimentally by studying the effect of a major phase to phase insulation failure. Two 45 kV cables with an outer diameter of 30 mm were mounted in a test rig and allowed to touch. A 40 kA current was injected through deliberately damaged insulation for 100 ms. The copper conductor was vapourized, and some vapourization and deformation of the XLPE took place. The experiment demonstrated that no fire developed. The vaporized insulation is almost entirely combusted. CO2 is the major product, with only small amounts of CO and hydrocarbons formed.


When an internal arc develops within the winding, considerable energy is released. The high internal pressure built up in connection with an arc may lead to a tank rupture in an oil-filled transformer.

In Dryformer, the energy is taken up by the XLPE cable, which stretches, and the air surrounding the unit. Most importantly, the wall surrounding Dryformer can be made to deform in a controllable fashion, directing the energy away from areas where personnel may be located.

Changing from liquid insulation, transformer oil, to solid insulation, XLPE, changes the failure mode dramatically. Pressure build up occurs in a quite different manner in a liquid, which is incompressible compared to a gas. In liquid, a shock wave rapidly propagates to the tank wall, which may subsequently rupture. For the same source energy, the force at a given point from the source is much less for the case involving air than for the case involving oil.

Design principles

The Dryformer is based on a spherical design, which distributes stress resulting from heat and vibration evenly around the structure. It also makes the flux path of the electric current smoother.

ABB has produced two versions of Dryformer: a 20MVA, 6.6kV unit for indoor installations, and a 16MVA, 11kV unit for outdoor use.

As with all transformers, losses are minimized by optimization of the transformer's design and selecting the best possible materials for the electrical components. One way to minimize losses is to site the transformers as close as possible to the point of energy consumption in order to get the benefits of high-voltage transmission for as long as possible.

Life cycle assessment

During the development of Dryformer, the goal was to design a transformer with as low environmental impact and resource consumption as possible, seen from a life cycle perspective. For this purpose, a life cycle assessment (LCA) was carried out. The study compared the environmental impact between Dryformer and a conventional system at a rated output of 40 MVA, 69/24 kV. The systems considered were:

The environmental impact during operation is indirect, resulting from electrcity losses in the apparatus. For Dryformer, the calculations carried out have indicated that the average unit would have, over a 30-year life, have no-load losses of 28.5 kW, and load losses of 104 kW. For the conventional transformer, the no-load losses are 19.5 kW, and the load losses 165 kW.

When the product has served its time, it is assumed that all metallic materials in the systems are recycled and the polymers and the transformer oil are incinerated. Dryformer is, in general, easier to recycle. One of the reasons is fewer components.

The main differences between the systems are in the effect on aquatic ecotoxicity (ECA), due to the absence of oil in Dryformer, and the risk of oil spillage for the conventional transformer. A percentage of important substances contribute to the ECA. For this specific case, a transformer oil leakage in the conventional system corresponding to 1 per cent of the oil volume during a 30 year lifetime was assumed.


To acquire a total reliability picture of the transformer, failure rates for individual components must be collected. The statistical reliabilities of each component are combined, and a failure rate for the total transformer can be established, as shown in Table 2.

As the components used in Dryformer have a long history, it is possible to predict the expected reliability.

Overload capacity

Overload capacity of conventional oil insulated transformers is limited by the life consumption of the oil-impregnated paper surrounding the conductors, and by the life of the oil itself. These transformers are designed to have hot spot temperatures not exceeding 98OC. The maximum temperature is 140OC.

For Dryformer, the maximum permitted overload is not limited by ageing of the insulation system, but by the temperature above which softening of XLPE reduces mechanical strength of the winding. The design of Dryformer permits an upper temperature limit of 80OC for limited time operations, and 70OC for continuous load. At these temperatures, ageing of XLPE is negligible. Thus Dryformer can reach maximum temperature without any loss of working life. The hot spot in oil insulated transformers is calculated from winding mean temperature rises, but the hot spot in Dryformer windings is directly measured from the top of the unit. This enables a more accurate control of the hot spot to be carried out than for oil-insulated transformers.

To compare Dryformer overload capacity to that for oil-insulated transformers, some common overload cases have been applied.

Case 1: This is a short duration emergency case suggested by Vattenfall. The normal 70 per cent load is interrupted by a 140 per cent overload for 0.5 hours.

Case 2: This is a repeated (24 h cycle) overload case that is taken from the IEC354 loading guide, involving a 70 per cent load lasting 22 hours, and interrupted by a 134 per cent overload lasting 2 hours. This case is designed to give a normal life consumption for the oil-insulated transformer.

Case 3: The overload (at 30OC ambient) at which windings reach steady state hot spot temperatures of 70OC is calculated. This is done to show how much overload can be permitted continuously without affecting the mechanical properties of the windings at all.

The resulting hot spot temperatures for the different cases are shown in Table 3 below. These show that Dryformer can be operated at these overload conditions without reaching dangerous temperature levels.

World's first solid insulation distribution transformer


Table 1 Comparison of total flammable materials
Table 2 Failure rate comparison
Table 3 Overload case comparison
Life cycle assessment

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