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Lottefors goes dry

The world’s first dry oil-free, cable-would power transformer has been inaugurated at a utility in Sweden. PEi analyses the technology and its implications for the power market.

In July last year, ABB Transformers introduced, Dryformer, a high voltage transformer that does not need any oil for cooling or dielectric insulation. The cable technology is the same as that used in its ground breaking Powerformer which was launched in 1998.

The first unit has now been installed at the Lottefors hydropower power plant near Arbra in central Sweden and is expected to demonstrate a number of benefits to utilities which until now could not be considered.

ABB first began working on the idea of a dry transformer in the mid 90s in response to the industry’s needs for a more environmentally friendly, safer transformer.

The company began talking to Swedish utility Birka Kraft at the beginning of 1998. The involvement of the utility allowed ABB to benefit from the customer’s viewpoint at an early stage.

The technology

According to ABB, Dryformer represents a technological revolution and is part of ABB’s technology shift in the high voltage field. Its use of a cable with cross-linked polyethylene (XLPE) polymer insulation in place of paper-insulated copper conductors, has its roots in an idea conceived by Professor Mats Leijon, a researcher at ABB in Sweden.

This technology was first applied in the high voltage generator, Powerformer, which was launched in 1998. Powerformer adopts the revolutionary idea of using windings with circular conductors instead of traditional stator windings with rectangular conductors.

A cylindrical conductor gives an even electric field distribution as can be derived from Maxwell’s equations.

The result is a generator that produces power at grid voltage levels and consequently the omission of the need for a step-up transformer. This in turn reduces electrical losses by 30 per cent.

Research on the “Dry Transformer” first started in the Spring of 1996. The difference, compared with conventional oil-cellulose insulated transformers, is that the windings in the Dryformer are made from dry polymer cables with cylindrical conductors.

In a high-voltage polymer cable, the electric field remains inside the cable while its surface is held at earth potential. The magnetic field however, which is needed for the transformer’s function is not adversely affected by the cable.

In a transformer, the cable technology opens up completely new possibilities for the optimal design of electric and magnetic fields, mechanical forces and thermal stresses. This enables the designer to better meet specific customer demands.

During initial development at ABB Corporate Research, a test transformer – a 10 MVA, single phase unit – was designed, built and tested at ABB Transformers in Ludvika, Sweden.

The Dryformer is currently available for primary voltages between 36 kV and 145 kV, and for power ratings up to 150 MVA.

Modernising Lottefors

The unit installed at Lottefors, which serves as a step-up generator transformer, is rated at 20 MVA and 140 kV. It was inaugurated in January of this year. Despite being a small (13 MW) power plant located in the heart of a Swedish forest, Lottefors has been periodically revamped to become a very modern plant. A revamp in the 1980s led to the installation of modern remote controlled turbines and in 1996, the complete control system was replaced. The power plant is now fully automated and controlled via satellite.

Electricity from the plant is transmitted via underground cables and the step to install Dryformer and a compact urban substation was seen as a natural progression. The old switchyard and 70 kV overhead lines at the power station can now be demolished.

Birka Kraft chose Dryformer because it does not need any oil for cooling or electrical insulation. This eliminates the risk of oil leaking into the soil or ground water. It also eliminates the risk of fires and explosions.

This environmental friendliness is the key feature of the new transformer. ABB has made a complete life-cycle analysis showing the environmental impacts over the unit’s whole life and compared it with conventional oil-insulated transformers. The results show that Dryformer can be recycled to the same extent as a conventional unit.

With the elimination of oil and the controlled exposure to electric fields, Dryformer brings new possibilities in terms of where it can be located. The only exposure to voltage is at the cable terminations, which can be kilometres away from the transformer.

Dry cable terminations with silicon rubber outer insulation replace porcelain bushings. This means there is no danger from cracked porcelain or oil vapour.

The reduced amount of flammable material consequently reduces the need for extensive fire fighting equipment. This in turn means that the units can be sited indoors or in highly populated urban areas.

According to ABB, Dryformer is easier to maintain than conventional transformers and has higher availability. The only requirements for safe operation are thermometers in the windings and control of the cooling equipment. The traditional transformer’s oil pit, oil level gauge, gas alarm and oil thermometer are completely unnecessary. This simplifies installation, which only involves making the cable connections and installing the cooling equipment – with no oil related components.

Apart from the windings, the cooling system for the Dryformer is the other major component that is considerably different from that found in a conventional transformer. Two separate fan systems are available to draw air through the layers of the winding to maintain the correct temperatures. It is designed so that if one fan system fails, the other has the capacity to cool the transformer. If a total power failure occurs at the station, there is no danger to the insulation as the main source of heat, the losses, are not generated. Also, because air is the cooling medium (which is continually replenished) no degradation of that medium occurs.

First installation

The first installation at Lottefors went smoothly and has operated as expected since the end of 1999. On the back of this success, the second dryformer unit will be installed at another swedish hydro power station shortly.

ABB is continuing to develop Dryformer together with customers such as Birka Kraft. During the first years, it will be aimed at locations such as city centres and areas which are environmentally sensitive.

Further technical development work is also being done on a dry tap-changer, improved cable terminations and alternative cooling arrangements. But ABB sees its most important work in developing and adapting Dryformer, along with utility customers, for its function on the grid and its role in the substation.

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