Powerformer, record performer

In 1998 the very first Powerformer was commissioned in the Porjus Hydro Power Centre in the northern part of Sweden. This first machine has a rated voltage of 45 kV, a rated capacity of 11 MVA and a rated speed of 600 rpm. It is connected directly to the 45 kV busbar without any step-up transformer. ABB Generation is now carrying out an extensive test programme to verify the performance of the concept. So far the test results verify calculations or show that Powerformer achieves better performance than expected.

In 1998, Eskilstuna Energi och Miljö awarded ABB Generation a contract for the second Powerformer, to be installed in a new CHP plant in Eskilstuna in central Sweden. This turbo machine has a rated voltage of 136 kV, a rated capacity of 42 MVA and a rated speed of 3 000 rpm. It will be in commercial operation at the end of 2000.

The Porsi project

After the successful demonstration of the Powerformer concept, Vattenfall, the largest electric power utility in Sweden, decided to use the technology for the rehabilitation of a generating unit at its Porsi hydropower plant. This will be third Powerformer installation.

Vattenfall owns a number of hydropower plants in the central and northern part of Sweden. Many of these plants were taken into service before 1970. The improved reliability and lower maintenance cost of Powerformer makes it an attractive solution in Vattenfall's rehabilitation programme. Based on first hand experience of Powerformer, Vattenfall now wants to be able to broaden their experience of Powerformer and verify the performance for general application.

Porsi hydropower plant is situated at the river Lule älv about 120 kilometres NW of the town Luleå in the far north of Sweden. The gross head is 33 metres and the mean annual flow is close to 500 cubic metres per second. The initial construction in Porsi hydropower plant started in 1958 and the first two Kaplan turbine units were ready by 1962. A third generating unit was added during the 1980s. At that time a new three-winding transformer replaced the step-up transformer of the first unit and connected the third generator to the transmission network.

Now Powerformer will replace the first generator and the step-up transformer of the second unit. At the same time, Vattenfall will replace the first Kaplan turbine by a propeller turbine. This third Powerformer will have a rated voltage of 155 kV, a rated capacity of 75 MVA and a rated speed of 125 rpm. The rated power factor will be one. The work on site will start in May year 2000. The commissioning will be finalized early in 2001.

Advantages of Powerformer

As described in previous articles in ModernPowerSystems (March and May 1998) Powerformer enables direct connection of a generator to the transmission network without any intervening step-up transformer, with far-reaching consequences for the electrical industry.

The stator winding in a conventional generator has rectangular conductors. The shape of these conductors causes an uneven electric field distribution with high electric field strengths at the corners. Advanced measures are necessary to control the electric field in the end regions of conventional generators. The classical design philosophy has resulted in generators with a rated voltage no greater than 36 kV. However, the voltage of power transmission networks has reached much higher levels, so there is a need for step-up transformers to connect conventional generators to transmission networks.

The winding of Powerformer has cylindrical conductors, providing an even electric field distribution. The stator winding of Powerformer is based on technology used for power cables. Available insulation materials and production techniques offer reliable cables at operating gradients of the order of 10 kV/mm and even more. Such high electrical fields are not acceptable for conventional coil insulation based on mica and epoxy.

The idea of connecting generators directly to the transmission network is not new. Already in 1905 the efforts of Professor Mengarini and engineers of the Ganz Company resulted in two three-phase 30 kV, 5.2 MVA, 450 rpm and 45 Hz generators in the Subiaco hydropower station, Italy. The power was transmitted to Rome, a distance of 55 kilometres.

One remarkable step was taken in 1928 by Sir Charles A. Parsons and J. Rosen , when they built a three-phase turbo generator of 25 MVA, 33 kV and 3 000 rpm. At least eight such directly connected turbo generators were built in the years 1928-34.

Among the benefits of direct connection are: