Powerformer - four years on

Four of the Powerformer reference projects to date are in Sweden, while in August 2001 orders were placed for a machine in Japan and one in Canada. With the exception of the biomass-fuelled Eskilstuna CHP plant, all are in hydro power applications.

In the Powerformer concept, round XLPE high voltage cables are used in the stator winding instead of normal rectangular stator conductors. The cables that are used are the same type as the ones used for power transmission and distribution. The technology makes it possible to deliver power to the grid without the need for medium voltage switchgear or step-up transformers, and it also opens the way for new system thinking. Among the anticipated advantages are up to 2 per cent improvement in plant efficiency, higher reliability and availability, lower maintenance costs, reduced environmental impact, and overall life cycle cost savings.

June 1998 saw inauguration of the first Powerformer, at the Porjus hydro plant in northern Sweden. This unit is primarily intended for research and development. As of March 2002 it had logged more than 15 000 operating hours. It has been subjected to an extensive test programme, including short circuit tests from full voltage. Operating experience has exceeded expectations and further tests are planned.

In December 2000 the Eskilstuna unit was synchronised to the grid and the plant was inaugurated in March 2001, supplying the grid at 136 kV without an intermediary transformer. This unit operated successfully until mid November 2001, clocking up around 4400 hours.

But then the plant had to be shut down when high temperatures were encountered in the generator.

The problem is associated with the water cooling system of the stator and not associated with the Powerformer technology per se. The plant hopes to be fully operational in time for its next operating season, which runs from October.

Problems have also been encountered at Vattenfall's Porsi hydro plant, on the Lule River, also in northern Sweden, the largest Powerformer installation to date. But again, the root cause is not attributable to the Powerformer system itself, says Alstom.

The 75 MVA Porsi Powerformer began supplying electricity to the grid on 5 May last year - at a voltage of 155 kV, believed to be a record voltage for direct supply of power to a grid. A few days earlier the plant had established another record when it produced a voltage of 202 kV during overexcitation testing.

However, in the summer of 2001 there was a turbine bearing failure, followed by two generator bearing failures resulting in rotor to stator contact.

Following bearing repair the plant has now been recommissoned and was reconnected to the 155 kV grid on 24 February. Heat run and load tests have succesfully been carried out and at the time of writing the unit was undergoing a trial run, expected to last until 3 April when hand-over was scheduled.

The owner, Vattenfall, is operating the Powerformer remotely from a control room in the town of Vuollerim near Lule Älv.

Extensive investigations have been undertaken on site by GE Hydro and Alstom and it has been concluded that the failure was due to mechanical problems in the conventional technology, not Powerformer.

Meanwhile, the Powerformer unit at the Höljebro hydro power station in Sweden, which was synchronised to the grid on 23 August 2001 - 15 months from placing of the order - is performing very well, with more than 2500 operating hours recorded.

The Höljebro plant is situated on the Ljusnan river close to the town of Soderhamn. It is new-build, rather than a rehabilitation project like Porsi and Porjus. The plant, which is connected directly to the 80 kV network, has an estimated added value of about 2 million euro. The Powerformer unit used about 12 km of cable. Sound levels during operation have been very low.

Miller Creek and Katsurazawa

The first two Powerformer units outside Sweden were those for the Miller Creek hydro power plant in Canada and the Katsurazawa hydro power plant in Japan.

The Miller Creek power plant is situated about 140 km north of Vancouver in the province of British Columbia and is owned by EPCOR, Edmonton Power Corporation.

The Katsurazawa hydropower plant, in the town of Hokkaido, dates from the 1950s. One unit will be replaced by a Powerformer generator.

The Miller Creek and Katsurazawa Powerformer units are currently being manufactured at the Alstom factory in Bilbao, Spain. Both core stacking and winding of cable will be performed there before the units will be transported to site.

Planned commissioning date is late autumn 2002 for Miller Creek and spring 2003 for the Katsurazawa unit.

Meanwhile, discussions are underway on future projects in the USA, Canada and Norway.

New system thinking

Since the beginning of the Powerformer project one of the main aims has been to increase the voltage and build machines for direct connection to the transmission grid, ie eliminate the transformer. However, the use of high voltage cable wound generators need not be limited to direct connections. They can also be used in conjunction with transformers, with some interesting consequences.

Generation at higher voltage opens up the possibility of moving the transformation further away from the generation, as well as opening up other transformation options. For instance, transformation is less limited to unit bound connections since the short circuit current is less of a limitation on higher voltage levels. It is also possible to use auto-transformers, which for a given MVA rating normally have a lower investment cost and lower losses.

For rotating machines voltage is the main cost driver while for transformers apparent power is generally the main cost driver.

A key objective is therefore to find the best voltage for each individual Powerformer and plant.

In Sweden most of the generators are relatively small (in terms of MVA) and slow speed, because of the low head. On the other hand the distance from the production in the north to the load in the south is very long and requires high transmission voltage (in Sweden the national grid is 220-400 kV). This contrast has initiated some new thinking, raising such fundamental questions as:

• Where is the load?

• Where is the production?

• What is the most efficient connection point for a machine?

In Sweden, as you move closer to the coast and closer to the south the load increases and the sizes of the generators decrease. This presents an opportunity because the number of systems using lower system voltages also increases closer to the load. The voltage is flexible and the transformation may not be a part of the power plant but a part of the power system.

Many new and interesting opportunities have been identified in Swedish market just because of this simple proposition.

Consider the example of a Swedish plant where the efficiency gain could be more then 1500 kW just by eliminating the single unit transformer and connecting the units at 45 kV instead of 130 kV, as it is today. Among the benefits would be:

• higher efficiency, improved power flow;

• production of power at the same voltage level as the users want it;

• higher availability because the units are connected to different outgoing lines and do not depend on a single transformer;

• one transformer is eliminated;

• no need to accommodate a transformer in the cavern; and

• no need for generator switchgear with very high installed short circuit power.
Tables

Table 1. Powerformer reference units to date