Variable-Speed Drives Raise Energy Plant's Output

A Swedish energy company has increased the amount of energy it can sell by 35GWh/year thanks to the installation of ABB medium-voltage drives. Malarenergi AB is a city-owned electric power and district heating provider based in Vasteras, Sweden. Malarenergi was looking for ways to be more environmentally friendly and more efficient in the production of electricity and heat. The more effectively the plant can convert the fuel's thermal energy - either to electricity or to heat - the higher its efficiency.

In recent years the focus has been on increased electricity production. Malarenergi had been using resistors connected to slip-ring motors to control the speed and hence the flow in the district heating pumps. The heat from the resistors was used to produce district heat. This method of using the heat created by the losses is comparable to using electricity to produce district heating, an expensive method with today's high electricity costs.

To improve this situation, the company invited ABB to perform an energy appraisal, which revealed a lot of energy could be saved by upgrading the pump and fan applications with the latest variable-speed control technology. ABB replaced the resistors and slip-ring motors on the district heating pumps with variable-speed drive systems, including drives, high-efficiency motors and transformers. The total system involves seven ACS 1000 and one ACS 6000. They control four district heating pumps (4 x 1765kW), a boiler feed pump (5750kW), an accumulator pump (800kW) and a fan and a pump for a new bio-fuelled boiler.

Since the installation of the variable-speed drives, the losses caused by the inefficient flow-control method have been considerably reduced. Although this has reduced the production of district heat from the resistors, this has been replaced by a higher production of electrical energy in the process that generates both heat and electricity. The losses were removed from the district heating system, which increases the cooling water-temperature difference across the heat exchangers in the district heating circuit. This results in an increase in saleable electricity of about 35GWh/year.

The increased plant efficiency also led to reduced CO2 emissions. Since the installation of the ACS 1000 drives and a new control system, the differential pressure in the district-heating pumps is controlled automatically. This gives more accuracy, stability and a lower temperature of the return water, which allows a better utilisation of the heat by the consumers. The new method has also led to reduced maintenance costs, as the existing motors were old and in need of major maintenance overhaul.

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