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VSDs increase power plant output

03 June 2010

A Swedish energy company has increased the amount of energy it can sell by 35GWh/year thanks to a production efficiency drive involving the installation of medium voltage drives

Malarenergi AB is a city-owned electric power and district heating provider based in Vasteras, Sweden. Like other companies operating in this sector, its business is affected by issues arising from various EU directives, electricity certificates and the Kyoto protocol, so the search was on to look for ways to be more environmentally friendly and more efficient in terms of electricity and heat production. Essentially, the more effectively the plant is able to convert its fuel’s thermal energy – either to electricity or to heat – the higher the overall efficiency of the plant.

The company had been using resistors connected to slip-ring motors to control the speed, and hence the flow, of district heating pumps. The heat generated by the resistors was used in the production of district heat. But this method of using heat created by losses is comparable to using electricity to produce district heating in the first place – an expensive option given the current high cost of electricity.

As part of a strategy to improve the efficiency of its operations, Malarenergi invited ABB to perform an energy appraisal of the plant. This revealed that a lot of energy could be saved simply by upgrading the pump and fan drives using variable speed control technology. The resistor banks and slip-ring motors that previously controlled the speed of the district heating pumps were subsequently replaced with new variable speed drive configurations comprising drives, high-efficiency motors and transformers supplied by ABB.

In all, seven ACS 1000 and one ACS 6000 drives were installed to control four district heating pumps (4 x 1,765kW), a boiler feed pump (5,750kW), an accumulator pump (800kW) plus a fan and a pump for a new bio-fuelled boiler.
As a result of this installation, losses previously experience with the slip-ring motor/resistor flow control method have now considerably reduced. And while the resistors no longer contribute energy directly to the district heat supply, this element has been replaced by a higher production of electrical energy in a process that generates both heat and electricity.

The losses were thus removed from the district heating system, which increases the cooling water temperature difference across the heat exchangers in the district heating circuit and resulting in an increase in saleable electricity of about 35GWh/year. Increased plant efficiency has also resulted in reduced CO2 emissions.

Differential pressure in the district heating pumps is now fully automatically controlled, improving accuracy and stability, and lowering the temperature of the return water, ensuring that consumers are better able to utilise the heat delivered to their homes. The new motors have also had a positive impact in terms of plant maintenance requirements.

Malarenergi admits that at the start of this project, it had a few problems understanding just how much the overall efficiency of its plant was going to be improved by this retrofit. However, ABB’s initial survey was able to shed a lot of light and its subsequent technical support throughout the project has clearly paid off. As Malarenergi’s systems engineer, Sven Olof Kindstedt says: “We have today a more efficient operation, an improved heat rate and a better balance in the district heating network.”

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