Affärsverken, which owns the cogeneration plant, discovered the attack on Friday morning and reported the incident to the police.
The cyber attack should not affect customers and socially critical activities.
We are working to deliver the heat as we should and we do that without any deviation, says Anna Claesson, press officer at Affärsverken.
She does not want to comment on whether any demands have been made on the company in connection with the cyber attack.
What kind of attack it is or how long it has been going on is not known.
We have investigators with expertise in the area. They are working full-time to find out what has happened and who is behind it, says the police's press spokesperson Filip Annas.
