The operator announced in September a tender to secure up to 300 megawatts of reserve capacity to replace ageing gas turbines it said were becoming increasingly unreliable.
Reserve capacity units need to be available to start production at 15 minutes notice to meet power demand and avoid blackouts in the event of outages at regular plants.
Vattenfall will supply 50 MW of reserve capacity and Fortum some 60 MW for a 12-month period starting on June 1, Svenska Kraftnat said.