LONDON (Reuters) - Britain's electricity network operator National Grid (>> National Grid plc) has brought forward by a year a scheme to tap additional power capacity over the winter following a number of unexpected plant outages.

National Grid launched a tender for its Supplemental Balancing Reserve (SBR) on Tuesday, asking power generators how much spare capacity they could provide this winter to fill a potential supply gap.

The network operator decided to take SBR forward after a series of unplanned shutdowns at large power plants worsened the supply outlook for this winter since its last capacity assessment in June, it said.

Britain is already facing a power capacity crunch as ageing and polluting stations are shutting down, while new plants are slow to start operating because of a government overhaul of energy policy.

"At this stage we don't know if these reserve services will be needed, but they could provide an additional safeguard," said Cordi O'Hara, National Grid's Director of UK Market Operations.

Fires at E.ON's (>> E.ON SE) Ironbridge and SSE's (>> SSE PLC) Ferrybridge power plants have reduced output, while the Barking power plant has closed and there have been production problems at EDF Energy's (>> EDF) Heysham and Hartlepool nuclear power stations.

If the network operator decides SBR is necessary this winter, it will launch a competitive tender for a specific amount of spare capacity.

Power generators would have to be available to provide additional electricity between 07:00 a.m. - 21:00 p.m. BST from November to February, the months with the highest demand.

The new back-up capacity mechanism is running in parallel with a programme that allows National Grid to ask contracted users, mostly factories, to reduce their electricity demand when the system is strained.

(Reporting by Karolin Schaps; Editing by Michael Urquhart)

Stocks treated in this article : EDF, E.ON SE, SSE PLC, National Grid plc