UK National Grid tenders again for next winter power cover

* National Grid (LSE: NG.L - news) seeks 1 GW of reserve capacity for 2015/16

* Contracts from second tender to be offered in May (Updates throughout)

LONDON, March 2 (Reuters) - National Grid opened its second tender round on Monday for up to 1 gigawatt (GW) of energy capacity reserve in Britain for next winter as a precautionary measure if power supplies are tight.

The contracts from the second round will be offered in May this year, the grid operator said in a statement. The first tender round was held in December, which secured 700 megawatts of additional reserve.

Last year, Britain faced tight electricity supply in the winter after several power plants were hit by unexpected outages, which forced National Grid to introduce schemes to cut energy use and make extra capacity available.

National Grid introduced schemes to pay companies not to use electricity during peak hours and to encourage utilities to make idle capacity available.

For the winter of 2014-15, the grid operator procured extra reserve capacity which added a 1.1 GW cushion to the electricity system, even though milder temperatures, continental power imports and high levels of renewable power generation meant it was not needed.

"Although we did not need to use the additional reserve we secured for the winter just gone, contracting additional capacity was a sensible insurance policy to take out, given that margins had tightened," Cordi O'Hara, National Grid's Director of UK Market Operation, said in a statement.

"We already know that the margin between supply and demand next winter could tighten further," she added.

(Reporting by Nina Chestney; editing by Susan Thomas and William Hardy)