Iberian prompt power falls further on wind, demand

* Spain day-ahead 33.32 euros ($49.71)/MWh, down 0.78

* Portugal prompt drops 0.88 to 33.40 euros/MWh

MADRID, Oct 15 (Reuters) - Iberian prompt power extended declines on Thursday when the prospect of a surge in cheap wind power coupled with weak demand offset the impact of a nuclear power plant halting for unscheduled maintenance.

The 1,000 megawatt Asco II nuclear plant was disconnected from the grid on Wednesday evening to repair a back-up diesel generator. Operators could not say how long the work would take. [ID:nMDT008675]

Wind farms, however, were forecast to more than fill the gap left by Asco II by ramping up output to more than 8,000 MW later in the day from 5,000 MW in the morning, when the Omel exchange fixed the day-ahead "pool" rate, the market benchmark.

Demand for electricity has been declining for more than a year in Spain due to the recession. Consumption is also seasonally low in October, when mild weather means neither heating nor air-conditioning is in demand.

Forwards retreated slightly and bucked additional gains by Brent crude LCOc1 to almost $74 a barrel in late trade.

Dealers say this was due to low demand and because benchmark calendar year 2010 is seldom traded in Spain and they did not want to be caught out by a sudden correction in oil.

Deals in baseload calendar year 2010 were reportdely closed at 38.40 euros/MWh, down 0.45 on the day.

Data from the CSN nuclear regulator and national grid operator REE (REE.MC) showed seven of Spain's eight nuclear plants were running normally and generating 5,254 MW between them, or 15.1 percent of total demand.

Apart from the halt at Asco II, the 1,000-MW Cofrentes nuclear is currently refuelling and not due back online until next week.

Spanish power stations were meanwhile emitting 9,439 tonnes per hour of carbon dioxide. (Reporting by Martin Roberts; Editing by Keiron Henderson)