MADRID, Sept 24 (Reuters) - Spain's 1,000 megawatt Vandellos II nuclear power station halted off schedule early on Thursday after operators detected a hot spot in the main transformer, a statement from the CSN nuclear watchdog said.
The fault was detected at 0000 GMT and plant was disconnected from the grid to allow for repairs.
"It won't happen overnight. It'll take several days," a spokeswoman said at the plant, which is near the northeastern port of Tarragona.
Spain's number-two power utility, Enel-owned Endesa has a 72-percent stake in Vandellos II, and the country's biggest utility Iberdrola, owns the remaining 28 percent.
Two of Spain's eight nuclear power stations are now out of action, because the 1,000 MW Cofrentes plant is refuelling and not due back on line until next month.
Data from national grid operator REE showed that the six working nuclear reactors were generating 5,260 MW by 0930 GMT, or 15.1 percent of total demand. That compares to 7,300 MW when all eight are running normally.
Nuclear power tends to sway wholesale prices on the spot market because utilities can sell to the pool at a discount to gas- or coal-generated power.
Spain's nuclear plants supply around 20 percent of Spain's electricity and in recent years have worked on an average at 86 percent of capacity, although the proportion has fallen as the reactors have aged.
Nuclear power is unpopular in Spain and the Socialist government has vowed to phase them out in favour of booming renewable energy sources. However, REE and analysts say most of the plants may be around for decades to come. (Reporting by Paul Day and Martin Roberts; Editing by Toby Chopra)