Bulgaria's official electricity system operator ordered producers to lower capacity on Thursday to prevent the shaky system from overheating amid sharply dropping consumption and exports.

Citing "an impossibility to maintain the balance between production and consumption", the operator ordered cuts on all power plants.

It did not say how much capacity should be cut or for how long.

The country's sole nuclear power plant at Kozloduy announced shortly afterward that it had lowered capacity at its two 1,000-megawatt units to 750 and 550 megawatts.

It also said it would now advance a planned shutdown of one of the units for maintenance and refuelling.

The operator already ordered last week the temporary shutdown of eight thermal power plant units and the economy and energy ministry outlined a set of urgent measures to safeguard the country's badly overheating electricity grid.

Shrinking industrial activity and low household use have caused consumption to more than halve in recent weeks, while exports were just a tenth of what they were at the same time last year, according to energy ministry data.

Problems have been brewing in Bulgaria's electricity sector for years, due in part to a drop in exports, increased production -- without the necessary demand -- following a boom in renewables and the global economic crisis.

But these troubles have accelerated in the past few months, and the sharp imbalances threatened safety, experts warned.

In February, high electricity bills and anger against power monopolies sparked nationwide rallies that later turned to poverty and corruption, finally forcing the government to resign.