FRANKFURT • Germany's drive to harness wind and solar power is producing so much electricity that it is spilling over into neighbours' grids and raising the threat of blackouts.

Poland and the Czech Republic are spending US$180 million (S$244 million) on equipment to protect their systems from German power surges, while Austria is curbing some trading to prevent regional networks from collapsing.

The overflow can exceed the output from four atomic reactors.

Germany's fivefold increase in green energy in the past decade has outpaced investment in power lines to move it across the country.

Electricity is looping through Poland and the Czech Republic to reach southern Germany, where supply is constrained as Chancellor Angela Merkel shuts nuclear plants in the wake of the 2011 Fukushima disaster in Japan.

The disruptions show the limits to the European Commission's vision of a single power market.

"A huge accumulation of overflow increases the threat of a blackout," Mr Zbynek Boldis, head of trade and international relations at Czech grid CEPS, said in an interview in Budapest. "The root of the situation is allowing a huge amount of electricity to be generated regardless of the capacity of the grid."

German power companies plan to spend at least €22 billion (S$33 billion) on high-voltage power lines as they upgrade their grids to accommodate more solar and wind power before the last of the eight remaining reactors is closed in 2022.

Nine units generating almost 10 gigawatts - enough to power 20 million European homes - have been shuttered since 2011, with the latest on June 27. Nuclear power now accounts for 16 per cent of Germany's electricity, compared with 26 per cent for renewables.

German power overflows are forcing Poland to protect its grid from overloading and triggering a blackout. The nation's grid operator had to double the amount of last-minute changes to power plant output to balance the surges in the first quarter from a year earlier.

On the windiest days, sometimes more than 50 per cent of the power sent from northern Germany to its southern states and Austria travels through Poland and the Czech Republic, according to CEPS.

Polish grid operator PSE says the surges take up so much of its import capacity that there is not enough remaining for it to carry lower-cost German power that could be made available to commercial users.

German electricity for delivery next year is about 18 per cent cheaper than Polish power, close to the widest gap since at least 2008.

The Poles and Czechs will, by the end of next year, finish installing transformers on two power lines connecting with Germany to control the unplanned flows and help free up capacity that can be traded or auctioned off to domestic users.

While that will ease the overflow pressure, the Polish and Czech grids still face congestion from the demand for cheap German power in Austria, which shares a wholesale electricity market with Germany. For more than a decade, Austrian traders have bought German electricity when it is cheap to sell to other countries at higher prices.

This aggravates grid congestion as the amount traded has "significantly exceeded" the planned physical capacity between the countries, said Mr Jochen Homann, president of the German grid regulator.

"We're in an absurd situation," said German Economy Minister Sigmar Gabriel at a utility conference in Berlin last month. "We produce cheap power in the north and can't ship it south." BLOOMBERG