* Scheme to compensate for nuclear closures may run longer
* Lines to transport renewable power are not built in time
* Weak point in planned transition to low-carbon economy
ESSEN, Germany, Feb 9 (Reuters) - Germany is not expanding power transport networks quickly enough and may need to extend a back-up power scheme beyond its planned expiry in 2017, the country's energy regulator said on Monday.
Known as the winter reserve because that is the time of year when demand is highest, the scheme became necessary after Germany shut 40 percent of its nuclear power station capacity in one go in 2011 after the Fukushima nuclear disaster in Japan.
"I think we will need the instrument longer (than 2017)," said Jochen Homann, president of the federal network agency (Bundesnetzagentur) at a conference in Essen.
"That is based on the admission that we aren't keeping up with the necessary grid expansion," he added.
Homann's agency has to ensure electricity grid operators (TSOs) provide enough capacity so transmission lines are always balanced to guarantee supply, especially as the share of volatile renewable power has grown to a quarter of all output.
This winter, the regulator has contracted 2,500 megawatts of standby power, and the TSOs have secured reserves from German, Austrian and Italian utilities.
However, the amount of standby power may have to rise because utilities such as RWE and E.ON which have had to shut nuclear plants are now also closing coal- and gas-fired ones due to the huge expansion of renewable power.
The government in the past assumed that enough new power transmission lines would be constructed by 2017 to transport especially wind power from the north to the industrial south, which in the past relied to a large extent on nuclear.
But Homann said that of 1,900 kilometres of new power lines planned in 2009, only 463 km had been built. In 2014 alone, it was just 140 km.
TSOs expected only 40 percent of the 2009 plan to be built by 2016, Homann said.
Moreover, a separate national plan dating from 2013 -- when the government legislated for entirely new high voltage point-to-point lines in recognition of the nuclear switch-offs -- has yet to materialise into infrastructure.
The energy regulator received applications for two direct current "power highways" as they have been dubbed just before Christmas. But consultations with the public about where they will run as well as constructing and commissioning them will take years.
Homann said time was running out.
"I am convinced that we won't succeed with the energy transition if we don't make enough progress with the network expansion," he said. (Reporting by Vera Eckert; Editing by Mark Potter)