* Power networks more unstable since nuclear drop-out
* Renewable power swings trigger need for intervention
* New lines, more reserve power needed
By Vera Eckert
FRANKFURT, Nov 27 (Reuters) - German power networks needed nearly three times the intervention of technicians last year as in 2010, a cartel office report showed, highlighting the strain on the grid ahead of the 2012/2013 winter.
Compared with a year earlier, the number of threats to the normal running of the network in 2011 rose to 4,548 hours from 1,403 hours in the previous year, said the report on Tuesday, which the authority issued jointly with the network regulator.
Crises are costly as more manpower is required and the risks of a break-down rise, potentially leaving consumers and industry without electricity in Europe's biggest economy.
A press statement issued by the two bodies blamed the "unhindered expansion of renewable energies coinciding with only linear additions to network infrastructure," demanding that network expansions be urgently prioritised.
Grids have become overstretched after Germany shut 40 percent of its nuclear capacity last year, following a political decision to rely more on renewable power, which tends to be more volatile due to changing weather.
By 2020, Germany aims to derive 35 percent of its power from green sources, such as wind or sunshine.
This has prompted a rapid rise of new installations, with solar power output in Jan-Sept up by more than 50 percent from the same period a year earlier.
But networks suffer because special coping measures are needed when high winter demand coincides with either cold or calm weather, or when low demand combines with gusty winds or heat waves.
In such cases, grid managers may have to ask generators to wind up or down their feeds into the network, or in the worst case ask customers or exporters to curb consumption.
Details of the report showed that on one line between Sottrum in Lower Saxony and Borken in the state of Hesse, the length of tight network situations saw a 13-fold increase to 319 hours last year.
The Economy Ministry has updated a plan to expand the high voltage grids that transmit electricity across the country. The plan is meant to become law by the end of the year but is already less ambitious than was foreseen when its strategy was initially set out in May.
In the meantime, construction delays are adding to the sense of urgency.
Of 1,834 kilometres of new lines deemed required in 2009, only 214 km were built by end-2011 and only 35 km will be added in 2012, the report issued on Tuesday said.
The energy regulator meanwhile has secured reserve capacity of 2,500 megawatts (MW) from old power stations that can be brought in to stabilise the system during shortfalls. (Editing by Mike Nesbit)