* Most of 36 national grids prepared
* Risks most pronounced in second January week
* Oil-plant closures tighten France's availability
FRANKFURT, Nov 29 (Reuters) - Sustained cold spells and low generation availability could cause problems for electricity networks in several European countries this winter although most are well prepared, a power network lobby said on Wednesday.
Power systems last winter struggled to cope with unforeseen cold weather in January at a time of tight nuclear capacity in France, prompting the group of transmission system operators, ENTSO-E, to look closely at possible worst-case scenarios.
The biggest risks in 2017/18 were seen in Britain, France, Belgium, Poland and Italy and looked most severe in the second week of January, ENTSO-E said in its biannual "adequacy report" on 36 countries.
It covered the outlook from Nov. 29 through April 1 and stretched from central European countries as far as Turkey, Kosovo, Malta, and Burshtyn Island in Ukraine.
Grids at the height of winter season can require more network usage to cover demand rises when consumers switch on heating and lighting more often, while stable thermal generation volumes are declining in a long-term transition to renewables.
At the same time, if unneeded variable green power generation coincides with weak demand, for example should night temperatures be mild, there can be risks around the management of grid curtailment, which ENTSO-E said could happen in Ireland and some southern Italian regions.
To cope with unforeseen power shortages, network managers can ask consumers to curb demand, or selectively shut them off.
Weather watchers so far expect no particular problems.
Most of Europe will likely experience above-average temperatures from December to February, the coldest months of the year, the Weather Company said recently.
ENTSO-E noted that after a dry year, hydroelectric power reservoir levels that point to potential supply from pumped storage power stations, in Italy and Spain were close to the historical minimum.
Those in France and Switzerland had recovered to near average values, and in Austria higher than average in October.
It also said France, which is exposed to temperature risks, had overcome some nuclear availability problems but shut sizeable oil-fired power plants this year.
Germany, Europe's default exporter with huge nominal overcapacity, may be tight if a long cold spell coincides with low river water levels in the south, according to ENTSO-E.
But it has various reserve schemes in place to meet potential shortfalls. (Reporting by Vera Eckert, editing by David Evans)