Towards a Baltic Sea Region Strategy in Critical ... - Helsinki.fi

CRITICAL INFRASTRUCTURE PROTECTION IN THE BALTIC SEA REGION
2.3 ELECTRIC POWER SYSTEM DISTURBANCES
The majority of the serious electricity blackouts are caused by different weather
phenomena, such as storms, snow and ice. Distribution interruptions caused by
technical problems are usually restricted to small areas and have a short duration.
The power lines in Finland have been designed to withstand normal storms.
Interruptions are primarily caused by trees falling over the lines and usually affect
distribution networks. The transmission and sub-transmission networks are treeproof
and therefore not vulnerable to the same extent.
Aerial lines are often struck by lightning, in which case short circuits or earth
faults may occur. Lines or transformers may also be damaged, sometimes
permanently. The underground cables are seldom damaged by lightning; however,
if they are, the recovery times are longer compared to damages in the aerial lines.
If the power stations or distribution networks are also damaged, the interruptions
may affect widespread areas.
During winter, snow may cause power interruptions, mainly by pressing tree
branches onto the lines. On rare occasions, snow masses may press entire lines to
the ground. No severe problems caused by ice (such as the 1998 ice storm in
Canada) have been reported in Finland.
Floods and heavy rainfall may cause disruptions in the underground cable
networks, mostly by damaging transformers. Aerial lines are not affected to the
same extent.
Subzero temperatures may damage disconnectors, switches or protective
fixtures, or even break entire lines. It is important to point out that even shorter
power distribution disruptions may have severe consequences to the customers in
subzero weather.
Disruptions of non-weather related causes, such as structural or operational
errors, are usually short-lived and isolated to a smaller area. However, damages in
transmission or sub-transmission network (e.g. due to a broken component) may
cause widespread distribution disruptions. Fortunately, these are not very common.
The duration and number of electricity blackouts in Finland are constantly low.
The reliability of the electricity distribution in 2006 was over 99%.
In 2006, the electricity consumers in Finland had, on average, only a few
electricity blackouts whose duration was less than hour. In rural areas, where the
electricity grid consists mainly of aerial lines, electricity blackouts occurred more
often and were also longer lasting compared to the populated areas and cities.
Especially falling trees in the high winds and storms can damage the aerial lines
seriously, requiring long lasting repair efforts. In rural areas, the average
interruption time due to electricity blackouts was three and a half hours in 2006. In
populated areas, the average interruption time was a little above half an hour and
in cities about ten minutes. The number of blackouts per consumer in 2006 was 13
in the rural areas, three in the populated areas and only one in the cities. (Finnish
Energy Industries, 2007)
The dependency on electricity in modern society is high and will not be
reduced in the future. The increase of extreme weather conditions caused by the
climate change is a threat to the electricity distribution in the future, and the
warmer atmosphere will contain more humidity, increasing the possibilities of
heavy rainfall. Consequently, heavy rains with floods, similar to those found in
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