21-05-2013, 04:40 PM
Wind Power in the German Network
ABSTRACT
During the last fifty years, UCTE has created an integrated electric system which is one of
the biggest in the world and which has reached the highest level of security and quality of
supply. This development, mainly motivated by the wish to increase the security of the
system and also by the opportunities to exchange electricity on a commercial basis between
vertically integrated companies, has been a long and controlled process. In opposite to other
systems like Norway with it’s hydro power generation, in Central Europe the electrical
energy is mostly generated with thermal power plants. In the last years the development of
natural energy is progressing in around the world due to increase in the conventional energy
cost and conservation of natural environment. Especially the wind generating technology is
successfully growing. Best conditions for generation and supply of wind energy are given
in Germany, because of the political support of natural energy generation by law and
subsidies. Therefor the centralised structure and also the type of energy generation in
Germany is changing to a system with smaller decentralised units using renewable energy
resources. Additional re-dispatching measurements of the thermal power plants in Germany
will lead to a new situation concerning stability and security aspects in planning and
operation.
The German interconnected power system is made up of the extra high voltage networks of
the four German transport system operators (TSO’s). The German power system situated in
the centre of Europe is thus connected through powerful links to the networks of it’s
numerous neighbouring countries. Due to technical and economical advantages of an
increasing energy exchange between the Norwegian hydro power plants and the thermal
power generation in Central Europe HVDC connections to the Scandinavian countries are
existing for many years. This geographical situation opens up great economic opportunities
to German companies. The step by step integration of the increasing generation of wind
energy in the North of Germany is an additional challenge for energy trading, but also for
planning and operation of the transmission system keeping security and stability limits.
Since 2002 more than 10000 MW generated wind energy have been installed in Germany.
The windfarms have been supplied approximately 4% of the public demand of electric
energy in Germany. At this time, no important outages concerning wind energy generation
happened in Germany. Wind energy is only generated on-shore mostly in the North of
Germany, which belongs to the transmission systems of E.ON and Vattenfall Europe. One
reason for the increase of these energy generation is the law for renewable energy
generation (EEG), which made it feasible to get a profitable return on investment because
of the political support and subsidies from the German government. The north of Germany
consists in the main of natural reservations and agricultural regions. The industrial regions
are mainly in the western part of Germany like the Cologne region or the valley of the river
“Rhein“. Therefor the windenergy has to transmitted from North to the centre of industry.
The German transmission system for electrical energy is not planned for large power
transits from the north to south. Especially the transmission and distribution system in the
regions “Schleswig-Holstein”, which is neighboured to Denmark is rather weak. Actually
the system is running to it’s limitations. A strong high voltage transmission system is
actually missing. The results are congestions in all parts of the weak northern part of the
E.ON transmission system. It will be expected that approximately 1000 km new overhead
transmission lines are necessary.
Wind turbines are introduced in electrical power systems in order to reduce the adverse
environmental impact of conventional electrical power generation. Wind turbines are
normally not based on a conventional grid coupled synchronous generator, as is the case
with most other types of generation. Instead, squirrel cage induction generators are used, of
generators that are controlled by power electronic converters. As a result wind turbines
interact with the power system in a way that differs from conventional generators.
Important features are the short circuit power contribution, primary and secondary control.
The properties of voltage control and frequency control have to be used not only with
conventional power plants but also with wind farms, based on modern concepts. Otherwise
windfarms are able to infeed active power only a few seconds after tripping and
synchronising, in opposite to thermal power plants which need a few hours for this
procedure. Therefore, the dynamic behaviour of a power system changes when the wind
energy penetration increases.