Wind Power in Power Systems

454 Areas with Limited Transmission Capacity
The Swedish Government proposed a bill (Governmental Bill 2001/02: 143) on
planning objectives for wind power. According to this bill, the objective of 10 TWh of
wind power production per year has to be achieved up to 2015. The bill was passed in
2002 and since then several studies have been conducted in order to draw up prerequisites
for the integration of wind power in Sweden.
Energimyndigheten, the country’s energy authority, has prepared a report pointing
out areas that have suitable conditions for the integration of wind power. One alternative
that was suggested was to develop large-scale wind power in the mountainous
area in the north of the country. Favourable wind conditions and better economic
conditions than for offshore wind farms make the area attractive for this purpose, even
though the transmission capacity is limited.
In 2002, the Swedish government commissioned TSO Svenska Kraftna¨ t to define
general prerequisites for an integration of large-scale wind power (10 TWh) in the
mountain area and offshore. The resulting report considers a case with 4000 MW of
installed capacity with a utilisation time of 2500 h in northern Sweden (Arnborg, 2002).
According to the report, five new 400 kV transmission lines at a total cost of SEK20
billion would be required to guarantee electricity transmission available for 100 % of the
year (for the exchange rate, see Footnote 1, page 444).
Assumptions and results
The following assumptions are made:
. Data from 2001 were used to arrive at the probability distributions.
. Only the inner transmission limit is considered. It is assumed that exports to neighbouring
countries are not affected.
. The transmission limit is assumed to be constant.
. Power generated in northern Sweden can be consumed in the central and southern
parts of the country.
. Only active power flows are considered, assuming that reactive power compensation
can be controlled in hydro power plants.
. Variations of active power output at wind farms are not compensated by regulation at
hydro power plants.
In addition, wind speed measurements available from the existing wind turbines in
Sourva (northern Sweden) were used. The expected power output is obtained from a
typical power curve for a pitch-regulated wind turbine. The calculated power output was
then scaled up to represent different levels of installed capacity.
Figure 20.10 illustrates the percentage of wind energy that has to be spilled at various
wind power penetration levels. The results were obtained by usind the probabilistic and
the simplified method. For a comparison, the calculation is also perfomed to obtain
hourly values in 2001. Here, wind power production is modelled using hourly wind
speed measurements from Sourva and scaling up the power output in order to represent
different levels of installed capacity. Actual hourly power transmission from 2001 is
included in the analysis. The expected power transmission with wind power is calculated
by summing these two variables for each hour. The resulting power transmission is
compared with the transmission limit of 7000 MW and the annual energy spillage is
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