Wind Power in Power Systems

44 Wind Power in Power Systems: Introduction
The consequence is that there will be more variations that have to be balanced by
primary and secondary control. Experience from Europe shows that even very high
wind power penetration levels (up to 20 %) may not require additional primary control
capacity as long as the installed wind power capacity is geographically distributed over a
wide area (see also Chapter 8). The smoothing effect related to geographical distribution
will result in low short-term variations in wind power production. However, owing to
current limitations in wind speed forecast technologies, any mismatch between forecasted
wind power production and actual wind power production has eventually to be
handled by the secondary control capacity (on wind power forecasts, see Chapter 17; on
the experience in Denmark, see Chapter 10; for additional discussion, see Chapters 8, 9
and 18). The requirements for secondary control capacity are therefore significantly
influenced by the wind power penetration level.
The additional system requirements for keeping the system balanced at all times
depends very much on the individual system; that is, it depends on the load characteristics,
the flexibility of the existing conventional power plants and the wind power penetration as
well as the geographic distribution of the wind farms. The cost of meeting the increased
requirements depends on the type of power plant (i.e. PG), the size of interconnections to
neighbouring systems and, of course, on the additional requirements.
3.7.3.4 Wind power requirement 2: network availability on demand
As already mentioned, wind farms want to sell into the power system whenever wind
power production is possible. Depending on the power system design and the wind
power penetration this may cause transmission congestions as well as stability issues
(i.e. voltage stability). For a more detailed discussion of these issues we refer the
reader to Chapter 20 as well as to Part 4 of this book, in particular to the introduction
of Part 4, Chapter 24.
3.7.3.5 Customer requirement 3: economical power supply
Again, we will first assume there is no wind power connected to the power system shown
in Figure 3.9. In general, power system design must analyse the costs and benefits of a
certain reliability level. No power system will be built with a 100 % reliability, as the
costs will be in no relation to the benefits. Also, the cost of increasing reliability is very
high if one already has a high reliability. There are two issues that have to be taken into
account.
First, the power system must have a sufficient amount of power capacity, PG, to meet
maximum demand, PD þ PL. We assume that we have a reliability of 99.9999 % [i.e.
during 1 hour per year (the expected mean value) there is not enough installed capacity
to meet the load]. If we want to increase reliability, we have to build a new plant that is
used only during one hour per year. In this case it is sometimes considered economically
more efficient to disconnect consumers (paid voluntary disconnections or forced disconnections)
than to build a new power plant that is used only 1 hour per year.
For the dimensioning of a power system, the so called N 1 criterion is usually
applied. The N 1 criterion means that an outage in the largest power plant should not
cause the disconnection of any consumer. An outage in a large plant causes a decrease in
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