Wind Power in Power Systems

392 Economic Aspects
cutoff behaviour, for instance, using a linear power reduction instead of a sudden
shutdown of the wind turbine (for details, see Chapter 3). This way, the costs for
possible additional primary control capacity can be avoided.
The second case occurs when a network fault causes frequency or voltage variations
which lead to the disconnection of a very large number of wind turbines during times
with high wind speeds (i.e. during times with high wind power production). New grid
codes, however, require that wind turbines are able to stay connected to the network
during certain faults (fault ride-through capability; see Chapter 7). Hence the likelihood
of such situations is considerably reduced as a result of the new grid codes.
In summary, it is hardly possible clearly to determine the costs that wind power might
cause regarding primary control. To reduce the likelihood of additional costs for
primary control capacity caused by wind power, a certain shutoff behaviour for wind
turbines can be required and/or new grid codes can be implemented. Whether this leads
to the best overall economic solution depends very much on the specific case. As
mentioned in Chapter 7, fault ride-through capability may increase the investment costs
for wind turbines by up to 5 %. In power systems with sufficient primary control
capacity, such additional investment costs may not be economically justified.
18.3.2 Treatment of system operation costs
Here, system operation costs are defined as the costs that are independent of network usage
but that are essential for reliable system operation. Such costs are related to primary control
and primary control capacity, black-start costs and investments in system robustness. The
primary control costs are included because they can usually not be linked to any market
participant. System operation costs should in principal be divided between all power system
users based on their network usage, e.g. transmitted energy (kWh).
In some countries (e.g. England and Wales, and Australia) wind power is currently
not required to contribute to these costs. Similar to the case of network upgrade costs, it
can be argued that wind power is a measure to create a sustainable energy supply from
which all market participants gain. Hence, these costs should be covered by market
participants, excluding wind power generation.
The English and Welsh regulator Ofgem, however, concluded that such treatment
may be considered unfair. Hence Ofgem is currently considering the implementation of
a new tariff system that requires wind farms to contribute to the system operation costs
(Ofgem 2003).
18.3.3 Secondary control issues
If there is an imbalance between generation and consumption, the primary control will
jump into action to reduce the imbalance. This means that primary control reserves
will be partly used up after the imbalance is restored and/or that the primary control will
reduce the imbalance but the restored frequency will deviate from 50 Hz or 60 Hz.
Secondary control is then used to solve this problem by freeing up capacity used for
primary control and moving this required capacity under a secondary control regime
(for an illustration, see Figure 8.3, page 148). Secondary control capacity should usually
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