Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 122

code compliance a lengthy process during which a wind farm can only be entitled
to temporary connection permission.
Chapter 5 explores the need to spill wind in the context of system frequency
management. Wind is a free resource once the wind farm costs have been met. There
is thus a reluctance to spill wind rather than save fossil fuel to achieve spinning
reserve. Nonetheless, with high penetration of wind, especially on smaller systems,
there may be a need to ensure that the system still has adequate active power
reserves. There is no major technical problem with this requirement other than the
constantly varying response available depending on wind strength. However, taken
over a large number of wind farms and a wide enough area, statistically the resource
should be available. There is a code issue relating to how the response is specified.
Ideally the wind farm would track the instantaneous wind speed and deliver a fixed
margin below its calculated instantaneous output, keeping the remainder in reserve
for regulation. Some codes seek to specify fixed amounts in MW, which means that
at times when the wind is decreasing this amount is not available and at other times
more reserve would be available. There are commercial implications and the matter
should be resolved if the resource is to be deployed seriously. Whilst not within the
scope of this textbook, if efforts are made to provide more real-time firmness of
wind farm output by linking it with energy storage of various types, it may be more
efficient to call on the energy storage to provide reserves than to spill free wind.
It is worth noting that there is a purely economic argument for operating wind
farms at various levels below potential to provide reserve. Such operation is justi-
fied when the value of reserve exceeds that of energy, as may happen at times of
low demand and high wind potential. A theoretical framework to facilitate such
operation is provided in Tang et al. (2014).

4.6.2 Transient response
Three-phase power systems are prone to various kinds of fault, e.g.

single line to ground

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line to line

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double line to ground

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three lines to ground

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The detailed analysis of these fault types may be found in various standard
texts (Weedy et al. , 2012). The real power and voltage are both involved in the
resultant system dynamics following an event. The important issue here is to
understand the behaviour of a WTG under the most severe system fault that may
arise - namely a three-phase-to-ground fault in the transmission network. Such an
event is rare, but it is the worst credible case for system stability and happens to be
quite easy to analyse. If this fault is adjacent to the wind farm and distribution line
of interest, the voltage seen by the wind farm will be virtually zero. Generators
close to the fault cannot supply load energy due to the collapsed voltage. Since the
load represents a braking torque on the generator, which has suddenly been
removed, the generator will accelerate.