Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 120

Wind farm output
target

Droop lines
with settable slopes

Dead-band
Settable limits

Normal set point

47 Hz Frequency 52 Hz

Figure 4.11 Frequency governor characteristic for typical grid code

have settable values. Typical values of dead-band setting are 101-99%, although
in Denmark wind farms are required to take no action between 102% and 98%.
Figure 4.11 shows a generic grid code requirement for frequency response. Specific
grid code requirements in a number of jurisdictions are summarised in Table 4.4.
Philosophies differ on how the frequency response should be used. Much of
this depends on how response is dealt with in markets. If traditional plant is con-
tracted outside a market mechanism, then it will be commercially important to plan
to use that contracted resource before asking wind farms to move into frequency
regulating mode. At the other end of the spectrum, if the response is totally a
market quantity, then wind farms might be free to participate on a price and sta-
tistical availability basis. It is clear that wind farms can always reduce output, but
can only increase output in response to a frequency transient if operating below
potential.
Utilities carry out compliance testing to ensure that new or re-commissioned
plant meets the grid code requirements on the principle that plant must be capable
of sharing the pain of a system disturbance. In the case of traditional plant, precise
conditions are established and the governor set-point is adjusted up by say 0.5 Hz to
simulate a frequency fall. The plant response is timed and values for, say, 3 s, 10 s
and long-term load-lift are recorded. The latter gives the instantaneous droop at the
plant pre-loading test point.
The problem with carrying out these tests for a wind turbine is that the source
power is varying. It may therefore be necessary to record the plant performance
over a range of real system events and assess average performance against accu-
rately clocked wind speed while in spilled-wind operating mode. This makes grid