Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 196

A

100

B

C

80

60

40

20

D

E

0

47 48 49 50 51 52 53

Frequency (Hz)

Figure 5.31 Wind turbine power-frequency capability chart

two sets of settings defined offline, switchable within 1 minute. Under normal
conditions, a 4 per cent droop characteristic is required above 50.15 Hz, with a high
frequency trip setting of 51.5 Hz. In constrained mode, however, turbine output
should be restricted to 50-100 per cent of available output at 50 Hz, while operating
on a 4 per cent droop characteristic above and below this frequency. Constrained
mode is likely to be requested by the TSO during summer nights when the system
demand is low, and a reduced number of governor controlled conventional genera-
tion units may be online.
It may be noted that provision of reserve by wind farms is often justified on pure
economic grounds. It has been shown that when wind penetration is high, the mar-
ginal cost of reserve can exceed the marginal cost of energy (Tang et al. , 2014). In
these circumstances, it makes perfect sense to reduce wind farm power output below
potential, with most of the difference then being available as primary reserve. Wind
farms participating in such 'active' wind curtailment should benefit from the margin
of reserve over energy price. As always, the relevant market arrangement needs to
reflect the underlying economic reality. The wind farm reserve would be delivered
on the basis of the 4 per cent droop characteristic mentioned above, similar to
thermal generation. The frequency-sensitive power could then be adjusted by the
power control loop shown in Figure 3.27 for a variable-speed wind turbine generator.

5.3.6.4 Wind curtailment
When a power system is stressed, one option to help alleviate existing problems is
to curtail wind generation output. Curtailment may be required to reduce local
voltage, provide enhanced system security at times of minimum load (by avoiding
switching off a conventional generator), limit the ramp rates on conventional gen-
eration (during the daily morning rise) or avoid transmission system overloading