Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 204

50.1 400

Wind generation
System frequency

50

300

49.9

200

49.8

100

49.7

49.6 0

0 5 10 15 20 25

Time (s)

Figure 5.36 Fixed-speed turbine inertial response

There is thus the danger of widespread nuisance tripping, whereby ROCOF
protected wind farms, and other embedded generation, may be tripped unnecessa-
rily. The severity of the initial event may therefore be increased significantly,
leading possibly to load shedding. Figure 5.36 illustrates the response of a fixed-
speed turbine to a frequency transient on the Ireland system. Although an inertial
response (see Section 5.3.7) is provided initially, the high ROCOF ( 0.18 Hz/s), as

the result of a generator loss elsewhere in the system, causes the ROCOF relay
(0.125 Hz/s threshold) to activate incorrectly. (This particular relay was installed
before the threshold was changed to 0.5 Hz/s.)
Vector shift protection offers an alternative to ROCOF protection, and is
preferred on more recent wind farm installations. Again relying on a power
imbalance causing the island frequency to change, the period of the electrical cycle
is monitored. The recommended UK setting is a vector shift of 6 at 50 Hz,
equivalent to a 0.33 ms change in the period. Assuming that the vector shift is
sustained (following a frequency transient) this figure equates to a ROCOF of
0.83 Hz/s - just about conceivable on the Ireland system. However, as discussed
earlier in Section 5.3.7, variable-speed wind turbines do not naturally provide an
inertial response, since the turbine rotor rotational speed is independent of the
system frequency. Any reduction in the effective system stored energy (due to
variable-speed turbines displacing conventional generation) will thus further

increase the ROCOF, and the likelihood of widespread nuisance tripping.

5.4 Energy balance

When wind energy penetration reaches around 15 per cent in an isolated power
system, wind curtailment is almost inevitable. The issue is best understood
by considering potential wind generation for a typical system demand pattern.