Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 177

relative size of the generators to that of the power system itself. However, consider
a case during the evening period when the load falls off naturally and conventional
generation reduces its output accordingly. If wind power is also increasing at the
same time, is the conventional generation plant sufficiently flexible to balance the
system load? Should wind output be constrained in advance? Should conventional
generation be de-committed early? Similar questions can be posed during the
morning rise in demand. In general, conventional generation is able to cope with
worst-case ramp rate scenarios, over time scales up to 1 hour. (Over longer time
horizons, stand-by generation may be required.) Prudence would suggest that, as
wind penetration increases, strict limits must be placed on the rate of change of
wind power production. Such limits should apply under all conditions, i.e. turbine
start-up, normal operation and shutdown. Power variability from an individual
wind farm may also result in local voltage problems, as was seen in Chapter 4.
Staggered connection prevents several turbines starting at the same time, and
hence a reduction in the initial loading rate (see also Section 4.2). Some grid codes,
for example EirGrid (Ireland) and E.ON (Germany), require that ramping rate
requirements are complied with during start-up, in addition to normal running. It is
also advisable that wind turbines should not be permitted to start if the frequency is
high - indicative of excessive generation supply. In Ireland, a ramp frequency
controller is set to prevent ramping upwards when the frequency is 50.2 Hz or
higher, implicitly including turbine start-up.
Staggered shutdown is the natural complement to staggered start-up, although
not quite so straightforward to introduce. Turbines are likely to be shut down
because of high wind speeds, and hence any delay in doing so implies increased
maintenance costs. A typical cut-out speed for modern turbines is 25 m/s. Above
this speed the turbine shuts down and stops producing energy. A hysteresis loop
and a programmable delay are usually introduced in the turbine control system,
such that small changes in wind speed around the cut-off threshold do not require
the turbines to persistently stop and start. Restart of the turbine may require a
(hysteresis) drop in wind speed of 3-4 m/s. Recent turbine designs have focussed
on continuous operation during such high wind speed conditions, with electrical
output gradually curtailed as wind speed approaches 35 m/s, as shown in
Figure 5.27. Such an approach can mitigate wind variability, while increasing
energy capture under extreme conditions.
A TSO may require that individual turbines within a wind farm have distinct
(but similar) cut-out speeds, providing a gradual, rather than sudden, 'wind-down'
of wind farm production. Two thousand and five hundred megawatts of wind
generation was lost in the German E.ON grid on 26 February 2002 due to high wind
speed protection over several hours. As a result, Energinet (Denmark) and Svenska
Kraftn¨t - SvK (Sweden), for example, both require that high wind speed must not
cause simultaneous cut-out of all wind turbines. There may also be benefits in
signalling early warning to the regional control centre of a potential overspeed
condition. At a higher level, prediction methods and weather warnings can provide
indication of imminent high wind and/or storm conditions. A phased shutdown over
30 minutes, a requirement of the former Scottish grid code, would ensure that the