Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 178

Rated output

0 4 25 35

Wind speed (m/s)

Figure 5.27 Modified wind turbine power curve

downward ramp rate is not excessive. However, just as for the case of staggered
shutdown in low wind conditions, any advance in this action implies a loss of
energy production.
When the turbines are operational, the positive ramp rate can be controlled
easily by adjusting the rotor pitch angle (see later, Figure 5.30a). This operation can
be implemented independently for each turbine or co-ordinated across the entire
wind farm. In contrast, the output of stall-controlled (passive) wind turbines cannot
be readily controlled. However, above rated wind speed, and depending on the
configuration and design of a particular pitch control system, stall-controlled wind
turbines commonly present power fluctuations of lower amplitude. The German
(E.ON) maximum ramping rate specification is 10 per cent of turbine rating per
minute, while in Ireland two settings are specified - ramp rate per minute and ramp
rate over 10 minutes. The 1-minute ramp rate is set currently at 8 per cent of regis-
tered capacity per minute (not less than 4 MW/min and not higher than 12 MW/min)
while the 10 minute ramp rate is 4 per cent of registered capacity per minute (not less
than 1 MW/min and not higher than 6 MW/min). In Great Britain, the ramping
requirements are defined by the size of the wind farm - no limit for wind farms up
to 300 MW capacity, 50 MW/min between 300 and 1,000 MW capacity, and
40 MW/min beyond 1,000 MW in size. With sufficient notice, the ramp rate should
be adjustable by the TSO with increasing wind penetration. In Ireland, for example,
both settings (per minute and per 10 minutes) should be independently variable
over the range 1-30 MW/min. In Energinet.dk (Denmark), the ramp rate should be
adjustable within the range of 10-100 per cent turbine rating per minute.
This can become a useful tool for the TSO following the daily peak demand
period, for example, when the remaining generating plant are required to track the
general fall off in demand. At higher wind penetration levels, there may also be
concern should wind variability approach the maximum ramping rates of conven-
tional generation. Finally, limiting the negative ramp rate must be considered more
challenging, as it requires a degree of forecasting. Due to the possibility of a sudden
drop in wind speed, it would be difficult to constrain to a maximum negative
ramp rate on the time scale of minutes. However, in theory this should be more