Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 198

Before wind curtailment can be implemented, individual wind farms will
require integration with a centralised SCADA-type system. Turbine control
systems should be capable of receiving an external power setpoint from the TSO,
and the ability, under extreme conditions, to be constrained off remotely. Grid
codes are requiring these facilities for new wind installations. In E.ON, for exam-
ple, a wind farm, upon instruction from the TSO, should be able to reduce its output
to a reference value at a ramp rate exceeding 10 per cent of the rated capacity per
minute without tripping. Similarly, wind farms in Ireland should be capable of
receiving an external setpoint, curtailing output. Denmark and Sweden also require
that it should be possible to reduce output to less than 20 per cent of registered
capacity with 2 seconds (Energinet.dk) and 5 seconds (SvK) - this facility is
thought to be required mainly under fault conditions and prevents rotor overspeed.
E.ON and Energinet.dk also require that wind turbines can be connected and dis-
connected remotely.
Future wind farms may also be sized with the probability, rather than the
possibility, of curtailment. Network investment can often lag behind wind farm
expansion, even though areas with strong wind regimes may suggest high wind
farm concentration. Consequently, expensive upgrading of weak networks should
be balanced against the probability of distribution, or even transmission, conges-
tion. In other words, does the benefit of a larger wind farm in terms of increased
average energy production exceed the lost opportunity cost of brief periods of
curtailment? This is likely to occur when wind power production is high, local load
is low and consequently the local voltage is unacceptably high (Dinic et al. , 2006).
At such times the energy price will undoubtedly be relatively low.
For most utilities there are unresolved issues regarding who should suffer the
financial costs of lost wind power production. Does the potential need for curtail-
ment suggest that wind generation should be centrally dispatched or profiled?
There is, however, a consensus that spilling of wind should only occur when no
other alternative strategies exist. Minimum system demand, when generating units
are already operating close to their operational limits, is often considered as a
period when high wind production may lead to curtailment. However, the value of
generation at such times is likely to be low. Market incentives can confuse the
matter further because the wind farm operators may receive payment for green
certificates, in addition to energy (see Chapter 7 - Electricity Markets). In countries
where an ancillary services market exists it may be possible to profit from the
imposed curtailment by supplying spinning reserve and/or load following duties.

5.3.7 Wind turbine generator inertial response
Following the loss of a major system infeed, the initial ROCOF will depend on the
magnitude of the lost generation, and the stored energy of the system, as expressed
earlier in (5.1):

df

dt ¼ D P f 0

Hz = s

2 E system