Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 197

(following network faults or other failures). More dramatically, wind generation
may be reduced gradually in advance of a forecast storm front - before wind farm
protection trips out individual turbines due to the high wind speeds. Additionally,
considering mainland Europe as an example, uneven growth in wind generation
could lead to fluctuating power exchanges across international tie lines. Excessive
export of power can be curtailed by taking conventional generators offline, at the
expense of reduced dynamic response and system fault level. With wind farms of
increasing size, rapid curtailment of individual sites becomes feasible, rather than
communicating with a distributed network of small turbines.
For most power systems wind curtailment is a backup control option being
considered for the future when wind penetration levels are likely to be much higher.
However, in Crete, wind curtailment has been required for stability reasons for
some years at times of high wind output and low system demand, in order to
maintain a minimum number of committed conventional units. Fortunately,
high winds tend to coincide with the high demand summer period, implying that
curtailment is mainly required during the winter. In 2001 curtailment was already
6 per cent of annual wind production, rising to 11 per cent by 2002 (Papazoglou,
2002). During winter (November-January) curtailment is accentuated, rising to
almost one quarter of wind power production. The concentration of wind farms on
the east of the island, and hence a lack of diversification, accentuates the difficul-
ties. With nearly 400 MW of new wind farm proposals, and a system capacity of
581 MW, these figures were expected to grow considerably from the installed wind
capacity of approximately 70 MW (Kaldellis et al. , 2004). In California, wind
production tends to be high during the late spring months. At the same time,
melting mountain snow also causes a peak in hydroelectric generation (Piwko
et al. , 2005). During the night, when the demand is low, there can be an excess of
generating capacity, on occasion requiring the curtailment of wind generation.
Even in mainland Europe, a requirement for wind curtailment is not that
uncommon. In Spain, concerns about transient stability, particularly during periods
of minimum demand, has led to wind being curtailed several times to maintain
sufficient online conventional generation. Denmark presents an equally challenging
problem of meeting heating demand and electrical demand requirements, particu-
larly during cold, windy periods. Within the Energinet.dk system, 1,500 MW of
CHP plant, in addition to 2,400 MW of wind power, feeds the distribution network,
providing almost 50 per cent of the annual energy production (Bach, 2005). The
distributed CHP plant are not dispatchable, instead they follow heating tariffs and
local supply contracts. This can cause difficulties when wind production is high,
while heating demand is also high but the electrical demand is low. Again, wind
production must be curtailed so that sufficient centralised generation can remain
online to provide frequency regulation and other ancillary services. Since January
2005, however, legislation has required distributed CHP units ( > 10 MW) to
respond to (electricity) market signals. In conjunction with measures such as elec-
tric water heating and flexible demand, discussed later in Section 5.4, it has been
suggested that further wind curtailment could be avoided until its penetration
reaches 50 per cent (Lund, 2005).