Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 151

states must ensure that transmission system operators (TSOs) and distribution sys-
tem operators (DSOs) guarantee the transmission and distribution of electricity
produced from renewable energy sources, subject to the reliability and safety of
the grid. TSOs are also required to give priority dispatch to renewable energy
sources, insofar as operation of the power system permits. The costs associated
with grid connection and grid reinforcement should also be transparent and
non-discriminatory, while charges associated with use of the transmission and dis-
tribution network should not discriminate against renewable sources.
So, if significant wind generation is now introduced into the plant mix,
displacing conventional thermal plant, the task of load-frequency control will be
affected. Generator loading levels and ramping requirements, availability of
spinning reserve, amongst other issues, also need to be addressed. However, it is
worth remembering that, irrespective of current and future levels of wind genera-
tion, power systems are already required to cope with significant variability and
intermittency concerns. For example, on Wednesday 11 August 1999, following a
mid-morning solar eclipse over the United Kingdom, and as people resumed their
normal daily activity, the system demand in England and Wales rose from 33.2 to
36.2 GW (a jump of 3,000 MW) in just 5 minutes. In comparison, the wind farm
capacity connected to the National Grid (England and Wales) network in 2011,
12 years later, was comparable at 3,050 MW.
Furthermore, generator forced outage rates of 4-5 per cent can be considered
typical, although highly dependent on a variety of factors including plant age,
implemented technology and operator practice (Armor, 2003). Thus, unit com-
mitment is undertaken daily with the expectation, rather than the possibility, that
unplanned outages of the conventional generation plant may occur, leading to the
need for various categories of reserve (see Section 5.2). In general these outages are
not predictable, resulting in a rapid reduction from full output to zero output.
In order to reduce the immediate impact, some utilities, such as EirGrid (Ireland),
practice a slow wind-down, where possible, of the affected generator. Clearly, the
abrupt loss of a wind infeed, perhaps due to a problem internal to the wind farm, or
as a result of external network problems, could cause similar problems to the loss of
a thermal unit - the frequency would fall, and the remaining synchronised units
would increase their output to restore a demand-generation balance. The fact that a
wind farm consists of multiple turbines, whose individual operation is largely
independent of each other, significantly increases the overall availability of the
wind farm. For economic and possibly technical reasons (see Section 5.3.6),
the wind farm will probably not be scheduled to provide spinning reserve. Hence
the loss of a wind farm should be less significant than the loss of a thermal gen-
erator of similar rating. So, assuming that the largest wind farm block, likely to be
an offshore wind farm, does not represent the largest infeed, then the short-term
emergency reserve targets for the system will be unaffected.
Indeed, it has been argued that a power system's natural ability to cope with
unexpected events and existing load variations, through the provision of spinning
reserve and the part loading of units, ensures that low levels of wind generation
can be safely accommodated without undue cost or difficulty. In Germany,