Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 174

5.3.3.1 System interconnection
A convenient method of reducing regional wind variability is through system
interconnection. Of immediate interest, synchronised interconnection permits
aggregation of loads and generation over a wide area, increases significantly the
rotating inertia of the system during severe transients and reduces the reserve
burden that individual regions must carry. It is, therefore, believed to follow that
large-scale integration of wind power will be easier than for isolated, or asyn-
chronously connected, power systems. The logic proceeds as follows: wind
variability decreases as the area of interest increases and the output of individual
wind farms are aggregated together. So, while Ireland may possess significant
variability in wind power production, consideration along with Great Britain
reduces the effective variability. Applying the same process further, for example
to mainland Europe, results in a resource of increasing time invariance - the wind
always blows somewhere! Hence it is asserted that, with more interconnection,
wind power can be wheeled from areas of high production to areas of low
production.
Denmark is a prime example of the benefits, being a member of the European
continental grid network. Transmission links are in place with neighbouring
countries such as Germany, Norway and Sweden, which have enabled Denmark to
achieve 28 per cent wind penetration (Wiser and Bolinger, 2013). About 60 per cent
of the remaining generation is provided by CHP plant, much of which operates
according to the heat demand and time-of-day tariffs, rather than electrical demand
requirements. It therefore falls upon the small fraction of conventional generation
to provide grid stabilisation and reserve duties. As a result, Denmark uses its
external links to both export its increasing surplus of electricity production and
import spinning reserve capability (Bach, 2005).
For a number of reasons, however, an expansion of system interconnection
cannot solve the issues surrounding wind variability. Considering the example of
the European continental network, power trading occurs between national grids, as
defined by contracts agreed over 24 hours in advance. Transmission capacity
will be reserved well in advance to meet these agreements. Automatic generation
control (see Section 5.2) is applied within control areas to ensure that cross-
boundary power flows are as specified. The temptation may exist to exploit the
interconnection capacity to spill excess wind generation or fill wind generation
shortfalls. Assuming that scarce interconnector capacity would even be made
available to accommodate wind power imbalances, penalties are likely to be
imposed for not complying with agreed power exchanges. It will probably be more
economic to spill excess wind power when transmissions networks are congested,
rather than construct additional interconnection capacity to access the small amount
of additional energy involved. In Denmark, for example, it is realised increasingly
that relying on external sources for regulating power can be risky and expensive,
while limiting the export transfer capability. Domestic sources of reserve have been
examined, with the focus on centralised CHP units to decrease electrical production
during periods of high wind, and use of small-scale distributed CHP plant for
load-following duties (Lund, 2005).