Environmental Engineering Reference
In-Depth Information
countries of Denmark, Germany and Spain which are of greater interest. Clearly,
Denmark has the highest wind farm density, but this represents about 75 per cent of
the interconnection capacity. At the end of 2004, Germany and Spain had installed
wind capacities of 16.6 and 8.3 GW respectively. However, while this translates into
a relative export capability of approximately 85 per cent for Germany, the equivalent
figure for Spain is 250 per cent. In other words, Spain has reduced opportunity for
transient support from neighbouring countries. In 2005, wind power capacity in
Spain grew by a further 1,764 MW, second only in Europe to Germany (1,808 MW).
The Portuguese and Spanish TSOs have carried out joint studies into the effects of a
sudden loss of wind generation following a system disturbance. The increase in
power flow from neighbouring areas can cause overloading of the limited inter-
connection capacity, included ties from Spain to France, leading to a requirement for
wind curtailment under certain operating conditions (Pe¸as Lopes, 2005). Ireland
also presents an interesting example. Although connected by a submarine HVDC
cable to Scotland (and later to Wales), market arrangements have militated against
frequency control and reserve provision being enabled (SEI, 2004). So, except under
emergency conditions, Ireland is dynamically isolated from Great Britain.
The low capacity factor (typically between 30 and 40 per cent) of wind
generation also counts against further interconnection. Except countries such as
Denmark and Scotland where annual energy production exceeds local demand,
periods of excess wind production can occur during low demand periods when
electricity prices are low. The potential value of large-scale wind power import/
export is therefore unlikely to justify investment in transmission. So, given the
economic and environmental difficulties of building additional transmission capa-
city to facilitate occasional large-scale wind imbalances, it is likely that individual
countries with ambitious wind production targets must manage the uncertainty and
variability of the ensuing generation largely within their own networks. However,
there is a potential caveat to this statement. The European Commission identified a
lack of system interconnection as one of the main barriers to effective competition
in European power markets. In 12 out of the 14 EU-15 member states the top three
utilities controlled more than two thirds of the market, while the average capacity
share of the generation market of the top three utilities was 76 per cent (EWEA,
2005). Amongst various solutions proposed by the European Commission and
European Council is that the volume of interconnection capacity should equate to
10 per cent of the installed production capacity. Increased import should, of course,
reduce dominant positions, and almost as a side effect facilitate increased wheeling
of wind power from Spain to northern Germany say, or vice versa.
5.3.3.2 Wind turbine ramping control
Grid codes normally specify a minimum ramp rate for conventional generation.
In Ireland, the requirement is set at 1.5 per cent of registered capacity per minute, or
90 per cent per hour, while in Holland (TenneT), forming part of the interconnected
continental system, the equivalent target is 7 per cent of registered capacity per
minute, ensuring full plant availability after 15 minutes. The ramping requirements
will have been based upon the known variability of the system demand, and the
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