Environmental Engineering Reference
In-Depth Information
response by fast increase in power from part loading through blade pitch control in response
to drops in frequency and through the same mechanism provide high frequency response
through fast reduction in power in response to increases in frequency.
As wind power capacity has increased, to the extent that at times it is the dominant form
of generation in parts of Denmark and Northern Germany, there is an increasing demand for
wind capacity to be dispatchable and to behave more like conventional generation. Very large
wind farms are now expected to conform to connection standards that limit ramp rates for
increase in power and also to contribute to frequency regulation under times of high network
stress. These requirements are increasingly included in the national grid codes that regulate
access to the public networks. In these early days it is unclear to what extent this will result
in wind power being curtailed, for example to comply with given ramp rates, and to what
extent such constraints add value to the system operator.
Conventional steam generation plant assist the network frequency stability at the onset of
a sudden imbalance of demand over supply by slowing down. Wind turbines respond differ-
ently. The stored energy is in the rotor inertia and fi xed speed turbines will provide a limited
benefi t from their inertia provided that the voltage and frequency remain within their operat-
ing limits. Variable-speed wind turbines will not normally provide this benefi t as their speed
is controlled to maximize the energy production from the prevailing wind.
Large wind turbines are now almost always of the variable speed type and as they increas-
ingly displace conventional generation the total system inertia from such generation will
decrease. Consequently the rate of change of frequency and the depth of the frequency dip
caused by a sudden loss of generation will both increase. However, variable speed wind tur-
bines could be controlled in principle to provide a proportionately greater inertial energy to
the system than conventional plant of the same rating. Such sophisticated control arrange-
ments to support system functions are likely to be requested by utilities as wind penetration
increases.
Finally, grid codes require wind turbines to maintain power infeeds to the system even
under transient local voltage reductions. Such reductions are usually due to fault conditions
in the vicinity of the wind farm. It can be shown that maintenance of power infeed from all
generators is essential to ensure system recovery after a fault clearance.
3.6.2 Biofuels [9]
Traditional thermal plant could be described as capacity limited , i.e. capable of theoretically
generating its rated output continuously, as gas, coal, oil or fi ssionable material is abundantly
available on demand. In contrast, an energy crop based plant could be described as energy
limited because the locally harvested fuel is limited in nature and may or may not be capable
of sustaining all year round continuous plant generation at full capacity. Transporting biomass
fuel from remote areas would not be economical.
A biomass plant would be expected to operate as a base-load generator running as far as
possible at full output. Such plant would be able to contribute to continuous low or high fre-
quency response services similarly to a conventional plant. For a low frequency response the
plant would need to run part-loaded, a convenient strategy providing extra income if, say,
due to a low crop yield year the stored fuel would not be capable of servicing continuous full
output.
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