Environmental Engineering Reference
In-Depth Information
is needed regarding the demand that is assumed to be concurrent with the generation in cal-
culations. Ideally this should represent the worst case scenario and does not necessarily occur
when the maximum expected input from the renewable energy source(s) is concurrent with
the maximum demand. Second, overhead lines are naturally cooled by wind, which increases
their capacity during periods of high wind. Consequently lines installed primarily to carry
wind power can be undersized relative to conventional design guides.
6.3.2 Transformers
Determining the thermal limits for transformers is more complicated. The rating of a trans-
former is generally given in kVA or MVA, but there may be three different fi gures quoted,
corresponding to natural (convective) cooling, fan cooling and cooling by pumped oil. Fur-
thermore, transformers have long thermal time constants and thus can be overloaded for short
periods of time without causing overheating or signifi cant damage. Calculation of the damag-
ing effect of such transient temperatures can be diffi cult.
Transformers are generally chosen to match the expected maximum demand and are nor-
mally operated at a signifi cant proportion of their thermal limit, partly because standing (no
load) losses are signifi cant and so the effi ciency of a lightly loaded transformer is poor. Thus,
in areas with very high penetrations of embedded generation, thermal limits of transformers
can restrict further installation. In round fi gures, this only occurs when the rating of an embed-
ded generator connected below a particular transformer, less the minimum load in the same
area, exceeds the maximum load in that area.
Most transformers can accommodate reverse power fl ow, up to the normal forward rating,
without a problem. However, there are some (not many) on-load tap-changers that have very
limited reverse-power capability. Furthermore, some automatic voltage regulators (control
relays) associated with on-load tap-changers can be affected by reverse power fl ow.
Thus, the thermal limits of existing transformers are not normally a limiting factor in the
installation of distributed generators, unless the installed capacity of generators exceeds the
maximum demand in the area served. Such high penetrations are not common, at least in the
UK, although there is one example in Wales where two wind farms are connected below one
substation. Power fl ow through this substation is frequently reversed and occasionally genera-
tion has to be curtailed when the reverse power fl ow reaches the transformers' thermal
limits.
6.4 Other Embedded Generation Issues
6.4.1 Flicker, Voltage Steps and Dips
Flicker
Incandescent light bulbs can cause annoyance by fl ickering due to rapid voltage variations.
The human eye is particularly sensitive to fl icker at around 8 Hz. Sensitive electronic equip-
ment may also be affected. Such voltage variations are generally caused by rapid variations
of active or reactive power fl ows within the network. The classic causes of unacceptable
fl icker are sawmills in rural areas and arc furnaces. Distributed generators can also contribute
