WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–13
A loop feed, on the other hand, allows electrical power to reach a transformer in one of
two paths, as was shown in Fig. 2. A short in a buried distribution line can be isolated in
a loop so that repair work can proceed without power production from any turbine being
interrupted. A loop connection is therefore very desirable if it can be justified economically.
A windfarm with two transformers T 1 may not be able to justify a loop but one with ten or
more transformers would almost certainly need a loop. This adds a total of $600 to the cost
of each T 1 , as seen in Table 9.3.
The switches in a loop are typically of the LBOR (loadbreak oil rotary) type. There are
two switches, one for each direction of the loop. Each switch has two positions, on and off.
If one switch is on and the other is off, power is coming to the transformer from only one
direction and the loop is open at this point. If both switches are off the loop is open and the
transformer and the associated wind turbines are not energized. If it is desired to have the
loop closed at this point but for the transformer to not be energized, then the circuit between
the loop and the transformer must be opened with another device, typically a fuse on the
transformer itself. Two of these LBOR switches costs an additional $500 over the base cost
of the transformer.
Also important are lightning arresters, which are devices connected between a phase conductor
and ground which start to conduct when the voltage exceeds some rated value, as will
happen when lightning strikes the conductor. In a windfarm, lightning will probably hit one
of the wind turbines rather than a padmounted transformer or a buried distribution cable,
so arresters may not be absolutely essential at this point. However, the windfarm location
is likely to be the highest point within several miles, with poorly conducting soils (or rocks).
Every thunderstorm is likely to produce several lightning strikes within the confines of the
windfarm. Given the unpredictability of lightning, it is probably wise to put arresters on each
transformer, at an additional cost of $400.
The column labeled NLL indicates the No Load Losses for each transformer. The 750 kVA
transformer consumes 1112 W continuously while energized, even when no power is flowing
through the transformer. This means that 9741 kWh will be dissipated as heat in a one
year period, if the transformer is continuously energized. If electricity is worth $0.05/kWh,
this amounts to $487/year, or almost 5% of the initial cost of the transformer. Less efficient
transformers can be manufactured at a lower price, but these can rarely be justified by a
careful economic analysis.
The next column, labeled LL, shows the Load Losses in watts for full load conditions.
These are the copper losses for the transformer. (We refer to I 2 R losses as copper losses even
if the transformer is actually wound with aluminum wire.) The economically optimum ratio
of LL/NLL depends on the price of electricity and on the duty factor of the transformer. A
transformer which is only occasionally operated at full load can have a somewhat higher value
of LL as compared with the transformer being operated with a very high duty factor. The
values given in Table 9.3 are close to the economic optimum for a typical utility in 1991.
Suppose that the combined rating of a cluster of wind turbines is not exactly equal to a
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001