WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–25
size of around 350 kcmil. Beyond that point, the wire cost increases more rapidly while the
energy saved becomes smaller, so the cost per kWh begins a slow increase.
The low voltage wire loss can be determined from Fig. 5, which shows a single turbine
connected to a transformer T 1 by underground conductors in a trench of length D T .Allthe
low voltage wire in a windfarm carries approximately the same current, so the total loss can
be found by using the total low voltage trench length. There are two possibilities, either single
circuit or double circuit, shown as Fig. 5b and Fig. 5c. Each of the three conductors in the
single circuit case will carry the full turbine current I. The additional ampacity of the double
circuit case is obtained by simply paralleling two conductors for each phase. There are now
six conductors in the trench, each carrying a current I/2.
The power dissipation in the single circuit case is
while the power dissipation in the double circuit case is
P s =3I 2 R s (10)
P d =6(I/2) 2 R d =1.5I 2 R d (11)
where R s and R d are the resistances of a conductor of length D T . Itistemptingtoassume
that P d is smaller than P s because the multiplying factor is half as large (1.5 rather than 3).
But it should be remembered that R d is larger because smaller wire is used in the double
circuit case. In fact, the losses will be exactly the same if the conductor size in the double
circuit case is half the size of the single circuit conductor. It does not make any difference
in losses if the total conductor area of 500 kcmil, for example, is obtained from a single 500
kcmil conductor or from two conductors, each of 250 kcmil area.
Example
A wind turbine rated at 220 A is located 300 ft from its circuit breaker CB1 and transformer T1.
Assume that the trench for a single circuit costs $1/ft and $2/ft for a double circuit. Wire costs for
600 V conductors are given in Table 9.5. What is the cost/ft for the single and double circuit cases
and what is the power loss at rated current for each case?
As mentioned above, the wire must have a rating of 1.15(220) = 253 A. We see in Table 9.2 that
3/0 aluminum has a rating of 272 A in the single circuit case. Each of the individual conductors in
the double circuit case must have a rating of 253/2 = 126.5 A. This is met by 2 AWG, rated at 151 A.
The cost per ft for the single circuit case is
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001