WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–15
I = S wf
√
3V
(7)
where S wf is the total three-phase VA of the turbines connected to each loop and V is the
high side line to line voltage. For the case of 10,000 kVA and a high side voltage of 12,470 V,
this is a current of 464 A. The conductor in the loop type circuit needs to be sized to handle
all the current of all the generators in case one of the circuit breakers CB2isopen.
The appropriate tables in the NEC then need to be consulted. These are Tables 310-81
and 310-82 of the 1990 NEC for copper and aluminum conductors, respectively. The ampacity
values for conductors rated at 15 kV are summarized in Table 9.4. The table also includes
ampacities for three conductors in a trench or six conductors in two adjacent trenches (or a
single wide trench), similar to the low voltage case discussed in the previous section.
Table 9.4. Ampacities of buried
conductors, 15 kV class
AWG COPPER ALUMINUM
one two one two
kcmil circuit circuits circuit circuits
6 130 120 100 95
4 170 160 130 125
2 210 195 165 155
1 240 225 185 175
1/0 275 255 215 200
2/0 310 290 245 225
3/0 355 330 275 255
4/0 405 375 315 290
250 440 410 345 320
350 535 495 415 385
500 650 600 510 470
750 805 740 635 580
1000 930 855 740 680
The required wire size for a 10,000 kVA collection of turbines (rated current 464 A at 12.47
kV) would be a single circuit of 500 kcmil aluminum, or a double circuit of 3/0 aluminum. The
double circuit case would have about half the aluminum of the single circuit case because of
better heat transfer characteristics. For typical costs of aluminum conductors and trenching,
the single circuit configuration is least expensive up to wire size AWG 4/0, which represents
akVAratingof √ 3(12.47)(315) = 6804 kVA. For a slightly higher rating, say 7200 kVA, it is
cheaper to use a double circuit with wire size AWG 1.
Table 9.4 shows that the maximum allowable current is 680 A per circuit for the double
circuit case, or a total of 1360 A. This would correspond to a kVA rating of √ 3(12.47)(1360)
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001