WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–16
= 29,400 kVA. A windfarm with a rating larger than this would need to have two loops, each
loop having two circuit breakers CB2 connected to the low voltage bus on transformer T 2 .It
might be cheaper and more efficient to design for two loops for power levels well under this
level. Two single circuit loops of approximately the same length as one double circuit loop
would certainly be cheaper. The question would be whether the savings in cable would offset
the cost of two additional circuit breakers.
Costs for underground conductors in dollars per 1000 ft are given in Table 9.5. The table
includes both the 600 V and the 15,000 V rating. The former would be used between the
turbine and the transformer T 1 and the latter after T 1 . The thicker insulation of the 15
kV conductor obviously makes it substantially more expensive. There will be three of these
conductors in a three-phase circuit (six for the double circuit case), so these prices must be
multiplied by three (or six) to get the circuit cost per foot. The price of the 15 kV wire
includes a neutral wrapped around the insulation of each conductor, so no additional neutral
wire needs to be purchased.
In addition to the cost of conductors, we have the cost of trenching. This will cost about
one dollar per foot for the single circuit case in good trenching situations and more where
there are rocks or other problems. The cost of trenching will approximately double for the
double circuit case.
Table 9.5. Underground aluminum conductor
cost in dollars per 1000 ft of conductor
Size 600 V 15 kV
6 $107
4 135
2 178 $690
1 253 780
1/0 287 860
2/0 303 1070
3/0 363 1280
4/0 425 1500
250 585 2060
350 799 2800
500 1100 3850
750 1600 5300
1000 2063 7220
The total trench length required for both the low voltage and distribution voltage circuits
can be best calculated from a sketch of the site layout. For cases where several turbines are to
be connected to one unit transformer it would be wise to check total trench length for more
than one possible configuration. For example, if eight turbines are to be connected to one
transformer, is it better to put all eight in one row, or should the transformer be connected
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001