WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–19
Distribution voltage trenches are shown which do not cross the low voltage trenches. This
is not an absolute technical requirement, but doing it will certainly remove the hazard of low
voltage cable crossing over high voltage cable.
After the trench lengths are determined, the circuit breaker CB2 must be selected. It is
quite possible that a good design will result in a rated loop current of 500 A or less. When we
go to the catalog to find a breaker with this rated current, however, we are surprised to find
that the minimum rating is 1200 A, with only two other choices, 2000 and 3000 A. The reason
for this is the strength requirements for withstanding large fault currents. Depending on the
impedance of the source, a circuit breaker may be required to interrupt between 10,000 and
50,000 A. It takes a finite amount of time to detect the fault, send a signal to the breaker,
mechanically move the contacts, and extinguish the arc, and during this time the contacts
must withstand this fault current. Contacts large enough to withstand such fault currents are
large enough to handle at least 1200 A on a continuous basis. The breaker can be operated
on any value of current less than this, of course. Estimating prices for circuit breakers are
shown in Table 9.6.
The transformer T 2 will step up the windfarm distribution voltage to the value necessary
to tie into the utility’s transmission network. Transformers of this size are not shelf items.
Many options must be specified at the time of ordering, and then a specific price can be
quoted. We shall present the basic procedure used by Westinghouse to illustrate the concept.
The list price for a Westinghouse three-phase transformer with rating 2500 kVA or larger
is given by the formula
C = C 1 M 2 M 3 M 4 (1 + PA/100) + C 6 + C 7 (8)
where C 1 is the base list price, M 2 is an efficiency multiplier, M 3 is an operating voltage
multiplier, M 4 is a frequency multiplier, PA are the percentage adders, C 6 is the cost of load
tap changing equipment, and C 7 is the cost of various dollar adders. We shall briefly discuss
each of these items.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001