WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–14
nominal transformer rating. Should we select the next larger size of transformer, or might we
get by with the next smaller size?
Example
Assume that we have five turbines rated at 50 kVA each. Should we select a 300 kVA transformer
or a 225 kVA transformer?
The smaller transformer would be operated at 11 percent over its rated value during the times
when all turbines were operating at full power. We would save $6500 - $6000 = $500 of initial cost,
and 554 - 481 = 73 W of no load losses, amounting to 648 kWh/year. The load losses are higher, of
course. Copper losses are proportional to the square of the current, and the current is proportional to
the load kVA (since voltage is essentially fixed). Therefore the copper losses at 250 kVA would be, for
each transformer,
P loss,225 =
( ) 2 250
(1476) = 1822 W
225
( ) 2 250
P loss,300 = (1872) = 1300 W
300
The transformers must be operated at this power level for over 1200 hours per year before the extra
copper losses exceed the reduced eddy current and hysteresis losses (no load losses) of the smaller
transformer. The larger transformer will always have smaller load losses than the smaller transformer
for the same load, but the total loss will be less for the smaller transformer whenever the load is less
than about 20% of rated. This will be the situation for more than half the time at most wind farms,
so a detailed economic study could easily show the smaller transformer to be the economic choice.
But what about damage to the transformer by operating it in an overloaded condition?
It turns out that these transformers can be operated at 113 percent of rated power for up
to four hours in ambient temperatures of 40 o C (104 o F) without a reduction in normal life.
This would be a no wind condition, but full power operation in a windfarm would always be
accompanied by strong winds with resultant cooling. Also it would be rare indeed for full
power to be maintained for over four hours at temperatures as high as 40 o C. Lower ambient
temperatures would also increase the allowable overload. Therefore, it may be appropriate to
select a transformer with a rating up to about 10 percent smaller than the generator rating,
rather than the next size larger. If this is done, it may be necessary to monitor the transformer
temperature during extended periods of high power operation. If the transformer temperature
should exceed a safe level, one of the turbines can be shut down until the transformer has
cooled down.
5 SELECTION OF SIZES, DISTRIBUTION VOLTAGE EQUIP-
MENT
The next step is to select the wire size on the high voltage side of T 1 . The rated current is
determined from
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001