WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–28
power for 20% of the time and is off the remaining time, or is producing 20% power all the
time. In the first case, the yearly energy loss would be
W s1 =(0.2)(8760)(3)(I) 2 R s (12)
and in the second case, if we assume the current drops to 0.3I for 20% power,
W s2 =(1.0)(8760)(3)(0.3I) 2 R s =0.45W s1 (13)
W s1 is an upper bound for conductor losses. Depending on the variation of current with
power, W s2 is a reasonable estimate for the lower bound of losses. An assumption of 0.6W s1
or 0.7W s1 should be adequate for most purposes.
A similar argument can be made for the load losses of transformer T 1 . The current will
be the same as in the low voltage conductors, and the resistance will just be that of the
transformer windings. An estimate of (0.6)(CF)(8760)P LL should be quite acceptable.
The no load losses of T 1 can be found by multiplying the no load power from Table 1 by
the number of hours per year that the transformer is energized. Significant amounts of energy
can be saved by opening the circuit breakers CB2 during long periods of low winds. These
circuit breakers are not intended for frequent cycling, but a few times each week should be
acceptable.
The losses of the distribution voltage conductors can be determined in a manner similar
to that of the low voltage conductors. Actually, the losses in these conductors will be much
lower than the low voltage conductor losses and can be ignored without significant error. The
reason for this is that the distribution voltage conductors must be sized for the worst case
condition of one circuit breaker open and all the loop current flowing through the other one.
In this case, the loop current increases from zero next to the open circuit breaker to rated at
the operating circuit breaker. The average current in the loop would be approximately half
the rated current, with losses on the order of one fourth the losses we would expect if the
entire loop carried the same current. In normal operation, however, both circuit breakers CB2
will be operating, so the maximum current in the loop will be half the rated current. At some
point around the midpoint of the loop, the current will actually be zero. The losses in this
case will be on the order of one tenth the expected losses for a uniform current throughout
the loop. This will usually be less than 0.5% of the energy produced by the windfarm, hence
is not very significant.
8 PROTECTIVE RELAYS
The circuit breakers are operated by a variety of protective relays which sense various operating
conditions that may be harmful to the utility, the windfarm, or to operating personnel. Some
functions, such as overcurrent, would be common to all the circuit breakers. Others, such as
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001