WIND ENERGY SYSTEMS - Cd3wd

Chapter 6—Asynchronous Generators 6–34
Figure 19: Phasor diagram for circuit in Fig. 18 at balanced operation at unity power factor.
generator described in the previous section. The capacitance C = 0.860 pu (actual value is
550 µF). The resistance R L was omitted so all the power is being delivered to the utility. The
generator voltage of 230 V line to line is used as the base, but the available utility voltage was
actually a nominal 208 V. This explains why V ab always has a value less than 1.0 pu, since
the utility connection did not allow the generator voltage to reach 230 V.
At values of P m near zero, the current I a being supplied to the utility is also near zero. This
forces I b and I c to have approximately the same magnitudes. As P m increases, I a increases in
an almost linear fashion. The voltage V bc across the capacitor and the current I c through it
remain essentially constant. The current in phase b decreases at first and then increases with
increasing P m . The voltage V ab to the transformer increases from 0.92 pu to 0.99 pu as P m
increases, due to voltage drops in the transformer and wiring.
The current I a reaches the machine rating at a value of P m of about 0.6 pu. As mentioned
earlier, a generator should supply up to two-thirds of its three-phase rating to a single-phase
load, but because of lower efficiency the generator limit will be reached at a slightly lower
value. For this particular machine, the three-phase electrical rating is about 32,500 W. Rated
current was reached at 21,000 W as a single-phase machine or 0.646 of the three-phase rating.
This is just slightly under the ideal value of 0.667.
The efficiency drops if a larger capacitor is used. This increases I c , which increases ohmic
losses in that winding without any compensating effect on the losses due to imbalance. A
value of I c of about half of rated seemed to give the best performance over this range of input
power. This makes its value close to the average values of I a and I b for this range of P m ,
which is probably close to the optimum value.
If there is enough power in the wind to drive the generator above two-thirds of its rating,
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001