WIND ENERGY SYSTEMS - Cd3wd

Chapter 6—Asynchronous Generators 6–35
Figure 20: Variation of line-to-line voltages and line currents of 40-hp induction generator
connected in circuit of Fig. 18. R L = ∞; C = 0.860 pu.
then a resistance R L can be added to draw off the extra power. As the total generator power
increases, so does the reactive power requirements so utility power factor is improved if some
additional capacitance is added in parallel with R L . Figure 21 shows the variation in voltages
and currents for R L = 3.72 pu (actual value of R L is 8.92 Ω) and C = 1.09 pu (actual value of
C is 700 µF) for the circuit in Fig. 18. The voltage V bc and the current I c stay nearly constant,
as before. The current I b has a minimum at P m = 0.35 pu and now I a has a minimum at P m
= 0.2 pu rather than increasing monotonically as before. The important item to note is that
the three currents and the three voltages are nearly equal at P m = 0.6 pu. This indicates the
generator is operating close to balanced conditions at this power level. Adjusting R L and C
will move this balance point either right or left. The balance point shown here is less than that
for rated conditions, which means the generator will again be unbalanced when rated current
is reached in one of the generator windings. In this particular case rated current is reached
in line a for a total electrical power of 27,100 W or 0.834 of three-phase rating. A smaller
resistance and a larger capacitance would be necessary to move the balance point toward that
for rated conditions.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001