Page 544 - Industrial Power Engineering and Applications Handbook

16/518 Industrial Power Engineering and Applications Handbook
I, . z, = /, ' z,

Y-' '
IC '
R, = R2 = R (assumed)
-.---*
\
\
ab = active component remains the same Le., 21 remains the same
I. cos g= 1;. cos g;= 1;. cos g;
bc, bc, and bc, = only reactive components vary

Figure 16.21 Variation in the load currents with a change in the field excitation on load

system frequency is required to remain constant at fb Asbuming that the above change in the excitation
it will be essential to reduce the power input to PM2, causes the following changes in the basic parameters
so that the drooping curve of PM2 shifts below to
0°C' and the load line to CC' at the original frequency (i) I, to Z; at a p.f. cos 4,'
fb, delivering the same total load yet again. The load (ii) l2 to 1; at a p.f. cos @;
shared by G2 is now still lower than before.
The load sharing by the two machines can thus be then 7; = 7, + 7,
varied by shifting the drooping curves of the prime I;cos@;=I, COSQ, =IcosQ
movers by altering their power input.
2 By changing the excitation (field current) Now I; = I, - r,
that the engine input is not varied, there is no variation
in the load sharing by the two machines. It is the I; cos 4; =I2 cos $2 = ICOS Q
basic theory of change in the field excitation. Since a The active components will thus remain same, and
change in the excitation causes a variation in the only the reactive components I( sin 4' and
generated e.m.f.s (E, and E2), the variation in voltage I; sin $2' will vary.
causes a corresponding rise or fall in the reactive
component of the current. EI cos Q will remain the Note
same, except the variation in the copper losses (I,'R), If the machines were loaded unequally before making a change in
which may vary the load sharing, marginally up or the excitation, the same ratio of loading would continue even after
down, depending upon whether it supplies I, or the change in the excitation of both machines provided that V,
receives. For instancc, at higher excitation, the e.m.f. remains the same, i.e
will rise and so will the load current, but at a lower
p.f. the generator will have to feed extra losses and
thus share a marginally lower load than previously.
To illustrate the above, consider Figures 16.15 and
16.21. Assuming that both machines were equally If E, rises further, it will do so at still lower p.f.s and El will have
to be further reduced to maintain the same V, at yet higher p.f.s to
loaded, an increase in excitation of G, will increase maintain the same level of active component and vice versa.
E, to E;, which will tend to increase vh as noted in
equation (1 6.7). A corresponding decrease in excitation Conclusion
of C2 from E2 to E; can, maintain the same level of With the change in excitation, only the reactive power,
vh, as illustrated in Figure 16.21. The phasor difference kVAr and the terminal voltages El and E2 altered can be
between E,' and E; Le. E,, will give rise to a without altering the active components of the load currents
circulating current I,-, lagging E, by almost 90°, as or the power shared by the two machines. As discussed
noted above and illustrated in Figure 16.21. The load in Section 16.3.2, a generator is designed for a particular
sharing can now be computed as follows. p.f. (0.8 lagging), having a defined value of kVAr. A