94 HANDBOOK OF ELECTRICAL ENGINEERING
Figure 4.5 Open-circuit voltage in per unit versus exciter field voltage in per unit. For use in determining the
SE75 and SE100 parameters of the exciter.
S 3 =
Va 31 −Va 32
Va 32(4.15)
UseVfd 1 as the reference output voltage and convertVfd 3 into a per unit factor ofVfd 1 ,
denoted asVfd3pu.
Vfd3pu=Vfd 3
Vfd 1( 4. 16 )
By simple proportions,
Va 32 =Va 12 .Vfd3pu ( 4. 17 )
Choose a factoru>1, such that,
u=Va 32
Va 22=
Va 12 .Vfd3pu
Va 22(4.18)
Therefore,
Va 22 =Va 12 .Vfd3pu
u(4.19)
Substitute (4.19) into (4.14), and (4.17) into (4.15),
S 2 =
(
Va 21 −Va 22
Va 12 Vfd3pu)
u=AeBVf d^2 (4.20)