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Fig. 5. Stationary short-circuit characteristic.
1 gs stationary short-circuit current (per-unit
values)
lf field current (per-unit values)
Fig. ti. Transient reactance.
x’d transient reactance
If,,, transient short-circuit current (per-unit values)
the entire stationary short-circuit characteristic is
linear. In other cases saturation appears on the
short-circuit characteristic too.
Knowing the field current, the exciter voltage is
then
Em = Rf lf (16)
where Rf denotes the inherent resistance of the field
circuit.
Fig. 5 shows the stationary short-circuit
characteristic of the generator, the voltage-rise curves of
which are given in fig. 1. Saturation has no influence
in this case.
Influence of saturation *
Although the conditions for short-circuit current
have been derived for unsaturated machines, the
results are valid also for saturated machines, when
the experimental curves, figs. 1 and 5, are used.
When calculating the value of the short-circuit
current (or the external reactance), the influence
of saturation on the transient reactance3 must,
however, be taken into consideration, fig. C.
Flying excitation
The calculation procedure for flying excitation is
shown in the following example.
A short-circuit is to be produced with the
following data:
Ego = 0.8; I go = 4.0; P.F. = 0
(Voltage and current given in per-unit values related
to rated voltage and steady state short-circuit current
at rated no-load voltage.)
Transient reactance of the generator (fig. 6)
afd = 0.069
Inherent resistance of the field circuit (measured)
Hf = 0.14
Reactance required by the short-circuit current
External reactance required xe = x x/,i = 0.115
-0.069 = 0.026
Field current required (fig. 5) If — 4.0
Exciter voltage required (eq. 16) Em — RsIf =
= 0.14 • 4 = 0.56
Magnetization time required (fig. 1) tm = 0.82 sec.
Super-excitation
At super-excitation the main problem is to
determine the series resistance Hs in the field circuit.
Having obtained the required exciter voltage from
fig. 5 and eq. (16), it is possible to determine this
resistance. The no-load characteristic of the
generator gives the relationship between the field current
at no-load (//„) and the required generator voltage
Eg0. The value of the series resistance is then
fis=pl-Rf (17)
’fo
where Rf denotes the inherent resistance of the field
circuit.
Two-phase sliort-circuit
The method developed for three-phase
short-circuits may be applied also to two-phase
short-circuits by replacing the values of the three-phase
transient reactance by the two-phase reactances and
using the two-pliase stationary short-circuit
characteristic. Although fig. 1 refers to the no-load
condition of the generator, it may still he used for
determining the magnetization time tm.
References
1. Christie I, I.eyburn II and Fenn R W: A new testing station for
high-power circuit breakers. J.I.E.F. (1955) vol. 102 p. 700.
2. Rüdenberg R: Transient performance of electric power systems.
McGraw-Hill Rook Comp. Inc. (1950) p. 147.
3. Kii.gore L A: Effect of saturation on machine reactances. Trans.
A.I.E.E. (1935) vol. 54 p. 545.
Kraft och Ljus, organ för elektrotekniken i Finland, har
utkommit med ett specialhäfte om vattenkraft (juli—
augusti 1959, 48 s. redaktionell text). Innehållet är skrivet
av ledande finska elektrotekniker och behandlar Finlands
vattenkraftförening och framtiden, det nya
vattenlagsförslaget, regleringsbolag, Finlands utbyggda vattenkraft,
vattenkraften i Pielisjoki, Kaltimo vattenkraftverk,
kontroll av vattenkraftsituationen samt den internationella
dammkommissionens 26:e exekutivmöte i Helsingfors.
.132 ELTEKNIK 1959
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