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Magnetizing Performance of
Short-Circuit Generators
Laszlo Fogaras, ASEA, Ludvika
Vid högeffektprov, speciellt brytprov, är det av
intresse att kunna hålla kortslutningsströmmen
konstant under den tid provet varar — storleksordning
0,2 s. Detta kan ske med hjälp av en kraftig
övermagnetisering under själva kortslutningstiden,
över-magnetiseringen skall väljas så att den kompenserar
ankarreaktionens avmagnetiserande verkan, och kan
antingen åstadkommas genom kortslutning av ett
motstånd i fältkretsen eller medelst s.k. flygande
magnetisering. Den senare metoden innebär att
generatorn kortslutes medan spänningen ännu
befinner sig i stigande efter det att fältet slagits till. Vid
rätt inställning ger båda metoderna samma resultat.
För stora maskiner är dock flygande magnetisering
att föredra eftersom generatorn här endast behöver
vara magnetiserad under mycket kort tid, med lägre
förluster och möjlighet till mer ekonomisk
maskindimensionering som följd. Denna metod användes
vid Aseas högeffektlaboratorium i Ludvika.
Utomlands är metoden med resistans i fältkretsen
vanligare, i den mån utrustningarna överhuvud taget
tillåter övermagnetisering.
I artikeln beskrives hur inställningarna i
magneti-seringskretsen skall väljas för att konstant ström
skall erhållas under kortslutningstiden.
Efforts arc made to keep the transient short-circuit
current constant during the short-circuit time in
order to obtain high interrupting powers while
carrying out high-power tests. This can be achieved
by means of some form of over-excitation. In this
case the adjustment of the magnetizing performance
is essential and one of two methods may be adopted
for this purpose, the so-called super-excitation or
flying excitation. With super-excitation the exciter
voltage and a resistor in the field circuit are
controlled, and with flying excitation the exciter voltage
and the magnetization time are controlled.
With super-excitation the generator is excited to
the required voltage before the test. At the instant
of the short-circuit a resistor lying in the field
circuit is also short-circuited1, thus giving an increase
in the field current. With flying excitation the
exciter, which has been previously excited to a
certain voltage, is switched on to the field coil of the
generator. The short-circuit is then produced at a
predetermined time while the generator voltage is
still rising. The first method ensures a precise
adjustment of the generator voltage before the test, but the
test itself results in high power losses, since the
generator must be excited to the required voltage
621.313.1.064.073
before the short-circuit. The second method is
preferable because the losses in large sets will be
reduced. With proper adjustment the two methods
give, however, exactly the same increase in
excitation during the short-circuit time.
The short-circuit current
According to Rüdenberg2, the short-circuit current
If, is defined by the following expression:
hj = /^ jeos at- ^ [e ~eiTcosv/t-O-oje^cosv/r]j (1)
where
I,js = the steady-state short-circuit current
o— the leakage coefficient
i’i = an angular frequency of very small value
(d.c. part)
i’i" = an angular frequency which nearly equals
the rotational frequency
o)— the rotational frequency
r = the time from the beginning of the
short-circuit
£>i and Qi = constants dependent on the dimensions
of the machine. Their values are derived
in the following.
The equation is valid for constant excitation and
machines with laminated rotors; the rapidly
decaying subtransient component occuring with solid
rotors is neglected.
Disregarding the d.c. term and applying r.m.s.
values, eq. (1) yields
l’, = hs\ 1 +
1 - o -
where
with
Q2 — Q—Q
, Q’2 (l-o)
(2)
(3)
Hi, Hj denote the stator and rotor resistances and
Lv L2 the stator and rotor inductances.
The second term on the right-hand side of eq. (3)
with to2 in the denominator may be neglected for
large machines. Thus eq. (3) gives as a good
approximation
Q 2f
HL
oL2
1
o T0
(4)
ELTEKNIK 1959 1 129
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