The mechanical effects of short-circuit currents in - Montefiore

power converter in a three phase bridge connection,
which is supplied from the mains through a 380 V/28 V
transformer. Smoothing is accomplished by inductor
and capacitor. Energy is stored by a storage battery. The
with motor-generator converter connected is loaded an
ohmic resistance.
Shorting is induced by a thyristor in order to avoid
bouncing of the power breaker contacts when the circuit
is closed. The short-circuit is switched off by the power
breaker.
The short-circuit current characteristics are recorded by
a transient recorder, where rapid current variations of up
to 60 kA/ms are measured. The data gained are
converted and processed on a computer.
Figure 6.3 shows short-circuit current characteristics of
the power converter. Curve “a” does not include a
smoothing inductor. The LG/LN ratio of the d.c. and
three-phase inductances is relatively low; the RN/XN
ratio of active resistance to three-phase reactance is
high. After about 10 ms there appears the steady-state
short-circuit current. Curve “b” shows lower LG/LN and
RN/XN ratios; this has been accomplished by introducing
a smoothing reactor into the three-phase side: In this
case, there appears a marked peak of the short-circuit
current, which is considerably higher than the steadystate
short-circuit current. With RN → 0 and LN → 0, it
may theoretically reach twice the value of the steadystate
short-circuit current. In the case of curve “c”, a
relatively high smoothing inductor was introduced into
the d.c. side; there is no marked peak of the short-circuit
Figure 6.2 Circuit diagram of the modelling system.
98
current. The steady-state short-circuit current appears
much later than in curves “a” and “b”. In curve “d”, a
rapidly acting control limits the short-circuit current to
100 A. In practice, this control often works relatively
slow, so that peak as well as steady-state short-circuit
currents may flow.
Figure 6.4 shows short-circuit characteristics of fully
charged, stationary batteries. The short-circuit current
rises rapidly to peak value and thereafter drops at first at
a low, later at a high time constant. The final value of
the short-circuit current therefore depends upon the
duration of the short circuit. In the case of a battery
which has been discharged to minimum admissible cell
voltage, there still flows a considerably high shortcircuit
current of a similar shape having a magnitude up
to 30 to 40 per cent of that of the current of the fully
charged battery.
Figure 6.5 shows the short-circuit current of smoothing
capacitors of different capacities. Short-circuit duration
is very short. With high capacities, peak short-circuit
currents may reach high values.
The short-circuit current of d.c. motors is shown in
Figure 6.6. With large motors, the peak short-circuit
current may reach 10 times the rated current, and will
appear within 10 to 50 ms. Reduction in magnitude and
the decay of the current depend upon the overall
moment of inertia as well. Owing to the excitation
voltage being constant after the occurrence of the short
circuit, separately excited motors show higher shortcircuit
currents.
1 transformer 4 smoothing capacitor 7 ohmic load 10 d.c. motor
2 three-phase bridge 5 d.c. transformer 8 power breaker 11 d.c. generator
3 smoothing inductivity 6 lead-acid accumulator 9 short-circuit SCR 12 load resistor