Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 18)

Taking into account the aforementioned facts, the switchgear should be dimensioned so that
ƒ it does not weld at the high making currents and
ƒ that no unacceptable temperature rise occurs during continuous duty.
1.5.2.1 Switching-on single capacitors
If a capacitor with a specific capacity is connected to the power supply, then the making current
is largely determined by the transformer size and by the network impedance to the capacitors,
i.e. from the prospective short-circuit current at the installation site of the capacitor.
The loading of the switchgear increases as
ƒ the capacitance of the capacitors increases,
ƒ as the rated power of the supplying transformer increases and hence its short-circuit
impedance decreases,
ƒ decreasing impedance of the connecting lines.
Table 7 in IEC 60947-4-1 states the below derivation of capacitor switching capacity I
eAC-6b
from
the rated operational current I
eAC-3
in relation to the prospective short-circuit current i
k
:
2
2
) 6 (
) 1 (
−
⋅ =
−
x
x
i I
k b AC e
at
k
AC e
i
I
x
) 3 (
3 . 13
− ⋅ =
valid for
) 3 (
205
−
⋅ >
AC e k
I i
1.5.2.2 Switching of long, screened lines
Long screened lines have comparatively large capacitances and therefore create high transient
current loads during switching. Typical applications are variable frequency drives. The peak
currents to be expected should be taken into account when selecting switchgear to the same
extent as for the switching of single capacitors.
1.5.2.3 Switching capacitors of central compensation units
If individual capacitors of capacitor banks are switched – for example in reactive power control
units - especially adverse conditions occur at closing of the switchgear contacts as the capaci-
tors already connected to the power supply represent an additional source of energy.
The inrush current is limited by the impedance of the circuit (conductors, capacitor inductance,
inductances between the individual capacitor branches).
The loading of the switchgear is therefore determined by
ƒ the power ratio of the switched capacitors to those already connected to the power supply
and
ƒ the impedance of the individual circuit branches
For avoiding welding of the switching contacts of the contactors the switchable capacitance can
e.g. be increased, by additional inductances in the capacitor branches (e.g. a few winding turns
of the connecting wires).
With special capacitor-contactors or capacitor-contactor-combinations that connect capacitan-
ces to the power supply via pre-charging resistances, a very high switchable capacitance at a
minimum of interference with the supplying network can be achieved, as the making currents
are specifically limited by the resistances and strongly reduced.
AC-6b is defined in IEC 60947-4-1 as the utilization category for the switching of capacitor
banks.
LVSAM-WP001A-EN-P - April 2009 1-8