Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 17)

In the case of motors, the capacitors can be connected up- or downstream the motor protection
unit (
Fig. 1.5-2 ). In most cases the capacitor will be connected parallel to the motor (case 1). In
this case the motor protection unit should be set to a smaller setting current I
e
than the motor
rated current I
N
as the magnitude of the line current falls due to the compensation:
Case 1 Case 2
Fig. 1.5-2
Individual compensation of motors
I
e
= (cos φ
1
/cos φ
2
) · I
N
cos φ
1
= power factor of the uncompensated motor
cos
φ
2
= power factor of the compensated motor
1.5.1.2 Group compensation
For group compensation each compensation device is assigned to one consumer group. This
may consist of motors or also for example of luminescent lamps that are connected to the mains
via a contactor or a circuit breaker (
Fig. 1.5-1 b).
1.5.1.3 Central compensation
Mostly reactive power control units are used for central compensation which are directly
assigned to a main- or sub-distribution station (
Fig. 1.5-1 c). This is especially advantageous if
many consumers with differing power requirements and variable on-times are installed in the
network.
Central compensation also offers the advantage that
ƒ the compensation device is easy to monitor due its central location,
ƒ any retrospective installation or extension is relatively simple,
ƒ the capacitive power is continuously adapted to the reactive power requirement of the
consumers and
ƒ making allowance for a simultaneity factor a lower capacitance is often required than for
individual compensation.
See IEC 61921; Power capacitors – Capacitor batteries for correcting the low-voltage power
factor
1.5.2 Switching of capacitors
Capacitors form oscillator circuits together with the inductances of the lines and the transform-
ers. During closing, very high transient currents with higher frequencies may flow. Typical
values are 10 ... 30 times the capacitor rated current at frequencies of 2 ... 6 kHz. For this
reason, the switching of capacitors represents a very heavy load on switchgear and can result in
increased contact burn-off or under adverse conditions even welding of the contacts. Especially
when capacitors are switched by contactors, it should be ensured that they are discharged
before switching-on to avoid even higher transient currents and welding of the contacts in case
of adverse phase angles.
A harmonic component in the supply voltage leads to increased current consumption by the
capacitors and results in additional heating of the current carrying circuits. To prevent any
undesired temperature rise, the rated operational current of the contactors, load switches and
circuit breakers shall be higher than the capacitor rated current. Generally this should only be 70
… 75 % of the rated current of the circuit breaker.
LVSAM-WP001A-EN-P - April 2009 1-7