Capacitor Switching in APFC Panels
The switching of capacitor banks is a
special and challenging application in Automatic Power Factor Correction (APFC)
panels. The selection of appropriate switching device for such applications is
based on two criteria:
® Ability to carry rated capacitor
current continuously
® Ability to make the peak-inrush current
of capacitor
It is simple to calculate the capacitor
rated current and select the switching device to be able to carry rated
capacitor current (2.5 to 3 times the capacitor rated current to take care of
overload, harmonics, supply voltage variation and capacitor value tolerance).
However, it is a little difficult to select the switching device which is able
to make the peak- inrush current. This is because the peak inrush current for
capacitor switching application depends upon various factors such as:
® The inductance of the network
(including cables, switchgears and transformer)
® Thetransformer power rating and %
impedance ® Methods used for power factor correction: O fixed capacitor bank
O multi-stage capacitor bank with steps
of equal ratings
O multi-stage capacitor bank with steps
of unequal ratings
® |n multi-stage capacitor bank, the nos.
and rating of steps already switched on
In most of the installations, the
multi-stage capacitor banks are used with steps of unequal ratings. The bigger
steps of higher kVAr ratings being switched on initially and smaller steps are
switched on periodically, for achieving the targeted power factor. In such
cases, the value of inrush-current peak will be far higher and hence the
smaller capacitors will be heavily stressed.
Capacitor switching can be done by
various ways such as:
Power contactor:
® Normal power contactors will simply
allow the inrush current to flow through them and because of this, contactors
and capacitors are heavily stressed. Contactor selection should be such that
it withstands the heavy inrush current
for which some amount of derating would be required.
® Power contactors should be used along
with inrush current limiting reactors, which will increase the cost and size of
the APFC panel.
® |Instead of de-rating power contactors
and using current limiting reactors, capacitor duty contactors shall be used.
Capacitor duty contactor:
® MO C range of capacitor duty contactors
can be used to limitthe inrush current to less than 10*|.
® (Capacitor duty contactors have
pre-contacts with currentlimiting resistors (of 4 Q).
® Capacitor duty contactors are employed
where the frequency of switching is less i.e., the load fluctuation is not
often. The capacitor requires atleast 60 seconds to discharge to a nominal
value (50 V). So capacitor duty contactors cannot be used when load
fluctuationis heavy.
® MO C contactors can be used for
switching capacitors upto 80 kVArin each branch.
Thyristor switching module:
® TSMis used for dynamic power factor
correction i.e., wherever the load fluctuation is heavy (welding, steel
rolling, etc.)
® Rapid switching (5 ms) is possible with
TSM along with Quick Discharge Resistor (QDR). Existing discharge resistor in
capacitors shall be replaced with QDR.
® There will be no inrush current while
using TSM (zero voltage switching and zero current switching). So frequent
switching will not affect the life of capacitors and there may not be a need to
use extra currentlimiting reactors.
® TSM has thermal cutoff, which will
switch off when temperature exceeds beyond a certain limit. It will
automatically switch on when optimum temperature is attained.
® Panel design becomes critical while
using TSMs and adequate cooling arrangements need to be employed.
