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Power
Factor Control Input
Signals
Before
automatic power factor correction equipment is purchased,
the input signals required for the power factor controller
should be considered. These input signals can be derived
from several locations in an existing facility without
the need for the purchase of new equipment (i.e. purchase
of a capacitor bank or filter bank with a potential
transformer and the installation of a current transformer).
This bulletin discusses the external signal requirements
for the power factor controller (PFC) that NEPSI offers
and some of the possible locations from which the signals
can be derived.
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PFC
Signals
The
required control signals for the PFC are system
voltage and line or load current. These signals
in general are derived from potential transformers
and current transformers which steps (scale) the
system voltage and current from system levels
down to standard control voltage and current levels
of 120VAC and 5AMPs maximum. The signals must
be derived from the electrical system such that
the PFC will sense a 90° lead in current when
the power factor of the load is at unity. For
a 90° lead in current, the PT must be connected
from phase-C to Phase-B and the CT in line with
Phase-A. Figure 1A illustrates the required connection
for a 90° lead in current measurement. Note
that the load has been placed down stream from
the CT with the capacitor
bank between the CT and the load. This arrangement
is required for most PFC's to operate properly. Some
new controllers have the capability to perform
internal phase rotation changes, but it is still
advisable to connect those controllers up with
the connections as shown in Table 1.
Other
CT and PT connections are possible. Some
of these connections are shown in the in the table
to the left.. The table shows five other connections
that are possible. In fact their are many other
connections which are possible if the secondary
leads of the PT and CT are swapped. This is a
very common problem associated with the start-up
of an Automatic Power Factor Correction capacitor
bank. In general the PT connection must be made
across any two phases with the CT in line with
the third phase. If this condition is met, it
is possible to get a proper phase shift in current
to voltage by changing the polarity on the secondary
leads that are located at the PFC terminal block.
Existing
Signals
For
many situations, it is possible to utilize existing
Relay Signals and/or Metering CT and PT circuits
for the input signals to the PFC if they are located
near the proposed capacitor bank installation
and sense the corresponding load current. These
signals are advantages to use because they require
no additional cost for installation and in many
cases no shut down time for PT and CT connections.
A new CT on a main incoming cable for example,
requires that the plant process be shut down momentarily
so that the CT may be installed by breaking the
main cable connection and inserting the CT over
the cable jacket. In many cases a low voltage
split core type CT may be used if the cable is
fully insulated and exposed. In this case a plant
shut down may not be necessary.
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Existing
CT and PT circuits may be readily available to cut cost
and prevent plant shutdown since one of the following
circuits may exist.
1. Over
current relay protection circuits (CT only) is widely
used as primary protection or back-up protection
for transformers, utility tie circuits, and cable
mains.
2.
Differential relay protection circuits such as transformer
differential (CT only) and bus differential (PT
only).
3.
Demand meter and watt-hour meter circuits (PT and
CT).
Figure
2 illustrates a typical industrial tie from the utility
source. It shows both differential (87 device) and over
current protection (50/51 device) and metering of voltage,
current and KW. Rather than use a separate CT for the
PFC for example, the CT secondary circuit of the 50/51
device could be broken and brought through the PFC.
This would prevent the need for the tie to be isolated
or plant to be shutdown for the installation of
the CT and the need for a new CT. The PT circuit, could
also be utilized for the voltage signal that the PFC
requires.
NEPSI Recommendations
NEPSI
recommends that the voltage sensing device be derived
from the NEPSI capacitor bank since the control power
is also required for controlling contactors or switches
for capacitor switching. The control power demands that
are required for the switches are beyond what should
be taken from any metering or protection circuit.
If
the customer has auxiliary control power for the switching
needs of the capacitor bank, then the voltage signal
for the PFC to derive PF may be obtained from an existing
PT circuit and the control power from the customer’s
auxiliary power.
The
CT circuits which are used for overcurrent protection
are less complex then those of differential protection
system. If plant policies allow the protection circuit
to be broken and used in conjunction with the PFC, additional
cost and down time can be avoided with an existing metering
or protection system.
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