27-10-2012, 05:01 PM
Current Transformer Principles of Operation
Current Transformer Principles of Operation.pdf (Size: 76.02 KB / Downloads: 25)
Current transformers (CT's) provide a simple, inexpensive and yet accurate means of
sensing current flow in power conductors. They are available in 3 basic configurations:
1. Ring Core CT's are available for measuring currents from 50 to 5000
amps, with windows (power conductor opening size) from 1" to 8"
diameter.
2. Split Core CT's are available for measuring currents from 100 to 5000
amps, with windows in varying sizes from 1" by 2" to 13" by 30". Split
core CT's have one end removable so that the load conductor or bus bar
does not have to be disconnected to install the CT.
3. Wound Primary CT's are designed to measure currents from 1 amp to
100 amps. Since the load current passes through primary windings in the
CT, screw terminals are provided for the load and secondary conductors.
Wound primary CT's are available in ratios from 2.5:5 to 100:5 (Models
189 and 190 are examples of wound primary CT's).
Installing CT's
Window type CT's should be mounted with the H1 side of the
window towards the power source. The X1 secondary
terminal is the polarity terminal (Figure 3).The polarity marks
of a current transformer indicate that when a primary current
enters at the polarity mark (H1) of the primary, a current in
phase with the primary current and proportional to it in
magnitude will leave the polarity terminal of the secondary
(X1).
If a CT test switch is used, the switch must have a "make-before-break" contact pattern to
assure that the CT is not open-circuited during transition.
Normally CT's should not be installed on "Hot" services. The power should be
disconnected when the CT's are installed. Many times this is not possible because of
critical loads such as computers, laboratories, etc. that cannot be shut down. Split core
CT's should not be installed on "Hot" uninsulated bus bars under any conditions.
On existing cable installations where the monitoring point is not close to a cable end,
small ring core CT's can be shunt installed on #8 cable or smaller by using Hot Tap
insulation piercing connectors similar to those made by 3M. A parallel shunt through the
CT is connected to the power cable as shown in (Figure 4). Tape all bare ends.