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| Current Transformer |
Current
transformers and Potential transformers are widely used in industries to
measure alternating currents and voltages, respectively, of high magnitudes and
to operate and control protection devices. Current transformers and Potential
transformers, like other transformers, have two windings i.e., Primary and
Secondary windings. The output of these transformers, i.e., secondary winding
remain connected to other instruments like energy meters and other protection
devices. Current transformers and Potential transformers reduce high value
alternating currents and voltages, respectively, flowing in main transmission
line or supply system to very low values in proportions specified. Thus, these
transformers provide economic, accurate, easy, simple and safe way of handling
supply currents and voltages to get monitored and controlled. The magnitude of
outputs of these transformers depends upon the ratio of the transformer. These
transformers are also known as Instrument Transformers. Current transformer and
potential transformer are also commonly called CT and PT respectively.
CONSTRUCTION OF CURRENT TRANSFORMERS ;
Current
transformers have primary windings, secondary windings and a core of magnetic material like normal transformers. Function of current transformers has to
reduce transmission or supply current to low values in specific proportions to
be sensed by other display, metering and protective instruments. For this
reason, primary windings of current transformers have very few turns and some
CTs have only one turn whereas, secondary windings of current transformers have
large number of turns. Conductors of primary windings of CTs are of thicker
gauges or heavy wires of higher diameters to handle high transmission currents.
Primary windings are connected in Series of transmission lines. Some current
transformers do not have primary windings instead a transmission line or supply
line is passed through a hole of magnetic core wrapped over with conductors of
secondary windings. These transformers are available in many shapes and sizes.CONSTRUCTION OF POTENTIAL TRANSFORMERS ;
Construction
of Potential transformers is similar to Current Transformers, as discussed above, except the number of
turns in secondary winding are much less than primary winding just like any
step down transformer.Types of Instrument Transformers ;
Some
of the common types of Instrument Transformers are Wire-wound and Ring or
Toroidal type transformer. Rectangle CTs & Split-core CTs are also available,
so that CTs can be fixed without removing or opening connections. Standard
output of Instrument CTs is 1 Amp or 5 Amp and standard output of Instrument
PTs are 110 V or 220 V. CTs can be of Oil-immersed as well.
Important
characteristics of Instrument transformers, which should be taken into
consideration while selecting Instrument transformers are ; i) Input &
Output ratio or Transformer Ratio, ii) Standard Load, iii) Accuracy, iv) Rated
Voltage, etc.
Some
of the Applications of Instrument transformers are ; i) Metering Inputs, ii)
Monitoring Loading and Un-loading of power transformers, heavy motors, etc., iii)
Protection against over-loading of various equipment like power transformers, to operate protective relays, circuit breakers
and switch gears, iv) Earth fault or leakage protection, etc.
Current Transformer and Potential Transformer together can be used to measure Power with the help of Watt Meter.
SECONDARY OF CURRENT TRANSFORMERS CANNOT BE LEFT OPEN OR KEPT SHORT CIRCUITED ;
Secondary
terminals of Current transformers should not be left open because of high
induced e.m.f. in secondary windings. Current transformers work as Step-up
transformers which increase input e.m.f. as primary windings have only one or
two turns of conductors whereas secondary windings have many hundreds of turns
of conductor wrapped around magnetic core thereby magnify e.m.f. or voltage to
many times which can be dangerous and source of accident. So secondary
terminals of these transformers are kept short circuited when not connected with
helping instruments.
In the end, instrument transformers prove to be very useful in industries in sensing current flow and e.m.f. between two or more terminals of high magnitudes very efficiently, economically, safely and easily.
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