CURRENT TRANSFORMER, POTENTIAL TRANSFORMER OR CT, PT

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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