INSTRUMENT TRANSFORMERS ( CT, PT)

   INSTRUMENT TRANSFORMERS (CT, PT)

The high voltages and very large currents met within A.C. systems has led to the adoption of

instrument transformers. By their use voltages and currents are 'scaled-down' or stepped down

to a level suitabie for handling by standardised types of instrument movement. They are

suitable only for A.C. measurements.

• They are classified as:

(1.) Current transformers.

(2.) Potential transformers.

• The CT PT are known as instrument transformers because these are used with measuring

instruments for measurement of electrical quantities or in other words these are used for

metering.

* Current Transformers: Construction_

For lower current ranges, square shaped or shell-type cores are generally used. Ring type cores

are used or currents above about 100 amperes. The current carrying conductor (current to be

measured) is passed through the central hole of the transformer core. The secondary is wound

uniformly on the core, or in case of square and shell type in close magnetic contact with the

primary to minimize the magnetic leakage between the windings.

• The cross-section of core is of more area and its length is small, the material used being silicon

steer or mumetal having high permeability and low loss. The ring type core gives low reluctance

due to no Joints ana low leakage reactance.

• To enable currents at high voltages to be dealt with on primary the windings are adequately

insulated trom each other, oil immersion or compound filling being used where voltage is at or above 6600 volts.

• Portable current transformers have their winding split into a number of equal sections and large

number of ratios being obtained by adopting series, series-parallel connections.

In all cases the standard value of secondary current is 5 amperes or 1 ampere.

*Theory:

• Connections of a ammeter for measuring current with current transformer are shown in

Fig. a. The primary winding which is of one turn or a few turns is connected in series with the

main circuit, hence carries the current to be measured. It is coupled magnetically by à closed

circuit, of silicon- steel, to a secondary winding consisting of many turns than the primary and

the instrument (ammeters) is connected.

The maximum value of current of this instrument being fixed, the ratio of secondary to primary turns is adjusted in order that maximum current of primary is stepped down to  5 amperes, or 1 ampere as desired. If I the base m is full-scale instrument current and if N the base p and N the base s are the turns of primary and secondary winding then we can write the following realation

 I N the base p = I the base m× N the base s

As primary mmf is equal to secondary mmf neglecting losses, thus N the base s/ N the base p= I/ I the base m.

Figure. a. Construction of current transformer 

• The primary winding of this current transformer is in the form of single conductor or bus bar which carries a high current. The primary ampere tumhara ( N the base 1, I the base 1) producers flux in the core. The secondary winding has large number of turns. The secondary also produces its own ampere turns ( N the base 2, I the base 2). These secondary a produce flux in opposite direction to that of primary ( basic transformer action.)

• If secondary winding is kept open when primary carries current, the prime ampere turns ( N the base 1, I the base 1) will setup a high in the core ( because I the base 1 is high ) . This value of  flux may saturate that core and at the same time it links with secondary winding producing very high induced voltage across secondary. This high voltage may damage the insulation of the winding, and it is not safe operator.

• Therefore current transformer secondary should always be shorted when primary is energized or carries current . Thus secondary winding will carry current and produce its ampere turns to keep the flux in core to a safe value.

* Advantages and Disadvantages_

1. Cannot be used with D.C

2. Its use enable the instrument manufacturer to standardises on a single ammeter to carry 5 amperes or 1 ampere.

3. Using long leads, the instrument can be isolated from high voltages and stay fields, increasing the safety in use. 

4. It enables more than one standard instrument to be used with one current transformer, which must than be suitable designed for multiple use.

5. Its current ratio is not constant but change with current and power factor .

6. The secondary current does maitain a phase difference of 180° with the primary current. This introduces error in measurement of power.

7 Netural of secondary is to be connected to earth as a safeguard.