Instrument Transformer

INSTRUMENT TRANSFORMER

We have studied earlier, that a low‒range ammeter can be used to measure a high value current by using shunts. Similarly, the measuring range of a voltmeter can be extended by using multipliers. However, there are some limitations to use of shunts and multipliers for measurement of high current and high voltage respectively by using low‒range ammeters and voltmeters.

For measurement of high current and high voltage, voltmeters and ammeters of higher ranges are not used. Instead, for such measurement current transformers (CT) and potential transformers (PT) are used along with low range ammeters and voltmeters. Transformers used  for measurement of current is called current transformer (CT) and the transformer used voltage measurement is called a voltage transformer or potential transformer (PT)

Types of Instrument Transformer

1. Current Transformer (CT)

Generally a transformer is a device which consists of two windings called primary and secondary and transfers the energy from one side to another by changing the voltage and current. Current transformer is a device used to decrease the current level by stopping up the voltage and keeping the energy as constant. Fig. 6.26. shows the circuit of a current transformer in which the primary winding of the CT is connected in series with line carrying the current to be measured and therefore the primary current is dependent on the load connected to the system whose current is to be measured. The secondary winding of the CT is connected to the low range ammeter. As the secondary voltage of CT is higher than the primary voltage, secondary winding has more number of turns compared to the primary winding. In case of CT, the secondary current is less than the primary current.

In case of Current Transformer, n = N₂/N₁ = I₁/I₂= V₂/V₁

Where,

I₁ be the primary current in A

I₂ be the secondary current in A

V₁ be the primary voltage in V

V₂ be the secondary voltage in V

For example, if the range of CT is 500:5

I₁ = 500 A

I₂ = 5 A

n = 500/5 = 100

V₂/V₁ = 100

V₂ = 100 V₁

From the above equation, it is noticed that in order to decrease the secondary current by 100 times, the secondary voltage should be decreased by 100 times.

2. Potential Transformer (PT)

Potential transformers are similar to two winding transformers except that the secondary volt‒amp loading is very low. The primary winding is the high voltage winding which is connected across the lines whose voltage is to be measured and the secondary is connected to the low range voltmeter coil. One end of the secondary winding is always grounded for safety purpose. Fig. 6.27 shows the circuit of a potential Transformer.

In general potential transformer acts as step down transformer. The secondary winding voltage of PT is lesser than the primary voltage. The secondary current of PT is more than that of primary current. The turns ratio of PT can be expressed as

 n = N₁/N₂ = V₁/V₂

Advantages and Disadvantages of Instrument Transformer

Advantages

1. High voltage and high current can be measured using low range voltmeter, ammeter along with the instrument transformer.

2. The rating of low range meter can be fixed in respective of the value of high voltage or high current to be measured.

3. Instrument transformer ensure the safety of the operator and make the handling of the equipments very easy.

4. They can be used for operating many types of protecting devices such as relays or pilot lights.

Disadvantages

Instrument transformers can be used only for ac circuits and not for dc circuits.