Instrument Transformer: Advantages of Instrument Transformers

Instrument Transformer
Instrument Transformers are used in AC system for measurement of electrical quantities i.e. voltage,
current, power, energy, power factor, frequency. Instrument transformers are also used with protective
relays for protection of power system.
Basic function of Instrument transformers is to step down the AC System voltage and current. The
voltage and current level of power system is very high. It is very difficult and costly to design the
measuring instruments for measurement of such high level voltage and current. Generally measuring
instruments are designed for 5 A and 110 V.
The measurement of such very large electrical quantities, can be made possible by using the Instrument
transformers with these small rating measuring instruments. Therefore, these instrument transformers
are very popular in modern power system.
Advantages of Instrument Transformers

The large voltage and current of AC Power system can be measured by using small rating measuring
instrument i.e. 5 A, 110 – 120 V.
By using the instrument transformers, measuring instruments can be standardized. Which results in
reduction of cost of measuring instruments. More ever the damaged measuring instruments can be
replaced easy with healthy standardized measuring instruments.
Instrument transformers provide electrical isolation between high voltage power circuit and measuring
instruments. Which reduces the electrical insulation requirement for measuring instruments and
protective circuits and also assures the safety of operators.
Several measuring instruments can be connected through a single transformer to power system.
Due to low voltage and current level in measuring and protective circuit, there is low power
consumption in measuring and protective circuits.
Types of Instrument Transformers
Instrument transformers are of two types –
 Current Transformer (C.T.)
 Potential Transformer (P.T.)

Current Transformer (C.T.)
Current transformer is used to step down the current of power system to a lower level to make it
feasible to be measured by small rating Ammeter(i.e. 5A ammeter)
Connection of current transformer or ct
Primary of C.T. is having very few turns. Sometimes bar primary is also used. Primary is connected in
series with the power circuit. Therefore, sometimes it also called series transformer. The secondary is
having large no. of turns. Secondary is connected directly to an ammeter. As the ammeter is having very
small resistance. Hence, the secondary of current transformer operates almost in short circuited
condition. One terminal of secondary is earthed to avoid the large voltage on secondary with respect to
earth. Which in turns reduce the chances of insulation breakdown and also protect the operator against
high voltage. More ever before disconnecting the ammeter, secondary is short circuited through a
switch ‘S’ as shown in figure above to avoid the high voltage build up across the secondary.
Potential Transformer (P.T.)

Potential transformer is used to step down the voltage of power system to a lower level to make is
feasible to be measured by small rating voltmeter i.e. 110 – 120 V voltmeter. A typical connection
diagram of a potential transformer is showing figure below.
Connection of potential transformer or pt
Primary of P.T. is having large no. of turns. Primary is connected across the line (generally between on
line and earth). Hence, sometimes it is also called the parallel transformer. Secondary of P.T. is having
few turns and connected directly to a voltmeter. As the voltmeter is having large resistance. Hence the
secondary of a P.T. operates almost in open circuited condition. One terminal of secondary of P.T. is
earthed to maintain the secondary voltage with respect to earth. Which assures the safety of operators.
Difference between C.T. and P.T.
Few differences between C.T. and P.T. are listed below –
Sl. No. Current Transformer (C.T.)
1 Connected in series with power circuit. Potential Transformer (P.T.)

.
1) Connected in Parallel with Power
2 Secondary is connected to Ammeter. circuit.
3 Secondary works almost in short 2) Secondary is connected to Voltmeter.

circuited condition.
3) Secondary works almost in open
4 Primary current depends on power circuited condition.
circuit current.
4) Primary current depends on secondary
5 Primary current and excitation vary burden.
over wide range with change of power circuit
current 5) Primary current and excitation variation
are restricted to a small range.
6 One terminal of secondary is earthed to
avoid the insulation break down. 6) One terminal of secondary can be
earthed for Safety.
7 Secondary is never be open circuited
7) Secondary can be used in open circuit
condition

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Instrument Transformer

Advantages of Instrument Transformers

Types of Instrument Transformers

Instrument transformers are of two types –

 Current Transformer (C.T.)

 Potential Transformer (P.T.)

Potential Transformer (P.T.)

Connection of potential transformer or pt

Sl. No. Current Transformer (C.T.)

1 Connected in series with power circuit. Potential Transformer (P.T.)

3 Secondary works almost in short 2) Secondary is connected to Voltmeter.

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