2015
Instrument Transformers
VIPIN KUMAR
ELECTRICAL ENGINEER
8/12/2015
Instrument Transformers
Table of Contents
Introduction .................................................................................................................................................. 2
Advantages of Instrument Transformers ...................................................................................................... 2
Types of Instrument Transformers ............................................................................................................... 3
Current Transformer ..................................................................................................................................... 3
Potential Transformer (P.T.) ......................................................................................................................... 4
Difference between C.T. & P.T. ..................................................................................................................... 5
References .................................................................................................................................................... 5
VIPIN KUMAR
1
Instrument Transformers
Introduction
Instrument Transformers are used in A.C. 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 A.C. System voltage &
current. The voltage & current level of power system is very high. It is very difficult & costly to
design the measuring instruments for measurement of such high level voltage & current. Generally
measuring instruments are designed for 5A & 110V. 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
(I)
The large voltage & current of A.C. Power system can be measured by using small
rating measuring instrument i.e. 5 A, 110 – 120 V (Sawhney,2010).
(II)
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.
(III) Instrument transformers provide electrical isolation between high voltage Power circuit
& measuring instruments. Which reduces the insulation requirement for measuring
instruments & protective circuits & also assures the safety of operators.
(IV)
Several measuring instruments can be connected through a single transformer to power
system.
(V)
Due to low voltage & current level in measuring & protective circuit, there is low power
consumption in measuring & protective circuits.
VIPIN KUMAR
2
Instrument Transformers
Types of Instrument Transformers
Instrument transformers are of two types –
1. Current transformer (C.T.)
2. Potential transformer (P.T.)
Current Transformer
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). A typical connection diagram of a Current transformer is shown in figure below.
.
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 &
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.
VIPIN KUMAR
3
Instrument Transformers
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.
Primary of P.T. is having large no. of turns. Primary is connected across the line (generally between
on line & earth). Hence, sometimes it is also called the parallel transformer. Secondary of P.T. is
having few turns & 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.
VIPIN KUMAR
4
Instrument Transformers
Difference between C.T. & P.T.
Few differences between C.T. & P.T. are listed below –
Sl.
Current Transformer (C.T.)
No.
1
Connected in series with power circuit.
Connected in Parallel with Power circuit.
2
Secondary is connected to Ammeter.
Secondary is connected to Voltmeter
3
Secondary works almost in short circuited Secondary works almost in open circuited
condition.
condition.
4
Primary current depends on power circuit Primary current depends on secondary
current.
burden.
5
Primary current & excitation vary over
wide range with change of power circuit
current
One terminal of secondary is earthed to
avoid the insulation break down.
Primary current & excitation variation are
restricted to a small range.
Secondary is never be open circuited.
Secondary can be used in open circuit
condition.
6
7
Potential Transformer (P.T.)
One terminal of secondary can be earthed for
Safety.
References
Bakshi, U.A. 'Electrical Measurements And Instrumentation". Pune: Technical Publications, 2009. English.
Cooper, Helfrick and. “Modern Electronic Instrumentation and Measurement Techniques”. Prentice-Hall
of India, 1988.
Golding, E.W. “Electrical Measurement and Measuring Instruments”,. Sir Issac Pitman and Sons , 1960.
Jones, B.E. “Instrumentation Measurement and Feedback”. Tata McGraw-Hill, 1986.
Morris, Alan S. "Measurement and Instrumentation Principles" . 2001.
Sawhney, A. K. "Electrical and Electronic Measurement and Instrumentation". Delhi: Dhanpat Rai Co. (P)
Ltd., 2010. English.
VIPIN KUMAR
5