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INSTRUMENT TRANSFORMERS
These are special type of transformers used for the measurement of voltage, current, power and energy
...
Current Transformer (or Series Transformer)
2
...

Potential transformers are used where the voltage of an AC circuit exceeds 750 V as it is not possible to provide
adequate insulation on measuring instruments for voltage more than this
...
To insulate the high voltage circuit from the measuring circuit in order to protect the measuring
instruments from burning
2
...

These instrument transformers are also used in controlling and protecting circuits, to operate relays, circuit
breakers etc
...

Measurement of current as CT
The primary winding is so connected that the current to be measured passes through it and the secondary is
connected to the ammeter
...

The following three types of core constructions are generally employed :
1
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Shell type
3
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The laminations are of L shaped assembled together
...

The main advantage of this type of core is that sufficient space is available for insulation and is suitable
for high voltage work
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•

Ring type core is commonly used when primary current is large
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•

It is a joint less core and there is very small leakage reactance
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The function of PT is to steps down the voltage to the level of voltmeter
...
The measuring instruments can be placed for away from the high voltage side by connecting long wires
to the instrument transformer
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2
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3
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4
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5
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Phasor Diagram of CT

Steps to draw Phasor Diagram of CT
Take flux φ as the reference vector
m

o

The induced e
...
f
...

1

The induced e
...
f
...

2

1

The magnitudes of e
...
f
...

The no load primary current I is drawn which has two components, magnetising component I
o

and
m

the working component I
w

The secondary current I2 lags behind the emf E2 by an angle of γ
...

The secondary current I2 is now transferred to the primary by reversing I2 and multiplied by K where K
is the turn ratio
...

E2 = V2 + I2 (R2

+ j

X2)

Ratio Error of CT
The transformation ratio I1 /I2 should be constant and within the limits
...
So this error is known as Ratio Error
...
This is the angle by which the
secondary current, when reversed, differs in phase from the primary current
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This is possible only
by using the material of the core of high permeability, short magnetic path and large cross section area
of the core
...
Therefore to avoid the joints in building of
core, the cores are made if two types,
•

Ring type core

•

Spiral type core

By providing a suitable turn ratio i
...
number of turns of the secondary can be reduced by one or two
turns
...
Therefore the two windings should be closed to
each other to reduce the secondary winding leakage reactance
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It also reduces phase angle error
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Current transformers are widely used in power measuring circuits
...
Current transformers are also used in power houses, sub stations etc
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Phasor Diagram of PT

Steps to draw Phasor Diagram of CT
Take flux φ as the reference vector
m
o

The induced e
...
f
...

1

The induced e
...
f
...

2

1

The magnitudes of e
...
f
...

The no load primary current I is drawn which has two components, magnetising component I
o

and
m

the working component I
w

The secondary current I2 lags behind the emf E2 by an angle of γ
...

The secondary current I2 is now transferred to the primary by reversing I2 and multiplied by K where K
is the turn ratio
...

E2 = V2 + I2 (R2

+ j

X2)

V1 = E1 + I1 (R1

+ j

X1)



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