instrument transformers

1. CONTENTS
1. Introduction
2. Instrument Transformers
3. Need of Instrument Transformers
4. Applications of instrument Transformers
5. Current Transformers
6. Parts of Current Transformers
7. Working of CT’S
8. Let’s learn more about CT’s
9. Calculation of CT’s
10. Applications of Current Transformers
11. Safety of CT’S

2. INTRODUCTION
 Transformers is a device-
1. transfers electric power from one circuit to another
2. it does so without change of frequency
3. it accomplishes this by electromagnetic induction and
4. It is divided into core & shell types of transformers
5. for measurement purpose we uses INSTRUMENT
TRANSFORMRS .

3. INSTRUMENTS TRANSFORMER
 These are special types of transformers used for the
measurement of voltage , current ,power & energy. As the
name suggests , these transformers are used in conjuction with
the relevant instruments such a ammeters , voltmeters
,wattmeters & energy meters.

5. NEED OF INSTRUMENT TRANSFORMERS
 For the purpose of measurement of large currents and high
voltage in transformer we uses instrument transformer.
 It also make it possible to measure it with low range voltmeters
and low range ammeters.
 For these purpose ,specially constructed accurate ratio
instrument transformers are employed in conjunction with many
measuring instruments.
 They are basically of two types –
1. Current Transformers
2. Potential Transformers

6. APPLICATIONS OF INSTRUMENT
TRANSFORMER
• It would not be economical to have a ammeter to measure
600amps in a conductor directly , but we can have an ammeter
range 0-5amps which corresponds to 0-600amps.
• Similarly , for measuring voltage of 14,400volts directly it would
not be economical , so we place 0-120 volts will be corresponds
to 0-14,400volts & economical.
 The degree to which an instrument transformer produces a
current or voltage i. e. proportionate to the one to be monitored
is to it’s accuracy.

7. CURRENT TRANSFORMER
 They generally used to step
down current in a very
predictable fashion with
respect to current and phase
.
 The current (or series)
transformer has a primary
coil of one or more turns of
thick wire connected in series
with the line whose current is
to be measured.
 The secondary consists of a
large number of turns of fine
wire & is connected across
the ammeter terminals.

8. PARTS OF CURRENT TRANSFORMER
 Like any transformer, a current transformer has a primary
winding, a core and a secondary winding, although some
transformers, including current transformers, use an air core. In
principle, the only difference between a current transformer and
a voltage transformer (normal type) is that the former is fed with
a 'constant' currentwhile the latter is fed with a 'constant'
voltage, where 'constant' has the strict circuit theory meaning.
 Typically current transformers consist of a silicon steel ring core
wound with many turns of copper wire .

9. WORKING OF CT’S
 The alternating current in the primary produces an alternating
magnetic field in the core, which then induces an alternating
current in the secondary. The primary circuit is largely
unaffected by the insertion of the CT.
 Accurate current transformers need close coupling between the
primary and secondary to ensure that the secondary current is
proportional to the primary current over a wide current range.

10.  The current in the secondary is the current in the primary
(assuming a single turn primary) divided by the number of turns
of the secondary.
 The conductor carrying the primary current is then passed
through the ring; the CT's primary therefore consists of a single
'turn'. The primary 'winding' may be a permanent part of the
current transformer, with a heavy copper bar to carry current
through the core.

11. LET’S LEARN MORE ABOUT CT’S

12. CALCULATION OF CT’S
 If the current transformer has
primary to secondary current
ratio of 100:5,then it steps up
the voltage 20 times whereas
it steps up the voltage 20
times whereas it steps down
the current to 1/20 of its
actual value.
 Hence , if we know current
ratio (I1/I2) of the transformer
& the reading of the a.c.
ammeter , the line current
can be calculated.
 For eg; 1. A100:5
transformer is used in
conjunction with a 5-amp
ammeter . If the latter reads
3.5A,find the line current.
 Sol: As we know that
I1/I2=100/5 , now primary
on line –current =3.5*(100/5)
=70A

13. APPLICATIONS OF CURRENT TRANSFORMERS
 Current transformers are used
extensively for measuring current and
monitoring the operation of the power
grid.
 Current transformers can be mounted on
the low voltage or high voltage leads of a
power transformer; sometimes a section
of a bus-bar can be removed to replace
a current transformer.
 Often, multiple CTs are installed as a
"stack" for various uses. For example,
protection devices and revenue metering
may use separate CTs to provide
isolation between metering and

14. SAFETY OF CURRENT TRANSFORMERS
 Regarding safety issues ,it is considered important as the
secondary of a current transformer should not be disconnected
from its burden while current is in the primary, as the secondary
will attempt to continue driving current into an effective
infinite impedance up to its insulation break-down voltage and
thus compromise operator safety.

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