1. Instrument
Transformers
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2. Group
Good For Everything
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3. Group Members
Sl
No.
NAME
ID
TOPIC
Introduction of
Instrument X
former
01
Mehedi Hasan
11105140
02
Abu Jauad Khan Aliv
(Leader)
11105137
Design & Function
of CT
03
Farzana Farzin Rimi
11105192
Types & Accuracy
Limit of CT
04
Ab-E-Zamzam
11105199
05
Prodip Kumar Das
11105136
06
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Humayun Kabir
11105167
Equivalent Circuit,
Safety & Usage of
CT
PT Design &
Function
Error & cause of
Error Of PT &
conclusion
3

4. Overview
1.
2.
3.
4.
5.
6.
7.
8.
9.
Introduction to Instrument Transformer
Its Classification
Design & Function of CT
Construction, Accuracy Limit & Equivalent CKT
Safety Precaution & Usage of CT
Design & Function of PT
Construction & Error of PT
Cause of Error in PT
Conclusion
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5. Introduction
Instrument transformers are necessary for
Isolating the protection,
control & measurement
equipment from the high
voltages of a power
system
Supplying the equipment
with the appropriate
values of current and
voltage
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6. Types of Instrument Transformers
Two Types of Instrument X-formers
Current Transformer (CT)
Potential Transformer (PT)
Current transformer & Potential transformer
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7. Current Transformer (CT)
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8. Design
 Like any other transformer, a current
transformer has a primary winding, a
magnetic core and a secondary
winding
 The primary winding of a current
transformer is connected in series
with the power circuit
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 The impedance is negligible
compared with that of the power
circuit
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9. Design
 The alternating current flowing in the
primary produces a magnetic field in
the core & induces a current in the
secondary winding
 The CT's primary circuit consists of a
single 'turn' of conductor, with a
secondary of many tens or hundreds
of turns
 The primary winding may be a
permanent part of the current
transformer, with a heavy copper bar
to carry current through the
magnetic core
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10. Function
 Reduce power system current
to lower value for
measurement
 Insulate secondary circuits
from the primary
 Permit the use of standard
current ratings for secondary
equipment
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11. Current Transformer Construction
BAR PRIMARY
WOUND PRIMARY
Primary
Secondary
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12. Bar-Primary Type CT
Laminated ‘strip’ wound steel
toroidal core
Insulation to stop flash-over from
HV primary to core & secondary
circuit
1000 turns sec
‘Feeder’ or ‘Bus-bar’ forming
1 turn of primary circuit
RELAY
1A
1000A
Generator, or system
voltage source
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Insulation covered
wire, giving inter-turn
insulation & secondary to
core insulation
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13. Accuracy
The accuracy of a CT is directly related to a
number of factors
 Burden
 Rating factor
 Load
 External electromagnetic fields
 Temperature
 Physical configuration.
 The selected tap, for multi-ratio CTs
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14. Accuracy
• Burden
The secondary load of a current transformer is
usually called the "burden" to distinguish it
from the load of the circuit whose current is
being measured
• Rating factor
Rating factor is a factor by which the nominal
full load current of a CT can be multiplied to
determine its absolute maximum measurable
primary current
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15. C.T. Equivalent Circuit
ZCT
Ip
Is
P1
Ip/N
Ie
S1
N
Ze
Zb
Vt
Es
= Primary rating of C.T.
Ie
= Secondary excitation current
N = C.T. ratio
Zb = Burden of relays in ohms
Is
Es
= Secondary current
= Secondary excitation voltage
(r+jx)
ZCT = C.T. secondary winding
impedance in ohms (r+jx)
Ze = Secondary excitation
impedance in ohms (r+jx)
Vt = Secondary terminal voltage
across the C.T. terminals
Ip
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16. Safety Precaution
 The secondary of a current transformer is not
disconnected from its load while current is flowing in
the primary
 The transformer secondary will attempt to continue
driving current across the effectively infinite impedance.
 This will produce a high voltage across the open
secondary
 This high peaks of voltage may not be measured by
conventional voltmeter. But these high peaks of
induced voltage may breakdown the CT insulation, and
may case accident to personnel
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17. Usage
 Monitoring the operation
of the power grid
Current transformers
are used:
 Measuring current
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A CT for operation on a 110 kV grid
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18. Potential Transformer (PT)
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19. Design
 A Potential Transformer theory is just like
theory of general purpose step down
transformer
 Primary of this transformer is connected across
the phases or and ground depending upon the
requirement
 PT has lowers turns winding at its secondary
 In an ideal Potential Transformer when rated
burden connected across the secondary the
ratio of primary and secondary voltages of
transformer is equal to the turns ratio
 But in actual transformer there must be an
error in the voltage ratio as well as in the phase
angle between primary and secondary voltages
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20. Types of Potential Transformer
Two main basic types are available:
 Electromechanical VT`s
 Similar to a power transformer
 May not be economical above 132kV
 Capacitor VT`s (CVT)
 Used at high voltages
 Main difference is that CVT has a capacitor
divider on the front end
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21. Construction of PT
 The construction of a voltage transformer takes into
account the following factors
a. Output – Seldom more than 200-300VA. Cooling
is rarely a problem
b. Insulation – Designed for the system impulse
voltage level. Insulation volume is often larger
than the winding volume
c. Mechanical Design – Not usually necessary to
withstand short-circuit currents. Must be small
to fit the space available within switchgear
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22. Function
 Provides isolation from high
voltages
 Must operate in the linear region
to prevent accuracy problems Do not over specify VT
 Must be capable of driving the
burden, specified by relay
manufacturer
 Protection class VT will suffice
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23. Errors in PT
 The difference between the
ideal value Vp/KT and actual
value Vs is the voltage error
or ratio error in a potential
transformer
% voltage error = Vp −
KT.Vs /Vp X 100 %
 The angle ′β′ between the
primary system voltage Vp
and the reversed secondary
voltage vectors KT.Vs is the
phase error
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Is - Secondary Current
Es - Secondary induced emf
Vs - Secondary terminal voltage
Ip - Primary current
Ep - Primary induced emf
Vp - Primary terminal voltage
KT - numbers of primary turns/number of
secondary turns
Io - Excitation Current
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β - phase angle error

24. Cause of Error in PT
 The voltage applied to the
primary of the potential
transformer first drops due to
internal impedance of primary
 Transformed voltage across
secondary winding will again
drops due to internal
impedance of secondary
before appearing across
burden terminals
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25. Conclusion
Current Transformer
 Measuring current
 Monitoring the operation of
the power grid
• CT secondary should not be kept open
Potential Transformer
 Measuring Voltage
 Provides isolation from high voltages
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