Presentation on "Instrument Transformer" (Summer vocational training presentation) from BHEL Jhansi

2. Instrument Transformer
Guided By :
Sr. Er. S. K. Roy
Instrument Transformer
Manufacturing Department
BHEL Jhansi
Presented By :
Rishi Kumar
1604320042
B.Tech (EE)
A
Summer Vocational Training Presentation
On
2
Head of Department :
Dr. D. K. Srivastava
Electrical Engineering Department
BIET Jhansi

3. Overview
1. Introduction to Instrument Transformer
2. Its Classification
3. Design & Function of CT
4. Construction, Accuracy Limit & Equivalent CKT
5. Safety Precaution & Usage of CT
6. Design & Function of PT
7. Construction & Error of PT
8. Cause of Error in PT
9. Conclusion
10. Reference

4. Introduction
Instrument transformers are necessary for
Isolating the protection, control &
measurement equipment from the
high voltages of apower system
Supplying the equipment with the
appropriate values of current and
voltage

5. Types of Instrument Transformers
Two Types of Instrument Transformers :
Current Transformer (CT)
Potential Transformer (PT)
Current transformer & Potential transformer

6. Current Transformer (CT)

7.  Like any other transformer, a current transformer has a
primary winding, a magnetic core and asecondary winding
 The primary winding of a current transformer is
connected in series with the powercircuit
 The impedance is negligible compared with that of the
power circuit
 Thealternating current flowing inthe primary produces a
magnetic field in the core & induces acurrent in the
secondary winding
 TheCT'sprimary circuit consists of a single 'turn' of
conductor, with a secondary of many tens or hundreds of
turns
 Theprimary winding maybe a permanent part of the
current transformer, with aheavycopperbar to carry
current through the magnetic core
Design

8.  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
Function

9. CurrentTransformerConstruction
BARPRIMARY WOUNDPRIMARY
Primary
Secondary

10. RELAY
1A1000A
1000 turnssec
Insulation covered wire,
giving inter-turn insulation
& secondary to core
insulation
Generator, orsystem
voltagesource
‘Feeder’ or ‘Bus-bar’forming
1 turn of primary circuit
Insulation to stop flash-over
from HVprimary to core&
secondary circuit
Bar-Primary Type CT
Laminated‘strip’woundsteel toroidal core

11. Accuracy
The accuracy of a CT is directly related to a number of
factors :
 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 whichthe nominal full load current of
a CT can be multiplied to determine its absolute maximum measurable
 Load
 External electromagnetic fields
 Temperature
 Physical configuration.
 The selected tap, for multi-ratio CTs

12. C.T. Equivalent Circuit
ZbN
P1
ZCT
S1
VtZe
Is
Ip
Ip/N
Ie
Es
Ip = Primaryrating of C.T. Ie = Secondaryexcitation current
N = C.T.ratio Is = Secondarycurrent
Zb = Burdenof relays in ohms Es = Secondaryexcitation voltage
(r+jx) Vt = Secondaryterminal voltage
ZCT= C.T.secondary winding
impedancein ohms (r+jx)
acrossthe C.T. terminals
Ze = Secondaryexcitation

13. 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

14. Usage
 Current transformers are used:
A CT for operation on a 110 kV grid
Monitoring the operation of
the power grid
 Measuring current

15. Potential Transformer (PT)

16. 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
 PThaslowers turns winding at itssecondary
 In an ideal Potential Transformer when rated
burden connected across the secondary the
ratio of primary and secondary voltages of
transformer is equal to theturns ratio
 But in actual transformer there must be an error
in the voltage ratio as well as in the phase angle
between primary and secondaryvoltages

17. Types of Potential Transformer
Two main basic types are available :
 Electromechanical PTs
 Similar to a power transformer
 May not be economical above 132kV
 Capacitor PTs (CPT)
 Used at high voltages
 Main difference is that CPT has a capacitor divider on the
front end

18. 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

19.  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
Function

20. Errors in PT
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
β - phase angle error
 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

21.  The voltage applied to the primary
of the potential transformer first
drops due to internal impedance of
primary
Cause of Error in PT
 Transformed voltage across
secondary winding will again
drops due to internal impedance
of secondary before appearing
across burden terminals

22. 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

23. References
1. "Measurement Canada Standard Dwg. No.3400 D3 Delta Connected CTs" (PDF).
Measurement Canada. Retrieved 12 December 2012.
2. "Limits of the 0.3 and 0.6 accuracy classes for measuring current transformers".
Measurement Canada. Retrieved 18 April 2013.
3. "PS-E-15 — Provisional Specifications for Approval of Electronic Voltage
Transformers". Measurement Canada. Retrieved 18 April 2013.
4. Monsef, H.; Ghomian, T. (2006). "Modified quadrature method for accurate voltage
measurement in optical voltage transducer". IEE Proceedings - Generation,
Transmission and Distribution. 153 (5): 524. doi:10.1049/ip-
gtd:20050189. ISSN 1350-2360.
5. Network Protection & Automation anshu, AREVA 2002