1. PROTECTION OF TRANSFORMERS
Mini Project Presentation on
submitted by
P.KIRANMAYI
R.HEMALATHA
N.MOHAN SAI KRISHNA
K.LAKSHMI THULASI
R.N.ADITHYA SHARMA
11121A0292
11121A0293
11121A0282
12125A0208
10121A0281
Under the esteemed guidance of
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
SREE VIDYANIKETHANENGINEERINGCOLLEGE
(AUTONOMOUS)
SREE SAINATHNAGAR, A.RANGAMPET-517102
External Guide
Mr. S.V.KRISHNA ,
ADE Chandragiri,
33/11kV substation.
Internal Guide
Dr. P.NAGARAJU MANDADI, M.Tech, Ph.D,
Professor,
SVEC.

2. OBJECTIVE
 The main objective of this mini project is to study
the need for protection of transformer, various
schemes employed to provide continuous and
reliable power supply.
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3.  Introduction
 Transformer
 Power transformers
 Need for protection of Transformers
 Faults in Transformers
 Protection of Transformers
 Analysis of Transformer Protection at substation
 References
 Conclusion
CONTENTS
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4. INTRODUCTION
Electrical power system have a dynamic and complex behavior.
Different types of faults can interrupt the healthy operation of the
power system.
Some of the major Electrical faults are phase faults that include
phase to phase faults and phase to ground faults and three phase
faults.
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5. TRANSFORMER
A transformer is a static electrical device that transfers power
from one side to other side without change in frequency.
It works on the principle of Faraday’s Laws of Electro Magnetic
Induction.
Fig.1: Transformer
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6. TYPES OF TRANSFORMERS
Transformers are classified as two categories:
 Power Transformers
 Distribution Transformers
Power Transformers
Power transformers are used
in transmission network of
higher voltages
Fig.2: Power Transformer
Contd.,
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7. Contd.,
Distribution transformers:
Transformers of rating upto 200kVA,
used to step down the distribution
voltage to a standard service voltage
are known as distribution
transformers
Fig.3: Distribution Transformer
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8. NEED FOR PROTECTION OF TRANSFORMER
 Electricity plays a crucial role in our lives today. The electrical
power system has several components and equipments to
transform and regulate the current flow.
 Transformers are crucial equipment among them. Hence
transformer must be protected to:
• minimise loss of revenue due to outage.
• Reduce inconvenience to the consumers.
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9. FAULTS IN TRANSFORMER
External Faults Internal Faults Incipient Faults
• Short Circuits • Phase to Earth • Short Circuit in
laminations
• High Voltage, High
frequency disturbance
• Phase to Phase • Core Bolt Insulation
Failure
• Over Load Condition • Inter Turn • Local heating due to
clogging of oil
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Table No: 1

10. External Faults Internal Faults Incipient faults
•Atmospheric voltage surges •Insulation breakdown
between windings and
Earth
•Improper formation of the
core
•Switching operation •Insulation breakdown
between different phases
•Insufficient cooling ducts in
the winding
•Overloading
•Sudden disconnection of
large load
•Insulation breakdown
between adjacent turns
•Bad quality of raw material
CAUSES FOR THESE FAULTS
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Table No: 2

11. PROTECTION OF TRANSFORMER
The protection of transformer consists of:
Relays
Circuit Breakers
Surge Arrestors
Thermostat
Pressure Relief Valve
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12. RELAYS
A relay is a device that detects the fault and initiates the operation
of the circuit breaker to isolate the defective elements from the
rest of the system.
Percentage differential protection relay and Buchholz relay are
used in the substation.
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13. Fig.5: Percentage Differential Protection of Transformer
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PERCENTAGE DIFFERENTIAL PROTECTION

14. BUCHHOLZ RELAY:
This relay being filled with oil, displaces the oil for the case of gas
collection due to internal fault and issues alarm contact and as per
the severity of gas collection, tripping results. This is mechanical
tripping device.
Fig.6: Buchholz Relay
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15. POSITIONING OF BUCHHOLZ RELAY:
Fig.7: Positioning of Buchholz Relay
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16. CIRCUIT BREAKERS
 An automatically operated electrical switch designed to protect
an electrical circuit from damage caused by overload or short
circuit.
Fig.8: Vacuum circuit breaker Fig.9: SF6 Circuit Breaker
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17. SURGE ARRESTORS
 Provides protection from voltage surges such as those caused by
lightning.
 Presence of higher voltage- they provide a path to the ground.
Fig.10: surge arrestors in substation
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18. THERMOSTAT
 Component of a control system which senses the temperature
so that the system’s temperature is maintained near a desired
set point.
Fig.11: Temperature in thermostat
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19. PRESSURE RELIEF VALVE
 Over pressure caused by internal faults can be instantaneously
relieved through the valves, thus avoiding greater damages such
as the deformation or even the burst of the tank .
 It is necessary to protect the transformer tank with a suitable
equipment capable of almost instantaneously discharging
overpressure already at the development, before the structure
integrity is damaged and the safety valves has just this function.
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20. Power Transformer Protection
Abnormal condition Protection Remarks
Earth Faults Differential Protection For Transformers of above
5MVA
Over Loads 1.Thermal Overload Relay
2.Temparature Relays
sound alarm
Temperature increase is
indicated on control board
Through Faults Graded time lag over
current Relay
Protection of distribution
Transformers
High voltage surges due to
lightning, switching
1.Horn gaps
2.Surge arresters
1.Not favoured
2.In addition to arresters
for incoming lines.
Saturation of magnetic
circuit
1.Over fluxing protection
2.Overvoltage protection.
For important generator
transformer and feeder
transformers.
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Table No: 3

21. Abnormal conditions Protection Remarks
Large internal faults ph-ph,
ph-G, below oil level.
Faults in tap-changer
1.Buchholz relay trips the
CB
2.Percentage differential
protection
1.It is too slow and less
sensitive
2.It is used for
transformers of above
5MVA
Incipient faults below oil
level resulting in
decomposition of oil, faults
between phases and
phases and grounds
Buchholz relay sounds
alarm
Buchholz relay is used for
transformers of rating
500KVA and above
Contd.,
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Table No: 4

22. ANALYSIS OF TRANSFORMER PROTECTION
AT 33/11 kV SUBSTATION LOCATED IN
CHANDRAGIRI, TIRUPATI
 The substation is an outdoor air insulator switch gear (AIS)
type.
 The input to this substation is a 33kV line from 132/33kV
Chandragiri substation.
 The protection is given to the two 8MVA, 33/11kV
transformers on both primary and secondary side.
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23. ON 33kV SIDE:
 The protection for the input side i.e., H.V side includes
isolators, lightning arresters and circuit breakers.
 For the purpose of metering CT’s and PT’s are used.
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24. Equipment Ratings & Function
Lightning Arrestors •Metal oxide surge arrestors(3)
•Rating: 30kV/10A
•For protection from over voltages during lightning
Circuit Breakers •SF6 circuit breakers
•Rating: 36kV, 1600A,31.5kA and 3sec.
•To connect or remove sections of substation from supply
Isolators •Isolators with earth switch(2) & without earth switch(3)
•Rating: 36kV, 1600A, 31.5kA, 1sec
•The isolators used should be in tune with the circuit breakers.
CT’s & PT’s •Ratings:CT-33kV,400A
•Ratings:PT-33kV/110V
•A pair of CT and PT is used for energy metering by the
APSPDCL.
•Another CT with 3 primaries is used for differential protection
•One more CT is used near the transformer.
Conductors •ACSR zebra conductor.
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Table no: 5

25. ON 11kV SIDE:
The protection for the output side i.e., LV side includes
isolators, lightning arresters and circuit breakers of small
ratings.
For the purpose of metering CT’s, PT’s are used.
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26. Equipment Ratings & Function
Lightning Arrestors •Metal oxide surge arrestors
•Ratings:10kV,10kA
•Protection against voltage surges
Circuit Breakers •Vacuum Circuit Breaker
•Ratings:12kV, 1600A
•To connect or remove sections of substation
from supply
Isolators •Isolators without switch
•Ratings: 12kV, 1250A, 31.5kA, 1sec.
•The isolators used should be in tune with
the circuit breakers.
Relays •Differential Relay
•Buchholz relay(mechanical relay)
CT’s • CT Ratings:11kV,20kA
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Table no: 6

27. CONCLUSION
 Thus the transformer protection is studied at 33/11kV in
Chandragiri and various schemes of protection are
analysed.
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28. REFERENCES
[1]. Sunil S Rao, “Switchgear and Protection”, 11th edition, Khanna Publishers, 2005
[2]. Badri Ram, D .N. Viswakarma, “Power System and Protection and Switchgear” ,
18th reprint, Tata McGraw-hill Publications,2005.
[3]. Bogdan Kasztenny, Michael Thompson and Normann Fischer, “Fundamentals of
Short- Circuit Protection for Transformers”, Schweitzer Engineering Laboratories,
Inc, March 2009.
[4]. B. L. Soni, Guptha, Bhatnagar, Chakrabarthy, A Text Book on “Power System
Enginnering”, Dhanpat Rai & Co, 2007.
[5]. www.electrical4u.com
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