The document discusses industrial oriented mini project work carried out by six students from Sindhura College of Engineering and Technology at Ramagundam Thermal Power Station. The students completed a project on "Transformer Protection and Maintenance" under the guidance of an Assistant Engineer from May 15-28, 2014. Their conduct during the project period was found to be satisfactory.

1. ANDHRA PRADESH POWER GENERATION CORPORATION LIMITED
RAMAGUNDAM THERMAL STATION, RAMAGUNDAM.
From To
The Superintending Engineer, The Head of Department,
Operation & Maintenance, Electrical Engineer Department,
RTS-B, Ramagundam. Sindhura College of Engg & Tech,
NTPC.
Sir,
Sub:-Industrial Oriented Mini Project Work At RTS-B to The Students of 3rd
year, B.Tech (Electrical).
Ref:-SE/O&M/RTS/DE/AT&P/AM(HR)/SUB ENG2/F.PROJECT
WORK/D.No. 292/14, Date: 15.05.2014
With reference to the above it is to inform that the following students
of “Sindhura College of Engineering & Technology”, NTPC,
Ramagundam, have carried out a Industrial Oriented mini Projects training on
“TRANSFORMER PROTECTION & MAINTENANCE” at RTS-B Station,
Ramagundam during the period “15.05.2014 to 28.05.2014” under the guidance
of Sri. K. Sridhar (Assistant Engineer /EM).
1. ZEBA ANJUM
2. G.RAJKUMAR
3. O.BHARGAV
4. MD.KHAJA MUNWAR SHARIEF
5. K.SAINATH REDDY
6. A.PRADEEP
Their conduct during this period is found satisfactory.
SUPERINTENDING ENGINEER/O&M,
RTS-B, RAMAGUNDAM.

2. ANDHRA PRADESH POWER GENERATION CORPORATION LIMITED
RAMAGUNDAM THERMAL STATION, RAMAGUNDAM.
CERTIFICATE
This is to certify that the following students of “ Sindhura
College of Engineering & Technology ”, NTPC, Ramagundam, have undergone
project work on “ TRANSFORMER PROTECTION & MAINTENANCE ” at
RTS-B, Ramagundam during the period “15.05.2014 to 28.05.2014” under the
guidance of Sri. K. Sridhar (Assistant Engineer/EM).
1. ZEBA ANJUM
2. G.RAJKUMAR
3. O.BHARGAV
4. MD.KHAJA MUNWAR SHARIEF
5. K.SAINATH REDDY
6. A.PRADEEP
Their conduct during this period is found satisfactory.
Project Guide:
(K. Sridhar)
Assistant Engineer,
Electrical Maintenance,
RTS-B, Ramagundam.

3. ACKNOWLEDGEMENT
We are very thankful to Mr. B. Surya Narayana,
Superintending Engineer R.T.S ‘B’ STN, RAMAGUNDAM
for giving this opportunity of doing the project work.
We are very grateful to Mr. K. Sridhar,
Assistant Engineer EM Division R.T.S ‘B’ STN under
whose guidance we are able to complete the project.
We are also grateful to Sub Section
Engineers of EM and MRT section especially Mr. D.
Shankaraiah, Assistant Divisional Engineer R.T.S ‘B’ STN
who gave their valuable suggestions and cooperation.
We are grateful to our principal Mr. P. Sushanth
Babu, Sindhura college of engineering and Technology,
who give their kind cooperation and allowed us to do the
project outside the college campus.

4. ABSTRACT
A power transformer is most costly and essential equipment of
an electrical transformer. It is well known fact that power transformers are the
heart of the power systems which enable us to establish very large power
systems networks. Failure of any transformer will create critical situation
hence extra attention is required in commissioning, maintenance & protection
of these transformers. The periodical maintenance practice should improve
the life period of transformer.
For getting high performance and long functional life of the
transformer, it is desired to perform various maintenance activities. Not only
that, a power transformer also requires various maintenance actions including
measurement and testing of different parameters of the transformer.
This synopsis deals with power transformers, (especially 75
MVA, 13.8/132 KV transformer and 10 MVA,132/3.3 kV Station Transformer,
available at RTS) maintenance, overhaul & protection.
The Electrical Engineer of any power plant, Sub station or
switching station regularly monitors the transformer in his day to day walk
down check list and act accordingly to rectify problems that are encountered.
The power transformer is subjected to many internal or external
faults during its life, depending up on the intensity of the fault; it deteriorates
the winding, insulation and core of the transformer. Hence different types of
protection like Differential, Over Current, and earth fault protection are
provided for the transformer to reduce the damage, caused by the faults.
Transformer oil testing, DGA (Dissolved Gas Analysis) are some
other methods to identify gradually developed faults and take necessary
measures to improve the oil properties by filtration or complete oil
replacement.

5. PROFILE OF RTS.B, STATION
Ramagundam thermal power station (R.T.P.S) of Andhra
Pradesh state electrical board (APSEB) is situated at Ramagundam in the
district of Karimnagar. The plant is situated at 60 Km from Karimnagar and 4
Km from Ramagundam railway station. The power station is about 0.5 Km
from the state high way connecting Hyderabad – Mancheriyal. The power
station has only one unit of 62.5 MW.
The unit was commissioned in 1972. The plant was
financed under AID scheme and unit comprises of boiler of CE, USA & turbine
generator of GE USA.
The coal is received at the power station by road and rail
from Godhavarikhani of Singareni collieries. Adequate facilities are provided
for unloading coal from the rail wagons. However at present the entire coal
requirement is received at plant by road by means of trucks.
The raw water to the plant is drawn from the river Godavari
situated about 8 Km from the plant. The water from the river is pumped to a
reservoir on the top of the hill near the plant. Water is supplied by gravity to
DM plant through clarifier and directly to CW to cooling tower basins as make
up.
The ash from the boiler is disposed to the ash disposal area
situated about 1 Km from the plant towards east of the plant. This area is
getting filled up & extension of the area has to be developed.
The present coal quality as reported is about 3700 K
cal/Kg and ash content 42%. Due to ageing deterioration of equipment,
controls and non-availability of spares the performance has deteriorated. The
boiler was designed for a coal quality of 4050 K cal/Kg with ash content of
38.7%. At present the unit is operating at a PLF of 65% and heat rate has been
reported to be 2660- 2730 K cal/ KWh, while design heat rate is 2616 K
cal/KWH. The unit has been adequate residual life and also for improving the
performance by implanting renovation and modernization works. There fore
APGENCO (erstwhile APSEB) has decided to carry out renovation &
modernization (R & M) works on the unit to restore the unit to operate at its
rated capacity and at designed efficiency. The Unit was taken for R&M on
20.09.2006 and continuing till today. The major R&M works includes Boiler,
Turbine & Generator, and Generator transformer.

6. CONTENTS
CHAPTER-1 INTRODUCTION
1.1 Introduction
CHAPTER-2 CONSTRUCTION FEATURES OF
TRANSFORMERS
2.1 Core
2.2 HV & LV winding
2.3 Conservator
2.4 Bucholtz relay
2.5 Vent pipe & Diaphragm
2.6 Breather
2.7 Bushings
2.8 Tap Changer
2.9 Cooling
CHAPTER-3 RECOMMENDED MAINTENANCE
SCHEDULE FOR POWER TRANSFORMERS
4.1 Limits of insulation resistance of windings of power transformers
CHAPTER-4 POWER TRANSFORMER MAINTENANCE
SCHEDULE
CHAPTER-5 LOSSES IN TRANSFORMER
5.1 Core Losses
5.2 Copper Losses
5.3 Stray Losses
5.4 Dielectric Losses
CHAPTER-6 TRANSFORMER PROTECTION
6.1 Types of faults.
6.1.1. Through faults
6.1.2. Internal faults
6.2 Different types of relays
6.2.1 Buchholz relays.
6.2.2 over fluxing relays
6.2.3 REF relays
6.2.4 O/C & E/F relays.
6.2.5 Differential relays.
CHAPTER-7 TESTINGS
7.1 Ratio test
7.2 Open Circuit and Short Circuit
7.3 Transformer Oil testing
7.4 Dissolved Gas Analysis
7.5 Magnetizing current test
7.6 Magnetic balance test

7. POWER TRANSFORMERS:
PROTECTION & MAINTENANCE
CHAPTER-1
INTRODUCTION:
Power transformers are the basic building blocks of the
power system, the capital investment involved in power system for the
generation transmission and distribution of the electrical power is so great
that proper precautions must be taken to ensure that the equipment not only
operates as nearly as possible to peak efficiencies, but also that it is protected
from accidents.
Power Transformer
The power transformer could be used in a power station or power
system could be a bank of three single phase transformers connected in either
star/delta or star/star etc., or could be a single three phase transformer with
single core. Normally for large capacity transformers, a three phase is used
because of it lighter weight, cheaper in cost, occupies less space and more
efficient. The only disadvantage is that any thing that effects the winding of
one phase will effect the other also, whereas in single phase transformers this
is not so, as one transformer can be replaced and the operation can be
continued.
The power generated at power stations is stepped up and transmitted
on extra high tension lines of 132 KV or 220 KV. The voltage is again step
down to 33 KV or 11 KV at various distribution transformer were voltage is
stepped down to 440/400 V before supply is made available at consumer
installation. It is roughly estimated that the power generated is transformed 3
or 4 times before it reaches a consumers system is 10 - 12 times its generating
capacities have to be provided at various stages. In the distribution net work a
transformer is most common of all electrical equipment.

8. CHAPTER-2
CONSTRUCTION FEATURES OF TRANSFORMERS:
2.1 CORE:
Core is used to support the windings in the transformer.
It also provides a low reluctance path to the flow of magnetic flux. It is
made up of laminated soft iron core in order to reduce eddy current loss
and hysteresis loss. The composition of a transformer core depends on
factors as voltage, current and frequency. Diameter of the transformer core
is directly proportional to copper loss and is inversely proportional to the
iron loss. If the diameter of the core is decreased, the weight of the steel in
the core is reduced which leads to less core loss of the transformer and the
copper loss increase. The vice versa happen when the diameter is
increased.
Laminated Steel Transformer Core
2.2 HV & LV WINDINGS:
The LV & HV windings are generally circular and
concentrically arranged. When a transformer is opened the HV coils are seen
first. When the HV coils are lifted LV coils are seen.
The LT coil is normally of copper strip insulated by
manila paper. In between LT coils & HT coils places concentrically: A
separator is used made of leatheroid paper on a bakelite cylinder. The HV
coils are normally of double paper covered or double cotton covered or
enameled copper wire of suitable guage wound in the layers. In between layers
press pan paper & manila paper is used for insulation for 2 - 16 nos. of coils in
each HV winding are used in which two or tapped coils the connection leads
between coils and from the coils to tapping switch are insulated by sleeves.

9. High Voltage Windings Low Voltage Windings
2.3 CONSERVATOR:
This is a reservoir for oil. Whenever the oil in the
transformer contracts during low temperature the oil is drawn from this and
when the temperature is high the oil expands and the excess volume of oil
goes into this and is store.
Conservator Tank
2.4 BUCHOLTZ RELAY:
It consists of a case in which two spherical floats are provided. Each
assembly of floats is arranged in such a way that when the transformer oil is
completely filled and ready for service, the contact of both the switches are
open when minor fault cause, e.g., some insulation break down between the
turns or core is over heated or transformer has been over loaded and raising
the temperature of oil, small bubble of gas due to vaporization oil will pass
through the relay and gradually go on accumulation above the assembly of
float to alarm. Circuit when gas pressure becomes sufficient, float alarm is

10. forced to move down wards and thus close the circuit of alarm. This alarm
circuit will also operate if the oil in the transformer is insufficient for cooling
(i.e. oil might have leaked out).
When serious internal short circuit between the phases, earth
faults due to break down of insulation, puncture of bushings, etc., generation
of gas will be rapid owing to high current. Due to this, oil will rush suddenly
through the pipe line causing the trip circuit to short circuit of the two contact
points of trip circuit and hence a relay operates and it isolates the
transformer.
*Bucholtz relay and Conservator of 10 MVA, 132/3.3 kV
Transformer
2.5 VENT PIPE & DIAPHGRAM:
The vent pipe is pressure relief device for the main tank
provided for oil to gush out when ever fault develops in transformer. This is
only safety device to avoid major damage inside or to prevent the tank from
bursting. The vent pipe is closed at the end by a diaphragm. In fact for some
makes of transformer to diaphragms are provided on at the bottom and other
at the mouth of the vent pipe. The diaphragm gets broken when pressure is
developed in the tank & oil gushes out. It is to be ensured that this diaphragm
is intact and air-tight as otherwise moisture may enter through this and cause
damage to the oil in the transformer.
2.6 BREATHER:
The insulating oil of transformer is provided for cooling
and insulating purpose. Expansion and contraction of oil during the
temperature variations cause pressure change inside the conservator. This
change in pressure is balanced by the flow of atmospheric air into and out of
the conservator. Transformer breather is a cylindrical container which is filled
with silica gel. Insulating oil reacts with moisture can affect the paper
insulation or may even may lead to internal faults. So it is necessary that the
air entering the tank is moisture free. It consists of silica gel contained in a
chamber. For this purpose breather is used. When the atmospheric air passes
through the silica gel breather the moisture contents are absorbed by the silica
crystals. Silica gel breather is acts like an air filter for the transformer and
controls the moisture level inside a transformer. It is connected to the end of
breather pipe.

11. Silica Gel Breather
2.7 BUSHINGS:
Up to 33 KV voltages ordinary porcelain bushings are
used. Above this voltage condenser and oil filled terminal bushings or a
combination of both are employed.
Bushings
2.8 TAP CHANGER:
The output voltage may vary according to the input voltage and
the load. During loaded conditions the voltage on the output terminal fall and
during off load conditions the output voltage increases. In order to balance the
voltage variations tap changers are used. Tap changers can be either on load

12. tap changer or off load tap changer. In on load tap changers the tapping can
be changed without isolating the transformer from the supply and in off load
tap changers it is done after disconnecting the transformer. Automatic tap
changers are also available.
Tapings in transformer
2.9 COOLING:
The cooling of a transformer is carried out by following methods.
ON: Majority of transformers are oil immersed with natural cooling that is
the heat developed in the cores and coils is passed on to the oil and hence to
the tank valves for which it is dissipated. Thus has an advantage that moisture
can not easily affect insulation.
OB: In this method the cooling of an ON type transformer is improved by air
blast over the outside tank.
OFB: For last transformer artificial cooling may be used. This method
comprises forced circulation of oil to a radiator were oil is cooled and again let
in to the transformer.
OW: An oil immersed transformer of this type is cooled by the circulation of
water in cooling tubes.

13. CHAPTER-3
RECOMMENDED MAINTENANCE SCHEDULE FOR
TRANSFORMER
Items to be
inspected
Inspection notes frequency Action required
1. Ambient
temperature
Daily
2. Winding
temperature &oil
temperature
Check the
temperature
Daily Shutdown
transformer &
investigate if found
abnormal.
3. Load & voltage Check against rated
figures
Daily
4. Oil level Weekly If low top up with dry
oil, examine
transformer for
leakage
5. Oil level in
bushing
Weekly If low top up with dry
oil, examine
transformer for
leakage
6. Relief diaphragm Monthly Replace if cracked or
broken.
7. Dehydrating
breather
Check for air passage
color of the agent
Monthly If found pink change
by spare charge or
old charge may also
reactivated.
8. Bushing Examine for cracks &
dirt
Quarterly Clean or replace
9. Oil Check for the di
electric strength &
water content
Half yearly Take suitable action
to restore quality of
oil
10. Cooler fans,
bearings motors &
control mechanism
Lubricate bearings,
check gear box,
examine contacts,
controls & interlocks
Half yearly Replace burnt or
warm contact or
other parts
11. Oil in coolers Test for pressure Half yearly
12. Oil in
transformer
Check for sludge Yearly Filter or replace
13. Oil filled bushing Test oil Yearly Filter or replace
14. Gasket Yearly Tighten bolts evenly
15. Cable box Check for ceiling
arrangements,
examine compound
cracks
Yearly Replace
16. Relays, alarms &
circuits
Examine relays &
alarm contacts
Yearly Clean components,
replace contacts &
fuses if necessary

14. * 17. Earth resistance Yearly Take suitable action
if resistance is high
18. O.L.T.C over
hauling
Check O.L.T.C.
R.T.C.C. of proper
functioning
Quarterly Clean & grease all
moving contacts
check oil in diverter
arrangements
19. Bucholtz relay
contacts
Check contacts &
floats
Monthly Rectify or replace
defective
20. IR test of
windings
Measure by
MEGGER
Yearly Take suitable action
if found low
21. Overall
inspection including
lifting of core
Once in 15 years Wash by hosting
down with clean dry
oil
22. Sludge Oil for all values Once in 10 years Replace if tests
values are not
attained.
• Permissible values of Earth resistance at
Power stations 0.5 ohm
Major substation 1.0 ohm
Small Sub stations 2.0 ohms
LIMITS OF INSULTATION RESISTANCE OF WINDINGS OF POWER
TRANSFORMERS
Rated
voltage of
the winding
Minimum safe insulation resistance in mega ohms at winding
temperature of given above
30°C 40°C 50°C 60°C
66 KV &
above
600 300 150 75
33 KV 500 250 155 65
6.6 KV & 11KV 400 200 100 50
Below 6.6 KV 200 100 50 25
CHAPTER-4
POWER TRANSFORMER MAINTENANCE SHEDULE

15. The following maintenance schedule is followed in power stations
and switching stations in the A.P power system.

16. PARTICULARS PERIOD REQUIRED SATISIFACTORY
RESULTS
1 Checking of oil level in conservator & bushing,
examining for leaks.
Daily shift No leaks
2 Checking for unusual noise. Daily shift 1. No unusual noise.
2. No sparks
3 Noting the loading in amp. Daily shift ----
4 checking for leakage of water into coolers
(forced cooling systems)
Daily shift ----
5 checking relief diaphragm for cracks Daily shift No cracks
6 Cleaning of bushings Monthly
or during
shutdown
7 Ensuring that oil comes out when air release
valve is opened
Monthly
or during
shutdown
Without air bubbles
8 Checking the color of silica gel (replacement or
recondition if necessary).
Monthly
or during
shutdown
Blue color
9 Inspection & cleaning of breather Monthly
or during
shutdown
Vent hole should not be blocked. Small
quantity of oil should be in the bottom
10 Measuring insulation resistance of windings
with 1000 V Megger
Monthly
or during
shutdown
See chart
11 Checking up of temperature bucholtz alarms for
correct operations
Monthly
or during
shutdown
Alarm should come when points of
thermostat touches set point
12 Noting the oil level tanks in the inspection glass
of bucholtz relay
Monthly
or during
shutdown
CC level shall be fault
13 Testing of oil from tank and conservator for di-
electric and testing strength (above
10000kVA).
Quarterly 30 KV -60sec.
40 KV- instant
40 KV -60sec.
50 KV-instant
14 Checking Bucholtz relay for any gas collection
and testing the gas collected
Quarterly If gas collected switch off & intimate TRE
30 KV -instant
15 Testing of oil for dielectric strength of tap
changer
Quarterly 30 KV -instant
16 Megger testing of motor of forced cooling
systems
Quarterly
17 Check transformer ground connection for
lightness
Quarterly
18 Cleaning of water jacket (forced cooling). Quarterly
19 Testing of oil in the conservator for dielectric
strength for transformer below 1000 KVA
(or before after wet season).
Yearly 30 KV -instantly
20 Checking up of gap setting of bushing of
transformers.
Yearly
21 Pressure testing of oil coolers (forced cooling
system)
Yearly
22 Testing motors, pumps & calibrating pressure
gauges etc (forced cooling).
Yearly
23 Calibration of temperature indicator by MRT. Yearly +/- 2.5%
24 Testing of oil in the conservator and tank acidity
(neutralization valve)
Yearly 0.3mg KOH/gm of air.
25 Testing the di-electric strength of oil in oil
bushing when ever the di-electric strength is
unsatisfactory filtering of transformer of oil
should be done.
Yearly 40 KV - 60sec
40 KV -60 sec
50 KV- instant
26 Checking operation of bucholtz relay by air
injection
Yearly alarm shall come
27 Tap changer maintenance. a. Over hauling.
b. Checking up of
contacts
c. Testing of oil for
acidity
d. Filtering or
renewal of oil (yearly or after 1000 operations
or when test results are poor)
Yearly
28 Major over haul (complete) of the transformers
with capacity and below should be done
whenever test results are unsatisfactory.
Yearly

17. CHAPTER-5
LOSSES IN TRANSFORMER
Losses can be considered as the difference between the Input
power and the output power. All electrical machines has certain losses. There
is no equipment which has zero loss or whose output power is equal to the
input. Losses occur in all electrical equipment and these losses are dissipated
in the form of heat.
Transformer is the most efficient electrical machine. Since the
transformer has no moving parts, its efficiency is much higher than that of
rotating machines. The various losses in a transformer are enumerated as
follows:
1. Core loss

18. 2. Copper loss
3. Load (stray) loss
4. Dielectric loss
5.1 Core loss:
When the core of the transformer undergoes cyclic magnetization,
power losses occur in it. There losses are together called as core loss. There
are two kinds of core losses namely hysteresis loss and eddy current loss.
Core loss is important in determining heating, temperature rise, rating and
efficiency of transformers. The core losses comprises of two components:
• Hysteresis loss
• Eddy current loss
Hysteresis loss
This phenomenon of lagging of magnetic induction behind the
magnetizing field is called hysteresis.
In the process of magnetization of a ferromagnetic substance through a
cycle, there is expenditure of energy. The energy spent in magnetizing a
specimen is not recoverable and there occurs a loss of energy in the form of
heat. This is so because, during a cycle of magnetization, the molecular
magnets in the specimen are oriented and reoriented a number of times.
This molecular motion results in the production of heat. It has been found
that loss of heat energy per unit volume of the specimen in each cycle of
magnetisation is equal to the area of the hysteresis loop.
The shape and size of the hysteresis loop is characteristic of each material
because of the differences in their retentivity, coercivity, permeability,
susceptibility and energy losses etc.
Hysteresis loop
Click thumbnail to view full-size

19. The net unrecoverable energy lost in the process is area of abco which is
lost irretrievably in the form of heat is called the hysteresis loss. the total
hysteresis loss in one cycle is easily seen to be the area of one complete
loop abcdefa.
If wh indicates the hysteresis loss/ unit volume, then hysteresis loss in
volume V of material when operated at f Hz is given by the following
equation.
Ph=whVf W
Steinmetz gave an empirical formula to simplify the computation of the
hysteresis loss based on his experimental studies. The formula given by
him is as follows:
Ph=khfBn
m W
where kh is a characteristic constant of the core material, Bm is the
maximum flux density and n is caller steinmetz constant
Permissible core losses in transformer
kVA Core loss (W)
16 155
25 195
40 260
50 295
63 350
75 385
88 400
100 500
125 570

20. kVA Core loss (W)
160 670
200 800
250 950
315 1150
400 1380
500 1660
860 1980
900 2400
1000 2800
All the above losses are subjected to positive or negative variation of 10%
Eddy current Loss
When the magnetic core flux varies in a magnetic core with respect
to time, voltage is induced in all possible paths enclosing the flux. This will
result in the production of circulating currents in the transformer core.
These currents are known as eddy currents. These eddy currents leads to
power loss called Eddy current loss. This loss depends upon two major
factors. The factors affecting the eddy currents are:
Resistivity of the core and
Length of the path of the circulating currents for a given cross section.
The eddy currents can be expressed as,
Pe =kef2
B2
W/m3
ke = ke'd2
/p
Where, d is the thickness of the lamination.
p is the resistivity of material of the core
Pe = ke'd2
f2
B2
/p W/m3
Hence from the above equations it is evident that Eddy current loss is
directly proportional to the square of the thickness of the lamination and
that of the frequency of supply voltage.
Total core loss
Total core loss = Hysteresis loss + Eddy current loss.
Reduction of Eddy Current Loss

21. Reduction of eddy current loss can be achieved by using core with
high resistivity and increasing the path of circulating currents.
By increasing the length of the path, the resistance offered by the material
to the induced voltages will increase, resulting in the reduction of Eddy
current loss.
High resistance can be achieved by using silicon steel cores. The resistance
of the steel can be increased by adding silicon to it. The cores can be
laminated along the flow of flux. Each lamination is insulated from the
adjoining one. This increases the path length of the circulating currents
with consequent reduction in Eddy current loss.
5.2 Copper Loss:
It is a well known fact that whenever there is a resistance to the flow
of current in a conductor, power loss occurs in the conductor due to its
resistance. Copper loss occurs in the winding of the transformer due to the
resistance of the coil. When the winding carries current, power loss occurs
due to its internal resistance. This loss is known as copper loss. The copper
loss can be expressed as below
Pcu = I2
R
Where, I is the current through the winding and R is the resistance of the
winding.
Copper loss is proportional to the square of current flowing through the
winding.
Permissible copper losses at 75 degree Centigrade
kVA Copper losses (W)
16 500
25 700
40 975
50 1180
63 1400
75 1600
88 1650
100 2000
125 2350

22. kVA Copper losses (W)
160 2840
200 3400
250 4000
315 4770
400 5700
500 6920
860 8260
1000 11880
All the above losses are subjected to positive or negative variation of 10%
5.3 Stray Loss:
Stray loss results from leakage fields including Eddy currents in the
tank wall and conductors. The winding of the transformers should be
designed such that the stray loss is small. This can be achieved by the
splitting of conductors in to small strips to reduce Eddy currents in the
conductors. The radial width of the strips should be small and they should
be transposed.
5.4 Dielectric Loss:
This loss occurs in the transformer oil and other solid insulating
materials in the transformer.
The major losses occurring in the transformer are Core loss and copper
loss. Rests of the losses are very small compare to these two. All the losses
occurring in transformer are dissipated in the form of heat in the winding,
core, insulating oil and walls of the transformer. Efficiency of the
transformer increases with decrease in the losses.
CHAPTER-6
TRANSFORMER PROTECTION
6.1 TYPE OF FAULTS:
The type of faults that the power transformers are subjects to are
classified as:
1. Through faults.
2. Internal faults
6.1.1 THROUGH FAULTS:

23. These are due to over load condition and external short circuits.
The transformers much be disconnected when such faults
only after allowing a predetermined time during which are other protective
gear would have operated. A sustained over load conditions can be detected by
thermal relay which gives an alarm so that the situation can be attended to or
the supply disconnected, if necessary. For the external short circuit
conditions, time graded O/C relay are generally employed. Fuses are provided
for low capacity transformers (distribution transformers).
6.1.2 INTERNAL FAULTS
The primary protection of a transformer is intended for the
conditions which arise as a result of faults inside the protected zone. Internal
faults are very serious & there is always the risk of fire. These internal faults
are classified into two groups.
Electrical faults which cause immediate serious damage but are generally
detectable by unbalance of voltage or current such as phase to phase faults,
short circuits between turns of high & low voltage winding etc.,.
Incipient faults: which are initially minor faults, causing slowly developing
damage? They include:
 A poor electrical connection of conductors or a core faults due to break
down of the insulation of the lamination bolts or clamping rings.
 Coolant failure which will cause a rise of temperature even below full
load operation.
 Possibility of low-oil content or clogged oil flow, which readily cause
local hot-spots on windings.
 Bad load-sharing between transformers in parallel, which can cause
overheating due to circulating currents.
Generally for group (1) it is important that the faulted
transformer should be isolated as quickly as possible after the fault has
occurred to limit the damage to the equipment. The faults of group (2)
through not serious in their incipient stage may cause major faults in the
course of time and should thus be cleared as soon as possible. It should be
emphasized that the means adopted for protection against faults of group(1)
or not capable of detecting faults of group(2), where as the means applicable
to detect the fault of group(2) may detect some faults in group(1) but are not
quick enough. These ideas are basic to transformer protection and the means
for protection against group (1 & 2) should not be treated as alternative but as
supplements to each other.
In A.P. system, the rating of power transformers at EHV
substations in general are as follows:
1. 220/132 KV 100 MVA auto transformers.
2. 220/33 KV 50 KVA & 31.5 MVA transformers.
3. 132/66 KV 40 KVA & 27.5 MVA transformers.
4. 132/33 KV 50 MVA, 31.5 MVA, 25, 16, 15 & 7.5 MVA
transformers.
5. 132/11 KV 16, 15 & 7.5 MVA transformers.
Most of the power transformers are of star-star type with neutral
solidity earthed. There are few transformers with delta-star windings (delta on
HV side).

24. Norms of transformer protection generally followed in A.P.
system are indicated below:
VOLTAGE
RATIO &
CAPACITY.
HV SIDE. LV SIDE COMMON
RELAYS
1. 132/33/11 KV
up to 8 MVA.
3 O/L relays + 1
E/L relay
2 O/L relays + 1
E/L relay
Bucholtz, OLTC
Bucholtz, OT, WT
2. 132/33/11 KV
above 8 MVA &
below 31.5 MVA.
3 O/L relays + 1
dir. E/L relay
3 O/L relays + 1
E/L relay
Differential,
Bucholtz, OLTC
Bucholtz, OT, WT
3. 132/33 KV,
31.5 MVA &
above.
3 O/L relays + 1
dir. E/L relay
3 O/L relays + 1
E/L relay
Differential over
flux, bucholtz,
OLTC bucholtz,
PRV, OT, WT.
4. 220/33 KV,
31.5 MVA &
220/132 KV, 100
MVA.
3 O/L relays + 1
dir. E/L relay
3 O/L relay + 1
dir. relay
Differential over
flux, bucholtz,
OLTC bucholtz,
PRV, OT, WT.
5. 400220 KV,
315 MVA
3 dir. O/L relays
(with dir.
Highest) + 1 dir.
E/L relays
restricted relay.
3 dir. O/L relays
(with dir.
Highest) + 1 dir.
E/L relays
restricted relay
Differential over
flux, bucholtz,
OLTC bucholtz,
PRV, OT, WT &
over load (alarm)
relay.
6.2 TRANSFORMER PROTECTION - DIFFERENT TYPES OF
RELAYS
 Bucholtz relays.
 Over fluxing relays.
 REF relays.
 O/L & E/L relays.
 Differential relays.
6.2.1Bucholtz relays:
When ever a fault in transformer develops slowly, heat is
produced locally, which begins to decompose solid of liquid insulated
materials & thus to produce inflammable gas & oil flow. This phenomenon has
be used in the gas protection relay or popularly known as bucholtz relay. This
relay is applicable only to the so called conservator type transformer in which
the transformer tank is completely filled with oil, & a pipe connects the
transformer tank to an auxiliary tank or “Conservator” which acts as an
expansion chamber. Figure shown as bucholtz relay connected into the pips
leading to the conservator tank an arrange to detect gas produced in the
transformer tank. As the gas accumulates for a minor fault the oil level falls &,

25. with it a floar ‘F’ which operates a mercury switch sounding an alarm. When a
more serious fault occurs within the transformer during which intense heating
takes place, an intense liberation of gases results. These gases rush towards
the conservator and create a rise in pressure in the transformer tank due to
which the oil is forced through the connecting pipe to the conservator. The oil
flow develops a force on the lower float shown as ‘V’ in the figure and over tips
it causing it contacts to complete the trips circuit of the transformer breaker.
Operation of the upper float indicates & incipient fault & that of the lower float
a serious fault.
ANALYSIS OF GASES IN BUCHOLTZ RELAY:
The gas collected from bucholtz relay is passed through the bottle
containing the 5% Silver Nitrate solution ( Ag No3) and allowed react with the
with it , Depending up on the color obtained from this reaction, the type of
fault in the transformer can be easily analyzed.
Color of the gas Identification
1. Colorless Air
2. White precipitate insulation gas of decomposed paper and cloth
3. Yellow gas of decomposed wood insulation
4. Gray gas of over heated oil due to burning
of iron
5. Black gas of decomposed oil due to electric
arch.
BUCHOLTZ RELAY OPERATION: CERTAIN PRECAUTION:
The bucholtz relay may become operative
not only during faults within the transformer. For instance when oil is added
to a transformer, air may get in together with oil, accumulate under the relay

26. cover & thus cause a false operation of the gas relay. For this reason when the
“gas” alarm signal is energized the operators must take a sample of gas from
the relay, for which purpose a special clock is provided. Gases due to faults
always have color & an order & are inflammable.
The lower float may also falsely operate if the oil velocity in the
connection pipe though not due to internal faults, is sufficient to tip over the
float. This can occur in the event of an external short circuit when over
currents flowing through the windings over heat the copper & the oil & cause
the to expand. If mal-operation of bucholtz relay due to over loads or external
short circuits is experienced it may be necessary that the lower float is
adjusted for operation still higher velocities.
In installing these relays the following requirements should be fulfilled.
1. The conductor connection the contacts to the terminals on the cover
must have paper insulation, as rubber insulation may be damaged by
the oil.
2. The floats must be tested for air tightness by for example, submerging
them in hot oil to create a surplus pressure in them.
3. The relay cover & the connection pipe should have a slope of 1.5 to 3%
& not have protruding surface to insure unrestricted passage of the
gasses into the conservator
A large number of faults gas protection operations may results from
failure to fully observe the above precautions.
6.2.2 OVERFLUXING PROTECTION: PRINCIPLES & RELAYS IN
A.P. SYSTEM:
The fundamental equation for generation of E.M.F in a transformer to give
flux
φ=K (E/F)
The over fluxing condition in transformer can occur during system over
voltage & or under frequency condition. This will cause an increase in the iron
loss & disproportionately great increase in magnetizing current. In addition
flux is diverted from the laminated core structure into steal structural parts. In
particular under condition of over excitation of core, the core bolts which
normally carry little flux may subjected to large component of flux diverted
from highly saturated & constricted region of core along side. Under such
condition, the bolts may be rapidly heated to temperature which destroys their
own insulation & will damage the coil insulation if the condition continues.
The over fluxing condition does not call for high speed tripping.
The tripping delayed for a minute or two by which time; the condition may
come too normally.
Of late the margins between the operating flux density & design
flux density are coming down due to economic consideration for the
manufacturer of the transformer. More over with sustained low frequency

27. operation, the transformer are naturally subjected to more than the rated
values.
These condition prompted provision of over fluxing relays from
80’s in the system.
6.2.3 RESTRICTED EARTH FAULT PROTECTION:
An earth fault in the winding is the most common type of
transformer fault and is best detected by using a “restricted” form of earth
fault protection. In this way time and current settings can be made
independent of other protection system, thus low settings and fast operating
times can be achieved.
The restricted scheme is a balanced system of protection and can
be applied to either star or delta windings. The scheme connections for either
type of windings are shown in figure. (1.4.2.4)
For the star winding, 3-line current transformer are balanced
against a CT in the neutral connection; while on the delta side, the 3-line CT’s
are connected in parallel.
An external fault on the star side will result in the line current
transformer of the affected phase and a balancing current in the CT’s, the
resultant current in the relay is therefore zero. During an internal fault, the
neutral CT only carries current & operation results.
The arrangements of residually connected CT’s on the delta side
of a transformer is only sensitive to earth faults on the delta side because zero
sequence is blocked by the delta winding. For example, on earth fault on the
star side transferred to the transformer appears on the delta as a phase fault.
There the arrangement is an inherently restricted earth fault scheme in this
application.
Modern practice is to employee a voltage operated (high
impedance principle) relay for this application. The relay is set to operate with
a certain minimum voltage across its terminals. The value of this operating
voltage is chosen to be slightly higher than the maximum voltage which can
possible appear across the relay terminals during external faults conditions.
6.2.4 BACKUP O/L & E/L RELAYS :
The following O/L & E/L relays are provided on transformers in
A.P. system.
Make of relay HV O/L & E/L (type) LV O/L & E/L (type)
EE/GEC CDG (with highest) +
CDD
CDG (with out highest)
+ CDG (CDD for
100MVA transformers)
ABB ICM 21P (with highest) ICM21 NP + ICM21NP

28. ER
ALIND
TJM1 (highest) + TJM12
TMAS311a + TMAS101a
+ TMWD (dir. Element)
TJM10 + TJM10
TMAS301a + TMAS 101a
6.2.5 DIFFERENTIAL RELAYS
A simple differential relay compares the currents at both ends of
a protected element as indicated below.
As long as there is no fault within the protected equipment the
current circulates between the two CT’s & no current flows through the
differential element. But for internal faults the sum of the CT’s secondary will
flow through the differential relay making it to operate.
PERCENTAGE DIFFERENTIAL RELAYS
Two basic requirements that the differential relay connections
are to be satisfied.
It must not operate for load or external faults.
It must operate for internal faults.
As on-load tap change facilities are invariably provided in the

29. grid transformers, any departure from the nominal tap position will results in
spill currents in the relay circuits. Further, the CT’s are often of different types
and have dissimilar magnetization characteristic, again resulting in spill
current during heavy through fault condition.
To avoid unwanted relays operation under the above two
conditions a “percentage bias” differential relays is used.
The operating characteristic of percentage bias differential relay
is shown in following figure.
In general the transformer primary current does not equal their
secondary current and the connections of the secondary winding do not
correspond to those of the primary. In order that the current flowing through
the relay should nearly equal zero during normal operating conditions and
when external short circuit appear, it is necessary to do every thing to have
secondary currents of the current transformers on the transformer primary &
secondary sides of equal order and coincide in phase. This is achieved by
accordingly selecting the current transformer ratios, having the method of
connection CT’s made in conformity with the vector group of three phase
power transformer and by the use of additional auxiliary CT’s in the scheme.
CURRENT TRANSFORMER RATIO & CONNECTIONS
FOR DIFFERENTIAL RELAYS:
A simple role of thumb is that the current transformer on any
star winding of a power transformer should be connected in delta and the CT’s
on any delta winding should be connected in star. Very rarely this rule is
broken. In case of winding connected in zigzag the CT’s will be connected in
star. This arrangement of CT connections will compensate for the phase shift
due to power transformer vector group connection.
The significant point is that, when grounded current can star
winding for an external fault, we must use the delta connection (or resort a
“zero phase sequence current shunt” that will be discussed late). The delta CT
connection circulates the zero sequence components of the currents inside the

30. delta and there by keeps them out of the external connection to the relay. This
is necessary because there are no zero phase sequence components of currents
on the delta side of the power transformer for a ground fault on star side;
therefore, there is no possibility of the zero phase sequence currents simply
circulating between the sets of CT’s and, if the CT’s on star side were no delta
connected, the zero phase sequence components would flow in the operating
coils and cause the relative to operate undesirably for external ground faults.
Transformer full load current:
In =Transformer capacity in MVA/√3*rate KV
If the CT’s are to be connected in star, the CT ratio will be IN / 1A.
If the CT’s are to be connected in delta.
The CT ratio will be: IN / 0.5775 A.
If the 0.5775A rated secondary core is not available, an auxiliary CT of 1 /
0.5775 A ratio can be used and its secondary connected in delta.
If the available CT’s on HV & LV side are not in inverse ratio of voltage,
auxiliary CT’s of suitable ratio have to be selected to match the currents to the
relay equal from both HV & LV side.
The Generator transformer at RTS, is protected for such internal fault by
General Electric make Differential Relay, type BDD 15B.
Settings adopted: 3.8 (132 kV side)
4.2 ( 13.8 kV)
These relays are meant for the overall protection of the Generator, UAT and
GT. This relay is provided with Percentage and Harmonic restraint and with a
sensitive polarized main unit as operating element.
Percentage restraint permits accurate discrimination between internal and
external fault of high currents and harmonic restraint enables the relay to
distinguish by difference in waveform, between the differential caused by
transformer internal fault and that caused by magnetizing inrush currents.
10 MVA Power Transformer Relay settings
132 kV/ 3.3 kV, CTR = 400/5, PTR 1200:1
S.No. Type/Make Protection Settings adopted
1 GE Differential 3.2 (LV)
5 (HV)
2 GEC Over current (HV) PS 2.5
TL 0.45
(LV) PS 6.0
TL 6.0
3 GEC Earth fault HV – PS 0.6

31. TL 0.10
MAGNETISING INRUSH CURRENT:
When a power transformer with its secondary circuit open, is
switched on, it acts as simple inductance and a magnetizing in rush current
which will be several times transformer full load current will flow. As the
inrush current flow in the primary of the transformer only, it appears to the
differential relay as an internal fault.
This relay is able to distinguish the difference between the
magnetizing inrush current and short circuit current by the difference in wave
shape. Magnetizing inrush current is characterized by large harmonic
components and that are not noticeably present in the short circuit current. A
harmonic analysis of typical magnetizing inrush current wave is shown in
table below.
HARMONIC COMPONENTS AMPLITUDE IN PERCENTAGE OF
FUNDAMENTAL
2nd
63.0
3rd
26.8
4th
5.1
5th
4.1
6th
3.7
7th
2.4
As seen from the above the 2nd
harmonic component is predominant in the
magnetizing inrush current.
A differential relay which extract the 2nd
harmonic current and fed to the
restraining coil to make relay inoperative due to magnetizing inrush current.
CHAPTER-7
TESTINGS
7.1 RATIO TEST:
Three phase AC supply voltage is applied to HV winding of the
generator transformer and by changing the tap positions the corresponding
changes in LV side voltage were observed
Tap
No.
Actua
l
Ratio
RY YB BR ry Yb br Measured
Ratio
1 10.52 395 395 395 37.5 37.5 37.5 10.53
2 10.38 395 395 395 38 38 38 10.39
3 10.25 395 395 395 40 40 40 9.87
4 10.11 395 395 395 42 42 42 9.40
5 9.98 395 395 395 42.5 42.5 42.5 9.29
6 9.84 395 395 395 43 43 43 9.18
7 9.70 395 395 395 44 44 44 8.97
8 9.57 395 395 395 45 45 45 8.77

32. 9 9.43 395 395 395 46 46 46 8.58
10 9.29 395 395 395 47 47 47 8.40
11 9.15 395 395 395 48 48 48 8.22
12 9.02 395 395 395 49 49 49 8.06
13 8.88 395 395 395 50 50 50 7.9
14 8.75 395 395 395 50.5 50.5 50.5 7.82
15 8.61 395 395 395 51 51 51 7.74
and are as shown below.
IR Value test:
IR values taken with 1 KV Megger at 25° C, by connecting
the Megger terminal as fallow:
 HV terminal to ground.
 LV terminal to ground
 HV to LV.
IR Values observed during test:
HV side:
R-body 150/200 MΩ
Y-body 150/200 MΩ
B-body 150/200 MΩ
LV side:
r -body 200/500 MΩ
y -body 200/500 MΩ
b -body 200/500 MΩ
HV to LV:
R-r 500/infinity
Y-y 500/infinity
B-b 500/infinity
7.2 Transformer Open Circuit and Short Circuit
S.C Test:
• Short Circuited HV terminals and voltage applied to LV side through a
Distribution transformer, so as to flow full load current (or certain
percentage of full load current, say 20%) in LV winding.
• Voltage and Ampere are measured on the secondary of the DTR (Across
CT & PT).
• V, W1, W2, A
• Copper losses will be calculated from the above

33. O.C TEST:
C T ratio adopted: 10/5=2
PT ratio adopted: 3300/110V=30
*Rated Voltage applied for LV winding with HV open from 11KV/440V,
100KVA DTR, and measured the following values,
V, W1, W2, A
Total Iron losses = (W1-W2) X MF
7.3 Transformer oil testing:
Transformer oil collected from the transformer form Top, Bottom and
Middle sample points and send to the laboratory for testing Oil
properties and Dissolved gases.
The following results are obtained:
TEST RESULTS
Name of the test Limit Result
Appearance *Amber/Clear &
transparent
Acidity (mg KOH/ g) 0.30 Max 0.005
B.D.V (KV) 40.0 Min *29/52
Density (gm/cm3
) 0.89 Max 0.86/
Flash point (°C)
Specific resistance (Ω-
cm)
145 Min
0.1E12 Min
150
2.64E12
Tan δ 1 Max 0.0093
Water content (ppm) 40 Max 5
* Before overhaul
7.4 Dissolved Gas Analysis:
Symbol Unit Result
132/3.3 kV
10 MVADTR
100 KVA
11kV/433

34. Total combustible
gas
Ml -
Hydrogen H2 ppm 2.1
Methane CH4 ppm 6.4
Ethane C2H6 ppm 2.3
Ethylene
Acetylene
C2H4
C2H2
ppm
ppm
3.8
ND
Carbon Monoxide CO ppm 165.6
Carbon Dioxide CO2 ppm 1126.0
PERMISSIBE GAS CONCENTRATION IN PPM
Service
life of the
equipmen
t in years
H2 CH4 C2H6 C2H4 C2H2 CO CO2
Up to 4 yrs 100/150 50/70 30/50 100/150 20/30 200/300 3K/3.5K
4 to 10 yrs 200/300 100/150 100/150 150/200 30/50 400/500 4K/5K
Above 10yrs 200/300 200/300 800/1000 200/400 100/150 600/700 9K/12K
GASES INVOLVED IN DIFFERRENT FAULTS
S.No Type of fault Gases involved
1 ARCING H2,C2H2,CH4
2 HOT SPOT H2,C2H4
3 PARTIAL DISCHARGE H2,CH4
4 INSL. DECOMPOSITION CO,CO2
7.5 MAGNETISING CURRENT TEST:
With the generator transformer kept under no load condition (LV side open),
three phase AC voltage applied to HV winding and the no load currents values
are taken.
Applied voltage = 415 V
Tap no.1
R-10.5mA
Y-9.5mA
B-10.5mA
7.6 MAGNETIC BALANCE TEST:
With the voltage applied across one phase of the winding, the
voltages induced in the other phases are observed at different tap positions.

35. The sum of the voltages (or fluxes) in other two phases should be
approximately equal to the voltage applied to the one phase.
Here any two phases acts as the return paths for the third path, for which the
voltage applied, with this test any defect in the magnetic circuit can be easily
identified.
RY YB BR
Tap no.1 400 240 150
230.5 400 160
190 200 400
Tap no.8 400 210 190
205 400 190
195 200 400
Tap no.15 400 200 195
200 400 200
195 200 400
CONCLUSION:
In all industrial countries the electrical power demand is ever
increasing, almost doubling its self approximately per decade. This
automatically demands for design, development and construction of
increasingly of high reliable in power transformers. Such large power
transformers maintenance and protection plays a vital role in power system.
It is a well known fact that “the prevention is always better than
cure”. Periodical maintenance and proper protection provides reliable and
qualitative power to the power system hence prevents it from block outs,
which in turn saves money and energy.
Now a days, with the invention of static relays/numerical the
protection of a power transformer became simple and easy. And a single and
small (small in size compared to earlier electromagnetic relays) relay can
provide the entire range of protection schemes for the power transformer.
Last but not the least, the testing of power transformer, its
auxiliaries like bucholtz relays, PRVs , CTs , PTs, Relays and oil will give early

36. signal about the transformer healthiness and alerts the maintenance engineers
to act immediately before the major problem took place.