Operation and maintenance of distribution transformers is important to prevent failures and repairs. Key aspects include: - Regularly check oil levels, insulation resistance, and test oil samples for breakdown voltage and acidity. - Maintain clean bushings, tight connections, and functional protection devices like fuses and lightning arrestors. - Common defects include oil leaks, overheating, and ineffective breathers. Causes may be corrosion, worn gaskets, overpressure, or inadequate ventilation/circulation. - Proper maintenance following schedules can maximize transformer life and minimize failures in the distribution system.

1. Operation & Maintenance of Distribution
Transformers, Prevention of Failures & Repairs

2. WHAT IS A TRANSFORMER?
Transformer is a static
device which transforms
A.C. Electrical power from
one voltage to another
voltage keeping the
frequency same by
electromagnetic induction.

3. Power Transformers
Transformers Capacity In A System:
Generally 6 to 8 times the installed capacity
Working Principle:
Static piece of apparatus used for transferring
power from one circuit to another without change
in frequency. It can raise or lower the voltage with
a corresponding decrease or increase in current.

4. Ideal Transformer : No Losses
EMF equation of a Transformer :
E1=4.44fN1øm
E2=4.44fN2øm
where N1=No. of turns in Primary
N2=No. of turns in Secondary
øm = Maximum flux in the core in Webers

5.  Voltage Transformation ratio (k) :
E2/E1 = N2/N1 = k
(a)Step up Transformer : k>1
(b)Step down Transformer : k<1
 Regulation :
%Regulation = ((E2-V2) / E2) * 100
where, E2 = Secondary Terminal Voltage at no
load
V2 = Secondary Terminal Voltage at load

6.  Losses in Transformers :
(a)No load losses = Hysteresis loss + Eddy current
loss
How can we minimize these losses ?
 By using steel of high silicon content with thin
laminations .
 The input power of a transformer, when on no load
measures the core loss.
(b)Load Losses :
I2R losses + Stray losses(losses occurring in the
mechanical structure and winding conductor due to
the stray fluxes).
These are measured by short circuit test.

7.  Efficiency:
% efficiency = ((output)/ input) * 100
 Condition of maximum efficiency : Iron loss = copper loss.
 Transformers are among the most efficient machines.
 In lower capacity ranges: Around 95% efficiency
 In higher capacity ranges : 99% is achievable.
 Auto Transformer :
Has a single continuous winding which is used for the
input and output voltages. Used where transformation ratio
differs a little from unity.

8. Components of Transformer
1. Windings
2. Core
3. Bushings
4. Conservator
5. Breather
6. Buchholz Relay
7. OLTC
8. Oil level Indicator
9. Thermometers
10.Various protective devices

9. Bushings
Two types:
(1)Porcelain : Upto 36 kv
(2)Condenser

10. Various Types of Cooling in
Transformers
Various Types of Cooling in Transformers:
(1)ONAN : OIL NATURAL AIR NATURAL
(2)ONAF : OIL NATURAL AIR FORCE
(3)OFAF : OIL FORCE AIR FORCE
(4)OFWF : OIL FORCED WATER FORCED
Type of cooling vs Rating :
 OFAF :100%
 ONAF :60%
 ONAN :40%

11. Parallel Operations of
Transformers
Satisfactory performance requires that they have :
 The same voltage ratio.
 The same per unit(or percentage) Impedance.
 The same polarity.
 The same phase sequence and zero relative
phase displacement.

12. (3) and (4) are absolutely essential.
(1) Must be satisfied to a closer degree.
(2) The more near , better will be the load division.
Tap Changers :
Two types:
(1) Off-Load Tap Changer.
(2) On-Load Tap Changer.

16. DISTRIBUTION TRANSFORMER
 TRANSFORMER WHICH IS USED FOR THE PURPOSE OF
DISTRIBUTION OF POWER.
 11KV/433V is the standard voltage rating.
 STANDARD KVA ratings are
25,63,100,160,200,250,315,400,500,630,750,1000,
1250,1500,2000,2500 KVA.
 IS-2026 is the INDIAN NATIONAL STANDARD.

17. MAIN FEATURES
Outdoor,oil cooled, 3 phase,50hz
Primary is delta connected and secondary is star
connected.
Naturally cooled (ONAN type).
Amongst all the types of transformers this is the
most required and most used type.

18. PARTS OF TRANSFORMER
MAIN TANK
RADIATORS
CONSERVATOR
EXPLOSION VENT
LIFTING LUGS
AIR RELEASE PLUG
OIL LEVEL INDICATOR
TAP CHANGER
WHEELS
HV/LV BUSHINGS
FILTER VALVES
OIL FILLING PLUG
DRAIN PLUG
CABLE BOX

30. ROUTINE TESTS
 Oil BDV TEST.
 Oil breakdown voltage is checked as per IS-335.
 100 mm L X 70 mm B X 80 mm Ht. glass pot.
 500ml Oil sample.
 Spherical electrodes with gap of 2.5 mm
 Recommended value : 60KV
 Equipment used : OIL BDV TEST SET.

31. MAINTENANCE OF TRANSFORMER
Transformer is the heart of any power system.
Hence preventive maintenance is always cost
effective and time saving. Any failure to the
transformer can extremely affect the whole
functioning of the organization.

32. MAINTENANCE PROCEDURE
OIL :
1. Oil level checking. Leakages to be attended.
2. Oil BDV & acidity checking at regular intervals.
If acidity is between 0.5 to 1mg KOH, oil should
be kept under observation.
3. BDV, Color and smell of oil are indicative.

33. MAINTENANCE PROCEDURE
1. Sludge, dust, dirt ,moisture can be removed by
filtration.
2. Oil when topped up shall be of the same make.
Otherwise it may lead to sludge formation and acidic
contents.
3. Insulation resistance of the transformer should be
checked once in 6 months.
4. Megger values along with oil values indicate the
condition of transformer.

34. MAINTENANCE
BUSHINGS:
 Bushings should be cleaned and inspected for
any cracks.
 Dust & dirt deposition, salt or chemical
deposition, cement or acid fumes depositions
should be carefully noted and rectified.

35. MAINTENANCE
 Periodic checking of any loose connections of the
terminations of HV & LV side.
 Breather examination. Dehydration of Silica gel if
necessary.
 Explosion vent diaphragm examination.
 Conservator to be cleaned from inside after every
three years.
 Cleanliness in the transformer yards with all nests,
shrubs removed.

36. PROTECTION OF TRANSFORMERS
 HT fuse & D.O. fuse.
 LT circuit breaker.
 HORN GAPS & Lightning Arrestor.
 Breather.

37. FAILURES & CAUSES
Insufficient Oil level.
Seepage of water in oil.
Prolonged Over loading.
Single Phase loading.
Unbalanced loading.
Faulty Termination (Improper sized lugs etc)
Power Theft.
Prolonged Short Circuit.
Lack of installation checks.

38. FAILURES & CAUSES
Faulty design
Poor Workmanship
Improper formation of core.
Improper core bolt insulation.
Burr to the lamination blades
Improper brazing of joints.
Burr /sharp edges to the winding conductor.
Incomplete drying.
Bad insulation covering.
Insufficient cooling ducts in the winding.

39. FAILURES & CAUSES
Bad Quality of raw material.
 Transit damaged transformers.
After failure, transformer is removed and
replaced with new/repaired one without
removing the cause of failure which
results in immediate or short time failure.

40. Specifications for Distribution
Transformers up to 100 KVA
 The distribution transformers shall comply with
the latest versions of Indian Standard
Specifications IS 1180(part1) and IS 2026
 The Standard Ratings shall be 16, 25, 63 and 100
KVA
 No load voltage ratios shall be as follows:
 (a) 11000/433-250 volts for 16, 25, 63 and 100
KVA
 (b) 10450/433-250 for 100 KVA only, whenever
specified
 No taps are to be provided in these transformers

41.  Winding Connections and Vectors
 The primary winding shall be connected in delta and the
secondary winding in star ( Vector symbols Dy 11), so as to
produce a positive displacement of 30 deg from the primary
to the secondary vector of the same phase. The neutral of the
secondary winding shall be brought to a separate insulated
terminal.
 The following standars fittings shall be provided
 (a) Two earthing terminals
 (b) Oil level gauge indicating three positions of oil, marked as
follows:
 Minimum( -5 deg C)
 30 deg C
 Maximum (98 deg c)
 (c) Lifting lugs

42.  The distribution transformer forms a link between the
consumers and primary distribution
 Dielectric strength of transformer oil shall be checked
once in a year
 Acidity of transformer oil shall be checked once in a
year
 Oil level shall be checked once in a month
 Over loading should be avoided by checking Tong
Tester Readings.

43. Distribution Transformers (DTRs)
 Most rugged of all electrical equipment
 All parts are tightly clamped
 No rotating or moving parts inside
 Shall be least prone to failure
 In a number of Indian Power Utilities, the rate
of failure is high (20 to 30% annually)

44. Equipment Life Cycle
 Specification – Design – Manufacture – Transport
– Erect – Commission – Operate – Protect –
Maintain

45. MAINTENANCE TECHNIQUES
 The maintenance techniques to be adopted should meet
the following criteria:
 Field staff should be able to carry out such tests and
interpret the test results
 Testing procedure to be evolved after detailed
discussions with the supplier of the equipment and
testing equipment
 Written procedure to be made available to the operating
staff

46. Maintenance of Equipment should
Include
 Diagnostics
 Overhauls
 Painting
 Mechanical Aspects
 Use of correct lubricants as specified by the manu-
facturer
 Availability of adequate spare parts

47. Responsibilities for Maintenance of
Distribution Transformers
S.No Name of the work to be
carried out
Person
responsibl
e to do the
work
Persons
responsible to
ensure that it is
done
1. Monthly maintenance
a. Maintaining distribution
transformer yard and
keeping the earth pits tidy
and watering of earth pits
Area L.M Area L.I
b. Cleaning the Transformer
including bushings
Area L.M Area L.I
c. Checking up of oil level and
reporting if it is low
Area L.M Area L.I

48. Responsibilities for Maintenance of
Distribution Transformers
S.No Name of the work to be carried
out
Person
responsible
to do the
work
Persons
responsible to
ensure that it is
done
1. Monthly maintenance
e Checking of earth connection Area L.M Area L.I
F Reconditioning breather by
reactivating silica gel or
replacement and maintaining
oil seal
Area L.M Area L.I
g Checking the LT Fuses and
renewing them
Area L.M Area L.I
h Topping up oil, where
necessary
Area L.M Area L.I

49. S.
N
o
Name of work to be carried out Person
responsible
to do the
work
Persons
responsible to
ensure that it is
done
2. Quarterly Maintenance
a. Renewing of H.G. fuses Area L.M Area L.I
b. Measurement of insulation
resistance and recording HV to
earth, LV to earth and HV to LV
with 1000 v megger and
recording along with temperature
of the winding
AE ADE
Responsibilities for Maintenance of
Distribution Transformers

50. S.
N
o
Name of work to be carried out Person
responsible
to do the
work
Persons
responsible to
ensure that it is
done
2. Quarterly Maintenance
c. Measurement of load currents R-
ph,Y-ph, B-ph and neutral
AE ADE
d Measurement of voltages at the
transformers and at tail end of LT
feeders and noting down
RY YB BR
RN YN BN
AE ADE
Responsibilities for Maintenance of
Distribution Transformers

51. S.No Name if work to be carried
out
Person
responsibl
e to do the
work
Persons
responsible to
ensure that it is
done
3 Annual Maintenance
F Lubricating AB switch and
checking for proper
operation
LM LI
g Checking line and earth
connection at AB switches
LM LI
h Checking line and earth for
lightning arrestor
LM LI
Responsibilities for Maintenance of
Distribution Transformers

52. S.No Name if work to be carried
out
Person
responsibl
e to do the
work
Persons
responsible to
ensure that it is
done
3 Annual Maintenance
d Checking connections for HV
and LV bushings including LV
neutral
LM LI
e Getting oil samples tested for
breakdown and acidity and
recording
LI AE
f Measurement of earth
resistance and recording
LI AE
Responsibilities for Maintenance of Distribution
Transformers

53. Maintenance Schedule of Distribution
Transformer
1. Cleaning of bushings and external surface of tank
Monthly
2. Checking of oil levels in the conservator and gauge
glass
Monthly
3. Checking of silicagel in the breather and replacement if
necessary
Montlhly
4. Checking of oil level in the oil seal of breather & top
up if necessary
Monthly
5. Checking of HG fuse & LT fuse and renew if necessary
(correct gauge shall be maintained)
Monthly
6. Checking of vent pipe diaphragm Monthly
7. Checking of terminal loose connections if any and
tightening the same
Monthly

54. Maintenance Schedule of Distribution Transformer
8. Checking for any oil leaks & rectification(including
replacement of oil seals if required)
Monthly
9. Taking tong tester readings during peak load hours
and remedial action
Quarterly
10
.
Noting down neutral currents and load balancing in
all the three phases
Quarterly
11
.
Measurement of IR values Half
yearly
12
.
Testing of oil for BDV, acidity Half
yearly
13
.
Checking of lightning arrestors and replacement if
required (once before monsoon)
Half
yearly

55. Maintenance Schedule of Distribution Transformer
14
.
Measurement of earth resistance, checking of
earthing system and rectification, if required
Half
yearly
15
.
Overhaul of transformer Once in 5
years

56. COMMON DEFECTS NOTICED AND THE
CAUSES
S.L
No
Part Defects Causes
1. Tank a. Leakage
of oil
b. Deformati
on
c. Overheati
ng
Corrosion/mechanical damage-
Gaskets worn out excessive internal
pressure-Improper circulation of
cooling oil and /or inadequate
ventilation.
2. Radiators a. Leakage
of oil
b. Deformati
on
c. Overheati
ng
Corrosion/mechanical damage-
Gaskets worn out excessive internal
pressure-Improper circulation of
cooling oil and /or inadequate
ventilation.

57. COMMON DEFECTS NOTICED AND THE
CAUSES
S.L
No
Part Defects Causes
3. Conservat
or
a. Leakage
of oil
b. Deformati
on
c. Overheati
ng
Corrosion/mechanical damage-
Gaskets worn out excessive internal
pressure-Improper circulation of
cooling oil and /or inadequate
ventilation.
4. Breather Ineffective Inlet choked-Silicagel saturated
5. Explosion
vent
Glass
Broken
Mechanical

58. COMMON DEFECTS NOTICED AND THE
CAUSES
S.L
No
Part Defects Causes
6. Core a. Loose
b. Increased
Losses
c. Excessive
Noise
Bolts loosening up-change in
characteristics due to heating,
vibration of stampings
7. Winding a. Short
circuited
b. Loosening
c. Insulation
Brittle
d. Open
circuited
Overloading-Air bubbles-loss of
insulation-shrinkage displacement,
overheating, decomposition, burn
out

59. COMMON DEFECTS NOTICED AND THE
CAUSES
S.L
No
Part Defects Causes
8. Oil a. Discolorati
on
b. High
acidity
c. Low BDV
d. Sludge
Contamination-Increased
moisture-Decomposition, chemical
action with other parts
9.
Terminal
Bushing
a. Breakage
b. Leakage of
oil
Strain-Gasket worn out-Loose fit

60. DISTRIBUTION TRANSFORMER FUSE
RATINGS LT MAINS AND CABLES
Capacity of
Transformer
in KVA
Full load
Amps
11Kv
side
Current
LT side
Size of To
fuse wire in
SWG
Size of single crore
PVC A1 cable in sq
mm
Size of LT
fuse units to
be used
11KV
side
LT
Side
For
mains
For feeders For
main(a
mps)
For
feede
r
(amp
s)
1. SØ 10 ---- ---- --
--
4 25
2. SØ 15 5 25
3. 25 1.31 33.4 38 20 50 1x50(2x25) 3x100 3x10
0

61. DISTRIBUTION TRANSFORMER FUSE
RATINGS LT MAINS AND CABLES
Capacity of
Transforme
r in KVA
Full load
Amps
11Kv
side
Current
LT side
Size of To
fuse wire in
SWG
Size of single core
PVC A1 cable in
sq mm
Size of LT
fuse units to
be used
4. 50 2.62 66.7 33 2x20 70 1x70(1x50
+1x25)
3x200 3x10
0
5 63 3.31 84.0 33 2x13 90 1x70 3x200 3x10
0
6. 75 3.94 100.0 33 2x1(1
x14)
95 1x95
(1x70+1x1
50)
3x200 3x20
0
7. 100 5.25 133.5 33 2.16 150 1x150(1x9
5+1x75
3x300 3x30
0
(3x2
00+3
x100
)

62. DISTRIBUTION TRANSFORMER FUSE
RATINGS LT MAINS AND CABLES
Capacity of
Transformer
in KVA
Full load
Amps
11Kv
side
Current
LT side
Size of To
fuse wire in
SWG
Size of single core
PVC A1 cable in sq
mm
Size of LT
fuse units to
be used
8.
150/160
6.4 213.4 30 2x14 2x120 2x120(1x1
50+1x95)
X300 6x30
0
9. 200 10.5 236.7 28 3x14 2x150 2x130(1x1
50+1x120
6x130 3x30
0
10. 250 13.12 334.4 28 4x14 3x120 1x120 9x300 9x30
0
11. 316 16.53 420.0 20
Note: 2x25 means 2 feeders of 25sq mm Cable

63. Permissible Values for Transformer Oil characteristics
Sl.N
o
Characteristic Voltage Class Permissible Values
1. Dielectric Strength
across 2.5 mm gap
145 KV and above
72.5 KV to 145KV
below 72.5 KV
60 KV (min)
50KV(min)
40KV (min)
2. Moisture Content 145 KV and above
72.5 KV to 145KV
below 72.5 KV
15 PPM
20 PPM
25PPM
3. Specific resistivity
at 90oc
1.0x1012 ohm Cm
(IS 6103)
4. Dielectric
dissipation factor
(tan δ) at 900C
0.005 (I.S 6262)
5. Interfacial tension 0.030 (I.S 6104)
6. Acidity 0.03 to 0.05 mg KOH/g of
oil

64. Some of the shortcomings in operation and
maintenance of Distributions Transformers:
• Transformers oil level is not maintained which may lead
to flash over.
• Not filtering the oil.
• Breaker for L.V. Protection removed or not
provided at the time of installation.
• Inadequate protection from overloads and short circuits
• Non-Maintenance of silica gel breather leading to
ingress of moisture.
• No factual data is collected on loading of the
transformers.

65. • Transformer is not properly earthed.
• Broken bushings are not replaced.
• Unbalanced loading on different phases due to single
phasing for load management or otherwise.
• Long LT lines, loose LT lines, trees touching LT lines cause
frequent short circuits and over load.
• Non provision of lightning arrestors or removal of L.As by
staff.
• Improper HV protection.
• Frequent blowing off of H.G. fuses used for HV protection
due to improper fitting(including wrong size of fuse
wire)and wind action resulting in mechanical damage.

66. The power transformers are usually provided with
adequate protection and also skilled staff is available to
look after the periodical maintenance and day to day
operations.
Distribution transformers are located in far flung areas of
small villages spread through out the country, which do
not receive any maintenance. The large scale
electrification of villages and massive energisation of
agricultural pump sets have resulted in installation of over
17 – 25 lakhs distribution transformers, which have very
little or no protection and practically no maintenance. The
overloading of the transformer goes unnoticed till the D.T.
fails.

67. A well planned maintenance schedule will bring
down the failure to a great extent.
OVERLOADING:-
The Distribution transformers are generally designed
to carry 10% overload continuously. But it is not
advisable. When the load in a D.T.reaches 80% ,it should
be strengthened by replacing with higher capacity or new
transformer installed.
Unbalance and overloading may cause the neutral to
be cut. The loads should properly be balanced and
overloading strictly avoided.

68. INADEQUATE / POOR DESIGN
1. Transformer tank size
Inadequate clearances for free circulation of oil
2. Percentage Impedance (Mechanical strength of the
coil)
Designing the transformers with increased impedance
to increase the short circuit withstand capacity.
Percentage impedance depends on two factors:
(a) size of wire used in H.V. coils
(b) Radial distance between H.V. & L.V. coils
Impedance affects the short circuit stresses.
3. Improper use of Aluminium Wires
4. Improper use of inter layer papers.
5. Use of inferior quality materials

69. The old standards specify that a transformer
should clear a short circuit within 2 seconds, if the
% impedance is 4 %.
But the present standards specify the short
circuit withstand time of 1 sec only irrespective of
capacity.
The Design of a transformer to counteract
mechanical stresses under short circuit conditions,
ultimately makes it more costly and hence a
compromise will have to be struck between the
capital cost and the short circuit capability of the
equipment.

70. INTELLIGENT DTRS
(A TECHNOLOGY SOLUTION)
• Failures of DTRs are being explained away
stating that they are scattered and
unattended
• Performance of DTRs can still be monitored
remotely by retrofitting intelligence
• Even load survey is possible for accurate
Distribution Planning
• Provide an Electronic meter with/without
load survey but with remote reading facility

71.  Obtain instant data of
3Phase currents
 3Phase voltages
 Phase Angle
 Active power & energy
 Reactive power
 Power factor, etc.
 Where meters have load survey facility data logging and storage
for last 30/45/60 days, obtain data remotely
 Conduct load survey
 Data very accurate and quickly transferable (portable)
 (contd..)
INTELLIGENT DTRS
(A TECHNOLOGY SOLUTION)

72. SPECIFICATIONS FOR TRANSFORMER
OIL-IS335-1983
Sl.N
o
CHARACTERISTI
CS
UNITS LIMIT IS335
1983
1. Appearance Clear free of
sediments and
Suspende
Matter
2. Density at 270C g/cm3 Max. 0.890
3. Kinematic
Viscosity 270C
mm2/
s
Max. 27.0
4. Flash Point(PM) 0C Min. 130
5. Pour point 0C Max. -6
6. Interfacil Tension
270C
N/M Min. 0.04
(contd…

73. SPECIFICATIONS FOR TRANSFORMER
OIL-IS335-1983
Sl.N
o
CHARACTERISTICS UNITS LIMIT IS335
1983
7. Neutralisation
Number
mgKOH
/g
Max. 0.03
8. Water Content Ppm Max. 50
9. Corrosion Sulfur Non corrosive
10. Inhibitor Not Present
11. Breakdown voltage
Unfiltered /Filtered
KV Min. 30
60
12. Dielectric Dissipation
factor tanδ at 90 0C
- Max. 0.005
(contd…

74.  The D.T. Repair and Maintenance centers should be set
up wherever necessary for maintenance and rectification
of the failed units.
The following need to be checked at regular intervals.
 Connections
 Fuses
 Oil level, oil leakage, and condition
 Bushings, Loadings, Earth resistance
 Rod gaps
 Protective equipment –LTCB, LAs, and A.B. Switches etc.
Breather and silica gel.

75. PREVENTIVE MAINTENANCE:
Ensures
• Proper Tree clearances
• Proper condition of LT lines (like avoiding loose spans,
maintaining clearances etc.)
• Oil level.
• Oil dielectric strength.
• Earth Resistance value (Less than 5 ohms)
• Effective operation of fuses
• No oil leaks
• No moisture ingress
Switching on and off operations during widely
varying load conditions cause disturbed movements in the
windings.

76. Purchase procedures:
SEBs adopt the method of lowest tender for ordering
transformers.
Vendor rating, by aiming at total minimum cost and not
merely the minimum initial cost is to be done.
Specification and Tests:
To procure high quality, reliable and long life transformers,
there is need to tighten the specifications and provide
some tests which would result in good quality
transformers.
Planning of Protection of DTs.
Failures due to overloading and inadequate protection
are large and REC has suggested the use of CSP
transformers and fail safe distribution transformers.