The document provides information on operation and maintenance of distribution transformers. It defines transformers and describes their working principle of mutual electromagnetic induction. It then discusses transformation ratios, the purposes of transformers, their advantages, types, parts, insulation, testing, and maintenance procedures. Key points covered include daily, quarterly and yearly maintenance checks, oil testing parameters, and common transformer tests like ratio, no load, short circuit and insulation tests.

1. A
PRESENTATION
ON
OPERATION & MAINTENANCE OF DISTRIBUTION TRANSFORMERS
BY
PREPAIRED BY- KAPIL SINGH
(ENGINEER-THERMAX LTD. C&H SSBU O&M)

2. Definition:
Transformer is a static Electro – magnetic equipment which
transforms alternating current from one voltage level to another voltage
level with out change in frequency. Transformer transfers power from one
winding to another winding via a common magnetic core.
Working Principle:
A transformer works under the principle of mutual electro magnetic
induction (Faraday’s laws of Electro-magnetic induction). It says that, when
ever a changing flux links with a coil an emf is induced in it and this induced
emf is proportional to the rate of change of flux and the number of turns in
the coils linking the flux.
Definition & Working Principle of Transformer

3. Equation for transformation ratio is,
E2/E1 = N2/N1 = k
k = >1 in step up transformer, where secondary turns are more and thus
voltage is more to reduce the transmission current.
k = <1 in step down transformer, where secondary turns are less than
primary and low voltage for consumer use.
If we include the current in transformation ratio the equation is,
E2/E1 = N2/N1 = I1/I2 = k
Transformation Ratio

4. Purpose:
a. Electrical energy may be transmitted economically over long distance by
stepping up of voltages to reduce the line losses.
b. To distribute the low voltages at consumer side by stepping down the
voltages.
Advantage:
a. Transformer is a static machine and losses are very less. There by
efficiency is high and about 95 to 98%.
b. Practically maintenance is very less.
Purpose & Advantage

5. Transformers can be classified into different groups and types based on the
following factors.
Types of Transformers
1. Type of core.
a. Core type transformer core.
b. Shell type transformer core.
c. Berry type transformer core.
d. Spiral type transformer core.
2. Method of cooling.
a. Natural cooling transformer.
b. Artificial cooling transformer.
c. Artificial cooling (water) transformer.
d. Mixed cooling transformer.
3. As per transformer ratio.
a. One to one transformer.
b. Step down transformer.
c. Step up transformer.
4. Based on number of phases.
a. Single-phase transformer.
b. Two-phase transformer.
c. Three phase transformer.
5. As per winding connection.
a. Star-star connected.
b. Star-delta connected.
c. Delta-delta connected.
d. Delta-star connected.
e. Open delta connected.
f. Scott connected.
6. As per the size of the transformer.
a. Distribution transformer.
b. Power transformer.
7. Based on function and utilization.
a. Auto transformer.
b. Potential transformer (instrument
transformer).
c. Current transformer (instrument
transformer).

6. Parts of the Transformer
LT Side
Buchlotz Relay
Neutral Bushing
Breather
Conservator Tank
OLTC Tank
Pr. Relief valve
Inspection
Window
Flanges for
Radiator
assembly
Thermo well for
OTI & WTI

7. Parts of the Winding
HT winding
Tapping's
of windings
for OLTC
Core
HT
termination
LT
termination

8. Insulation of Transformers
Two main kinds of Insulations:
1.Major Insulation:
 Insulating cylinder between LV & core
 Also between HV & LV
 Insulating barriers between limbs, coils & core yoke
 Oil impregnated paper used as it has high dielectric strength
2.Minor Insulation:
 Insulation between
individual turns & between layers
 Conductor Insulation: paper & cotton
 Air insulated Transformer
insulation used is glass tap

9. Taping of Transformers
Main two location:
1. Inside Tank
2. Outside Tank
Considered as integral part of transformer
Tapping is terminated just bellow oil level
External handle
Taping is done on HV winding
Provided in middle (axial & radial force)

10. Daily operational check of the Transformer
1. H.T. Voltage and current reading.
2. L.T. Voltage and current reading.
3. WTI current and maximum reading
4. OTI current and maximum reading
5. Oil level in conservator tank
6. Oil level in OLTC tank
7. Tap position
8. Tap changer counter reading
9. Tap changer motor MPCB and limit switch status.
10. PRV status.
11. Buchholz relay status.
12. Condition of silica gel in breathers.
13. Cleanliness of transformer and accessories.
14. Visual leakage / seepage of oil.
15. Humming/any abnormal sound of transformer and accessories.

11. Quarterly check of the Transformer
1)Cleaning of Transformer
2)Note down of Maximum temp of HV LV Winding. & LV Winding.
3)Checking of OLTC mechanism Step by step
4)Tightening of HV & LV connection
5)Checking of gas collection in Buchholz relay, cleaning of glass cover
6)Operation of tap changer from local and remote.
7) Condition check/replacement of silica gel.
8) Alarm/Tripping signal checking of OTI,WTI,Buchholz and PRV signal at panel.

12. Yearly check of the Transformer
1. Cleaning of Transformer
2. Oil leakages checks for all joint
3. Note down of Maximum temp of HV LV Winding. & LV Winding.
4. Checking of OLTC mechanism Step by step
5. Tightening of HV & LV connection
6. Checking of gas collection in Buchholz relay, cleaning of glass cover
7. Operation of tap changer from local and remote.
8. Condition check/replacement of silica gel.
9. Alarm/Tripping signal checking of OTI,WTI,Buchholz and PRV signal at panel
10. Oil dehydration
11. BDV of main Oil and OLTC oil
12. PPM of oil.
13. Ratio test in every tap
14. Magnetizing current test.
15. Magnetic balance
16. Winding resistance
17. Insulation resistance.
18. Other inter lock if any.

13. Tests on the Transformer
Routine Test
1. Ratio and Polarity test
2. Load Losses
3. Impedance measurements
4. Insulation resistance
5. Resistance of windings
6. No load losses
7. No load current
8. Voltage test
a. Separate source
b. Induced voltage
9. Core insulation test
Type Test
1. Temperature rise test.
2. Impulse voltage test.
3. Noise level test
Special Test
1. Partial discharge.
2. Radio interference.
3. Vibration test.
4. Short circuit withstand test.
5. Tan delta test.

14. Characteristics of Transformer Oil
Characteristics Requirement Method of testing Remarks
Appearance The oil shall be clear
and transparent and
free from suspended
matter of sediments.
A representative sample of the oil
shall be examined in a 100-mm
thick layer at 27°C.
Density at 29.5°C
max.
0.89 gm / cm3 IS-1448(P:16):1977
Kinematic viscosity max.
at
a. 27°C
b. 40°C
27 cSt
under consideration
IS-1448(P:25):1976
Interfacial tension at 27°C
min.
0.04 N/m IS- 6104:1971
Flash point penskymarten
(closed)
140°C IS-1448(P:21):1970
Pour point max. -6°C IS-1448(P:10):1970

15. Characteristics of Transformer Oil
CharacteristicsCharacteristics RequirementRequirement Method of testingMethod of testing RemarksRemarks
Neutralization valueNeutralization value
a. Total acidity maxa. Total acidity max
b. Inorganicb. Inorganic
acidity/alkalinityacidity/alkalinity
0.03 mg KOH/g0.03 mg KOH/g
nilnil
IS-1448(P:2):1967IS-1448(P:2):1967
IS-1448(P:2):1967IS-1448(P:2):1967
Alcoholic PotassiumAlcoholic Potassium
hydroxide solution ofhydroxide solution of
0.02 N should be in0.02 N should be in
place of 0.1 N Indicatedplace of 0.1 N Indicated
in test Method.in test Method.
Electric strengthElectric strength
(Breakdown voltage)(Breakdown voltage)
a. New unfiltered oil min.a. New unfiltered oil min.
b. After filtration min.b. After filtration min.
30 kV (rms)30 kV (rms)
If the above value is notIf the above value is not
attained the oilattained the oil
shall be filtered 60 kV.shall be filtered 60 kV.
IS-6792:1972IS-6792:1972 See noteSee note
mentioned belowmentioned below
Dielectric dissipationDielectric dissipation
factor (tanfactor (tan δδ) at 90) at 90°°CC
max.max.
0.0020.002 IS-6262:1971IS-6262:1971
Specific resistanceSpecific resistance
(resistivity)(resistivity)
a. At 90a. At 90°°C min.C min.
b. At 27b. At 27°°C min.C min.
35 * 1035 * 101212 ΩΩ-cm-cm
1500* 101500* 101212 ΩΩ-cm-cm
IS-6103:1971IS-6103:1971
Note: As a consequence of the tendency for water absorption to occur due to breathing on storage even when drums are sealed the oil shall be
filtered to remove moisture and particulate contaminates present in the original sample before the test as follows.
a. A sufficient quantity of oil is heated to 90 ± 2°C, then filtered hot under vacuum corresponding to an absolute pressure of about 2.5 kPa
through a sintered glass filter of porosity grade 4’.
b. A portion of filtered is cooled in a desiccator and used immediately to measure electric strength, if required, and specific resistance at 27°C.
The remaining hot filtrate is immediately used for measuring dielectric dissipation factor at 90°C and specific resistance at 90°C.

16. Characteristics of Transformer Oil
CharacteristicsCharacteristics RequirementRequirement Method of testingMethod of testing RemarksRemarks
Ageing characteristics
after
accelerated ageing
(open beaker method
with copper catalyst)
a. Specific resistance
at
27°C min. & at 90°C
min..
b. Tan δ at 90°C max.
c. Total acidity max
d. Total sludge max.
2.5 * 1012Ω-cm
0.2 * 1012Ω-cm
0.2
0.05 mg KOH/g
0.05% by weight.
IS-12177:1987
IS-6103:1971
IS-6262:1971
IS-1448(P:2):1967
IS-12177
Presence oxidation
inhibitor
The oil shall contain
antioxidant additives
IS-13631:1992 See note below
Note: For both phenol and amine types of indicators, qualitative methods shall be adopted. In case of ambiguity (marginal cases) in finding the
intensity of colour, a quantitativemethod shall be adopted. Value of 0.5 (max.) shall be treated as absence of DBPCPhenolic type inhibitor
(quantitative method for amine is under consideration).
Water content max. 50 ppm IS-13567:1992

17. Transformer oil Brands available in market
confirming to different
Standards/specifications as per products

18. Tests to be conducted on the Transformer
Test Purpose Item Required condition of transformer
IR value and PI
value.
Detects serious flaws, moisture
absorption and cleanliness of
winding. Winding. Winding has to be isolated.
Tan delta or dielectric
loss or power factor
or HV test.
Indicates insulation deterioration,
contamination and physical damage.
Winding, oil and
bushings.
Winding has to be isolated,
oil sample should be collected.
Excitation current at
high voltage.
Indicates defects in the magnetic core
structure, shifting or windings, failures
in turn to turn insulation. Winding Winding has to be isolated.
Turns ratio
Indicates short circuited turns and
internal connections Winding Winding has to be isolated.
Winding resistance Detects poor connections and
conductor shorts Winding Winding has to be isolated.
Core IR and
inadvertent grounds
Indicates deterioration of core
insulation system Core Winding has to be isolated.
Water contents Indicates moisture level in oil Oil Oil sample has to be collected
Total acidity,
neutralization number
Measures organic and inorganic
acids Oil Oil sample has to be collected
Dissolved
gas analysis
Indicates specific gases
generated Oil Oil sample has to be collected
Furanite compounds Indicates cellulose degradation Winding Oil sample has to be collected

19. First Stage: When we start the filtering process initially the temperature will be low, as the
insulation value is high. But as temperature increases the IR value starts to decrease because
the moisture entrapped in the coils are released due to rise in temperature and this causes the
IR value to go down.
Second Stage: Then comes the point where all the moisture is released and then will be no
decrease in IR value or rise in the temperature.
Third Stage: At this point the heaters are switched off. Now the moisture is removed by the oil
filters and the IR value goes up and as the heaters are off the temperature decreases.
Drying out of Transformer
150
700
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Time
IRVaule
25
Temperature
Stage 1
Stage 2
Stage 3
Temperature
At this point the heaters
are switched off
IR

20. Humming in the transformer is a sound, which is produced due to the
vibration of the cores in the transformer.
The vibrations are produced due to the change in polarity of an
alternating current or voltage and by the loose of lamination of the core.
Both can be minimized by tightening the core of the transformer.
Humming Sound of the Transformer

21. Ratio test - Apply 3ph, 415V to high voltage winding of the transformer and
measure the induced voltage on the low voltage terminals at all taps. Ensure
variation in secondary volts as the taps are changed and compare the results
with manufacturers test report or ratio mentioned on transformer nameplate. If
delta tertiary stabilizing winding is provided (usually provided for Y-Y
transformers), open the delta shorting link and measure the voltage across the
two terminals during ratio test - it should be negligible.
No load / magnetising current test - Remove the shorting between primary &
secondary and measure the currents in all the phases while maintaining the
supply at primary. This test is done at nominal tap, and at the minimum and
maximum taps.
Expected currents - Ir / Iy / Ib-- 1: 0.8: 1 for star transformer
- Ir / Iy / Ib-- 1: 1: 1.3 for delta transformer
Ratio Test and No Load Test

22. Short circuit test
With the transformer at nominal tap, apply 3ph ,415V to the primary and
adequate shorting on the secondary side( depending on % impedance );
ensure bushing CT secondary’s and WTI CT secondary’s are through.
a) Measure currents in primary and secondary
b) Check the currents in CT secondary of bushing CT, WTI CT, and LDC CT.
Repeat point (a) at maximum & minimum taps.
Core balance test
Apply 415V between phase-phase (for delta transformer) and phase-neutral (for
star transformer) on secondary side. Ensure that adequate safety precautions
are taken on the primary side, as high voltage will be induced.
Measure the current and voltages for the following phases--
For delta trafo - Vry, Vyb, Vbr
For star trafo - Vry, Vyb, Vbr, Vrn, Vyn, Vbn.
Short Circuit and Core Balance Test

23. Insulation test
Insulation tester of 2.5 or 5 KV may be used. Polarisation index is recorded by IR
value at 10/1 min ratio.
Insulation Test
PI Value
Insulation
Condition
Recommendation
1.0 to 1.5 Bad
Drying is
Mandatory
1.5 to 2 Doubtful
Drying is
recommended.
2.0 to 3.0 Good
No drying is
required.
3.0 to 4.0 Excellent
No drying is
required.

24. Transformer oil testing at site:
Prior to charging of transformer confirm the healthiness of the oil by testing the
dielectric strength of oil. The oil is to be tested at 2.5 mm gap. 3 readings have
to be taken and a average of these readings is the ‘breakdown voltage of the
oil’. BDV should be >60 KV.
Transformer Oil Testing
Transformer oil testing at Lab:
 Electric strength
 Water contents
 Dissolved gas analysis
 Dielectric dissipation factor (Dielectric dissipation factor (Tan δ)
 Total acidity max
 Total sludge max

25. In transformer there are mainly three types of losses
1-iron loss or core loss
2-Copper loss or I
2
R loss.
3-Stray and Dielectric loss.
These losses due to,
1. Resistance of the winding (copper losses).
2. Eddy current and Hysteresis in the iron parts and core (core and iron losses).
3. Losses due to leakage reactance (leakage flux).
At No load the copper losses and leakage flux losses are negligible due to the very
less primary current.
At loaded condition copper losses and leakage flux losses will exist in considerable
manner.
Copper losses are variable and can be calculated by Ip2
*Rp and Is2
*Rs.
(Where I-current , R-resistance p-primary winding s-secondary winding)
Losses in the Transformer

26. 1. Check oil leakage from bushings, valves, air release plugs, etc.
2. Direction of mounting of Buchholz relay, as per arrow on the relay; the arrow
should point towards the conservator
3. Oil level in condenser bushings
4. Oil level of Main Tank and OLTC Tank should be up to the filling mark on MOG
(Minimum Oil Level Gauge)
5. Transformer is earthed at two points
6. Stoppers provided for locking of wheels
7. Test taps on HV condenser bushings are fixed Valves on each radiator, both top
and bottom, should be open.
8. Valves on either side of Buchholz relay should be open
10. Tightness of conductors / cables on Trafo. HV side, LV side and Neutral.
11. Check resistance and IR value of NGR, if provided
12. Check that oil is filled in OTI and WTI pockets.
13. Check that colour of silica gel in breathers of main conservator and OLTC
conservator is blue.
14. Oil in breather cups to be filled up to level indicated
Commissioning Procedure for a Transformer
.

27. OLTC separate chamber type
OLTC in-built in the main tank
a) For in-built OLTC (diverter switch) ensure that the tap number on the Drive mechanism
(DM) is same as the one in the display on the main tank. Then connect the main
operating rod between the OLTC DM and diverter switch.
Conduct mechanical operations on the OLTC taking it through all the taps. Ensure that
the final end position (upper and lower limit) switches operate.
b) During the raise / lower operation note the direction of operating mechanism. Switch’ off’
the 3 PH AC supply and check operation of raise/lower contactor as per commands from
DM.
c) With OLTC at some intermediate tap, switch on the 3 phase supply and give either raise /
lower command to the OLTC. Check whether the mechanism is moving in the right
direction for the command given. If the direction is reverse, switch ‘off’ the AC & DC
supply, verify that the phase sequence of the supply is correct; if not correct, reverse the
phase sequence at the motor terminal box.
d) Observe the operation of OLTC and check blocking of operation at the end positions
e) Verify that insertion of manual operating handle cuts off electrical operation.
f) Test the Automatic Voltage Regulating Relay (AVRR), if provided. Perform automatic
operation from the AVRR.
Commissioning Procedure for an
OLTC of Transformer

28. 1. Cleaning of the transformer, especially the bushings. Ensure that all protections are
operative. Buchholz relay maybe tested by using the test screw, or by pumping in air with
a bicycle pump , or by draining oil from the relay after closing both side valves.
2. Air release from the transformer is to be done from air release plugs on the main tank,
radiators, HV &LV bushings, Buchholz relay and OLTC prior to charging of transformer.
Do not release air from the Condenser bushings as they are hermetically sealed.
3. Check operation of OTI/WTI, ensure oil in the temperature probe pots.
4. Check that all interconnecting valves are in open position.
5. Check IR values between each winding an earth, and between windings. Neutral
earthing is to be disconnected for this purpose.
6. Check that HV and LV neutral earthing has been re–connected after megger test.
7. Earthing and delta shorting of tertiary stabilizing winding (if provided), to be checked.
8. Proper setting of OTI & WTI to be done.
9. Perform trip test from all the transformer protections, including Buchholz relay, PRD, Oil
surge relay, OTI and WTI.
10. All protection relays to be properly set.
11. Reset the Maximum Reading Pointer (MRP) of OTI & WTI.
12. Ammeter selector switches should be selected to read some phase current.
13. All MCBs, fuses and switches in Marshalling boxes, CRPs, isolator cubicles and breaker
cubicles should be in circuit.
14. If the transformer is provided with an OLTC, energise it at Tap #1; if the transformer is
provided with an off-circuit tap changer, energise it at the nominal tap.
Prior to Energizing the Transformer

29. After energising the transformer :
1. If OLTC is provided, run the tap changer from Tap #1 up to nominal tap.
2. Check the phase sequence and magnitude of the transformer secondary
voltage at the incoming line PTs.
3. If the transformer is an extension to an existing system, do the phasing out
w.r.t. the existing PTs.
4. Check for any abnormal sound or vibration in the transformer.
5. Keep the transformer energised on no-load for 24 hours; take readings of
OTI, WTI and current.
After Energizing the Transformer

30. CONDITIONS LEADING TO FAULTS
Insulation breakdown
Aging of insulation
Overheating due to over excitation
Oil contamination and leakage
Reduced cooling
External circuit fault

31. NATURE OF TRANSFORMER FAULTS
Although an electrical power transformer is a static device, but internal stresses
arising from abnormal system conditions, must be taken into consideration.
A transformer generally suffers from following types of transformer fault-Over
current due to overloads and external short circuits,
Terminal faults,
Winding faults,
Incipient faults.
All the above mentioned transformer faults cause mechanical and thermal stresses
inside the transformer winding and its connecting terminals. Thermal stresses lead
to overheating which ultimately affect the insulation system of transformer.
Deterioration of insulation leads to winding faults. Some time failure of transformer
cooling system leads to overheating of transformer. So the transformer protection
schemes are very much required.
The short circuit current of an electrical transformer is normally limited by its
reactance and for low reactance, the value of short circuit current may be
excessively high.

32. TRANSFORMER PROTECTION
It is common practice to provide buchholz relay protection to all 0.5 MVA and above
transformers. While for all small size distribution transformers, only high voltage
fuses are used as main protective device. For all larger rated and important
distribution transformers, over current protection along with restricted earth fault
protection is applied. Differential protection should be provided in the transformers
rated above 5 MVA. Depending upon the normal service condition, nature of
transformer faults, degree of sustained over load, scheme of tap changing, and
many other factors, the suitable transformer protection schemes are chosen
Some Typical protection equipments are enlisted below
1.Buchholz relay
2.PRV
3.OTI
4.WTI
5.MOG
6.O/V current and E/F relay
7.Differential protection relay

33. ch
THANK YOU
C&H SSBU O&M REGION-DELHI