1. TESTING AND COMMISIONING
DET310
CHAPTER 4
THE POWER TRANSFORMER
THE POWER TRANSFORMER

2. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
3.1 INTRODUCTION
Substation transformers can range from the size of a
garbage can to the size of a small house; they can be
equipped with a wide array of gauges, bushings, and
other types of auxiliary equipment.
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3. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.2 TRANSFORMER NAME PLATE
The transformer nameplate contains most of the
Important information that will be needed in the field .
The nameplate should never be removed from the
Transformer and should always be kept clean and
legible.
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4. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
CONTINUE-
SERIAL NUMBER i) WEIGHT
CLASS j) DIMENSION
KVA RATING k) TAP CHANGER
VOLTAGE RATING
WINDING
VEKTOR GROUPING
PERCENTAGE IMPEDANCE
INSULATING FLUIDS
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5. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.2.1 TRANSFORMER CLASS
ONAN - Oil natural air natural
ONAF - Oil natural air force (use fan)
OFAF - Oil force air force ( use oil pump and fan)
OFWF - Oil force water force (use oil pump and
water cooling)
ODAF – Oil force and air force
Example
Transformer Rating: 1000 KVA 550C ONAN, which stands for the capability to supply 1000KVA with a temperature rise of 550degres celcius with normal oil circulation (no fans or pumps).
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6. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Hermetically sealed
ONAN
INSUALTION TEST OF ELECTRICAL EQUIPMENT
Conservator Type OFAF
Conservator Type
OFAF

7. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.2.2 VA RATING
Every transformer has a maximum output current that it can deliver at its standard output voltage. This VA rating (KVA or MVA for large power transformers) is dependent on the ambient temperature or cooling provided. Exceeding the VA rating will cause over-heating of the core and windings and subsequent damage
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8. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Continue-
The capacity of a power transformer is defined by full load power, in kVA it can produce.
INSUALTION TEST OF ELECTRICAL EQUIPMENT
Where S = Full load Power
Uph= rated voltage at no load (V)
Iph = rated output current (A)
For Star Connection: I phase= Phase Voltage=

9. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
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VOLTAGE RATING
Transformer rated voltage is usually specified by the voltage ratio
Example: 11/0.415kV ; Primary rated Voltage : 11kV
Secondary rated Voltage: 0.415kV
WINDING
EXAMPLE: PRIMARY : DELTA
SECONDARY: STAR
4.2.5 VEKTOR GROUP
The vector groups denotes denotes the way the transformers windings
are connected:
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10. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
INSUALTION TEST OF ELECTRICAL EQUIPMENT

11. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Continue:
Example: Dy11
D= HV delta winding
y= low voltage star winding
11=clock face (30 degrees lead) with reference to the HV 12 o’clock
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12. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Continue:
Winding falls into three main types: star, delta,
and zigzag
Can be categorised into four groups
Group1: 0 degress phase shift: Yy0,Dd0,Dz0,Zd0
Group 2: 1800 deg phase shift: Yy6, Dd6, Zd6, Dz6
Group 3: -30 deg phase shift: Dy1, Yd1, Yz1, Zy1
Group 4: +30 deg phaseshift:Dy11,Yd11,Yz11,Zy11
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13. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.2.6: IMPEDANCE
The impedance of transformers is express in percentage values to
Enable fault level calculations to be carried out
Example: 11/0.416kV Impedance: 4.75%
Fault level:
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14. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.2.7 TRANSFORMER OIL
Act as an insulation and thermal transfer agent
Mineral hydrocarbon oil to BS 148
Sometimes synthetic liquids, called askarels (pyroclor) are used
instead of oil as they are non-inflammable and non-explosive but
very expensive
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15. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.2.8 TAP CHANGER
TAP CHANGER
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Tap Changger

16. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
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TAP CHANGER
FUNCTION: ALLOWS THE NOMINAL VOLTAGE TO ADJUSTED TYPICALLY 2.5% AND 5% ABOVE AND BELOW NOMINAL VOLTAGE
EXAMPLE: 11kV 5 taps
Tap1: 5% below: V
Tap2: 2.5 % below V
Tap 3: Nominal Tap : 11kV
Tap 4: 2.5 % above: kV
Tap 5: 5% above: 11.5kV
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17. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.3 TRANSFORMER OIL
4.3.1 OIL DETERIORATION
Oxidisation of Oil-
- Oil reacts with oxygen to form a hydrocarbon sludge
- The sludge is deposited on the winding tank walls
- Causing heat to develop.
- Sludge may also block the cooling ducts.
- Oxidisation will results in formation of hydrocarbon and acid
- Causing corrosion of the various metals within the transformers
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18. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.3.2 Thermal Decomposition of Oil
Heating of a Transformer will results of thermal decomposition
Will lead to breakdown of oil insulation
Prolonged operation above 98 deg reduces transformers life
4.3.3 Contamination
Particularly from moisture which enters through transformer breather
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19. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.4 Oil System for Large Transformer
The conservator
- Oil reservoir and expansion vessel
- Exhaust through breathing device-usually fitted with
silica gel or drycol refrigeration dryer type
b) Explosion Vent
- Consists of a thin and relatively weak diaphragm
-Under internal fault, the excess pressure in the tanks burst
into explosion vent to release the pressure
- Under normal circumstances, seals the transformer from
the atmosphere
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20. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
CONSERVATOR TANK
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BREATHER

21. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
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RELIEVE
VALVE
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22. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
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4.5 THE BUCHOLZ
A mechanical relay connected through pipe and situated between main tank and conservator tank of a transformer. Only certain kVA transformer need to be installed with one, normally above 1MVA. It will be connected to the trip coil of the circuit breaker controlling the transformer or the installation
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23. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Continue
Principles of operation:
Electrical faults cause violent generation of gases, which flows
to the highest point, namely the conservator. This gases will
Trapped inside the Bucholz compartment causing the level of
oil to reduce thus causing the relay to initiate.
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24. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
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25. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
continue”:
Buchholz relay can be triggered by three operation
1) Air from atmosphere while filling the transformer oil
2) Mechanical failure
3) Insulation breakdown from oil
4) Winding insulation breakdown
Gas formed usually consists of hydrogen, Acetylene and carbon
monoxide.
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26. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Continue:
If the gas only contained less then 2% carbon monoxide, only
breakdown in oil occurs.
2) If the gas contained more then 20%, the fault are due to both
breakdown of oil and insulation
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27. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.6 COMMISSIONING TESTS
4.6.1 DATA COLLECTION
- manufacture and date of manufacturer
- serial Number
- rating-normal and assisted
- nominal voltages
- full load current at nominal tap
- percentage impedance
- vektor group
- tap changer type, range and no of tap
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28. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
General check and Inspection
Coordinating Gaps
Labelling
Earth Connections
Breathers
Valves in operational position
Tightness of fastening
Insulators undamaged and clean
No oil leaks
Correct oil levels in conservators
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29. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
4.6.3 Insulation Tests
Check Insulation between low voltage winding to earth with 1kV IR tester
Check Insulation between high voltage winding to earth with 5 kV IR
tester.
3. Check Insulation between HV and LV winding with 5 kV IR tester
Duration of all are one minutes
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30. POWER TRANSFOMER COMMISSIONING TEST
Insulation Test 1kv and 5kV:
Power Transformer
Hv Side
Lv Side E
Bacaan 100 M,ohms keatas
ARB

31. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Polarization Index Test.
Carry out IR test between Low Voltage to Earth with 1kV IR tester
Carry out IR test between HV to Earth with 5kV IR tester
Carry out IR test between HV to LV with 5kV IR tester
Duration of test is 10 minutes
Divide the value obtained with value for 1 min test
Polarization Index = R10/R1
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32. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Between
Insulation Resistance (MΩ) PI
Accepted value
1 Minute
10 Minute
Primary & Earth
4000MΩ
7000MΩ
1.75
Not less than 1.2
Secondary & Earth
3500MΩ
8000MΩ
2.29
Primary & Secondary
5100MΩ
10000MΩ
2.00
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33. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
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4.6.5 Ratio Test
Inject 415V three phase supply to HV side of transformer with the LV
side open circuited.
2)Measure the HV side and LV side voltage and calculate the ratio.
Compare the ratio with the name plate value and ensure that the ratio
is within the tolerance.
3)Repeat for all transformer taps.
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34. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
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Primary Voltage
Secondary output voltage
Tap No
Voltage
AB: 200
BC: 200
CA: 200 an bn cn
4.32 1
11550
4.43 2
11275
4.54 3
11000
4.66 4
10725
4.78 5
10450
INSUALTION TEST OF ELECTRICAL EQUIPMENT
Example:
a) At tap changer no 1:
VAB = 200V (Delta)
Therefore the ratio is correct.

35. TESTING AND COMMISIONING INSUALTION TEST OF ELECTRICAL EQUIPMENT
DET310
Impedance test:
Positive sequence impedance
Inject three phase supply to HV side of transformer with LV side
short circuited as shown below.
2)With the transformer at nominal tap, measure and record the current
and voltage at HV side.
3) Calculate the percentage impedance as given below:
% Impedance Where Vm = measured voltage
Im = measured current
Vr = rated voltage
Ir = rated current
Compare the results with declared value on the nameplate.
Repeat the above for transformer at lowest and highest tap positions.
INSUALTION TEST OF ELECTRICAL EQUIPMENT