Transcript Chapter 4_Power Transformer

TESTING AND COMMISIONING
THE POWER TRANSFORMER
DET310
CHAPTER 4
THE POWER TRANSFORMER
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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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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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CONTINUEa) SERIAL NUMBER
i) WEIGHT
b) CLASS
j) DIMENSION
c) KVA RATING
k) TAP CHANGER
d) VOLTAGE RATING
e) WINDING
f) VEKTOR GROUPING
g) PERCENTAGE IMPEDANCE
h) INSULATING FLUIDS
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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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Hermetically
sealed
ONAN
Conservator
Type OFAF
Conservator
Type
OFAF
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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 overheating of the core and windings and subsequent
damage
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ContinueThe capacity of a power transformer is defined by full load
power, in kVA it can produce.
S  U ph I ph x10 3 x 3
Where S = Full load Power
Uph= rated voltage at no load (V)
Iph = rated output current (A)
For Star Connection: I phase=

Sx10
3
3xU ph
Phase Voltage=
U ph
3
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4.2.3 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
4.2.4 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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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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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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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:
Transformer _ rating (V . A) x100%
Sec. _ Voltagex Im pedance(%)
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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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4.2.8 TAP
CHANGER
4.2.8
TAP CHANGER
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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: 10.45 V
Tap2: 2.5 % below 10.725 V
Tap 3: Nominal Tap : 11kV
Tap 4: 2.5 % above: 11.275 kV
Tap 5: 5% above: 11.5kV
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4.3 TRANSFORMER OIL
4.3.1 OIL DETERIORATION
a) 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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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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4.4 Oil System for Large Transformer
a) 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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CONSERVATOR TANK
BREATHER
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RELIEVE
VALVE
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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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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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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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Continue:
1) 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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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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4.6.2 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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4.6.3 Insulation Tests
1. Check Insulation between low voltage winding to earth with 1kV IR
tester
2. 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
Insulation Test 1kv and 5kV:
Power Transformer
Hv Side
Lv Side
E
Bacaan 100 M,ohms keatas
ARB
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4.6.4 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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Between
Insulation Resistance (MΩ)
PI
Accepted
value
Not less than
1.2
1 Minute
10 Minute
Primary &
Earth
4000MΩ
7000MΩ
1.75
Secondary &
Earth
3500MΩ
8000MΩ
2.29
Primary &
Secondary
5100MΩ
10000MΩ
2.00
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4.6.5 Ratio Test
1) 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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Primary
Voltage
AB: 200
BC: 200
CA: 200
Secondary output voltage
Tap No
Voltage
an
bn
cn
4.32
4.32
4.32
1
11550
4.43
4.43
4.43
2
11275
4.54
4.54
4.54
3
11000
4.66
4.66
4.66
4
10725
4.78
4.78
4.78
5
10450
Example:
a) At tap changer no 1:
VAB = 200V (Delta)
Vab  VAB x
Van 
1
433
 200 x
 7.5V (line )
a
11550
7.5
 4.32V ( phase)
3
Therefore the ratio is correct.
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4.6.7 Impedance test:
Positive sequence impedance
1) 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.