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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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CONTINUE-

a) 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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Conservator Conservator Type OFAFType OFAF

Hermetically Hermetically sealedsealedONANONAN

Conservator Conservator TypeTypeOFAFOFAF

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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 over-heating of the core and windings and subsequent damage

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Continue-

The capacity of a power transformer is defined by full load power, in kVA it can produce.

310 3 xxIUS phph

Where S = Full load Power

Uph= rated voltage at no load (V)Iph = rated output current (A)

For Star Connection: I phase= Phase Voltage=

phxU

Sx

3

10 3

3phU

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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:

(%)Im_.

%100).(_

pedanceVoltagexSec

xAVratingrTransforme

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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 CHANGER4.2.8 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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RELIEVEVALVE

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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:

Hv Side Lv SidePower Transformer

E

Bacaan 100 M,ohms keatasARB

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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

value1 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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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

Secondary output voltage Tap No Voltage

AB: 200BC: 200CA: 200

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)

Therefore the ratio is correct.

)(5.711550

433200

1lineVx

axVV ABab

)(32.43

5.7phaseVVan

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4.6.7 Impedance test:

Positive sequence impedance1) 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 voltageIm = measured currentVr = rated voltageIr = rated current

Compare the results with declared value on the nameplate.Repeat the above for transformer at lowest and highest tap

positions.