2. 1
ABSTRACT:
This report is about transformers, construction and working principles of
transformers, their types and the advancements in technology which has given
rise to compact and more efficient transformers. And the tests conducted on
transformers and oils used in them. And then, in the end, it contains information
on economics involved in sales of transformers such as capitalization of losses,
payback period etc.
3. 2
TABLE OF CONTENTS
ABSTRACT:...............................................................................................................................................................1
INTRODUCTION ........................................................................................................................................................5
WHAT IS TRANSFORMER? .....................................................................................................................................5
WORKING PRINCIPLE:.............................................................................................................................................5
BASIC COMPONENTS: ............................................................................................................................................6
PRIMARY WINDING:...................................................................................................................................................6
SECONDARY WINDING:..............................................................................................................................................6
PRINCIPAL TAPING: ...................................................................................................................................................6
TERMINALS:..............................................................................................................................................................7
SUB-TERMINALS: ......................................................................................................................................................7
CORE:......................................................................................................................................................................7
BUSHING: .................................................................................................................................................................7
DEFINITIONS OF SOME RELATED TERMS: ..........................................................................................................7
TURN RATIO: ............................................................................................................................................................7
PHASE DISPLACEMENT:.............................................................................................................................................7
COOLING MEDIUM TEMPERATURE:.............................................................................................................................8
THERMAL TIME CONSTANT: .......................................................................................................................................8
RATING: ...................................................................................................................................................................8
SERVICE CONDITION: ................................................................................................................................................8
CONSTRUCTION:......................................................................................................................................................8
CORE:......................................................................................................................................................................8
CORE TYPE CONSTRUCTION:.....................................................................................................................................9
SHELL TYPE CONSTRUCTION:....................................................................................................................................9
WINDINGS: ...............................................................................................................................................................9
TYPES OF TRANSFORMER USED IN ELECTRICITY TRANSMISSION: ........................................................... 10
POWER TRANSFORMER:......................................................................................................................................... 10
DISTRIBUTION TRANSFORMERS: ............................................................................................................................. 10
DISTRIBUTION TRANSFORMERS........................................................................................................................ 11
TYPES OF DISTRIBUTION TRANSFOMER.......................................................................................................... 12
DRY TYPE TRANSFORMES: ............................................................................................................................. 12
OIL IMMERSED TYPE TRANSFORMERS: ........................................................................................................ 14
SMART / SLIM TRANSFORMERS:..................................................................................................................... 15
BIO SLIM TRANSFORMERS: ............................................................................................................................. 15
ADVANTAGES OF SLIM® AND BIO-SLIM® TRANSFORMERS:........................................................................ 16
GREATER RELIABILITY, MORE FLEXIBILITY: ................................................................................................. 16
SAFETY:.............................................................................................................................................................. 16
RESPECT FOR THE ENVIRONMENT:............................................................................................................... 16
REDUCED NO-LOAD LOSSES: ......................................................................................................................... 16
MEETING SPECIFICATIONS: ............................................................................................................................ 17
OPENING NEW VISTAS ..................................................................................................................................... 17
4. 3
TRANSFORMER OIL TESTS:................................................................................................................................ 18
TYPE TESTS:.......................................................................................................................................................... 18
OXIDATION STABILITY TEST:........................................................................................................................... 18
SCOPE: ............................................................................................................................................................... 19
DISSIPATION FACTOR TEST: ........................................................................................................................... 19
SCOPE: ............................................................................................................................................................... 20
ROUTINE TESTS:................................................................................................................................................... 20
FLASH POINT TEST: .......................................................................................................................................... 20
POUR POINT TEST:............................................................................................................................................ 20
VISCOSITY:......................................................................................................................................................... 20
ACIDITY:.............................................................................................................................................................. 21
DIELECTRIC STRENGTH OF OIL: ..................................................................................................................... 21
CONCLUSION:.................................................................................................................................................... 22
INTER FACIAL TENSION OF TRANSFORMER OIL .......................................................................................... 22
TEMPERATURE RISE TEST OF TRANSFORMER OIL:.................................................................................... 22
SHORT CIRCUIT TEST ON TRANSFORMER....................................................................................................... 22
PROCEDURE .......................................................................................................................................................... 23
CONCLUSION:.................................................................................................................................................... 24
IMPULSE TEST OF TRANSFORMER ................................................................................................................... 24
CAUSE: ............................................................................................................................................................... 24
DAMAGE TO INSULATION:................................................................................................................................ 24
LIGHTNING IMPULSE......................................................................................................................................... 25
FULL WAVE:........................................................................................................................................................ 25
CHOPPED WAVE:............................................................................................................................................... 25
FRONT OF WAVE SHAPE: ................................................................................................................................. 25
SWITCHING IMPULSE:....................................................................................................................................... 26
PURPOSE: .......................................................................................................................................................... 26
PERFORMANCE OF IMPULSE TEST ................................................................................................................ 26
CONNECTION OF IMPULSE TEST:................................................................................................................... 27
GENERAL REQUIREMENTS FOR TRANSFORMER TESTS .............................................................................. 28
1. EQUIPMENT REQUIRED: ...................................................................................................................................... 28
2. GENERAL INSPECTION ........................................................................................................................................ 28
MECHANICAL CHECKS: ........................................................................................................................................... 28
CAPACITOR DIVIDERS TYPE:.................................................................................................................................... 28
ELECTROMAGNETIC TYPE:...................................................................................................................................... 28
ELECTRICAL CHECKS: ............................................................................................................................................ 28
POLARITY TEST................................................................................................................................................. 29
TRANSFORMER TURNS RATIO TEST.............................................................................................................. 29
GENERAL REQUIREMENTS OF TEST:- .............................................................................................................. 30
TYPE TESTS:- ........................................................................................................................................................ 30
ROUTINE TESTS:-.................................................................................................................................................. 31
TRANSFORMER WINDING RESISTANCE MEASUREMENT:-............................................................................ 31
TRANSFORMER TURN/VOLTAGE RATIO TEST:- .............................................................................................. 32
VECTOR GROUP TEST:-....................................................................................................................................... 32
5. 4
OPEN CIRCUIT TEST:............................................................................................................................................ 33
MEASUREMENT OF INSULATION RESISTANCE:-............................................................................................. 34
DIELECTRIC TEST OF TRANSFORMER:- ........................................................................................................... 34
TEMPRATURE TEST OF TRANSFORMER:-........................................................................................................ 35
SPECIAL TESTS: ................................................................................................................................................... 35
BIRD PROTECTION: .............................................................................................................................................. 35
PAY BACK PERIOD ............................................................................................................................................... 35
CAPITALIZATION:.................................................................................................................................................. 39
CAPITALIZATION OF LOSSES: ......................................................................................................................... 39
EXAMPLE OF TOC CALCULAIONS IN PAKISTAN ............................................................................................. 40
FOR LOW LOSS TRANSFORMER:............................................................................................................................. 40
FOR HIGH LOSS TRANSFORMER: ............................................................................................................................ 40
ADVANTAGES: ...................................................................................................................................................... 41
DISADVANTAGES: ................................................................................................................................................ 41
CONCLUSION......................................................................................................................................................... 42
6. 5
INTRODUCTION
WHAT IS TRANSFORMER?
Before we start our discussion about transformer let us take a brief view on its
history.
In 1884, Karoly Zipernowsky, Otto Blathy and Miksa Deri (ZBD), three engineers
associated with the Ganz factory, in their joint 1885 patent applications for novel
transformers (later called ZBD transformers), they described two designs with
closed magnetic circuits where copper windings were either a) wound around
iron wire ring core or b) surrounded by iron wire core. The two designs were the
first application of the two basic transformer constructions i.e. core form or shell
form in common use to this day. The Ganz factory had also in 1884 made
delivery of the world's first five high-efficiency AC transformers. This first unit had
been manufactured to the following specifications: 1,400 W, 40 Hz, 120:72 V,
11.6:19.4 A, ratio 1.67:1, one-phase, shell form.
So what are transformers and why they are an essential device to life? As in
present time everything runs on electricity it is essential to have such a device
which can transfer electricity to people at far which is transformer.
“A transformer is an electrical device that transfers electrical energy between
two or more circuits through electromagnetic induction. It can increase or
decrease the voltages of alternating current in electric power applications.”
WORKING PRINCIPLE:
Electromagnetic induction, the principle of the operation of the transformer, was
discovered independently by Michael Faraday in 1831. The relationship between
EMF and magnetic flux is an equation now known as Faraday's law of
induction, defined as follows:
7. 6
“The induced electromotive force in any closed circuit is equal to the negative of
the time rate of change of the magnetic flux enclosed by the circuit.”
Mathematically it is given by the following equation:
Where the magnitude of the EMF in Volts and ΦB is the magnetic flux through
the circuit in Webbers.
A varying current in the transformer's primary winding creates a varying magnetic
flux in the transformer core and a varying magnetic field impinging on the
transformer's secondary winding. This varying magnetic field at the secondary
winding induces a varying electromotive force (EMF) or voltage in the secondary
winding. Making use of Faraday's Law in conjunction with high magnetic
permeability core properties, transformers can thus be designed to efficiently
change AC voltages from one voltage level to another within power networks.
BASIC COMPONENTS:
Following are some basic components of the transformer:
Primary Winding:
That winding which receives energy from the supply.
Secondary Winding:
That winding which receives energy from the primary winding by electromagnetic
induction and delivers it as output energy to the load circuit.
Principal Taping:
The tapping which corresponds to the rated voltage of the winding. These are of
types: 1) Plus Tap 2) Minus Tap.
8. 7
Terminals:
That part of transformer intended to receive external connections.
Sub-Terminals:
A connecting point, other than a permanently jointed and insulated connection,
within the casing or tank serving the purpose of effecting connection to a
terminal, switch or any other auxiliary apparatus.
Core:
The core is the magnetic circuit upon which the windings are wound. It is usually
made of Ferro-magnetic material.
Bushing:
A bushing is an insulated device that allows an electrical conductor to pass safely
through a (usually) earthed conducting barrier such as the wall of a transformer
or circuit breaker. it must provide careful control of the electric field gradient
without letting the transformer leak oil. These are of two types: 1) HT Bushing 2)
LT Bushing.
DEFINITIONS OF SOME RELATED TERMS:
Turn Ratio:
The ratio of the number of primary turns to the number of secondary turns is
taken as equal to the ratio of the primary voltage to the rated secondary voltage,
multiplied by a phase factor depending on the method of connecting the windings
in the case of poly-phase transformers.
Phase Displacement:
The angular difference between the vectors representing the voltage induced
between the higher voltage and lower voltage terminals having the same marking
9. 8
letter and the corresponding neutral points, expressed with reference to the
higher-voltage side.
Cooling Medium Temperature:
The temperature of the available cooling medium above which the temperatures
rise is measured.
Thermal Time Constant:
The time which would be required for a transformer to reach a temperature rise
equal to that which would be attained under steady state specified loading
conditions in a cooling medium of constant temperature, if the initial rate of
temperature rise were maintained.
Rating:
Those quantities assigned to the transformer to define its operation under
conditions, specified in those recommendation and on which manufacture’s
guarantees and tests are based.
Service Condition:
Factors outside the control of the manufacturer which may affect the operation.
CONSTRUCTION:
The transformer is quiet simple in construction. It consists of two magnetically
linked windings, wound on two separate limbs of iron. It consists of the following
basic parts:
Core:
A transformer core can be constructed in two ways, depending upon the
arrangement of the primary and secondary windings. These two ways are:
10. 9
Core Type construction:
If the windings are wound around the core in such a way that they surround the
core ring on its outer edges, then the construction is known as the core type
construction of core.
Shell Type Construction:
In shell type construction of the core, the windings pass through the inside of the
core ring such that the core forms a shell outside the windings.
Windings:
Windings are wound on two separate limbs of iron to increase the magnetic flux
as iron is an efficient conductor and exhibits excellent magnetic properties.
These coils are also insulated from each other. Since both these coils are wound
on two separate limbs and due to the distance between them, flux leakages also
11. 10
occur which reduce our magnetic flux density and results in a reduced magnetic
coupling between the two coil windings.
In core type construction, these windings are arranged in a concentric manner
on the limbs, while in a shell type core construction, the same windings are
arranged in a sandwich like pattern.
Core type and Shell type Windings
TYPES OF TRANSFORMER USED IN ELECTRICITY
TRANSMISSION:
Following two are the type of transformers used in electricity transmission from
power plant to the consumers:
Power Transformer:
Power transformers are used to step up the generated voltage. Thus transfer
electric energy between the generator and Distribution primary circuits.
Distribution Transformers:
Distribution Transformers are used to step down the voltage. Thus used to
distribute energy from transmission lines and networks for local consumption.
12. 11
DISTRIBUTION TRANSFORMERS
A distribution transformer is a transformer that provides the final voltage
transformation in the electric power distribution system, stepping down the
voltage used in the distribution lines to the level used by the customer.
If mounted on a utility pole, they are called pole-mount transformers. If the
distribution lines are located at ground level or underground, distribution
transformers are mounted on concrete pads and locked in steel cases, thus
known as pad-mount transformers.
Distribution transformers normally have ratings less than 200 kVA, although
some national standards can describe units up to 5000 kVA as distribution
transformers. Since distribution transformers are energized for 24 hours a day
(even when they don't carry any load), reducing iron losses has an important role
in their design. As they usually don't operate at full load, they are designed to
have maximum efficiency at lower loads. To have a better efficiency, voltage
13. 12
regulation in these transformers should be kept to a minimum. Hence they are
designed to have small leakage reactance.
TYPES OF DISTRIBUTION TRANSFOMER
Distribution transformers are classified into different categories such as:
Dry type
Oil immersed type
Smart transformers
DRY TYPE TRANSFORMES:
Dry type transformers are transformers in which the insulation between the
turns of the winding is simply a high-grade paper. This design is relatively
inexpensive and works well for transformers that are rated for lower voltages and
that are installed indoors in relatively dry environments.
There are actually two versions of 'dry type' transformers. In the simplest (and
oldest) version, the insulation is simply dry paper. One of the issues with the so-
called 'open dry' arrangement is that the paper insulation can absorb humidity
from the atmosphere. Since the transformer is warm during operation, that's
normally not a problem. But if the transformer is cycled on and off, it is possible
for the insulation to absorb moisture when it is off, thereby reducing its effective
insulation strength when the transformer is turned back on. Also there is the
problem that water can damage the transformer, so there is the concern about
what happens if the basement floods, or the fire-suppression sprinklers go off.
Open-dry transformers are commonly used in commercial applications - office
buildings, shopping centers, etc.
There is a newer version in which the transformer is vacuum impregnated with an
epoxy material. This design is more expensive to produce, but has the
advantage that the transformer is protected from humidity. These designs are
commonly used in industrial applications.
Dry-type transformers can have their windings insulated various ways. A basic
method is to preheat the conductor coils and then, when heated, dip them in
varnish at a high temperature. The coils are then baked to cure the varnish. This
process is an open-wound method and helps ensure penetration of the varnish.
14. 13
Cooling ducts in the windings provide an efficient and economical way to
remove the heat produced by the electrical losses of the transformer by allowing
air to flow through the duct openings. This dry-type insulation system operates
satisfactorily in most ambient conditions are also sealed with an epoxy resin
mixture.
Another version of the dry-type transformer is a cast coil insulation system. It
is used when addition coil strength and protection are advisable. These type of
transformer are used in locations where environments are harsh, such as
cement and chemical plants and outdoor installations where moisture, salt
spray, corrosive fumes, dust, and metal particles can destroy other types of dry-
type transformers. These cast coil units are better able to withstand heavy
power surges, such as frequent but brief overloads experienced by transformers
serving transit systems and various industrial machinery. Cast coil units are now
being used where previously only liquid-filled units were available for harsh
environments.
Information on dry-type transformer loading from ANSI/EEE C57.96-1989
indicates that you can have a 20-yr life expectancy for the insulation system in a
transformer. For dry-type transformers having a 220 (degrees) C, insulating
system and a winding hot-spot temperature of 220 (degrees) C, and with no
unusual operating conditions present, the 20 yr. life expectancy is a reasonable
time fame. However, due to degradation of the insulation, a transformer might
fail before 20 years. Most 150 (degrees) C rise dry-type transformers are built
with 220 (degrees) C insulation systems. Operating such a transformer at rated
kVA on a continuous basis with a 30 (degrees) C average ambient should
equate to a "normal" useful life.
The life of a transformer increases appreciably if the operating temperature is
lower than the maximum temperature rating of the insulation. However, you
should recognize that the life expectancy of transformers operating at varying
temperatures is not accurately known. Fluctuating load conditions and changes
in ambient temperature make it difficult, if not impossible, to arrive at such
definitive information.
15. 14
Medium Voltage Dry-Type Transformer Cast Resin Dry Type transformer
OIL IMMERSED TYPE TRANSFORMERS:
Oil-immersed transformers also use high-grade paper as insulation between
turns, but the entire core and coil assembly is in a container that is filled with an
insulating oil. Traditionally, a high-grade mineral oil was used, and that's still the
preferred choice for high power, high voltage transformers that are to be installed
outdoors. But mineral oil is flammable and cannot be used indoors. For a number
of years, the industry used a non-flammable artificial oil that was sold under a
variety of trade names - Askarel, Pyranol, etc - all of which were polychlorinated
biphenyls. Today, the environmental hazards of those materials is well known,
and substitutes include silicon-based oils and even some high-grade vegetable
oils.
In addition to providing insulation, the oil in oil-immersed transformers is a heat
exchange medium - the oil can be circulated through a heat exchanger that
allows the transformer to be assigned a higher power rating. That is not possible
with dry-type transformers.
16. 15
Oil Immersed Type Transformers
SMART / SLIM TRANSFORMERS:
Smart/Slim transformers uses NOMEX as high temperature solid insulation and
Silicon – a high temperature liquid. They have high overload capacity.
In addition to near shore, off shore, onshore wind energy slim transformers are
used in number of other markets ranging from oil & gas industries and rail
infrastructure to buildings and energy distribution in critical areas.
BIO SLIM TRANSFORMERS:
Bio-SLIM® transformers share the main features of the SLIM® range. The added
value of the Bio-SLIM® comes from the use of a fully bio-degradable
transformer fluid, an ester, which allows it to be used in environmentally
sensitive locations, such as offshore and in water-catchments. Esters are closer
17. 16
to mineral oils than to silicone fluid in their electrical and cooling performance and
in their material compatibility.
ADVANTAGES OF SLIM® AND BIO-SLIM® TRANSFORMERS:
GREATER RELIABILITY, MORE FLEXIBILITY:
NOMEX® insulation withstands up to 220 °C with negligible thermal aging and
degradation. This means extra capacity to handle peak loads, high ambient
temperatures, current harmonics and other emergency situations. NOMEX®
thermal insulating technology boosts the thermal constraint of transformers by up
to 50 % over conventional transformers. The SLIM® design offers flexibility when
upgrading existing installations. Cost savings in peripheral equipment can result
from the use of existing connections, foundations and protection.
SAFETY:
The inherent self-extinguishing characteristics of NOMEX® plus a Class K3 (fire-
point >300 °C and net calorific value <32 MJ/kg) dielectric fluid results in high fire
safety, thanks to reduced fire load and limited flammability. This translates into
savings on safety-related protective equipment and insurance premiums.
RESPECT FOR THE ENVIRONMENT:
Pauwels’ advanced design uses fewer natural resources (copper, steel, up to 30
% less coolant). At the end of a transformer’s lifetime, material disposal is not a
problem: Class K3 dielectric fluid is environmental friendly, non-toxic and fully
recyclable; steel and copper can easily be recovered (thanks to the non-cast
windings); NOMEX® paper and pressboard can be handled as conventional non-
hazardous waste.
REDUCED NO-LOAD LOSSES:
The SLIM® design’s no-load losses are typically about one-half of the no-load
losses of dry-type transformers. With an electricity price of 0.038 EUR/kWh, 8760
hours a year and a 30-year life expectancy, a 1000 kVA SLIM® transformer
saves about half of its own initial purchase price, thanks to its 40 % lower no-load
losses (1.5 kW against 2.3 kW).
18. 17
MEETING SPECIFICATIONS:
Besides exceeding conventional specifications, SLIM® transformers also meet
the requirements of the new IEC 60076-14.
OPENING NEW VISTAS
SLIM® transformers will bring benefits in applications such as:
Public buildings
Office and residential blocks
Light and heavy industries
PCB replacement
Step-up converter transformers for wind industry (WTGT)
GSU transformers for power generation plants
Marine applications
Track-side units for railways, underground (metro) trains, trams
Hospitals and banks
Road and railway tunnels
Space-saving upgrades (e.g. a 2000 kVA SLIM® can offer the same
footprint as a conventional 1250 kVA unit)
Installations with demanding thermal, electrical or loading conditions where
dry type and conventional immersed transformers show a higher tendency
to fail.
SLIM TRANSFORMERS
19. 18
TRANSFORMER OIL TESTS:
The insulation oil of transformers fulfills the purpose of insulating as well as
cooling. Thus, the dielectric quality of transformer oil is a matter of secure
operation of a transformer.
As transformer oil deteriorates through aging and moisture ingress, transformer
oil should, depending on economics, transformer duty and other factors, be
tested periodically. Transformer oil testing sequences and procedures are
defined by various international standards.
Power utility companies have a vested interest in periodic oil testing since
transformers represent a large proportion of their total assets. Through such
testing, transformers' life can be substantially increased.
There are 2 types of tests for oil used in transformers
Type tests
Routine tests
Type tests include
1) Oxidation stability test
2) Dissipation factor test
TYPE TESTS:
OXIDATION STABILITY TEST:
The oxidation stability test of mineral transformer oils is a method for assessing
the amount of sludge and acid products formed in a transformer oil when the oil
is tested under prescribed conditions. Good oxidation stability is necessary in
order to maximize the service life of oil by minimizing the formation of sludge and
acid. Oils that meet the requirements specified for this test tend to minimize
electrical conduction, ensure acceptable heat transfer, and preserve system life.
There is no proven correlation between performance in this test and performance
in service, since the test does not model the whole insulation system (oil, paper,
20. 19
enamel, wire). However, the test can be used as control test for evaluating
oxidation inhibitors and to check the consistency of oxidation stability of
production oils.
SCOPE:
This test method determines the resistance of mineral transformer oils to
oxidation under prescribed accelerated aging conditions. Oxidation stability
measured by the propensity of oils to form sludge and acid products during
oxidation. This test method is applicable to new oils, both uninhibited and
inhibited.
DISSIPATION FACTOR TEST:
Dissipation factor (or power factor) – This is a measure of dielectric losses in an
electrical insulating liquid when used in an alternating electric field and of the
energy dissipated as heat. A low dissipation factor indicates low ac dielectric
losses. Dissipation factor or power factor may be useful as a means of quality
control, and as an indication of changes in quality resulting from contamination
and deterioration in service or as result of handling.
The loss characteristic is commonly measured in terms of dissipation factor
(tangent to loss angle) or of power factor (sine of loss angle) and may be
expressed as a decimal value or as a percentage. For decimal values up to 0.05,
dissipation factor and power factor values are equal to each other within about
one part in one thousand. In general, since the dissipation factor of power factor
of insulating oils in good condition have decimal values below 0.005, the tow
measurements may be considered interchangeable.
The exact relation between dissipation factor (D) and power factor (PF) is given
by the following equations:
𝑃𝐹 =
𝐷
√1+𝐷2
𝐷 =
𝑃𝐹
√1−𝑃𝐹2
The reported value of D or PF may be expressed as a decimal value or as a
percentage. For example:
D or PF at 25o
C = 0.002 or 0.2%
21. 20
SCOPE:
This test method describes testing of new electrical insulating liquids as
well as liquids in service or subsequent to service in cables, transformers,
oil circuit breakers, and other electrical apparatus.
This test method provides a procedure for making referee tests at a
commercial frequency of between 45 and 65 Hz.
ROUTINE TESTS:
FLASH POINT TEST:
Flash point of transformer oil is the temperature at which oil gives enough vapors
to produce a flammable mixture with air. This mixture gives momentary flash on
application of flame under standard condition. Flash point is important because it
specifies the chances of fire hazard in the transformer. So it is desirable to have
very high flash point of transformer oil. In general it is more than 140° (>10°).
POUR POINT TEST:
It is the minimum temperature at which oil just start to flow under standard test
condition. Pour point of transformer oil is an important property mainly at the
places where climate is extremely cold. If the oil temperature falls below the pour
point, transformer oil stops convection flowing and obstruct cooling in
transformer. Paraffin based oil has higher value of pour point, compared to
Naphtha based oil, but in India like country, it does not affect the use of Paraffin
oil due tits warm climate condition. Pour Point of transformer oil mainly depends
upon wax content in the oil. As Paraffin based oil has more wax content, it has
higher pour point.
VISCOSITY:
In few words, viscosity of transformer oil can be said that viscosity is the
resistance of flow, at normal condition. Obviously resistance to flow of
transformer oil means obstruction of convection circulation of oil inside the
22. 21
transformer. A good oil should have low viscosity so that it offers less resistance
to the convectional flow of oil thereby not affecting the cooling of transformer.
Low viscosity of transformer oil is essential, but it is equally important that, the
viscosity of oil should increase as less as possible with decrease in temperature.
Every liquid becomes more viscous if temperature decreases.
ACIDITY:
Acidity of transformer oil, is harmful property. If oil becomes acidic, water content
in the oil becomes more soluble to the oil. Acidity of oil deteriorates the insulation
property of paper insulation of winding. Acidity accelerates thee oxidation
process in the oil. Acid also includes rusting of iron in presence of moisture. The
acidity of transformer oil is measure of its acidic constituents of contaminants.
Acidity of oil is express in mg of KOH required to neutralize the acid present in a
gram of oil. This is also known as neutralization number.
DIELECTRIC STRENGTH OF OIL:
To assess the insulating property of dielectric transformer oil, a sample of the
transformer oil is taken and its breakdown voltage is measured. The transformer
oil is filled in the vessel of the testing device. Two standard-compliant test
electrodes with a typical clearance of 2.5 mm are surrounded by the dielectric oil.
A test voltage is applied to the electrodes and is continuously increased up to the
breakdown voltage with a constant, standard-compliant slew rate of e.g. 2 kV/s.
At a certain voltage level breakdown occurs in an electric arc, leading to a
collapse of the test voltage. An instant after ignition of the arc, the test voltage is
switched off automatically by the testing device. Ultra-fast switch off is highly
desirable, as the carbonization due to the electric arc must be limited to keep the
additional pollution as low as possible. The transformer oil testing device
measures and reports the root mean square value of the breakdown voltage.
After the transformer oil test is completed, the insulation oil is stirred
automatically and the test sequence is performed repeatedly. (Typically 5
Repetitions, depending on the standard) As a result the breakdown voltage is
calculated as mean value of the individual measurements.
23. 22
CONCLUSION:
The lower the resulting breakdown voltage, the poorer the quality of the
transformer oil.
Voltage breakdown during oil test
INTER FACIAL TENSION OF TRANSFORMER OIL
Inter facial tension between the water and oil interface is the way to measure
molecular attractive force between water and oil. It is measured in Dyne/cm or
mili-Newton/meter. Inter facial tension is exactly useful for determining the
presence of polar contaminants and oil decay products. Good new oil generally
exhibits high inter facial tension. Oil oxidation contaminants lower the inter-facial
tension (IFT).
TEMPERATURE RISE TEST OF TRANSFORMER OIL:
In this test we check whether the temperature rising limit of the transformer
winding and oil as per specification or not. In this type test of transformer, we
have to check oil temperature rise as well as winding temperature rise limits of an
electrical transformer.
SHORT CIRCUIT TEST ON TRANSFORMER
The connection diagram for short circuit test on transformer is shown in the
figure. A voltmeter, wattmeter, and an ammeter are connected in HV side of the
transformer as shown. The voltage at rated frequency is applied to that HV side
with the help of a variac of variable ratio auto transformer.
24. 23
Procedure:
The LV side of the transformer is short circuited. Now with the help of rheostat
applied voltage is slowly increased until the ammeter gives reading equal to the
rated current of the HV side. After reaching at rated current of HV side, all three
instruments reading (Voltmeter, Ammeter and Watt-meter readings) are
recorded. The ammeter reading gives the primary equivalent of full load current
IL. As the voltage applied for full load current in short circuit test on transformer is
quite small compared to the rated primary voltage of the transformer, the core
losses in transformer can be taken as negligible here.
Let’s say, voltmeter reading is Vsc. The input power during test is indicated by
watt-meter reading. As the transformer is short circuited, there is no output;
hence the input power here consists of copper losses in transformer. Since,
the applied voltage Vsc is short circuit voltage in the transformer and hence it is
quite small compared to rated voltage, so core loss due to the small applied
voltage can be neglected. Hence the wattmeter reading can be taken as equal
to copper losses in transformer. Let us consider wattmeter reading is Psc.
Where Re is equivalent resistance of transformer. If, Ze is equivalent
impedance of transformer.
25. 24
Therefore, if equivalent reactance of transformer is Xe
These values are referred to the HV side of transformer as because the test is
conducted on HV side of transformer. These values could easily be referred to
LV side by dividing these values with square of transformation ratio.
CONCLUSION:
Therefore it is seen that the short circuit test on transformer is used to determine
copper loss in transformer at full load and parameters of approximate
equivalent circuit of transformer.
IMPULSE TEST OF TRANSFORMER
CAUSE:
Lighting is a common phenomenon in transmission lines because of their tall
height. This lightning stroke on the line conductor causes impulse voltage. The
terminal equipment of transmission line such as power transformer then
experiences this lightning impulse voltages.
DAMAGE TO INSULATION:
Insulation is one of the most important constituents of a transformer. Any
weakness in the insulation may cause failure of transformer. To ensure the
effectiveness of the insulation system of a transformer, it must confirms the
dielectric test. But the power frequency withstand test alone cannot be adequate
to demonstrate the dielectric strength of a transformer. That is why impulse test
of transformer performed on it. Both lightning impulse test and switching
impulse test are included in this category of testing.
26. 25
LIGHTNING IMPULSE
The lightning impulse is a pure natural phenomenon. So it is very difficult to
predict the actual wave shape of a lightning disturbance. From the data compiled
about natural lightning, it may be concluded that the system disturbance due to
natural lightning stroke, can be represented by three basic wave shapes.
1) Full wave
2) Chopped wave
3) Front of wave
Although the actual lightning impulse disturbance may not have exactly these
three shapes but by defining these waves one can establish a minimum impulse
dielectric strength of a transformer.
FULL WAVE:
If lighting disturbance travels some distance along the transmission line before it
reaches the transformer, its wave shape may approach to full wave.
CHOPPED WAVE:
If during traveling, if flash-over occurs at any insulator of the transmission line,
after the peak of the wave has been reached, the wave may become in form of
chopped wave.
FRONT OF WAVE SHAPE:
If the lightning stroke directly hits the transformer terminals, the impulse voltage
rises rapidly until it is relieved by a flash over. At the instant of flash - over the
voltage suddenly collapses and may form the front of wave shape.
The effect of these wave forms on the transformer insulation may be different
from each other. Whatever may be the shape of lightning disturbance voltage
wave, all of them can cause insulation failure in transformer. So lighting
impulse test of transformer is one of the most important type test of
transformer.
27. 26
SWITCHING IMPULSE:
Through studies and observations reveal that the switching over voltage or
switching impulse may have front time of several hundred microseconds and this
voltage may be periodically damped out. The IEC - 600060 has adopted for their
switching impulse test, a long wave having front time 250 μs and time to half
value 2500 μs with tolerances.
PURPOSE:
The purpose of the impulse voltage test is to secure that the transformer insulation
withstand the lightning overvoltage which may occur in service.
PERFORMANCE OF IMPULSE TEST
The test is performed with standard lightning impulses of negative polarity. The
front time (T1) and the time to half-value (T2) are defined in accordance with the
standard.
28. 27
Standard lightning impulse Front time T1 = 1,2 μs ± 30% Time to half-value T2 =
50 μs ± 20%
In practice the impulse shape may deviate from the standard impulse when
testing low-voltage windings of high rated power and windings of high input
capacitance. The impulse test is performed with negative polarity voltages to
avoid erratic flashovers in the external insulation and test circuit. Waveform
adjustments are necessary for most test objects. Experience gained from results
of tests on similar units or eventual pre-calculation can give guidance for
selecting components for the wave shaping circuit.
CONNECTION OF IMPULSE TEST:
All the dielectric tests check the insulation level of the job. Impulse generator is
used to produce the specified voltage impulse wave of 1.2/50 micro seconds
wave. One impulse of a reduced voltage between 50 to 75% of the full test
voltage and subsequent three impulses at full voltage.
For a three phase transformer, impulse is carried out on all three phases in
succession.
The voltage is applied on each of the line terminal in succession, keeping the
other terminals earthed. The current and voltage wave shapes are recorded on
the oscilloscope and any distortion in the wave shape is the criteria for failure.
29. 28
GENERAL REQUIREMENTS FOR TRANSFORMER TESTS
1. Equipment required:
Following equipment is necessary to perform testings:
Polarity test kit
Megger 500-5000V
Ohmmeter
Multimeter
Autotransformers & Step-up transformer
2. General inspection
Mechanical checks:
General visual inspection and compliance with the drawings and manuals.
Check nameplate ratings and HV, LV terminal markings.
Check that all parts of the transformer are properly assembled and tight.
Check the HV connections are tight.
Check the cable connections on the LV side and the markings.
Check the oil levels and inspect for leakage. (Where applicable)
Capacitor dividers type:
Check that all parts of the transformers are properly assembled.
Electromagnetic type:
Check the installation of different sections.
Electrical Checks:
Check the equipment grounding (Continuity and connection)
Check the fuse rating of secondary side.
Perform the following operations
30. 29
POLARITY TEST
The polarity is checked using the flick method (application of direct current)
and check of deflection on a bi-directional milliammeter. The test is also used
to check primary and secondary circuit continuity.
When switch k is closed, the milliammeter pointer deflects positive.
When the circuit is opened, the milliammeter pointer deflects in the
negative direction.
TRANSFORMER TURNS RATIO TEST
A variable AC source is applied on the primary side. The primary and
secondary voltages are measured to determine the ratio V2/V1
31. 30
GENERAL REQUIREMENTS OF TEST:-
Electricity plays a crucial role in our lives today. We cannot without them as most
of our work and other related things we do every day depends upon them. This
electricity has several components and equipment helping humans to transform
and regulate the current flow in their usage. Transformers are crucial equipment
that comes along with the package. As the name suggest they are equipment
that allows us to either step up or step down the AC/DC current as well as
voltage supplied to several accessories we use every day. The equipment that
steps up the current or voltage is known as step up transformer whilst the
transformer that reduces the flow of current or voltage are known as step down
transformers. However, failure amidst this component is pretty rare but
nonetheless, it is very important for us to test it on regular basis to ensure its
longevity. And good equipment always sustains for longer periods without
providing much issues.
In order to determine the feasibility and reliability of the transformer, it has to go
through different tests. Some of them are done in the factory where transformers
built. Other are done in site. The tests that done in factory are categorises as
following.
Type Tests
Routine Tests
Special Tests
TYPE TESTS:-
To prove that the transformer meets client’s specifications and design
expectations, the transformer has to go through different testing procedures in
manufacturer sites. Some transformer tests are carried out for confirming the
basic design expectancy of that transformer. These tests are done mainly in a
prototype unit not in all manufactured units in a lot. Type test of transformer
confirms main and basic design criteria of a production lot. Different Type test
that done on transformer are as follows.
Transformer Winding Resistance measurement
Transformer Ratio Test
Transformer Vector group test
Measurement of impedance voltage/short circuit impedance (principal tap)
and load loss (Short circuit test).
32. 31
Measurement of no load loss and current (Open circuit test).
Measurement of insulation resistance.
Dielectric tests of transformer.
Temperature Test
Vacuum Tests
Tests on on-load tap changer
ROUTINE TESTS:-
Routine tests of transformer is mainly for confirming operational performance of
individual unit in a production lot. Routine tests are carried out on every unit
manufactured.
Transformer winding resistance measurement.
Transformer ratio test.
Transformer vector group test.
Measurement of impedance voltage/short circuit impedance (principal tap)
and
Load loss (Short circuit test).
Measurement of no load loss and current (Open circuit test)
Measurement of insulation resistance.
Dielectric tests of transformer.
Tests on on-load tap-changer.
Oil pressure test on transformer to check against leakages past joints and
gaskets.
TRANSFORMER WINDING RESISTANCE MEASUREMENT:-
In this test resistance of the copper wire is measured hence I2
R losses
calculated. It is also done in routine test that is in site area to ensure healthiness
of transformer e.g. to check loose connections, broken strands of conductor, high
contact resistance in tap changers, high voltage leads and bushings etc. There to
different techniques measure winding resistance of transformer enlisted as
follows.
Current Voltage Method
Kelvin Bridge Method
33. 32
Measuring winding resistance by Automatic Winding Resistance
Measurement Kit.
TRANSFORMER TURN/VOLTAGE RATIO TEST:-
The performance of a transformer largely depends upon perfection of specific
turns or voltage ratio of transformer. So transformer ratio test is an essential type
test of transformer. This test also performed as routine test of transformer. So for
ensuring proper performance of electrical power transformer, voltage and turn
ratio test of transformer one of the vital tests. The procedure of transformer ratio
test is simple. We just apply three phase 415 V supply to HV winding, with
keeping LV winding open. Then we measure the induced voltages at HV and LV
terminals of transformer to find out actual voltage ratio of transformer. We repeat
the test for all tap position separately.
VECTOR GROUP TEST:-
In three phase transformer, it is essential to carry out a vector group test of
transformer. Proper vector grouping in a transformer is an essential criteria for
parallel operation of transformers. There are several internal connection of three
phase transformer are available in market. These several connections gives
various magnitudes and phase of the secondary voltage; the magnitude can be
adjusted for parallel operation by suitable choice of turn ratio, but the phase
divergence can’t be compensated. So we have to choose those transformer for
parallel operation whose phase sequence and phase divergence are same. All
the Search transformers with same vector ground have same phase sequence
and phase divergence between primary and secondary. So before procuring one
electrical power transformer, one should ensure the vector group of the
transformer, whether it will be matched with his or her existing system or not. The
vector group test of transformer confirms his or her requirements.
34. 33
OPEN CIRCUIT TEST:
The secondary of the transformer is
left open-circuited. A wattmeter is
connected to the primary.
An ammeter is connected in series
with the primary winding.
A voltmeter is optional since the
applied voltage is the same as the
voltmeter reading. Rated voltage is
applied at primary.
If the applied voltage is normal
voltage then normal flux will be set
up. Since iron loss is a function of applied voltage, normal iron loss will occur.
Hence the iron loss is maximum at rated voltage. This maximum iron loss is
measured using the wattmeter. Since the impedance of the series winding of the
transformer is very small compared to that of the excitation branch, all of the
input voltage is dropped across the excitation branch. Thus the wattmeter
measures only the iron loss. This test only measures the combined iron losses
consisting of the hysteresis loss and the eddy current loss. Although the
hysteresis loss is less than the eddy current loss, it is not negligible. The two
losses can be separated by driving the transformer from a variable frequency
source since the hysteresis loss varies linearly with supply frequency and the
eddy current loss varies with the square.
Since the secondary of the transformer is open, the primary draws only no-load
current, which will have some copper loss. This no-load current is very small and
because the copper loss in the primary is proportional to the square of this
current, it is negligible. There is no copper loss in the secondary because there is
no secondary current.
Current, voltage and power are measured at the primary winding to ascertain
the admittance and power-factor angle. Another method of determining the
series impedance of a real transformer is the short circuit test.
35. 34
MEASUREMENT OF INSULATION RESISTANCE:-
Insulation resistance test of transformer is essential type test. This test is carried
out to ensure the healthiness of overall insulation system of an electrical power
Transformer.
Technique of Insulation Resistance is as follows.
1. First disconnect all the line and neutral terminals of the
transformer.
2. Megger leads to be connected to LV and HV bushing
studs to measure insulation resistance IR value in between the LV and HV
windings.
3. Megger leads to be connected to HV bushing studs and transformer tank
earth point to measure insulation resistance IR value in between the HV windings
and earth.
4. Megger leads to be connected to LV bushing studs and transformer tank earth
point to measure insulation resistance IR value in between the LV windings
and earth.
DIELECTRIC TEST OF TRANSFORMER:-
Dielectric tests of transformer is one kind of insulation test. This test is
performed to ensure the expected over all insulation strength of transformer.
There are several test performed to ensure the required quality of transformer
insulation, dielectric test is one of them. Dielectric tests of transformer is
performed in two different steps, first one called Separate source voltage
withstand test of transformer, where a single phase power frequency voltage of
prescribed level, is applied on transformer winding under test for 60 seconds
while the other windings and tank are connected to the earth and it is observed
that whether any failure of insulation occurs or not during the test. Second one is
induced voltage test of Transformer where, three phase voltage, twice of rated
secondary voltage is applied to the secondary winding for 60 second by keeping
the primary of the transformer open circuited. The frequency of the applied
voltage should be double of power frequency too. Here also if no failure of
insulation, the test is successful. In addition to dielectric tests of transformer
there are other type test for checking insulation of transformer, such as lightning
impulse test, switching impulse test and partial discharge test.
36. 35
TEMPRATURE TEST OF TRANSFORMER:-
Temperature rise test of transformer is
included in type test of transformer. In this
test we check whether the temperature
rising limit of the transformer winding and
oil as per specification or not. In this type
test of transformer, we have to check oil
temperature rise as well as winding
temperature rise limits of an electrical
transformer.
SPECIAL TESTS:
There are some other test which are done
on extra components of the transformers such as
Measurement of acoustic noise level.
Measurement of the harmonics of the no-load current.
Measurement of the power taken by the fans and oil pumps.
Tests on bought out components / accessories such as buchhloz relay,
temperature, indicators, pressure relief devices, oil preservation
system etc.
These tests are known as special tests.
BIRD PROTECTION:
To prevent flashover caused by large birds, transformers cover must be
effectively coated with a weather proof permanent insulating material / powder
coating having a dry dielectric withstand voltage of not less than 8 KV to
withstand 8 KV dielectric test.
PAY BACK PERIOD
One of the most common ways to evaluate the economic value of a project is
with a simple payback analysis. This is just the ratio of the extra first cost ∆P to
the annual savings, S:
𝑺𝒊𝒎𝒑𝒍𝒆 𝒑𝒂𝒚𝒃𝒂𝒄𝒌 =
𝑬𝒙𝒕𝒓𝒂 𝒇𝒊𝒓𝒔𝒕 𝒄𝒐𝒔𝒕 ∆𝑷($)
𝑨𝒏𝒏𝒖𝒂𝒍 𝒔𝒂𝒗𝒊𝒏𝒈𝒔 𝑺 (
$
𝒚𝒓
)
37. 36
For example, an energy-efficient air conditioner that costs an extra $1000 and
which saves $200/yr in electricity would have a simple payback of 5 years.
Simple payback has the advantage of being the easiest to understand of all
economic measures, but it has the unfortunate problem of being one of the least
convincing ways to present the economic advantages of a project. Surveys
consistently show that individuals, and corporations alike, demand very short
payback periods—on the order of only a few years—before they are willing to
consider an energy investment. The 5-year payback in the above example would
probably be too long for most decision makers; yet, for example, if the air
conditioner lasts for 10 years, the extra cost is equivalent to an investment that
earns a tax-free annual return of over 15%. Almost anyone with some money to
invest would jump at the chance to earn 15%, yet most would not choose to put it
into a more efficient air conditioner. Simple payback is also one of the most
misleading measures since it doesn’t include anything about the longevity of the
system. Two air conditioners may both have 5-year payback periods, but even
though one lasts for 20 years and the other one falls apart after 5, the payback
period makes absolutely no distinction between the two.
38. 37
Sr.
No.
kVA
Conventional Losses Transformer Annual
saving
@
Rs:7.43/k
Wh &
70%
loading
Extra
price
paid
(PKR)
Payb
ack
(year
s)
No-load Load S.P. (PKR) T.O.C.
1 200 544.5 3751 452,790 1,161,160 - - -
Sr.
No.
kVA
Low Losses Transformer (As per Amendment
5)
Annual
saving
@
Rs:7.43/k
Wh &
70%
loading
Extra
price
paid
(PKR)
Payb
ack
(year
s)
No-load Load S.P. (PKR) T.O.C.
1 200 396 2728 595,000 1,110,178
56,274.05
142,210 2.53
Annual Saving = [(Iron Losses Reduced + Copper Losses Reduced x 0.70) x hours in a day x working
days x Energy Cost] / 1000
Loading
Unit Rate
per kWH
Working
Days
Interest
Rate
Energy
Cost
increase
per year
70% 7.43 365 17% 10%
Investment
amount (PKR) 142,210
Interest Rate 17%
Energy price
escalation 10%
Total Service
Life of T/F 15
Annual Saving
(PKR) 56,274
40. 39
CAPITALIZATION:
CAPITALIZATION OF LOSSES:
The capitalized cost (CC) of a transformer can be expressed as sum of the
purchase price (Ct), the cost of no load losses and the cost of the load losses, or
as a formula:
CC=Ct + K1 x P0 + K2 x PK
Where k1 represents the assigned cost of no load per watt P0. The value of no
load losses per watt K2. The assigned cost of load losses per watt and Pk the
value of the load losses per watt.P0 and Pk are transformer properties. K1 and K2
are properties that depend on the expected loading of the transformer and
energy prices. K1 and K2 are calculated as follows:
K1 =
(1+𝑖) 𝑛−1
𝑖.(𝑖+1) 𝑛
× 𝐶 × 8760
K2 =
(1+𝑖) 𝑛−1
𝑖.(𝑖+1) 𝑛
× 𝐶 × 8760 × (
𝐼𝑙
𝐼 𝑟
)2
Where:
i = interest rate (% per year)
n = lifetime (years)
C = kWh price (EUR/kWh)
8760 = numbers of hours in a year
IL = loading current
IR = rated current
41. 40
EXAMPLE OF TOC CALCULAIONS IN PAKISTAN
TOC= Purchase Price + K1 x (no load losses) + K2 x (load losses)
Transformer rating: 200 kVA
Where:
K1 = Rs. 195.99 per KW
K2 = Rs. 299.99 per KW
For Low Loss Transformer:
Purchase price= Rs. 580,165
No load losses = 396 KW
Load losses = 2728 W
TOC1 = Rs. 580,165 + 195,99 x (396) + 299.99 x (2728)
= Rs. 1476149.76
For High Loss Transformer:
Purchase price = Rs 387,000
No load losses = 495 W
Load loses = 3410 W
TOC2 = Rs. 387,000 + 195.99 x (495) + 299.99 x (3410)
= Rs 1,506,980.95
Benefit of using low loss transformer = TOC2 – TOC1
= 1,506,980.95 – 1,476,149.76
= Rs. 30,831.19
42. 41
ADVANTAGES:
Life cycle cost analysis is a method that encompasses not only the initial
purchase price but also the comparative costs of competing models,
equalized to present day dollars. Since the operating cost of a transformer
over its life may be many times its initial price the only fair comparison with
competing models must take operating costs into account.
Another benefit to owning a transformer with low life cycle cost, results from
the fact that it runs cooler. Loss in the form of heat reduces the life of a
transformer by causing damage to the insulation over time. It can also
cause transformers to fail. Consequently, a transformer with lower life cycle
cost would be expected to have a longer life and lower failure rate, as well
as lower losses.
A transformer with lower losses reduces the amount of power generation
needed to accommodate the losses. This in turn reduces the emission of
greenhouse gases, i.e. carbon dioxide produced by fossil fuel generators.
DISADVANTAGES:
The drawback of this process is, as mentioned, the difficulty in predicting
the future load profile and electricity costs and tariffs with any confidence.
On the other hand, these optimization efforts depend on material prices,
particularly active materials, i.e. conductor and core material. Dynamic
optimization makes sense when there is the different price volatility of
different materials like aluminum and copper or high and low loss magnetic
steel.
For large transformers, above a few MVA , the cost of losses are so high
that transformers are custom built, tailored to the loss evaluation figures
specified in the request for quotation for a specific project.
For distribution transformers, often bought in large batches, the process is
undertaken once every few years. This yields an optimum transformer
design, which is then retained for several years less so changed
dramatically. In fact the loss levels established in national standards reflect
established practice of preferred designs with respect to loss of evaluation
values.
43. 42
CONCLUSION
The result that can be drawn from this report is that the purchase of higher-
cost higher-efficiency unit instead of a lower cost, low efficiency unit will
result in significant savings over the life of the transformer. As for the
environmental benefits, the high efficiency copper wound transformer will
contribute to reducing greenhouse gas emissions by reducing the
consumption of fossil fuel necessary to accommodate excessive
transformer losses.
