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Index
Topic Page
Transformer 3
Workshop 4
Construction 4
a. Core 5
b. Winding 6
i. Layer winding
ii. Disc Winding
c. Tap changer
Testing of Transformer
a. On load test
b. Impedance Test
c. Lightening Impulse Test
d. Atmosphere
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Transformer
(Ali, 2012) Transformer:
Transformer is an electrical machine that transfers electrical energy from
one circuit to another by the principle of electromagnetic induction. While
transferring electrical energy, its frequency does not change. When the
transformer raises the voltage i.e. when the output voltage of transformer
is higher than input voltage, it is called step up transformer and when its
output is lower than input, then it is called step down transformer. The
distribution transformer is the step down transformer that reduces the
voltage to domestic use. Since, its basic construction requires no moving
parts, so it is often called static transformer and it is very rugged machine
requiring the minimum amount of repair and maintenance. Owing to the
lack of rotating parts there is no friction or winding losses. Further, the
losses are relatively low, so that the efficiency of a transformer is high.
Typical transformer efficiencies at full load lie between 96% and 97% and
with extremely large capacity of transformers the efficiencies are as high as
99%.
Process to Manufacture Transformer
1. Core cutting
2. Core laying
3. Winding (HV & LV)
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4. Semi Assembly
5. Final Assembly
6. Testing
Workshop
According to SIEMENS standard, SIEMENS manufactures distribution transformers
ranges from 500kVA to 4MVA for private customers. SIEMENS manufactures
10kVA, 15kVA, 25kVA, 50kVA, 100kVA, 200skVA, 400kVA & 630kVA for WAPDA
(Water and Power Development Authority). For KESC (Karachi Electric Supply
Company), SIEMENS manufactures 250kVA, 500kVA, 750 kVA, 1000kVA, 1500kVA
etc. In DT (Distribution Transformer) Production, transformers are manufactured
ranges from 10kVA to 4MVA. Transformers whose rating is greater than
(>250kVA) copper foil is used as LV. In PT (Power Transformer) Production,
transformers that are manufactured ranges from 4MVA to 160 MVA.
Construction
The main part of a transformer is its active part. Active part mainly consists of
three parts.
Core in steel frame
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1. Core:
The core, which provides the magnetic path to channel the flux,
consists of thin strips of high-grade steel, called laminations, which are
electrically separated by a thin coating of insulating material. The strips
can be stacked or wound, with the windings either built integrally around
the core or built separately and assembled around the core sections.
Core steel can be hot- or cold-rolled, grain-oriented or non-grain
oriented, and even laser-scribed for additional performance. Thickness
ranges from 0.23 mm to upwards of 0.35 mm. The core cross section can
be circular or rectangular, with circular cores commonly referred to as
cruciform construction. Rectangular cores are used for smaller ratings
and as auxiliary transformers used within a power transformer.
Rectangular cores use different width of strip steel, while circular cores
use a combination of different strip widths to approximate a circular
cross-section. The type of steel and arrangement depends on the
transformer rating as related to cost factors such as labor and
performance. The material that is used to in core is called electrical steel
(Si alloy) and is also known as CRGO (Cold-rolled grain oriented). When
voltage is applied to the exciting or primary winding of the transformer,
the magnetizing current flows in the primary winding to secondary
winding. This current produces the flux in the core. The flow of flux in
magnetic circuits is analogous to the flow of current in electrical
circuits. When flux flows in the steel core, losses occur in the steel. There
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are two components of this loss, which are termed “eddy” and
“hysteresis” losses. To reduce hysteresis loss that occurs due to
magnetizing & demagnetizing of the core, can be reduced by adding silicon
3 to 4%. Only that material is used that has low area of hysteresis loss. Eddy
currents can be reduced by laminating the core and decreasing thickness of
a sheet of core. The thickness of a sheet that is taken in DT (Distribution
Transformer) and in PT (Power Transformer) is 0.23mm to 0.27mm and
0.3mm to 0.35mm respectively.
The core consists of yoke, central leg and limb. Top & bottom sheets are
called yoke & the yoke has two holes. The central sheet is called central leg
and side sheets are called limbs.
2. Winding:
The windings consist of the current-carrying conductors wound around
the sections of the core, and these must be properly insulated,
Yoke
Yoke
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supported, and cooled to withstand operational and test conditions.
The terms winding and coil are used interchangeably in this discussion.
Copper and aluminum are the primary materials used as conductors in
power-transformer windings. While aluminum is lighter and generally
less expensive than copper, a larger cross section of aluminum conductor
must be used to carry a current with similar performance as copper.
Copper has higher mechanical strength and is used almost exclusively in
all but the smaller size ranges, where aluminum conductors may be
perfectly acceptable. In cases where extreme forces are encountered,
materials such as silver-bearing copper can be used for even greater
strength. There are two types of winding mostly used in transformer.
a. Layer Winding:
Layer windings are among the simplest of windings in that the insulated
conductors are wound directly next to each other around the cylinder
and spacers. Several layers can be wound on top of one another, with
the layers separated by solid insulation, ducts, or a combination.
Several strands can be wound in parallel if the current magnitude so
dictates. Variations of this winding are often used for applications such
as tap windings used in load-tap-changing (LTC) transformers and for
tertiary windings used for, among other things, third-harmonic
suppression. Figure shows a layer winding during assembly that will be
used as a regulating winding in an LTC transformer.
The layer winding is mostly used in low rating transformers especially in DT
(Distribution Transformer). Two types of layers are used i.e. one is LV (Low
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Voltage) and another is HV (High Voltage). The LV winding is the winding
through we get low voltage. In step up transformer, the primary is LV and in
step down transformer, the secondary is LV. The LV is may be of paper
insulated copper (10kVA-200kVA) or copper foil (>200kVA) layered with
DDP (Diamond Dotted Paper). HV winding is the winding through we get
high voltage. In step down transformer, the primary is HV and in step up
transformer, the secondary is HV.
The diameter of LV is greater than HV because electric current in LV is
greater than HV.
b. Disc Winding:
Disc winding is used in high rating transformers such as power transformer.
A disc winding can involve a single strand or several strands of insulated
conductors wound in a series of parallel discs of horizontal orientation,
with the discs connected at either the inside or outside as a crossover
point. Each disc comprises multiple turns wound over other turns, with
the crossovers alternating between inside and outside.
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Two more kinds of windings are used in SIEMENS that are disc layer winding
and disc tape winding that are used in PT-Production.
2. Tap Changer:
For many decades power transformers equipped with load tap changers
(LTC) have been the main components of electrical networks and
industry. The LTC allows voltage regulation and/or phase shifting by
varying the transformer ratio under load without interruption.
From the beginning of LTC development, two switching principles have
been used for the load-transfer operation, the high-speed-resistance type
and the reactance type. Over the decades, both principles have been
developed into reliable transformer components available in a broad
range of current and voltage applications to cover the needs of today’s
network and industrial-process transformers as well as ensuring optimum
system and process control. In distribution transformer, off load tap
changer is used. It is called off load tap changer because to change its
tapping, you have to denergize the transformer. In Power transformer,
on load tap changer is used that can be adjusted without unplugging the
generator.
Testing of Transformer
According to ANSI and IEEE standards [1], all tests on power transformers
fall into one of three categories: (1) routine tests, (2) design tests, and (3)
other tests. The manufacturer may perform additional testing to ensure the
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quality of the transformer at various stages of the production cycle. The
main difficulties encountered in testing of large transformers by direct loading
are (i) wastage of large amount of energy(ii) a stupendous (impossible for large
transformer) task of arranging a load large enough for direct loading. The
performances characteristics of a transformer can be conveniently computed
through from the knowledge of its equivalent circuit parameter which, in turn
may be determined by conducting simple tests called the open-circuit, no load
test and short circuit or impedance test involving very little power
consumption (power needed to supply the losses incurred).
The sequence in which the various tests are performed is also specified. An
example of test sequence is as follows:
Tests before tanking
Preliminary ratio, polarity, and connection of the transformer windings
Core insulation tests
Ratio and polarity tests of bushing-current transformers
Tests after tanking (final tests)
Final ratio, polarity, and phase rotation
Insulation capacitance and dissipation factor
Insulation resistance
Control-wiring tests
Lightning-voltage impulse tests
Applied-voltage tests
Induced-voltage tests and partial-discharge measurements
No-load-loss and excitation-current measurements
Winding-resistance measurements
Load-loss and impedance-voltage measurement
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On Load Test or Open-circuit Test:
The purpose of this test is to determine the core loss or iron or
excitation loss.
In this test, one of the windings (usually high voltage winding) is kept
open circuited and the rated voltage at rated frequency is applied to
the other winding. No doubt, the core loss will be same whether the
measurement are made on the winding or HV winding so long as the
rated voltage of that winding is applied to it but in case the
measurement are made on HV winding, the voltage required be
applied would often be inconveniently large while I0 would be
conveniently small.
Short Circuit Test (Impedance Test):
The purpose of this test is to determine the full copper loss and equivalent
resistance and equivalent reactance referred to metering side. In this test, the
terminals of secondary winding (usually of low voltage winding) are short
circuited by a thick wire or strip or through an ammeter (which may serve the
additional purpose of indicating secondary rated load current) and variable
low voltage is applied to the primary through potential divider. Clamp meter is
used to measure the current in the wire and multimeter is used to measure
the applied voltage to the circuit.
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Lightening-Impulse Test:
Impulse tests are performed on all power transformers. In addition to
verification of dielectric strength of the insulation system, impulse tests are
excellent indicators of the quality of insulation, workmanship, and processing
of the paper and insulating-oil system. For this purpose high voltage is
applied. The sequence of tests, test connections, and applicable standards is
described below.
Lightning-impulse voltage tests simulate traveling waves due to lightning
strikes and line flashovers. The full-wave lightning-impulse voltage
waveshape is one where the voltage reaches crest magnitude in.
Below is picture of the machine which is used in Impulse Test.
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Atmosphere
As a multinational company, the working atmosphere for workers is better
than other companies in Pakistan. The floor is always kept cleaned as no
accident occurs. Seeing workers working with modern equipment were
really a good experience & they make their job easy and efficient. I found
workers of SIEMENS very co-operative, polite and always ready to help &
tell. Due to the working of overhead cranes, core cutting machines and
other machines, there is always a lot noise in workshop and noise creates
irritation in the nature of man and reduces the ability of hearing. Seeing
worker without head phone was really unpleasant.
Supervisors were also very co-operative and helpful. Whenever I had any
query, they were always ready to tell and help.
