transformer report-siemens

SIEMENS Pakistan Internship Report

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Internship Report

By

Mahmood Ali Korai (BU-Tr)


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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.

transformer report-siemens

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