SUMMER PRACTICAL TRAINING REPORT

ON

MANUFACTURING

AND TESTING OF DISTRIBUTION TRANSFORMER

DONE FROM

J & G TRANSFORMERS

DHORKHA VILLAGE, GURGAON

SUBMITTED BY:

CHIRAG MANCHANDA ROLL NO. – 13163

CLASS – ECE I (A)

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DEPARTMENT - ELECTRONICS AND COMMUNICATION ENGINEERING

ACKNOWLEDGEMENT

I would like to sincerely thank my teacher “Mr. Harsh Gera” for guiding me through this training. I would have not been able to complete this training successfully without his guidance.

I also express my sincere thanks and gratitude to all the faculties who have helped me in all possible ways.

Chirag Manchanda

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Table of Contents

S.No. Name Of Topic Page No.

1. Introduction of Company 4

2. Transformers 5

3. Types of Transformers 8

4. Distribution Transformer 95. Uses 10

6. Connections 117. Parts of Transformers 14

8. Testing of Transformer 239. Failures and Causes 3210. References 33

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INTRODUCTION OF COMPANY

AND

PROFILE OF ORGANIZATION J & G Transformers is Gurgaon , Haryana India based firm. Manufacturer and supplier of Transformers. Unmatched Quality has always been the hallmark of J & G Transformers and we have already gained a reputation as leading manufacturers of Air Cooled, Oil Cooled Transformers and allied products. This apart we also undertake, CT /PT, CVT Transformer Oil Filtration and Repairing.

Our stringent adherence to international standards of quality coupled with our constant penchant for innovations has seen us come out with incredible ideas. We take pride in our world-class infrastructure. We firmly believe in investing in quality machinery to deliver globally viable products. This is enabled by our quality assurance team, who maintain quality standards, cutting across every stage of production, right from the procurement of the material to the actual delivery and installation of Transformers.

Our asset is our team of highly qualified, competent and skilled manpower, who share our vision of always meeting or exceeding customer’s expectations. Cutting edge technology is employed at our production unit to enable quicker turn around times. With a team of dynamic professionals at the helm of affairs, we have grown by leaps and bounds, from the time of our inception. With over a decade of highly specialized experience, no wonder, we are the number one choice of client’s, when it comes to choosing from a range of quality Transformers like Oil Cooled Transformers.

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INTRODUCTION TRANSFORMER

A transformer is a power converter that transfers AC electrical energy through inductive coupling between circuits of the transformer's windings. A varying current in the primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic flux through the secondary winding. This varying magnetic flux induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling.

If a load is connected to the secondary winding, current will flow in this winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. Transformers may be used for AC-to-AC conversion of a single power frequency, or for conversion of signal power over a wide range of frequencies, such as audio or radio frequencies.

In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus enables an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np. The windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used in power stations, or to interconnect portions of power grids. All operate on the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical.

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Layout of a Typical Transmission system showing the importance of TRANSFORMERS at various stages

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P rinciple of Operation

The transformer works on the principle of ‘MUTUAL INDUCTION’. An alternating flux in the primary coil will create an alternating flux in the transformer core, which is linked with the other coil which produces a mutually induced emf according to faraday’s laws of electromagnetic induction.

.

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TYPES OF TRANSFORMERS

1. DISTRIBUTION TRANSFORMER

2.POWER TRANSFORMER

3.CURRENT TRANSFORMER

4.POTENTIAL TRANSFORMER

5.FURNACE TRANSFORMER

6.BOOSTER TRANSFORMER

7.RECTIFIER TRANSFORMER

8.LOCOMOTIVE TRANSFORMER

9.MINING TRANSFORMER

10.PHASE SHIFTING TRANSFORMER

11.WELDING TRANSFORMER

12.HIGH VOLTAGE TESTING/SC TESTING TRF.

13.GROUNDING TRANSFORMERS

14.CONVERTER TRANSFORMER

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Distribution Transformer

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. Because of weight restrictions transformers for pole mounting are only built for primary voltages under 30 kV

Classification

Distribution transformers[1] are classified into different categories based on certain factors such as

Type of insulation - liquid-immersed distribution transformers or dry-type distribution transformers

Number of Phases - single-phase distribution transformers or three-phase distribution transformers

voltage class (for dry-type) – Low voltage distribution transformers or medium voltage distribution transformers

Basic impulse insulation level (BIL), for medium-voltage, dry-type.

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Uses of Distribution Transformer

Distribution transformers are normally located at a service drop, where wires run from a utility pole or underground power lines to a customer's premises. They are often used for the power supply of facilities outside settlements, such as isolated houses, farmyards or pumping stations at voltages below 30kV. Another application is the power supply of the overhead wire of railways electrified with AC. In this case single phase distribution transformers are used.

In North American utility practice, single-phase transformers are widely used to power individual homes, while in Europe three-phase transformers are more common, which can supply several buildings. Pad-mount transformers are used in urban areas and neighbourhoods where the primary distribution lines run underground. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes.

High voltage hobbyists often use these transformers in reverse (step-up) by feeding 120 or 240 volts into the secondary and drawing the resulting high voltage off the primary bushings, using it to power devices like Jacob's Ladders and Tesla coils, and many other high voltage experiments

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Connections

Both pole-mount and pad-mount transformers convert the high 'primary' voltage of the overhead or underground distribution lines to the lower 'secondary' voltage of the distribution wires inside the building. The primaries use the three-phase system. Main distribution lines always have three wires, while smaller "laterals" (close to the customer) may include one or two phases, used to serve all customers with single-phase power. If three-phase service is desired, one must have a three-phase supply. Primaries provide power at one of a wide range of standard voltages from 4 to 33 kilovolts, but the most widely used are 7,200 or 14,400 volts.

Primary

Three phase distribution transformer

The high voltage primary windings are brought out to bushings on the top of the case.

Single phase transformers, generally used in the USA system, are attached to the overhead wires with two different types of connections:

If a primary neutral wire is available, a 'wye' or 'phase to neutral' transformer can be used. This usually has only one bushing on top, connected to one of the primary phases. The other end of the primary winding is 'grounded' to the transformer's case, which is connected to the neutral wire of the 3 phase system, and also earth ground. This type of distribution system, called 'grounded wye', is preferred because the transformers present unbalanced loads on the

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line, causing currents in the neutral wire. With the 'delta' connection, this can cause variations in the voltages on the 3 phase wires.

If no neutral wire is available, a 'delta' or 'phase to phase' transformer must be used. This has two bushings on top which are connected to two of the three primary wires, so the voltage across the primary winding is the phase-to-phase voltage. This type is used on long distribution lines where it is uneconomical to run a fourth neutral wire.

Transformers providing three-phase secondary power, which are used for residential service in the European system, have three secondary windings and are attached to all three primary phase wires. The windings are almost always connected in a 'wye' configuration, with the ends of the three windings connected and grounded.

The transformer is always connected to the primary distribution lines through protective fuses and disconnect switches. For pole-mounted transformers this usually takes the form of a 'fused cutout'. An electrical fault causes the fuse to melt, and the device drops open to give a visual indication of trouble. It can also be manually opened while the line is energized by lineworkers using insulated hot sticks.

Secondary

The low voltage secondary windings are attached to three or four terminals on the transformer's side.

In the USA and countries using its system, the secondary is most often the split-phase 240/120 volt system. The 240 V secondary winding is center-tapped and the center neutral wire is grounded, making the two end conductors "hot" with respect to the center tap. These three wires run down the service drop to the electric meter and service panel inside the building. Connecting a load between either hot wire and the

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neutral gives 120 volts. Connecting between both hot wires gives 240 volts.

In Europe and countries using its system, the secondary is often the three phase 400Y/230 system. There are three 230 V secondary windings, each receiving power from a primary winding attached to one of the primary phases. One end of the 3 secondary windings are connected to a 'neutral' wire, which is grounded. The other end of the 3 secondary windings, along with the neutral, are brought down the service drop to the service panel. 230 V loads are connected between any of the three phase wires and the neutral.

Higher secondary voltages, such as 480 volts, are sometimes required for commercial and industrial uses. Some industrial customers require three-phase power at secondary voltages. To provide this, three-phase transformers can be used. In the US, which uses mostly single phase transformers, three identical single phase transformers are often wired in a transformer bank in either a wye or delta connection, to create a three phase transformer.

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CONSTRUCTION

Parts:

MAIN TANK

CORE

RADIATORS

CONSERVATOR

EXPLOSION VENT

BREATHER

WINDINGS

HV/LV BUSHINGS

BUCHHOLZ RELAY

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Main Tank

It contains all the parts of the transformer. Small capacity tanks are fabricated from sheet steel while larger ones are assembled with cast aluminium. For cooling purpose the tank is welded with cooling tubes.

Core

Core of the transformer is either in square or rectangular in size. It is further divided into two parts. The vertical position of the core is limbs and horizontal position of the core is yoke of the core. Core is made up of laminations to reduce the eddy current losses get minimized. This lamination is insulated by using insulations line varnish or thick paper. Paper insulation is used for low voltage transformer and varnish is used for high voltage transformer.

Shell type transformer Core type

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Radiators

Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function inautomobiles, buildings, and electronics. The radiator is always a source of heat to its environment, although this may be for either the purpose of heating this environment, or for cooling the fluid or coolant supplied to it, as for engine cooling. Despite the name, radiators generally transfer the bulk of their heat via convection, not by thermal radiation, though the term "convector" is used more narrowly.

One might expect the term "radiator" to apply to devices that transfer heat primarily by thermal radiation (see: infrared heating), while a device which relied primarily on natural or forced convection would be called a "convector". In practice, the term "radiator" refers to any of a number of devices in which a liquid circulates through exposed pipes (often with fins or other means of increasing surface area), notwithstanding that such devices tend to transfer heat mainly by convection and might logically be called convectors. The term "convector" refers to a class of devices in which the source of heat is not directly exposed.

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Conservator

Conservator tank consists of oil level which depends on the operation of the transformer. The oil expands in summer with the increase in load and the oil contracts with decrease of the load. Conservator is a small auxiliary oil tank that may be mounted above the transformer and connected to the main tank by a pipe.

Its main function is to keep the main tank of the transformer completely filled with the oil in all circumstances. When the oil in the tank contracts then the conservator gives the oil to maintain the oil level in the tank. When the oil in the tank expands then the conservator takes the oil.

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Explosion Vent

An explosion vent is a safety device to protect equipment or buildings against excessive internal, explosion-incurred pressures, by means of pressure relief. An explosion vent will relieve pressure from the instant its opening (or activation) pressure pstat has been exceeded.

Several explosion vent panels can be installed on the same process vessel to be protected. Explosion vents are available in the versions self-destructive, non-self-re-closing and re-usable, self-re-closing.

Explosion vent construction must balance the contradictory requirements "low inertia" and "high strength". Inertia negatively affects an explosion vent's efficiency. High strength is required to endure the considerable forces that move the vent's venting element in order to open the venting orifice. Unintended disintegration must not cause disintegrating parts turning into a missile.

The evaluation of an explosion vent's efficiency and its range of application are subject to rules.

During normal venting, the explosion is freely discharged, allowing flames to exit the process being protected. When the protected vessel or pipe is located indoors, ducts are generally used to safely convey the explosion outside the building. However, ductwork has disadvantages and may result in decreased venting efficiency. Flameless venting, in combination with explosion vents, can extinguish the flame from the vented explosion without the use of expensive ducting, limitations to equipment location, or more costly explosion protection.

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Breather

When the transformer gets warm the oil and gas expands the gas at the top of the oil is expelled out when the transformer cools when air is drawn into the transformer.

“The breather is mainly used to avoid moisture from the transformer. If the moisture enters into the transformer the insulation provided between the two windings is damaged and it begins to conduct and the windings are damaged.”

In this process moisturized air should be entered into the transformer. Breather consists of silica gel which absorbs the moisture from the air and this air is send into the transformer. This process is called as BREATHING. The material is blue when dry and a whitish pink when dam. Also the breather consists of oil cap which avoid entering dust particles into the transformer.

Silica gel Breather

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Windings

Conducting material is used in the windings of the transformer. Usually the windings are in concentrically to minimize the flux leakages. There are two types of windings. The coils are wound on the limbs and are insulated from each other in the basic transformer the two windings wound on the two different limbs. Due to this leakage flux increases which affects the transformer efficiency or performance so it should be necessary that the windings should be very close to each other to increase the mutual inductance and stray capacitance to improve the high frequency response.

Such cylindrical coils are used in core type transformers and sandwich coils are very commonly used in shell type transformer here each high voltage winding lies between two low voltage windings such subdivisions of windings into small portions reduce the flux leakages.

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HV/LV Bushings

The bushings consists of a current carrying element in the form of a conducting rod upto 33KV ordinary porcelain insulators can be used above this voltage ratings oil-filled are capacitor type bushings are used. The bushing is very important to the overall transformer because without it, conduction would not be possible. The bushings are necessary to complete the conductive energy output of the volts that are transformed within the transformer so that they can then move through mediums such as air and gas, including the grounding barriers that each unit is designed with. These are some figures of bushings.

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Buchholz Relay

Buchhloz relay is a gas and oil activated protective device. It is practically used in all oil immersed transformers having rating more than 750kVA. It is installed in the pipe joining between the conservator and is used to give alarm in case of minor faults and to disconnect the transformer from the supply mains in case of severe internal faults.

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TESTING

TESTING IS CARRIED OUT AS PER IS-2026.

ROUTINE TESTS

1.Measurement of winding resistance

2.Measurement of insulation resistance

3.Seperate source voltage withstand test (High Voltage tests on HV & LV)

4.Induced Over voltage Withstand test (DVDF test)

5.Measurement of voltage ratio

6.Measurement of NO LOAD LOSS & current.

7.Measurement of LOAD LOSS & IMPEDENCE.(EFFICIENCY & REGULATION)

8.Vector Group Verification

9.Oil BDV test.

10.Lightening impulse test

11. Short circuit withstand ability test

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Measurement of winding resistance

This test measures the resistance of the HV & LV winding. The values of resistance should be balance for all three phases and should match the designed values.

Equipment used : Digital resistance meter.

Measurement of insulation resistance

Measures the insulation resistance of HV & LV windings with respect to earth (body) and between LV & HV winding.

INSULATION TESTER OR MEGGER IS USED.

Recommended Values are 2000Mohms for HV & 500 Mohms for LV.

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Seperate source voltage withstand test

High Voltage tests on HV & LV)- This test checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary.

HV high voltage test : LV winding connected together and earthed. HV winding connected together and given 28 KV ( for 11KV transformer) for 1 minute.

LV high Voltage test : HV winding connected together and earthed. LV winding connected together and given 3 KV for 1 minute.

Equipment used : High Voltage tester ( 100KV & 3KV)

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Induced Over voltage Withstand test (DVDF test)-

This test checks the inter turn insulation.

For a 11KV/433V transformer,866 Volts are applied at the 433V winding with the help of a Generator for 1 minute. This induces 22KV on 11KV side. The frequency of the 866V supply is also increased to 100HZ.

Equipment used : MOTOR GENERATOR SET

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Measurement of voltage ratio

This test measures the voltage ratio as per the customer’s requirement.

V1/V2 = N1/N2

The voltage ratio is equal to the turns ratio in a transformer. Using this principle, the turns ratio is measured with the help of a turns ratio meter. If it is correct , then the voltage ratio is assumed to be correct.

Equipment used : Turns Ratiometer

Measurement of NO LOAD LOSS & current.

The iron losses and no load current are measured in this test. The 433V winding is charged at 433V supply & the 11KV winding is left open .The power consumed by the transformer at no load is the no load loss in the transformer.

Effect of actual frequency must be taken into account.

Equipment used : Wattmeters or power analyser.

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Measurement of LOAD LOSS &

IMPEDENCE.(EFFICIENCY & REGULATION)

This test measures the power consumed by the transformer when the 433V winding is short circuited and The rated current is passed through the 11KV winding.

Equipment used : Wattmeters or power analyser.

Vector Group Verification test

This test verifies the Dyn-11 vector group of a distribution transformer.

Equipment used : voltmeter.

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Oil BDV TEST.

Oil breakdown voltage is checked as per IS-335.

100 mm L X 70 mm B X 80 mm Ht. glass pot.

500ml Oil sample.

Spherical electrodes with gap of 2.5 mm

Recommended value : 60KV

Equipment used : OIL BDV TEST SET.

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LIGHTENING 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.

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Short circuit withstand ability test

This tests measures the ability of the transformer to withstand the mechanical and thermal stresses caused by the external short circuit.

HV terminals are connected to the supply bus of the testing plant. The LV is short circuited. The testing plant parameters are such adjusted to give the rated short circuit current.

Supply is made on and closed after specified duration of short circuit. The record of current wave form is noted.

There should not be any mechanical distortion, fire to the transformer during this test. Similarly no wave form distortion. The transformer should also withstand the routine tests after the short circuit test.

The reactance of the winding measured before and after the S.C. test should not vary beyond the limits stated in the IS2026.

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FAILURES AND CAUSES

Insufficient Oil level. Seepage of water in oil. Prolonged Over loading. Single Phase loading. Unbalanced loading. Faulty Termination (Improper sized lugs etc) Power Theft. Prolonged Short Circuit. Faulty operation of tap changer switch. Lack of installation checks. Faulty design Poor Workmanship Improper formation of core. Improper core bolt insulation. Burr to the lamination blades Improper brazing of joints. Burr /sharp edges to the winding conductor. Incomplete drying. Bad insulation covering. Insufficient cooling ducts in the winding. Bad Quality of raw material. Transit damaged transformers.

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REFERENCES

www.wikipedia.org www.electronicsforu.com Electromechanical Energy Conversion by J.B.Gupta Electronic Transformers and Circuits by Reuben Lee

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