PROJECT REPORT ON

MANUFACTURING & ASSEMBLING OF

POWER & DISRTRIBUTION

TRANSFORMERS

AT

DANKE ELECTRICALS LTD

WAGHODIA G.I.D.C , VADODRA

PREPARED BY :

VASA PRITEN M

PRIYESH DOBARIYA

(BE 4TH SEM )

Faculty of technology & engineering

department of electrical engineering

We are very thankful to DANKE POWER providing such a great opportunity here and given a platform for us to nourish our technicality rich with the industrial exposure . We also greet the all the employees production manager, production incharge including helpers and repairers for giving a guidance and core technical knowledge which will be helpful for us to build and establish strong career in this industry

Thanking you

Vasa Priten

Dobariya Priyesh

SIGNED AND CERTIFIED BY

Mr. Ketan patel Mr. Rajesh patel Mr. Pragnesh patel

Plant Manager Production Manager Production Incharge

Mr. S.K Joshi

H.O.D Electrical Department

Production flow chart manufacturing and assembly

RAW MATERIA

L

Raw material basically used is CRGO(cold rolled grained oriented)silicon steel.The purpose of using CRGO is to reduce the Hysteresis Losses.

CORE CUTTING

It is in the form of thin sheets & cut to size as per design.Generally three different shapes of core laminations are used in one assembly.

COIL WINDING

Coil Winding is of two types:-R-S COIL HELICAL COIL SPIRAL COIL

CORE FITTING

The core assembly is vertically placed with the foot plate touching the ground. the top yoke of the core is removed. The limbs of the core are tightly wrapped with cotton tape and then varnished.

ASSEMBLY

The coils as specified in the design may be of following types:L.V COILH.V COIL

SECONDARY PROCESS AFTER ASSEMBLY

TESTING COIL WINDINGCONNECTION

TANKING

Tanking is a procedure of embracement of complete unit (active unit) in a mild steel tank, the unit being enclosed includes (core entrapped with windings).

TESTING AT POWER LEVEL

ROUTINE TESTRATIO METER METHODDIELECTRIC TESTS IMPULSE TEST LEVELSTEMPERATURE RISE TEST

DISMANTLING & DISPATCH

After testing is completed and before dispatch from the factory all necessary work, e.g. removal of bushings etc. shall be performed.A shipping list is made concerning the General arrangement drawing which list out the external attachments of a power transformer to be send to customers .

RAW MATERIALS

INCOMING MAJOR RAW MATERIALS TESTING

CRGO/LAMINATION: As per Purchase OrderSpecifications / Drawing

One Sampleper Lot

COPPER STRIP / ALUMINIUM STRIP:

As per Purchase Order

Specifications

One SamplePer Lot

COPPER WIRE /ALUMINIUM WIRE

As per Purchase Order

Specifications

One Sample:Per Lot

TRANSFORMER OIL: As per Purchase Order

Specifications

One SamplePer Tanker

M.S.MATERIALS: As per Purchase Order

Specifications

One SamplePer Lot

RADIATOSR: As per Purchase Order

Specifications

100 %

BUSHING: As per Purchase Order 

Specifications

As perSampling Plan

METAL PARTS: As per Purchase Order

Specifications

As perIS Standards

PRESS BOARDS: As per Purchase Order Specifications

As per IS Standards

GASKETS: As per Purchase Order 

Specifications

As perSampling Plan

WELDING ELECTRODE/BRAZING ROD/ SOLDERING WIRE:

As per Purchase Order As perSampling Plan

Specifications PAINT/ PRIMER /

THINNERAs per Purchase Order

Specifications

100%

TRANSFORMERTANK & FITTING:

As per Purchase Order

Specifications/Drawing

100%

HARDWARE /DRIN VALVE/PLUGS CAPS:

As per Purchase Order

Specifications

As perSampling Plan

TANK & FITTINGINCLUDING CABLEBOXES:

Tightness, Cleanliness

CRGO LAMINATIONS

COPPER AND ALUMINIUM STRIPS

TRANSFORMER OIL

RADIATORS

bushing parts

Preesboards

Gaskets

Safety meters

The construction of a power transformer  varies throughout the industry. The basic arrangement is essentially the same and has seen little significant change in recent years, so some ofthe variations can be discussed in this article.

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 0.36 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 a single 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.

Just like other components in the transformer, the heat generated by the core must beadequately dissipated.While the steel and coating may be capable of withstanding higher temperatures, it will come in contact with insulating materials with limited temperature capabilities. In larger units, cooling ducts are used inside the core for additional convective surface area, and sections of laminations may be split to reduce localized losses.

The core is held together by, but insulated from, mechanical structures and is grounded to a single point in order to dissipate electrostatic buildup. The core ground location is usually some readily accessible point inside the tank, but it can also be brought through a bushing  on the tank wall or top for external access.This grounding point should be removable for testing purposes, such as checking for unintentional core grounds. Multiple core grounds, such as a case whereby the core is inadvertently making contact with otherwise grounded internal metallic mechanical structures, can provide a path for circulating currents induced by the main flux as well as a leakage flux, thus creating concentrations of losses that can result in localized heating.

The maximum flux density of the core steel is normally designed as close to the knee of the saturation curve as practical, accounting for required overexcitations and tolerances that exist due to materials and manufacturing processes.

For power transformers the flux density is typically between 1.3 T and 1.8 T, with the saturation point for magnetic steel being around 2.03 T to 2.05 T.

There are two basic types of core construction used in power transformers: core form and shell form.

In core-form construction,there is a single path for the magnetic circuit. Figure 1 shows a schematic of a single-phase core, with the arrows showing the magnetic path.

Figure 1 - Schematic of single-phase core-form construction.

Figure 2 - Schematic of three-phase core-form construction

For single-phase applications, the windings are typically divided on both core legs as shown. In three-phase applications, the windings of a particular phase are typically on the same core leg, as illustrated in Figure 2.

Figure 3 - 'E'-assembly, prior to addition of coils and insertion of top yoke

Windings are constructed separate of the core and placed on their respective core legs during core assembly. Figure 3 shows what is referred to as the “E” – assembly of a three-phase core-form core during assembly.In shell-form construction, the core provides multiple paths for the magnetic circuit. Figure 4 is a schematic o fa single-phase shell-form core, with the two magnetic paths illustrated.The core is typically stacked directly around the windings,which are usually “pancake” – type windings, although some applications are such that the core and windings are assembled similar to core form.

Figure 4 - Schematic of single-phase shell-form construction

Due to advantages in short-circuit and transient-voltage performance, shell forms tend to be used more frequently in the largest transformers,where conditions can be more severe. Variations of three-phase shell-form construction include five- and seven-legged cores, depending on size and application.

COIL WINDING

750kVA dry type transformer windings

Construction

The windings  consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, 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.

(COUTESY: DANKE ELETRICALS LTD 400 KVA HV LAYER WINDING)

While aluminum is lighter and generally less expensive than copper, a larger cross section ofaluminum   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.

The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors. Multiple strands can be wound in parallel and joined together at the ends ofthe winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends.

Individual strands may be subjected to differences in the flux field due to their respective positions within the winding, which create differences in voltages between the strands and drive circulating currents through the conductor loops.

Figure 1 - Continuously transposed cable (CTC)

Proper transposition ofthe strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents. A variation ofthis technique,involving many rectangular conductor strands combined into a cable, is called continuously transposed cable (CTC), as shown in Figure 1.In core-form transformers,the windings are usually arranged concentrically around the core leg, as illustrated in Figure 2, which shows a winding being lowered over another winding already on the core leg of a three-phase transformer.A schematic of coils arranged in this three-phase application was also shown in Figure 1 (article ‘Power Transformer Construction – Core’) .Shell-form transformers use a similar concentric arrangement or an inter-leaved arrangement, as illustrated in the schematic Figure 3 and the photograph in Figure 7.

Figure 2 - Concentric arrangement, outer coil being lowered onto core leg over top of inner coil

Figure 3 - Example of stacking (interleaved) arrangement of windings in shell-form construction

With an interleaved arrangement, individual coils are stacked, separated by insulating barriers and cooling ducts. The coils are typically connected with the inside of one coil connected to the inside of an adjacent coil and, similarly, the outside of one coil connected to the outside of an adjacent coil. Sets of coils are assembled into groups, which then form the primary or secondary winding.

When considering concentric windings, it is generally understood that circular windings have inherently higher mechanical strength than rectangular windings, whereas rectangular coils can have lower associated material and labor costs.

Rectangular windings permit a more efficient use of space, but their use is limited to smallpower transformers  and the lower range of medium-power transformers, where the internal forces are not extremely high. As the rating increases, the forces significantly increase, and there is need for added strength in the windings, so circular coils, or shell-form construction are used.

In some special cases, elliptically shaped windings are used.Concentric coils are typically wound over cylinders with spacers attached so as to form a duct between the conductors and the cylinder. The flow of liquid through the windings can be based solely on natural convection, or the flow can be somewhat controlled through the use of strategically placed barriers within the winding.

Figures 4 and 5 show winding arrangements comparing nondirected and directed flow. This concept is sometimes referred to as guided liquid flow.

Figure 4 - Nondirected flow

A variety of different types of windings have been used in power transformers through the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils; or they can be wound horizontally and placed upright upon completion.

As mentioned previously, the type of winding depends on the transformer rating  as well as the core construction. Several of the more common winding types are discussed below.

Figure 5 - Directed flow

Pancake Windings

Several types of windings are commonly referred to as “pancake” windings due to the arrangement of conductors into discs. However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers.The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically. Figure 6 illustrates how these coils are typically wound. This type of winding lends itself to the interleaved arrangement previously discussed (Figure 7).

Figure 6 - Pancake winding during winding process

Figure 7 - Stacked pancake windings

Layer (Barrel) Windings

Layer (barrel) 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 ifthe 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 8 shows a layer winding during assembly that will be used as a regulating winding in an LTC transformer.

Figure 8 - Layer windings (single layer with two strands wound in parallel)

Helical Windings

Helical windings are also referred to as screw or spiral windings, with each term accurately characterizing the coil’s construction.A helical winding consists of a few to more than 100 insulated strands wound in parallel continuously along the length of the cylinder, with spacers inserted between adjacent turns or discs and suitable transpositions included to minimize circulating currents between parallel strands.

Figure 9 - Helical winding during assembly

The manner of construction is such that the coil resembles a corkscrew. Figure 9 shows a helical winding during the winding process. Helical windings are used for the higher-current applications frequently encountered in the lower-voltage classes.

Disc Windings

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.

Figure 10 - Basic disc winding layout

Figure 11 - Disc winding inner and outer crossovers

Figure 10 outlines the basic concept, and Figure 11 shows typical crossovers during the winding process.Most windings of 25-kV class and above used in core-form transformers are disc type. Given the high voltages involved in test and operation, particular attention is required to avoid high stresses between discs and turns near the end of the winding when subjected to transient voltage surges.Numerous techniques have been developed to ensure an acceptable voltage distribution along the winding under these conditions.

CORE FITTING

CONSTRUCTION

1.THREE LEG (NORMAL) 2.FIVE LEG (TO REDUCE HEIGHT)

CLAMPING ARRANGEMENT

1.END FRAME & INSULATION 2.CLAMPING PLATES 3.YOKE BOLTS OR YOKE STEEL BAND (NO HOLE PUNCHING REQ )

PROPER TIGHTENING TO REDUCE NOISE LEVEL

VARNISH AT EXPOSED SURFACES

ASSEMBLY BEFORE TERMINALS GEAR STAGE

TOP YOKE REMOVAL

LEVELING OF BOTTOM SUPPORT BLOCK

LEG PREPARATION

VISUAL CHECKING OF COIL

LOWERING OF COIL IN PROPER ORIENTATION

TOP INSULATION & CLAMPING RINGS

MOUNTING OF ENDFRAMES

SHRINKAGE IN HEATER

TOP YOKE FITTING

BTG TEST FOR CHECKING CORRECTNESS

CORE COIL ASSEMBLY

-The "active" part of the Transformer consists of the magnetic core with windings and accessories. The windings are placed over the core limbs and necessary connections are made as per the tappings and vector group. Sufficient ducts are provided between the coils to ensure heat dissipation through circulation of oil. Best quality insulation is provided at all joints and gaps. The optimum design of Core-Coil Assembly is achieved by considering the required technical particulars including cooling, size compactness and tapping arrangement. All leads and conductors are rigidly supported by special wooden frames.

DRYING PROCESS OF POWER TRNANSFORMER

(courtesy danke electrical ltd.)

Introduction

The drying process of power transformers has a strong influence on the quality of manufacturedunits or the lifetime after repair. Water in the solid insulation is accelerating the ageing (1) and therefore decreasing the life expectance. The paper and pressboard consists of cellulose, which ages in service mainly due to hydrolysis. Water abets this chemical reaction, which results in further water. Therefore a proper drying after assembling, typically down to 0.5% water content, assures a long life expectance. Also after repair, drying of the solid insulation might be necessary. For repairs on active parts of power transformers, the tank is opened and the oil is drained so that the paper and pressboard is directly subjected to the ambient humidity. Water from the surrounding air can migrate into the insulation. This process is faster for high insulation temperatures or new cellulosic material, which is not oil impregnated. However, even for oil impregnated insulation at ambient temperatures the water content will increase significantly after a few days .Now a days it is still challenging to define an adequate drying time. An unnecessary long drying times should be avoided. The high temperature during the drying process might deteriorate the cellulosicmaterial and the degree

of polymerization is decreasing.The drying time strongly depends on the initial water content, which itself depends on several changing factors, like the humidity or temperature in the factory. Typical methods for determining the water content in the solid insulation are not applicable. The Karl-Fischer-Titration on the paper sample would provide the highest accuracy, but during the process no paper samples can be taken. Therefore a paper sample can be analyzed only after the drying process. In case of insufficient dryness, the drying process has to be restarted. This involves a high effort in time and energy .During the drying process, indirect methods like a measurement of dew point or water vapor pressure can be used. However, these methods are measuring the ambience in the vacuum chamber and not the insulation itself. The water content is often underestimated, since mainly the water in the outermost layer is contributing to the result. Measuring the abstracted water rate can only give a value for the remaining water content if the initial water content is known. Even more challenging is defining the drying time for aged transformers. They cannot be dried to the same water contents like new transformers. The achievable water content strongly depends on the condition of the solid insulation and is typically not known. Additionally different transformer sizes and designs need to be handled in repair shops. Due to the issues mentioned above, there is a need for a measurement method, which is able to monitor the water content during drying, directly measured at the insulation itself.

Removal of moisture from transformers in service

Introduction

The design life of power transformers is usually 30 to 35 years. In fact the typical Time to Failure of a large generator transformer (working at constant full load) is 18 to 24 years and a transmission or distribution transformer (working at half load or less) can be 40 to 60 years. The actual life of a transformer is determined by ageing of the cellulosic insulation in the form of paper on conductors and leads, and of pressboard used for inter-turn or intersection spacers. The insulation life is determined by three factors:

• Operating temperature

• Access to oxygen

• Water absorbed in the paper.

As cellulose ages the length of the glucose chain slowly reduces due to chain schism from 1200 molecules to about 200 molecules when it no longer has sufficient mechanical strength to be viable.

Operating temperature

Unless redundancy has been built into the transformer rating at the specification stage, a transformer is always likely to operate at or near the rated value. For every 6.5º C increase in operating temperature the insulation life will halve, based on a moisture content in the paper of 0.3% by weight. The remnant life of the transformer will be reduced whenever it is operating at high temperature. The rate of ageing will be higher (up to 50 times faster at moisture contents of 5% in the paper), and the remnant life will be much reduced.

Oxygen

Ageing is due to chemical reactions between the long-chain glucose molecule of cellulose and oxygen. With no oxygen present there can be no ageing of the paper, but oxygen is always present either as air dissolved in the oil or as water in the oil or paper. Attempts to replace air in the oil by nitrogen or an electro-negative gas have always failed, but a rubber-based membrane has been used with success to prevent direct contact between the oil and the atmosphere. Unfortunately the material of the membrane has a lifetime of only 10 to 15 years before is begins to allow air and water vapour to diffuse through it. In addition, water at up to 10% of the weight of cellulose is formed by the chemical process of ageing. Fitting a membrane will trap water formed during this process and moisture is locked into the system where it is a catalyst causing more rapid ageing of the insulation.

Water

Under normal conditions oil will dissolve 60 ppm water before it saturates. Cellulose saturates at 10% water content when it is dry and between 12 and 18% water content when it is oil-impregnated. The presence of moisture as a contamination in the oil-paper insulation system will compromise the dielectric strength of the paper and will act as a catalyst for rapid ageing of the insulation system. Moisture can enter the oil-paper system in several ways:

• It can remain absorbed in the insulation if the factory drying process has been inadequate.

• It can enter the transformer during service if the air drying system has not been properly maintained, diffuse through gasket material or enter through cracks in the tank (welding defects).

• It can enter through openings in the tank if the internal insulation has not been correctly during site erection operations or during service outages.

• In addition, water is generated internally in the transformer as the paper and pressboard materials age in service. The rate of ageing in serviceis accelerated by operation at high temperature, by the presence of oxygen as air dissolved in the oil or as water in the paper.

Mitigation to reduce ageing

The traditional means of protecting the insulation system of a transformer from the ingress of water is to fit a silica gel breather. These breathers need to be maintained as often as every two weeks and do not remove moisture generated inside the transformer by ageing of the insulation.

Refrigerated breathers based on Peltier devices are widely used in the UK to continuously remove water from oil in the conservator. These devices will slowly remove water from the cellulose insulation but are ineffective when water in the paper exceeds 2.5% by weight. Oil filtration plant based on heat and vacuum operations are effective in drying the oil, but as more than 99% of the moisture is absorbed in the paper, it quickly migrates into the oil and the oil remains wet. Molecular sieve drying devices are connected to circulate the main tank oil over a charge of molecular sieve material; this is a naturally occurring zeolite selected with a 4 Angstrom pore size to match the size of a water molecule. Water is trapped at up to 40% by weight of molecular sieve material through chemical bonding, and can only be removed by a heat and vacuum process to break the energy bonds holding water in the material. Molecular sieve drying devices can be used to slowly remove moisture from the cellulose insulation by removing water from the oil. The movement of water from cellulose to oil takes place at the same rate as the movement of water from oil into the molecular sieve material. High levels of moisture have been removed from the insulation of transformers over a period of weeks and3 months to reduce the risk of the transformers failing due to electrical surges or through mechanical faults associated with high through-fault currents.

An alternative water management scenario is to fit molecular sieve devices to new transformers, in combination with refrigerated breathers or diaphragm seals. The molecular sieve device absorbs water dissolved in the oil and removes water produced by degradation of the cellulose as it is formed. This combination of devices is effective in maintaining high integrity of the transformer insulation by eliminating the main catalyst for ageing and avoidinga reduction in the dielectric performance of the insulation structure.

Pressing of transformer windings during active part drying

The present invention relates to a method and arrangement for pressing of windings assembled onto a transformer. The method comprises applying pressing force on the windings, and maintaining the pressing force on the windings during drying of transformer active part. Thus, before the drying process commences, the windings are assembled onto the transformer core and pressing force is applied to the windings. This pressing force is then maintained during the process of drying the transformer active part. The windings will as a result advantageously be effectively compressed onto the core and stabilized. This will subsequently lead to less pressure relaxation and better winding clamping.

ADVANTAGES AND MAINTAINING PRECAUTIONS

It is important that transformer windings are well-clamped and robust during transport to site and subsequent operation. To obtain this, the windings are compressed axially and clamped in position on transformer core after drying transformer active part. Drying is undertaken since cellulose insulation of the winding must be free from moist. Moist in the insulation of the windings deteriorate transformer operation; first of all, the dimensions of the cellulose-clad windings change as moist leave the cellulose material. Second, the material may partly lose its insulating effect with subsequent electrical problems in the transformer.

However, notwithstanding transformer active part drying, the clamping force of the winding applied to the transformer core is reduced with time because of mechanical relaxation of the insulation material. This relaxation is accelerated by heating and cooling since temperature expansion of the cellulose insulation is many times greater than that of copper and steel. Traditionally, to reduce the relaxation, the windings are

pressed and thus compacted separately in the winding work shop before and after drying of the windings.

Study of transformer external component & fittings

Rubberized cork sheet Radiators Buchholz relay Oil temperature indicator Winding temperature indicators Magnetic oil level gauage Silica gel breather Pressure relief valve On load tap changer Off circuit tap changing switch Porcelain bushings

Radiators AND ITS TYPES

-t type radiators swan neck type header type

tubular type radiator

Buchholz relay & silica gel beather

What is a Buchholz relay?

Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil immersed

transformers having rating more than 500 kVA. Buchholz relay is not provided in relays having rating

below 500 kVA from the point of view of economic considerations.

Why Buchholz relay is used in transformers?

Buchholz relay is used for the protection of transformers from the faults occurring inside the

transformer. Short circuit faults such as inter turn faults, incipient winding faults, and core faults may

occur due to the impulse breakdown of the insulating oil or simply the transformer oil. Buchholz relay

will sense such faults and closes the alarm circuit.

Working principle

Buchholz relay relies on the fact that an electrical fault inside the transformer tank is accompanied

by the generation of gas and if the fault is high enough it will be accompanied by a surge of oil from

the tank to the conservator

Whenever a fault occurs inside the transformer, the oil in the transformer tank gets overheated and

gases are generated. The generation of the gases depends mainly on the intensity of fault produced.

The heat generated during the fault will be high enough to decompose the transformer oil and the

gases produced can be used to detect the winding faults. This is the basic principle behind the

working of the Buchholz relay.

Construction

Buchholz relay can be used in the transformers having the conservators only. It is placed in the pipe

connecting the conservator and the transformer tank. It consists of an oil filled chamber. Two hinged

floats, one at the top of the chamber and the other at the bottom of the chamber which accompanies

a mercury switch each is present in the oil filled chamber. The mercury switch on the upper float is

connected to an external alarm circuit and the mercury switch on the lower is connected to an

external trip circuit.

Operation

Operation of the Buchholz relay is very simple. Whenever any minor fault occurs inside the

transformer heat is produced by the fault currents. The transformer oil gets decomposed and gas

bubbles are produced. These gas bubbles moves towards the conservator through the pipe line.

These gas bubbles get collected in the relay chamber and displaces oil equivalent to the volume of

gas collected. The displacements of oil tilts the hinged float at the top of the chamber thereby the

mercury switch closes the contacts of the alarm circuit.

The amount of gas collected can be viewed through the window provided on the walls of the

chamber. The samples of gas are taken and analyzed. The amount of gas indicates the severity of

and its color indicates the nature of fault occurred. In case of minor faults the float at the bottom of

the chamber remains unaffected because the gases produced will not be sufficient to operate it.

During the occurrence of severe faults such as phase to earth faults and faults in tap changing gear,

the amount of volume of gas evolves will be large and the float at the bottom of the chamber is tilted

and the trip circuit is closed. This trip circuit will operate the circuit breaker and isolates the

transformer.

Advantages of Buchholz relay

Buchholz relay indicates inter turn faults and faults due to heating of core and helps in the

avoidance of severe faults.

Nature and severity of fault can be determined without dismantling the transformer by testing

the air samples.

Limitation of Buchholz relay

It can sense the faults occurring below the oil level only. The relay is slow and has a minimum

operating range of 0.1second and an average operating range of 0.2 seconds.

Cooling of transformer

Cooling of transformers

Cooling of transformer is the process of dissipation of heat developed in the transformer to the

surroundings. The losses occurring in the transformer are converted into heat which increases the

temperature of the windings and the core. In order to dissipate the heat generated cooling should be

done.

How to cool the transformer?

There are two ways of cooling the transformer:

First, the coolant circulating inside the transformer transfers the heat from the windings and the core

entirely to the tank walls and then it is dissipated to the surrounding medium

Second, along with the first technique the heat can also be transferred by coolants inside the

transformer.

The choice of method used depends upon the size, type of applications and the working conditions.

Coolants

The coolants used in the transformer are air and oil. In dry type transformer air coolant is used and

in oil immersed one, oil is user. In the first said, the heat generated is conducted across the core and

windings and is dissipated from the outer surface of the core and windings to the surrounding air. In

the next, heat is transferred to the oil surrounding the core and windings and it is conducted to the

walls of the transformer tank. Finally the heat is transferred to the surround air by radiation and

convection.

Methods of cooling of transformer

Based on the coolant used the cooling methods can be classified into:

1. Air cooling

2. Oil and Air cooling

3. Oil and Water cooling

1. Air cooling (Dry type transformers)

Air Natural(AN)

Air Blast (AB)

2. Oil cooling (Oil immersed transformers)

Oil Natural Air Natural (ONAN)

Oil Natural Air Forced (ONAF)

Oil Forced Air Natural (OFAN)

Oil Forced Air Forced (OFAF)

3. Oil and Water cooling (For capacity more than 30MVA)

Oil Natural Water Forced (ONWF)

Oil Forced Water Forced (OFWF)

1. Air cooling (Dry type transformers)

In this method, the heat generated is conducted across the core and windings and is dissipated from

the outer surface of the core and windings to the surrounding air.

• Air Natural(AN)

This method uses the ambient air as the cooling medium. The natural circulation of the air is used for

dissipation of heat generated by natural convection. The core and the windings are protected from

mechanical damage by providing a metal enclosure. This method is suitable for transformers of

rating up to 1.5MVA. This method is adopted in the places where fire is a great hazard.

AIR BLAST

• Air Blast (AB)

In this method, the transformer is cooled by circulating continuous blast of cool air through the core

and the windings. For this external fans are used. The air supply must be filtered to prevent

accumulation of dust particles in the ventilating ducts.

2. Oil cooling (Oil immersed transformers)

In this method, heat is transferred to the oil surrounding the core and windings and it is conducted to

the walls of the transformer tank. Finally the heat is transferred to the surround air by radiation and

convection.

Oil coolant has two distinct advantages over the air coolants.

It provides better conduction than the air

High coefficient of conduction which results in the natural circulation of the oil.

ONAN

• Oil Natural Air natural (ONAN)

The transformer is immersed in oil and the heat generated in the cores and the windings is passed

on to oil by conduction. Oil in contact with the surface of windings and core gets heated up and

moves towards the top and is replaced by the cool oil from the bottom. The heated oil transfers its

heat to the transformer tank through convection and which in turn transfers the heat to the

surrounding air by convection and radiation.

This method can be used for the transformers having the ratings up to 30MVA. The rate of heat

dissipation can be increased by providing fins, tubes and radiator tanks. Here the oil takes the heat

from inside the transformer and the surrounding air takes away the heat from the tank. Hence it can

also be called as Oil Natural Air natural (ONAN) method.

ONAF

• Oil Natural Air Forced (ONAF)

In this method, the heated oil transfers its heat to the transformer tank. The tank is made hollow and

air is blown to cool the transformer. This increases the cooling of transformer tank to five to six time

its natural means. Normally this method is adopted by externally connecting elliptical tubes or

radiator separated from the transformer tank and cooling it by air blast produced by fans. These fans

are provided with automatic switching. When the temperature goes beyond the predetermined value

the fans will be automatically switched on.

• Oil Forced Air Natural (OFAN)

In this method, copper cooling coils are mounted above the transformer core. The copper coils will

be fully immersed in the oil. Along with the oil natural cooling the heat from the core passes to the

copper coils and the circulating water inside the copper coil takes away the heat. The disadvantage

in this method is that since water enters inside the transformer any kind of leakage will contaminate

the transformer oil.

OFAF

• Oil Forced Air Forced (OFAF)

In this method the oil is cooled in the cooling plant using air blast produced by the fans. These fans

need not be used all the time. During low loads fans are turned off. Hence the system will be similar

to that of Oil Natural Air natural (ONAN). At higher loads the pumps and fans are switched on and

the system changes to Oil Forced Air Forced (OFAF). Automated switching methods are used for

this conversion such that as soon as the temperature reaches a certain level the fans are

automatically switched on by the sensing elements. This method increases the system efficiency.

This is a flexible method of cooling in which up to 50% of rating ONAN can be used and OFAF can

be used for higher loads. This method is used in transformers having ratings above 30MVA.

3. Oil and Water cooling

In this method along with oil cooling, water is circulated through copper tubes which enhance the

cooling of transformer. This method is normally adopted in transformers with capacities in the order

of several MVA.

OFWF

Oil Forced Water Forced (OFWF)

In this method, copper cooling coils are mounted above the transformer core. The copper coils will

be fully immersed in the oil. Along with the oil natural cooling the heat from the core passes to the

copper coils and the circulating water inside the copper coil takes away the heat. The disadvantage

in this method is that since water enters inside the transformer any kind of leakage will contaminate

the transformer oil. Since heat passes three times as rapidly from copper cooling tube to water as

from oil to copper tubes, the tubes are provided with fans to increase the conduction of heat from oil

to tubes. The water inlet and outlet pipes are lagged in order to prevent the moisture in the ambient

air fro condensing on the pipes and getting into the oil.

• Oil Forced Water Forced (OFWF)

In this method hot oil is passed though a water heat exchanger. The pressure of the oil is kept higher

than that of the water therefore there will be leakage from oil to the water alone and the vise versa is

avoided. This method of cooling is employed in the cooling of transformers with very larger capacity

in the order of hundreds of MVA. This method is suitable for banks of transformers. Maximum of

three transformers can be connected in a single pump circuit. Advantages of this method over

ONWF are that the transformer size is smaller and the water does not enter into the transformer.

This method is widely used for the transformers designed for hydro electric plants.

Silica gel bearther

SILICA GEL BREATHER CONSTRUCTION & FUNCTION

Silica Gel Breather of Transformer

Whenever electrical power transformer is loaded, the temperature of the transformer insulating oil increases, consequently the volume of the oil is increased. As the volume of the oil is increased, the air above the oil level in conservator will come out. Again at low oil temperature; the volume of the oil is decreased, which causes the volume of the oil to be decreased which again causes air to enter into conservator tank. The natural air always consists of more or less moisture in it and this moisture can be mixed up with oil if it is allowed to enter into the transformer. The air moisture should be resisted during entering of the air into the transformer, because moisture is very harmful for transformer insulation. A silica gel breather is the most commonly used way of filtering air from moisture. Silica gel breather for transformer is connected with conservator tank by means of breathing pipe.

Construction of Silica Gel Breather

The silica gel breather of transformer is very simple in the aspect of design. It is nothing but a pot of silica gel through which, air passes during breathing of transformer. The silica gel is a very good absorber of moisture. Freshly regenerated gel is very efficient, it may dry down air to a dew point of below − 40°C. A well maintained silica gel breather will generally operate with a dew point of − 35°C as long as a large enough quantity of gel has been used. The picture shows a silica gel breather of transformer.

Working Principle of Silica Gel Breather

Silica gel crystal has tremendous capacity of absorbing moisture. When air passes through these crystals in the breather; the moisture of the air is absorbed by them. Therefore, the air reaches to the conservator is quite dry, the dust particles in the air get trapped by the oil in the oil seal cup. The oil in the oil sealing cup acts as barrier between silica gel crystal and air when there is no flow of air through silica gel breather. The color of silica gel crystal is dark blue but, when it absorbs moisture; it becomes pink. When there is sufficient difference between the air inside the conservator and the outside air, the oil level in two components of the oil seal changes until the lower oil level just reaches the rim of the inverted cup, the air then moves from high pressure compartment to the low pressure compartment of the oil seal . Both of these happen when the oil acts as core filter and removes the dust from the outside air

Magnetic oil gauage

This device is used to indicate the position of transformer insulating oillevel in conservator of

transformer. This is a mechanical device. Magnetic oil level indicator of transformer consists of

mainly three parts-

1. One float,

2. Bevel gear arrangement and

3. An indicating dial.

Working Principle of Magnetic Oil Gauge or MOG

All oil immersed distribution and electrical power transformers are provided with expansion vessel

which is known as c onservator of transformer . This vessel takes care of oil expansion due to

temperature rise. When transformer insulating oil is expanded, the oil level in the conservator tank

goes up. Again when oil volume is reduced due to fall in oil temperature, the oil level in the

conservator goes down. But it is essential to maintain a minimum oil level in the conservator tank of

transforer even at lowest possible temperature. All large electrical power transformers are therefore

provided with a magnetic oil level indicator or magnetic oil gauge. In conventional conservator

tank, a light weight hollow ball or drum floats on the transformer insulating oil. The float arm is

attached with bevel gear as we already explained during the discussion on the construction of

magnetic oil gauge. Naturally the position of the float goes up and down depending upon the oil level

in the conservator and consequently the alignment of float arm changes. Consequently, the bevel

gear rotates. This movement of bevel gear is transmitted to the pointer outside the conservator, as

this pointer is magnetically coupled with the bevel gear. The pointer of magnetic oil level indicator is

also incorporated with a mercury switch. So it is need not say, when oil level in the conservator goes

up and down, the pointer moves on the MOG dial to indicate the actual level of transformer

insulating oil in conservator tank. As the alignment of mercury switch changes along with the pointer,

this switch closes and actuates an audible alarm when pointer reaches near empty position on the

dial of magnetic oil gauge. This event alerts us for topping up oil in electrical power transformer

Temperature Indicator of Transformer

These are generally precision instruments. A temperature indicator ofpower transformer is specially designed for protection of transformer in addition to its temperature indication and cooling control features. That means, this device performs three functions1) These instruments indicate instantaneous temperature of oil and windings of transformer.2) These also record maximum temperature rise of oil and windings.3) These instruments operate high temperature alarm at a predetermined value of allowable temperature limit.4) Temperature indicators of transformer can also trip the circuit breakers associated with the power transformer when the temperature of oil or winding reaches a predetermined limit.5) These devices also control the cooling system of transformer. Switch on the cooling equipment when the winding attains a preset high temperature and switch it off when the temperature drops by an established differential.

Types of Temperature Indicator of Transformer1) Oil temperature indicator (OTI)

2) Winding temperature indicator (WTI)

3) Remote temperature indicator (RTI)

On Load tap changerPower transformers equipped with on-load tapchangers (OLTCs) have been the main components of electrical networks and industrial applications for nearly 90 years. OLTCs enable voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. From the start of tap-changer development, two switching principles have been used for load transfer operation – the high-speed resistor-type OLTCs and the reactor-type OLTCs. Over the decades both principles have been developed into reliable transformer components which are available in a broad range of current and voltage applications. These components cover the needs of today’s network and industrial process transformers and ensure optimal system and process control [1]. The majority of resistor-type OLTCs are installed inside the transformer tank (in-tank OLTCs) whereas the reactor-type OLTCs are in a separate compartment which is normally welded to the transformer tank (fig. 1). This paper mainly refers to OLTCs immersed in transformer mineral oil. The use of other insulating fluids or gas insulation requires the approval of the OLTC manufacturer and may lead to a different OLTC design,

Compartment type In-tank type