Transformer Manufacturing Process training report

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Department of Electrical Engineer A.I.E.T-Jaipur

1 INTRODUCTION ABOUT ATLANTA

1.1 Company Profile And History:-

Incorporated in the year 1983,

―Atlanta Electricals Pvt. Ltd.”

, has consolidated its position in the power generation and transformer industry as a manufacturer of wide range of special application transformers that match national as well as international quality standards. Having ISO-9001-2008 from NABCB, ISO 14001-2004 & OHSAS 18001-2007 certification, the company today is proud of have designed, manufactured and successfully commissioned more than 4500 Transformers. Our clientele include various State Electricity Boards and other domestic as well as foreign Enterprises such as Private Electricity supplying companies, Steel Plants, Hydel Power Projects, Windmill Plants, Textile units, Oil units, etc.

Committed to highest level of services and excellence, the company initially emerged as a partnership firm under the Atlanta Electricals. The capability to develop world emerged as a distribution, and specialty transformer is credited to the creation of a world class infrastructure at Vithal Udyognagar near Anand, one of the developing industrialized cities of India.

The 'all-in-one' facility at Vithal Udyognagar works cover 26000 square meter land and 7000 Square meter built up area with a production capacity of 7000MVA per annum. Here production, fabrication, storage, painting, testing, oil handling, sales and admin functions are undertaken under one roof increasing production efficiency, reducing transport related emissions and most importantly for reducing costs & direct supervision on quality aspects.

We at Atlanta Electricals accentuate conscientious Corporate Citizenship and strongly follow the concept of CSR. Hence, we have tied-up with Government Women ITI, Anand, in form of a public-private partnership agreement.

Driven by passion for excellence and future vision Atlanta have successfully designed, manufactured, type tested and installed 31.5 MVA, 220 kV & 60MVA, 132kV Class Transformers. The factory is equipped for manufacturing 160MVA, 220kV Class Transformers.

This facility is equipped with world class modern equipment and managed by a highly skilled and experienced team of production personnel who consistently ensure that each and every production activity factors in an adhere to the high quality benchmarks established by the organization.

 In order to serve the customers with lower lead times and higher capacity & capability Atlanta has newly established 4000 Sq. meter built up area for manufacturing 160MVA, 220kV Class Transformers

 For expansion of capabilities up to 315MVA, 400kV Class

Transformers an additional land of 18000 Square meters has been purchased

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 The manufacturing facility is equipped with all the modern

machineries that are required for a manufacturing capacity of 7,000 MVA per annum

ATLANTA has been promoted and is managed by four technocrats, having an experience of more than four decades in the field of designing and manufacturing transformers. All the directors are engineers and have a complete understanding of this line of business. Apart from well-experienced directors, there is a team of skilled, dedicated and enthusiastic personnel who are dynamic enough to respond to the various challenges.

As one of India's leading transformer manufacturing companies, and one that is held in high esteem, a great deal of relevance is attached to living up to the image as a value based organization. The company is managed by skilled, dedicated and enthusiastic technocrats with an experience of more than four decades in the field of designing and manufacturing transformers.

Over the years, the company has successfully carved a nice for itself in the industry. Providing conducive environment for the professional and personal growth of employees Atlanta has inculcating a spirit of integrity both vertically and horizontally.

1.2 WORKING ENVIRONMENT IN COMPANY:-

Atlanta Electricals takes responsible interest in the environment impact of its business activities. Its products are designed to contribute to the improvement in power generation and distribution system whilst reducing environmental impact. Continuous improvements in the system have enabled the company to increase production and growth steadily with minimum damage to the surroundings.

The company aims for continuous improvement in the environment, prevention of pollution in compliance to environmental regulations. The company trains and motivates all its employees for implementing the environmental policy and enhances their performance in respective area of operation.

For sticking and adhering to the above point ATLANTA has adopted system and has accredited by ISO-14001: 2004

1.3 Safety In Company:-

Atlanta Electricals is an ethically responsible company operating with transparency. The company is committed to their employees for providing all kinds of services related to their health and safety from all hazardous/ accidental events that may occur at the plant.

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The employees are given training for safe working practices followed by the safety rules.

 Health treatment, which falls under company‘s scope.

 Providing safe and modern tools and equipments.

 By adopting human and earth friendly technology for the

manufacturing of products.

Atlanta has adopted OHSAS system and has been accredited by OHSAS-18001:2007.

1.4 Quality Policy Of The Company:-

 We are committed to manufacture, service & timely supply of Power, Distribution and special type of transformers conforming to the specifications as per the client requirements

 Our supplier chain is well assessed as per our QAP which serves as a solid foundation for us to put up the best quality product in the market

 Close inspection of incoming materials, controlling in-process

parameters, timely assessment, upgradation of supplier chain, thorough inspection and testing of the transformer ensure consistent high quality standards

 We are committed to continuous improvement in performance

through effective implementation of quality management system

1.5 Vision of the Company:-

The expertise of competent personnel and the benefits of a sound infrastructure directly translate into Quality products. Each of our transformers undergo various examinations at different stages of production and are tested for all routine tests conforming to IS-2026 (Equivalent to IEC-60076). Regular Quality Checks ensure that each ATLANTA transformer builds enormous goodwill for the company.

Our transformers up to and including 31.5 MVA 220kV class , 60MVA, 132/66 kV class are successfully type tested at national Accredited laboratories like E.R.D.A, Baroda and C.P.R.I(Bhopal, Bangalore).

To serve our esteemed customers, in still better way, in terms of Early Delivery & so also Higher range of Transformers, ―ATLANTA‖ has now successfully completed EXPANSION PROJECT for 160MVA, 220 kV class

Power Transformers and the production for 220 kV class transformers has

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1.6 Mission of the Company:-

Company mission is to become largest Power, Distribution and Special duty transformers manufacturer in India and global market, known for its quality technology, fully integrated range, innovative directions, ethical behaviours and business results.

Build long lasting customer relationship will make us preferred supplier.

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2 INTRODUCTION OF TRANSFORMERS

2.1 WHAT IS TRANSFORMER?

A transformer is a device that transfers electrical energy from

one circuit to another through inductively coupled conductors—the

transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive.

If a load is connected to the secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. 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 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.

2.2 PRINCIPLE OF TRANSFORMER

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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A current flowing through a coil produces a magnetic field around the coil. The magnetic field strength H, required to produce a magnetic field of flux density B, is proportional to the current flowing in the coil. Figure 1 shown below explains the above principle

Figure 1: Relationship between current, magnetic field strength and flux A transformer is a static piece of apparatus used for transferring power from one circuit to another at a different voltage, but without change in frequency. It can raise or lower the voltage with a corresponding decrease or increase of current. Vp = -Np A Vs = -Ns A =

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When a changing voltage is applied to the primary winding, the back e.m.fs generated by the primary is given by Faraday‘s law,

EMF = Vp = -Np A

--- (1)

A Current in the primary winding produces a magnetic field in the core. The magnetic field is almost totally confined in the iron core and couples around through the secondary coil. The induced voltage in the secondary winding is also given by Faraday‘s law

Vs = -Ns A

--- (2)

The rate of change of flux is the same as that in primary winding. Dividing equation (2) by (1) gives

=

In Figure 2, the primary and secondary coils are shown on separate legs of the magnetic circuit so that we can easily understand how the transformer works. Actually, half of the primary and secondary coils are wound on each of the two legs, with sufficient insulation between the two coils and the core to properly insulate the windings from one another and the core. A transformer wound, such as in Figure 2, will operate at a greatly reduced effectiveness due to the magnetic leakage. Magnetic leakage is the part of the magnetic flux that passes through either one of the coils, but not through both. The larger the distance between the primary and secondary windings, the longer the magnetic circuit and the greater the leakage. The following figure shows actual construction of a single phase transformer.

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Figure 3: Transformer construction

The voltage developed by transformer action is given by

E = 4.44×f×N×Bmax×Acore

Where, E = rated coil voltage (volts), f = operating frequency (hertz), N = number of turns in the winding,

Bmax = maximum flux density in the core (tesla), and Acore = cross-sectional area of the core material in Sq.

meters.

In addition to the voltage equation, a power equation expressing the volt-ampere rating in terms of the other input parameters is also used in transformer design. Specifically, the form of the equation is

VA = 4.44×f×N×Bmax×Acore×J×Acond

Where, N, Bmax, Acore and f are as defined above, J is the current density (A/ sq. mm), and Acond is the coil cross-sectional area (mm2) in the core window; of the conducting material for primary winding. J depends upon heat dissipation and cooling.

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2.3 TYPES OF TRANSFORMER

A. Oil filled Transformers

 Power Transformers

 Mobile Transformers

 Distribution Transformers

 Furnace Transformers

a. Induction Furnace Duty b. Arc Furnace Duty

 Motor Starting Transformers

 Neutral Grounding Transformers

 Rectifier Duty Transformers  Testing Transformer

B. Encapsulated & VPI transformers



Air Cooled Transformers



Cast Resin Transformers



Boosters & Voltage Regulators

C. Reactors



Air Core Reactors

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3 MANUFACTURING PROCESS OF

TRANSFORMER

 Winding Construction

 Core Assembly

 Core & Winding(Coil) Assembly (CCA)

 Tapping &Tap Changer

 Drying Process

 Tank Construction

 Tanking & Final Fitting  Fittings & Accessories

 Painting

 Oil Filling & Filtration

 Testing

3.1 Winding Construction:-

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.

Transformer windings are designed to meet three fundamental requirements, viz. mechanical, thermal and electrical. They are cylindrical in shape and are assembled concentrically. Paper insulated conductors of high conductivity & soft drawn E.C. Grade copper is used which comply with the latest Indian as well as international Standards.

Windings are made with great care by well experienced skilled workers in dust free & temperature controlled environment.

Insulation between layers and turns is based upon the electrical and mechanical strength level. Interlayer cooling ducts (Axial & Radial) are provided to minimize the temperature gradient between windings and oil, and hence the hot spot temperature is kept to a minimum. This also ensures that the rate of insulation deterioration is minimized and high life expectancy is achieved.

Transpositions are made in multiple conductor windings, to ensure uniform current distribution, minimize circulating currents, decrease eddy current loss and improve the lamination factor.

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3.1.1 TYPES OF WINDING:-

1) Helical Winding

2) Layer Winding

3) Disc Winding

4) Interleaved Winding

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Disc Winding Interleaved Winding

Helical/Layer windings are most suitable for low voltage windings of large power transformers to mitigate high current design requirement. For High voltage windings the disc coils with excellent mechanical strength are used to take the stresses due to voltage level. Special interleaved or shielded construction offers most uniform voltage distribution despite system transient. Specialized disc winding and inter-leaved disc windings are used having very high series of capacitance giving a very good impulse voltage performance.

Transpositions are made in multiple conductor windings, to ensure uniform current distribution, minimize circulating currents, decrease eddy current loss and improve the lamination factor.

Transformer windings are made almost exclusively of copper, or to be precise, high-conductivity copper. Copper has made possible much of the electrical industry as we know it today because, in addition to its excellent mechanical properties, it has the highest conductivity of the commercial metals. Its value in transformers is particularly significant because of the benefits which result from the saving of space and the minimising of load losses.

The load loss of a transformer is that proportion of the losses generated by the flow of load current and which varies as the square of the load current.

This falls into three categories:

 Resistive loss within the winding conductors and leads.

 Eddy current loss in the winding conductors.

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Resistive loss can be lessened by reducing the number of winding turns, by increasing the cross-sectional area of the turn conductor, or by a combination of both. Reducing the number of turns requires an increase in 8m, i.e. an increase in the core cross-section, which increases the iron weight and iron loss. So load loss can be traded against iron loss and vice versa. Increased frame size requires reduced winding length to compensate and thus retain the same impedance, although as already explained there will be a reduction in the number of turns (which was the object of the exercise) by way of partial compensation. Reduction of the winding axial length means that the core leg length is reduced, which also offsets the increase in core weight resulting from the increased frame size to some extent. There is thus a band of one or two frame sizes for which loss variation is not too great, so that optimum frame size can be chosen to satisfy other factors, such as ratio of fixed to load losses or transport height.

The paths of eddy currents in winding conductors are complex. The effect of leakage flux within the transformer windings results in the presence of radial and axial flux changes at any given point in space and any moment in time. These induce voltages which cause currents to flow at right angles to the changing fluxes. The magnitude of these currents can be reduced by increasing the resistance of the path through which they flow, and this can be effected by reducing the total cross-sectional area of the winding conductor or by subdividing this conductor into a large number of strands insulated from each other. (In the same way as laminating the core steel reduces eddy current losses in the core.) The former alternative increases the overall winding resistance and thereby the resistive losses. Conversely, if the overall conductor cross-section is increased with the object of reducing resistive losses, one of the results is to increase the eddy current losses. This can only be offset by a reduction in strand cross-section and an increase in the total number of strands. It is costly to wind a large number of conductors in parallel and so a manufacturer will wish to limit the total number of strands in parallel. Also, the extra insulation resulting from the increased number of strands results in a poorer winding space factor.

Compact size is important for any item of electrical plant. In transformer windings this is particularly so. The size of the windings is the determining factor in the size of the transformer. As explained above the windings must have a sufficiently large cross-section to limit the load losses to an acceptable level, not only because of the cost of these losses to the user but also because the heat generated must be removed by the provision of cooling ducts. If the losses are increased more space must be provided for ducts. This leads to yet larger windings and thus a larger core is needed to enclose them. Increasing the size of the core increases the no-load loss but, along with the increase in the size of the windings, also means that a very much larger tank is required which, in turn, results in an increased oil quantity and so the whole process escalates. Conversely, any savings in the size of windings are repaid many times over by reductions in the size of the transformer and resultant further savings elsewhere. As the material which most economically meets the above criteria and which is universally commercially available, high-conductivity copper is the automatic choice for transformer windings.

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Winding Construction

3.2 Core Assembly:-

Core is built with Cold Rolled Grain Oriented Silicon Steel, low loss silicon steel lamination . Bonded core design/ technique is used to eliminate hole punching and to minimize fixed losses and Magnetizing Current. Use of HiB grade & Laser scribed Laminations and Rigid clamps significantly reduce vibrations and noise level. Cooling ducts are provided in large transformers for efficient circulation of oil to keep temperature of core well within limit without affecting the flux distortion and also in the core suitable insulation paper are inserted between some laminations for the purpose of reducing eddy currents and also minimizing magnetic short circuit.

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.

The step lap or Mitred joints at the core corners ensure a stream line magnetic flux path. The core limb are held with resin bonded glass bands to eliminate limb bolts. Yokes are clamped by solid mild steel plates with yoke stud ensuring high rigidity for withstanding mechanical socks during transportation & Short Circuits. The leg core in a which hard wooden bars are inserted, are tighten with synthetic resin impregnated fibre glass tape.

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The transformer core is closed magnetic circuit built up of thin laminations of electrical sheet steel. It is intended to concentrate the main magnetic flux linking with the winding and consists of limbs which carry the windings and yokes which close the magnetic circuit. The core laminations are insulated from one another by a film of heat-resistant coating or varnish, or by a combination of both. There may be forms of magnetic circuit: the shell type and the core type.

A magnetic circuit of the shell types is branched: there are two yokes per limb, which encircle the limbs on both sides. As the magnetic flux leaves a limbs, it branches off into two parts, therefore, in shell-type transformers, the cross-sectional area of the limbs is twice that of the yokes. The limbs and yokes are rectangular in section, which necessitates the use of rectangular disk windings. Because of the insufficient strength of such windings in the event of short circuits, complications in assembly and also somewhat greater mass of the shell-type magnetic circuits as compared with the core types circuits using cylindrical windings, the shell type in the Soviet Union is employed only for single-phase transformers in household appliances and for some special-purpose transformers.

The core-type magnetic circuits of butt-joint or interleaved construction are used in power transformers. In such circuits, two or three vertical circuits are bridged over by two horizontal yokes the top and the bottom one so that a closed magnetic circuit is formed.

The core limbs and yokes are built up of separate laminations of electrical sheet steel 0.35 or 0.5 mm think.

The core is built horizontally by stacking laminations, usually two or three per lay, on a jig or stillage. The lay-down sequence must take account of the need to alternate the lengths of plates to provide the necessary overlaps at the mitred corners as shown in shows a large core being built in the manufacturer‘s works. The clamping frames for top and bottom yokes will be incorporated into the stillage but this must also provide support and rigidity for the limbs until the core has been lifted into the vertical position for the fitting of the windings. Without clamping bolts the limbs have little rigidity until the windings have been fitted so the stillage must incorporate means of providing this. The windings when assembled onto the limbs will not only provide this rigidity, in some designs the hard synthetic resin-bonded paper (s.r.b.p.) tube onto which the inner winding is wound provides the clamping for the leg laminations. With this form of construction the leg is clamped with temporary steel bands which are stripped away progressively as the winding is lowered onto the leg at the assembly stage. Fitting of the windings requires that the top yoke be removed and the question can be asked as to why it is necessary to build it in place initially. The answer is that some manufacturers have tried the process of core building without the top yokes and have found that the disadvantages outweigh the saving in time and cost of assembly. If the finished core is to have the lowest possible loss then the joints between limbs and yokes must be fitted within very close tolerances. Building the core to the accuracy necessary to achieve this without the top yoke in place is very difficult. Once the windings have been fitted the top yoke can be replaced, suitably interlaced into the projecting ends of the leg laminations, followed by the top core frames. Once these have been fitted, together with any tie bars linking top and bottom yokes, axial clamping can be applied to the windings to compress them to their correct length.

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The reason we laminate the iron cores in transformers is because we want to limit what are called eddy currents. Transformers are basically two coils of wire wrapped around a core of iron. They work by induction. Induction occurs when current flows in one conductor (or one set of windings in the transformer) and the magnetic field that forms around that conductor (that set of windings) sweeps the other conductor (the other set of windings) and induces a voltage. In order to increase the effectiveness of the transformer, we need to improve the way the magnetic fields are coupled from one set of windings to the other set. Iron conducts magnetic lines of force well, so we use that to help conduct the magnetic lines of force from coil A to coil B. Problem is, iron is also a conductor, and it's being swept by the magnetic field as well. If we didn't use laminations, the iron core would provide a place for the magnetic lines to produce (induce) current, and that current flowing in the core would heat the core up really fast and waste energy.

Before concluding the description of core construction, mention should be made of the subject of core earthing. Any conducting metal parts of a transformer, unless solidly bonded to earth, will acquire a potential in operation which depends on their location relative to the electric field within which they lie. In theory, the designer could insulate them from earthed metal but, in practice, it is easier and more convenient to bond them to earth. However, in adopting this alternative, there are two important requirements:

1. The bonding must ensure good electrical contact and remain secure throughout the transformer life.

2. No conducting loops must be formed, otherwise circulating currents will result, creating increased losses and/or localised overheating. Metalwork which becomes inadequately bonded, possibly due to shrinkage or vibration, creates arcing which will cause breakdown of insulation and oil and will produce gases which may lead to Buchholz relay operation, where fitted, or cause confusion of routine gas-in-oil monitoring results by masking other more serious internal faults, and can thus be very troublesome in service.

The core and its framework represent the largest bulk of metalwork requiring to be bonded to earth. On large, important transformers, connections to core and frames can be individually brought outside the tank via 3.3 kV bushings and then connected to earth externally. This enables the earth connection to be readily accessed at the time of initial installation on site and during subsequent maintenance without lowering the oil level for removal of inspection covers so that core insulation resistance checks can be carried out.

In order to comply with the above requirement to avoid circulating currents, the core and frames will need to be effectively insulated from the tank and from each other, nevertheless it is necessary for the core to be very positively located within the tank particularly so as to avoid movement and possible damage during transport. It is usual to incorporate location brackets within the base of the tank in order to meet this requirement. Because of the large weight of the core and windings these locating devices and the insulation between them and the core and frames will need to be physically very substantial, although the relevant test voltage may be modest.

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Core Assembly

3.3 Core & Winding(Coil) Assembly (CCA):-

L.V. Windings are normally placed near core over insulating cylinder and oil ducts. HV Windings are assembled co-axially placed with respect to LV. Spacers between coils are 'T' shaped for added firmness. Coils are assembled with best insulating materials and are adequately clamped. SPA methodology is now a day widely adopted to have it's special beneficial characteristics. The winding is rigidly supported by a common spacer ring of densified wood at the top and bottom for precise alignment. Well profiled angled rings are placed between LV & HV windings to reduce voltage stress level. The ends & tapping leads of all windings are connected by special extra flexible, insulated copper cables which are rigidly braced in position.

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Core & Winding(Coil) Assem

bly

3.4 Tapping &Tap Changer:-

Generally Taps are provided on HV Windings for HV Variation or LV Variation as specified by customer. These are brought up to a gang operated switch, suitable for external manual operation and can be locked in any desired position to avoid unauthorized operation.

All the moving contacts are spring loaded to ensure proper pressure and good contacts.

To achieve precise voltage regulation on load tap changer is used instead of OCTC. Usually Higher capacity transformers i.e above 5000 KVA ratings, can be supplied with On Load Tap Changer along with necessary controls to make it suitable for manual, local electrical or remote Electrical operation.

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Tap Changer

3.5 Drying Process:-

The core- coil assembly is placed in Vacuum Auto Enclave to eliminate moisture content which is targeted less than 0.5 % moisture. Drying process is to be carried out @ 90°C & respective vacuum cycle at rated interval to improve Insulation resistance and remove ingress of moisture in insulation material.

Drying of grain involves exposing grain to air with low relative humidity (RH) which will lead to evaporation of the moisture in the grain and then the moisture‘s removal away from the grain. Since drying practices can have a big impact on grain or seed quality, it is important to understand some fundamentals of grain drying.

3.5.1 Moisture removal

In paddy grain, moisture is present at two places: at the surface of the grain, ‗surface moisture‘, and inside the kernel, ‗internal moisture‘. Surface moisture will readily evaporate when grain is exposed to hot air. Internal moisture evaporates much slower because it first has to move from the kernel to the outside surface. As a result, surface moisture and internal moisture evaporate at a different rate. This difference results in a different ‗drying rates‘ for different period of drying. The drying rate is defined as the rate at which grain moisture content declines during the drying process. It

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is normally expressed in percent moisture removed per hour [%/hr]. Typical drying rates of rice dryers are in the 0.5%/hr to 1%/hr range.

A drying curve, as illustrated in the figure below, shows how the grain moisture content (MC) and grain temperature change over time. As can be seen in the chart, the drying rate is not constant but changes over time. The temperature of the grain equally changes over time.

Theoretical drying curves with different drying periods.

3.5.1.1 Drying periods and implications optimal drying

There are three different drying periods which will occur consecutively in time:

 I. Preheating period (drying rate is almost 0): When wet grain is exposed to hot air, initially only a very slight change in MC is observed. This happens because all the heat provided in the drying air is used to heat up the grain to the drying temperature.

 II. Constant-rate period (drying rate is constant in time):

Once the grain is at the drying temperature, water starts to evaporate from the surface of the grain. During this period, all the heat from the drying air is used to evaporate surface moisture and the amount of moisture removed from the grain is constant in time. It is therefore called the constant-rate period. During this period, grain temperature is constant as well.

 III. Falling-rate period (drying rate declines over time):

As time passes, it takes more time for internal moisture to appear at the surface, and evaporation of water is no longer constant in time. As a result, drying rate will decline, and some of the heat from the drying air will heat up the grain. For paddy grain, the falling-rate period typically occurs at around 18% grain moisture content.

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By using the 18% MC and the drying curve characteristics as a guideline, a few recommendations can be made in regard to grain drying procedures. These guidelines can be used regardless whether grain is dried in the sun or by using artificial grain dryers.

3.5.2 Drying rate and temperature

Above 18% MC the grain drying rate can be increased (that is, drying will occur faster) by providing a higher temperature or more drying air without major changes in grain temperature. Below 18%MC increase in drying air temperature will not increase the drying rate but will increase grain temperatures and potentially damage the grain. Therefore, higher drying air temperatures can be used to dry grain quickly down to 18% MC (to remove "surface moisture") but lower temperatures should be used to remove internal moisture from the grain.

For seed purposes, drying air temperatures should never exceed 43ºC, regardless of the MC, to avoid overheating of the grain which kills the germ. Exposing paddy to 60ºC for one hour can reduce the seed germination rate from 95% to 30%. Two hours at 60ºC will reduce the germination rate to 5%.

3.5.3 Uniform drying

During the drying process there is always variability in MC of individual grains. Especially in fixed-bed dryers the grains at the air inlet dry faster than at the air outlet resulting in a moisture gradient in the grain bulk at the end of the drying process. For production of good quality grain or seed, this variability should be kept as low as possible. Frequent stirring in sun drying, grain turning in fixed bed dryers or circulation in re-circulating batch dryers will improve uniformity of drying, minimize the re-wetting of dried grains and thus maintain grain quality.

3.5.4 Tempering

When the drying of grain is temporarily stopped the moisture within the grain equalizes due to diffusion. When drying is restarted, the drying rate becomes higher compared to continuous drying. The process of stopping intermittently is called tempering. In addition during tempering the moisture differences between grains equalize. Tempering therefore also ensures that moisture gradients in the grain bulk that develop during drying in certain dryer types are minimized.

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To maintain grain quality, including a tempering period is recommended to allow for redistribution of internal moisture in the grain. In modern re-circulating grain dryers, grain is not dried continuously but goes through a cycle of drying followed by tempering. This improves drying rates, grain quality and reduces energy costs.

3.6 Tank Construction:-

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. These are some types of transformer tanks.

Protection of active part in transformer is very important. While achieving the optimized size of transformer to suit the site condition for installation. The main role of the tank is to protect the active part and tank is manufacture to have sufficient strengths to withstand internal & external faults that may occur during operation. Tanks are fabricated from low carbon M.S. Sheet of best quality proceed by qualified welders. The tank is designed to withstand vacuum and pressure test as per Indian / International standards. A robust skid under base is provided, and guide bars are located inside the tank to securely fix the core and windings assembly in position, and to prevent any movement during transportation.

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3.7 Tanking & Final Fitting:-

Dried out Core & Coil Assembly is tightened before application of hot oil shrinkaging for 24 hours. The oil shrinkaging process avoids looseness of active parts during it's service at site. At last uniform pressing is done on Core & Coil Assembly. High mechanical rigidity is achieved by hydraulic pressing at circulated force and tightening all pressure screws. Pressed Core & Coil Assembly is put in to the tank with proper locating & locking arrangements which is of prime importance to achieve high resistivity against transient damages, vibrations during service and Forces develop during fault occurrence.

After completion of Core & Coil Assembly insertion in tank, hot, degassed oil is then allowed into the transformer tank under vacuum. This oil is then circulated through the transformer and the oil degassing plant until all gas trapped in the core, windings, and the insulation is removed. This ensures a high degree of stability in the insulation structure and early attainment of its mature condition, which would not otherwise be achieved until the transformer had been in service for some time.

The transformer is made ready for testing after assembly of bushings, conservators, radiators & all other protective devices .All the joints are gasketed to avoid leakage with the help of rubberized cork sheets/Nitrite Rubberized ORing, which can withstand high oil temperature and do not deteriorate nor contaminate oil in its contact.

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3.8 Fitting & Accessories:-

3.8.1 Rating & Terminal Marking Plate

The transformer is supplied with rating and terminal marking plate made out of non-corrosive metal. The plate contains information concerning the rating, voltage ratio, weights, oil quantity, vector group, etc. The plate also includes unit Sr. no. and year of manufacturing.

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3.8.2 Tap Changing Arrangement

3.8.2.1 Off-circuit Switch

The transformer is normally fitted with an off-circuit tap changing switch to obtain required voltage ratio. It can be hand-operated by a switch handle mounted either or tank cover or on the tank side. The locking device is fitted to the handle to lock in any tap position. The switch mechanism is such that it can be locked only when it is located in its proper position and not in any intermediate position.

The transformer must be isolated from all time the live lines, before operating the switch.

Operating the switch when transformer is energized, will damage the switch contacts due to severe arcing between the contacts and many damage transformer winding.

When switch handle is provided on the side wall, it is necessary that switch handle assembly is dismantled before undertaking.

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3.8.2.2 Off circuit ratio changing links

Some times links are provided inside the transformer tank to obtain required voltage ratio. Links are required to be loosened and fixed in new required position as given in R & D plate. Links are accessible from the inspection cover. In case of conservator units, oil level has to be lowered below the inspection cover before unbolting inspection cover.

3.8.2.3 On load tap changer

The on load tap changer is an optional fitting. The on load tap changers are provided with local manual control, local electrical control and remote electrical control. The automatic voltage regulation can also be provided as an optional fittings.

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For further details, please refer to the attached manual of OLTC and schematics.

3.8.3 Earthing terminals

The core laminations assembly is connected to core clamping frame which is in turn connected to the tank. Two earthing terminals are provided on the transformer tank. The earthing terminals should be connected to the earthing.

3.8.4 Lifting Lugs

Two or four lifting lugs of adequate capacity are provided on a tank sides/top cover to lift fully assembled transformer filled with oil.

All lugs are designed for simultaneous use and must be used according. Two or four lifting lugs are provided for undertaking the core and windings of larger capacity transformer.

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3.8.5 Valves

Every transformer is provided with drain cum filter valve at bottom of tank, and filter valve at top of the tank. Valves are fitted with plugs/blanking plates to stop oil coming out.

Mainly two types of valves are provided. 1. Wheel valves.

2. Butterfly valves.

The wheel valves are used either with female screw threads or with flanges. These are of gun metal/cast iron type.

Generally, one isolating valve also known as shut off valves is provided for transformer up to 2000KVA between conservator and buchholz relay.

The butterfly type cast steel valves with the machined flanges are used at points of connection between tank and detachable radiators.

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3.8.6 Bushings

3.8.6.1 Oil Communicating Type

Transformers windings are connected to the external circuit through terminal bushings. The bushings are installed on the cover or on side walls of the transformer tank. The lower end of the bushing protrudes in to the tank and at both their ends are provided with suitable fasteners to connect the line leads in side the transformer and external conductors out side it.

The shape and size of the bushings depends on the voltage class, type of current. Electrical performance of these bushings conforms to I.S 2099 and I.S 7421. dimensional details and associated parts generally conform to I.S 3 up to 36 KV class. Bushings of 1000 volts are of two piece construction with out arcing horns, whereas all other bushings is possible without disturbing the active part. For bushings of two piece construction, tank cover is required to be removed for necessary access to the inner (lower) end of the bushings. These bushings are not detached at the time of transportation.

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3.8.6.2 Condenser Bushings

Generally, condenser bushings are used for 72.5KV and above. These bushings contain their own oil and are sealed to retain the same. Whenever these bushings are mounted on bushings pockets or raised truncated portions, air vent pips are provided for carrying away air or gases from these pockets to Buchholz relay during service typical assembly.

These bushings are detached from the transformers and dispatched separately. They are packed as per manufacturer‘s instructions. The draw through type lead is coiled and kept temporarily below the bushing blanking plate. The equipment required for mounting the bushings are

(1) Rope slings.

(2) Flexible steel wire approx 2mm in diameter, of suitable length.

3.8.7 Cable Boxes

Cable boxes are designed for receiving & protecting cables ends. Insulating paper is most hygroscopic & all paper insulated cable ends must be protected by suitable insulating compound. These cable boxes are provided with brass wiping glands & designed with clearances insides the box suitables for compound filling. The cable box in such case must be filled with compound as marked as indicated in the drawling.

Cable boxes of PVC are XLPE cables are designed with air clearances and hence these boxes are not required to be filled with compound.

Cable boxes of 3.6 KV & above are provided with detachable gland plates. Earthing terminals are also provided on these cable boxes for earthing the amounting of individual cable when cable boxes are provided with disconnecting chambers they permit removal of transformers for servicing without disturbing cable terminations.

3.8.8 Bus-duct\trunkings

Some users prefer connections to load by means of bus-duct. Bus-duct is supplied by some other agency However, we provide suitable flanges/ trunkings around transformer bushings for receiving the bus-duct.

The level of the bus-duct flanges from ground/rail level is indicated in the general Arrangement drawings of the transformer. the complete details of bus-duct flange is furnished by us giving dimensional details for the matching flanges, bolt spacing, bushing terminal details, etc

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3.8.9 Marshalling Box

The transformer is provided with curtain fittings directly mounted on the transformer at various location these fittings are having electrical contacts or terminals which are required to be connected ton the protection schemes to give alarm/annunciation under abnormal condition and if further required to disconnect the transformer from mains. in order to facilitate connection of all such device to the protective scheme, the cable form all such contacts are wired up to a weather-proof terminal box. This box called marshalling box, is also used for housing oil Temperature indicator (OTI) and winding temperature indicator (WTI).

The marshalling box is made of sheet metal and is provided with a glass window for observing OTI & WTI.

It has a hinged door with locking facility to prevent un-authorised access. The capillaries from OTI & WTI come out from to the bottom of the marshalling through suitably recessed gland plate thus preventing ingress of dust.

3.8.10 Magnetic Oil level Gauge (MOG)

This is a dial type oil level indicting device provided on large transformers with conservator at relatively high levels from the ground. In large transformers conventional glass oil level indicators are difficult to observe due to their heights and color change/dust accumulation on the glass. Further, the low oil contacts provided on the magnetic oil level indicator can be used for automatic alarm when the oil level in the

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conservator falls to a low level. This Protection feature and clear visibility justify the cost of MOG on a bigger transformer.

It consists of two compartments

(a) The oil side compartment which is fixed on the opening in the conservator.

(b) The pointer side compartment.

These Compartments are sealed against leakage of oil by a metallic diaphragm. On the oil side compartment, there is a bevel gear wheel and it is positioned near the diaphragm. Movement of the float due to rise and fall of oil level in the conservator results into circular motion of the driving magnet. A follower magnet is positioned in the pointer side compartment near the diaphragm. This magnet has its poles face to face to the poles of driving magnet from the oil side compartment coupling them magnetically. The movement of float is, therefore, transferred through the diaphragm, eliminating direct oil light mechanical coupling.

At the other end of the axis of the driven magnet an indication pointer is fitted. The dial is calibrated to show the oil level in the conservator. The dial and the pointer area housed behind the front glass. The dial has three positions marked. The follower magnet has also a cam fitted on it which operates a mercury switch. When this magnet is at a position corresponding to low oil level the mercury switch closes the Normally open (NO) contacts. These contacts are normally wired to give audible alarm. The contacts are brought to a terminal box at the lower end of the dial, for external connections.

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Magnetic Oil level Gauge

3.8.11 Oil Temperature indicator (O.T.I.)

 Oil Temperature indicator (O.T.I.) is generally provided on all transformers except for very small ratings. The direct reading pointer arrangement in this Instrument greatly facilitates observation of working temp. of oil. It also helps, if need be, in deciding the permissible overloads in accordance with I.S. 6600-1972. Guide for loading of oil immersed transformers.

 A Typical - Oil temperature indicator consists of a

 Bourdon tube with a pointer arrangement mounted in a case

comprising of a reading dial and a glass cover. There is a temperature sensing bulb which communicated to the Bourdon tube through the armoured capillary.

 The oil temperature indicator is provided with two pointers and associated contacts for protection of transformers. Both the pointers are independently adjustable and can be set to desired temperature. Setting of these pointers at required temperatures can be done form outside through the knob by using special keys.  The OTI is generally housed and wire upto terminal strip in the

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observation. The length of capillary does not influence the accuracy of measurement and extra length of capillary tubing must not be cut, as it would break communication between bulb and Bourdon tube.

If the oil temperature increases beyond set limit due to overload or inadvertent closure of radiator valves or insufficient air draft, the indicating pointer touches the present alarm pointer actuates the alarm contacts. The alarm contacts, when duly wired give an alarm. If the alarm is not attended and there is a further increase of temperature, the trip contacts which are wired to the trip circuit will operate and isolate the transformer from mains.

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3.8.12 Winding Temperature Indicator

Generally for transformers of high power rating a Winding Temperature Indicator is also required and provided.

A typical - Winding Temperature Indicating (W.T.I.) arrangement provided in a transformer comprises of the following:

(a) W.T.I. Pot

(b) Image Coil

(c) W.T.I. C.T.

(d) W.T.I.

The W.T.I. Pot is mounted at the top of the transformer tank. Hence the oil in the W.T.I. pot is at a temperature of TOP OIL. The image coil is a heater coil and develops additional heat raising the temperature of the oil inside the heater coil. There is a W.T.I. C.T. mounted on one of the line leads with its secondary feeding the image coil. As the load on the transformer varies, the line current varies, the W.T.I. C.T. secondary current passing through the Image Coil varies, the heat developed by Image coil varies and hence the temperature of the oil inside the Image Coil varies. The bulb of the W.T.I. is immersed in the oil inside the Image Coil and as seen above the temperature of this oil is dependent on Top Oil Temperature and the Load on the transformer.

The W.T.I. Image coil is designed and calibrated to indicate the Winding Hot Spot Temperature (HST) because this is the temperature which decides the life of the transformer. Thus the winding Temperature Indicator (W.T.I.) reads temperature.

Ambient Air Top Oil

Temperature Temp. Rise 1.1 x GRADIENT

Winding Temperature Indicator is also housed in the marshalling box. W.T.I. also has an alarm and trip contacts which are wired up to terminal strip. For fan cooled transformers, the auxiliary contacts of W.T.I. are use for switching ON and OFF the fans.

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Winding Temperature Indicator

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3.9 Painting:-

Metal which has been pre-treated by means of shot blasting to remove rust and welding scale is thoroughly cleaned, and then a coat of epoxy zinc chromate primer paint is immediately applied to all external surfaces. This anticorrosive primer has rust inhibitive properties and excellent chemical resistance. Two coats of Epoxy or PU paint , which is highly resistant to chemicals and oil, are then applied. Inside surface of the tank is applied with HOR paint.

3.10 Oil Filling & Filtration:-

All the Transformers are supplied with first filling of oil conforming to IS 335. Before filling, oil is heated, filtered and vacuum treated in filter machine to remove any foreign particles, moisture and air.

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3.11 Testing:-

Before dispatch, each and every transformer is subjected to all routine tests as specified by IS/IEC/BS/ANSI standards.

3.11.1 TYPES OF TESTING

1. Partial discharge test

2. Noise level measurement

3. Temperature rise test

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3.11.1.1 Partial discharge test

Partial Discharges (PD) in the high voltage insulation are local breakdowns of the insulation which does not result in a complete failure of insulation. Hence, the discharges are called partial. The change in the PD value is measured in unit apparent charge pC.

Current transformers and Potential transformer are very important equipments of any power system for metering and protection. Failure of these equipments will cause

 Short circuit fault in the system,

 Damage to other surrounding equipment / switchgear and

 Cause Non-availability of the system.

3.11.1.1.1 Reasons for PD in Instrument Transformers

The instrument transformers when manufactured in factory, due to its manufacturing process workmanship, some voids are present. These voids over a period of time start increasing in size due to overvoltages in system or ageing. When a voltage is applied to the object the gaseous particles start getting ionizing. At a particular stage the void size increases causing the apparent charge (pC) value to increase and finally cause failure of the instrument transformer. The failure or increase in PD value can also be due to moisture or contamination on the external surface of the equipment which may cause tracking with respect to earth.

3.11.1.1.2 Test Description and Set-up

For this test rated phase to neutral voltage is applied across the object. A coupling capacitor (having low inductance) is connected across the test object which converts the input currents to low output voltage. The output of this coupling capacitor is fed to the PD measuring instrument which gives the PD value in Pico Coulomb and also indicates the discharges on the sinusoidal waveform. The most important requirement for this test is a PD free power source transformer. The circuit for PD testing is shown in figure below

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3.11.1.1.3 Limiting values and Testing schedule

As per Indian Standard (IS) 11322 – the limiting value of PD in Cast resin instrument transformer is < 50 pC

Manufacturers limit this value to less than 20 pC before dispatch from their factory.

PD testing of instrument transformers should be carried out

 Just before commissioning to have the base foot print values at site.

 Once in every 2 years to trend the ageing or increase in the pC value and



Depending upon the pC value measured over a period of

six or twelve months.

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3.11.1.2 Noise level measurement

The test procedure is in accordance with IEC Standard 60076-10. At first, the background noise level shall be measured. Then, the transformer shall be powered at rated voltage and frequency under no-load conditions (with the tap-changer selected on the principal tapping), in order to carry out transformer noise level measurements.

They shall be performed in several points located around the transformer, placed at a distance of 0,3 m from the machine unless, due to safety reasons or following agreement between supplier and purchaser, the distance is increased to 1m. Measuring positions shall be spaced at a distance of at most 1 metre one from another; anyway, a minimum of 6 positions is required. The measurements shall be carried out at half the equipment height, if this does not exceed 2,5m; otherwise, they shall be performed at 1/3 and 2/3 of the component height.

After performing transformer sound level measurement, the machine is de-energised and background noise level is measured again; At the end, the final transformer sound level shall result by applying a correction by taking into account the lower background noise level. In case there is a high difference between the transformer and background noise level (>8 dB) no correction shall be applied.

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3.11.1.3 Temperature rise test

The test is for verifying whether the temperature rise limits of the windings , as agreed at the time of the enquiry, are respected. The test can be carried out by means of two different methods:

 Simulated loading method (in accordance with. IEC 60726 p.21.1.3)  The method used is defined at the time of the enquiry.

The average temperature rise shall be determined by means of the variation of winding resistance. The core temperature rise shall be determined by use of a thermometer. All the carries at rated conditions shall be performed in accordance with IEC 60726.

3.11.1.3.1 Simulated loading method

Temperature rise test is made by utilising the rises obtained on two tests, one with no-load loss only, and one with load-loss only.

The no-load test, at the rated voltage and rated frequency, is continued until steady state conditions are obtained; individual winding temperature rises are then calculated by measurement of hot windings resistance, that shall be carried out as shortly as possible after is connection. The short-circuit run with rated current flowing in one winding and the other winding short-circuited, is started immediately following the no-load run, and continued until steady state conditions are obtained; individual windings temperature rises are then calculated as above mentioned. The total winding temperature rise of each winding, with rated current in the winding and normal exitation of the core, shall be calculated in accordance with IEC Standards 60726.

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3.11.1.4 Sweep Frequency Response Analyzer (SFRA) Test

The SFRA test is non-intrusive (non-destructive) test. SFRA is an OFF line testing and it can be carried out for any voltage rating of Power Transformer, Generator Transformer and Distribution Transformer. The measurement of SFRA can be a part of regular transformer maintenance. The SFR Analyzer identifies the following abnormalities in the transformer before they lead to failure,

a. Core movement

b. Winding deformation and displacement c. Faulty Core ground

d. Partial winding collapse e. Hoop buckling

f. Broken or loosened clamping structures g. Shorted turns and open winding

The Technique of SFRA is a major advance in transformer condition monitoring analysis. This is a proven technique for making accurate and repeatable measurements.

The test can be carried out,

a) First to obtain initial signature (record) of the transformer Sweep frequency response for future reference / comparison. b) Periodical measurement as a maintenance check, once in two

years.

c) Immediately after a major external Short Circuit, specially for faults electrically closer to transformer.

d) Transportation or re-location of transformer. e) Earthquakes.

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4 INSTALLATION & COMMISSIONING

This section illustrated procedure for installation & Commissioning of our transformers.

A format of ―Commissioning Report‖ is included at the end of this section. Results of various pre commissioning tests as well as well as confirmation of check point are to be recorded in this report. This Report then would saver as a handy record for future reference.

4.1 Installation

4.1.1 Location

The transformer should be kept in a well ventilated place, free from excessive dust, corrosive fumes etc. Adequate ventilation is necessary for tank radiators so that they can dissipate heat. There should be clear space of about 1.25 m on all sides of the transformers if it is enclosed in a room.

4.1.2 Foundation

Foundation should be firm, horizontal and dry. Where rollers are fitted, suitable rails should be provided.

4.1.3 Provision for oil draining

Necessary provisions for oil draining, in the event of a fire, should be made by way of oil soak pits. Fire separation walls should also be provided when necessary.

4.1.4 Assembly of Dismantled components

Various components dismantled for transportation should be duly assembled.

4.1.5 Main Tank

Keep the main tank in its permanent position of operation. Lock the rollers to prevent any accidental movement on rails. Draw an oil sample from the botton of the tank and test it for Break-Down-Voltage (BDV).

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Note this value in ―Commissioning Report‖

4.1.6 Bushing

Clean the bushings and check that there are no hair-cracks or other damages. Test IR value of each bushing with a 500V Megger. It should be 100 M ohms or greater. Note details of Bushings in the ―Commissioning Report.‖ Mount all the bushings as described in 2.6. Ensure that the test tap cap is fully tightened, thus positively grounding the same. Adjust the Arcing Horn Gaps in accordance with the Insulation Co-ordination Note these values in the ―Commissioning Report‖.

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4.1.7 Conservator & M.O.G.

Note details of M.O.G. in the ―Commissioning Report‖. If the M.O.G. is provided with a locking lever, it should be removed. Mount the conservator. When there is as OLTC its conservator is some times provided separately or by making a partitioned compartment in the main conservator. OLTC conservator, if separate, should also be mounted.

4.1.8 Buchholz Relay

Note details of ―Buchholz Relay‖ for the transformer and of ―Oil Surge Relay‖ for the OLTC in the ―Commissioning Report.‖ Buchholz Relay floats are tied to prevent transit damage. They should be released. Also if ‗ Test‘ lever is provided, it should be in the working position. Mount the ―Buchholz Relay‖ and the shut off valves as described in 2.10 Similarly mount ―Oil Surge Relay‖.

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4.1.9 Breathers

Note details of Breathers, in the ―Commissioning Report.‖ If OLTC is provided, it may have its own separate breather. Note details of these Breathers also in the ―Commissioning Report‖. Check that the colour of silica Gel in Main Breather is Blue. Remove the rubber cap closing the breather pipe and fit the breather. Fill oil in the oil cup and remove the seal which closes the breather opening. Similarly mount the OLTC breather.