The transformer is very simple in construction and consists of a magnetic circuit linking with two windings known as primary and secondary windings.
Besides magnetic circuit and windings, it consists of a suitable container for the assembled core and windings, such as a tank, a suitable medium for insulating the core and windings from its container such as transformer oil, suitable bushings (either oà porcelain, oil-filled or condenser type) for insulating and bringing out terminals of the windings from the container, temperature gauge for measurement of the temperature of hot oil or hottest spot temperature, and oil gauge to indicate the oil level inside the tank.
Some transformers are also provided with a conservator tank in order to slow down the deterioration of oil and keep the main tank full of oil, an emergency vent to relieve the pressure inside the tank in case the pressure inside the transformer rises to a danger point and gas operated relay (Buchholz relay) in order to give alarm to indicate the presence of gas in case of some minor fault and take out the transformer out of circuit in case of serious fault.
List Of Transformer Various Parts
1. Core Construction
2. Windings
- 1. Cylindrical Windings
- 2. Helical Winding
- 3. Cross-Over windings
- 4. Continuous disc winding
- 5. Sandwich Windings
3. Insulation
4. Insulating Oil
5. Main Tank
6. Conservator or Expansion tank
7. Elbow Relief device- Presure Relief Valve
8. Temperature Gauge
9. Oil Gauge
10. Breather
11. Gas operated relay- Buchholz relay
12. Leads and terminals
13. Bushings
14. Tappings
1. Core Construction:
Since core is magnetic link between the two systems connected to the transformer and it itself contains a lot of energy, therefore, it is not by any means the passive component as it appears.
A transformer core is the steel system which forms the magnetic circuit with all parts pertaining to its construction.
Those parts of the magnetic circuit, which carry the transformer windings are called the limbs or legs, and those parts which connect the legs and serve for closing the magnetic circuit are termed yokes.
The core material and its construction should be such that the maximum flux is created with
minimum magnetizing current and minimun core loss.
The use of steel in a magnetic circuit introduces iron or core loss but ensures a high permeability of the magnetic circuit. Because of high permeability the magnitude of exciting current necessary to create the required flux in the core is small.
The presence of steel core causes 100 percent of the magnetic flux created by the primary to be linked with the secondary. The magnetic frame (cores and yokes) of the transformer is built up of laminated electrotechnic steel, called the transformer grade steel consisting of 3.5 % silicon.
The higher content of silicon increases the resistivity of the core, thereby reducing the eddy current core loss.
High content silicon steel has a high permeability at low flux densities and narrow hysteresis loop,thereby making magnetizing current and hysteresis core loss small.
The silicon content affects the mechanical properties also, e.g. it increases tensile strength and impairs ductility.
The steel gets brittle, if the silicon content is increased beyond 5 %. The steel used for transformer cores may be hot rolled or cold rolled.
The hot rolled steel which permitted a maximum flux density of 1.45 T was in use for a considerable length of time. In recent years this type of steel has been completely superseded by cold rolled steel allowing much higher flux densities upto 1.8 T to be used because it has better magnetic properties in the direction of rolling.
Although, cold-rolled steel is 25–35 % more expensive than hot rolled steel and needs special methods of core assembly, the increase in the value of maximum flux density makes it possible to reduce the amount of core material, As the flux in the cores is pulsating one.
it becomes necessary that the transformer cores are laminated and the laminations should be insulated and made as thin as practicable in order to reduce the eddy current loss to a minimum. However, there is a practical limit beyond which the thickness of laminations cannot be reduced further on account of mechanical considerations.
The thickness of laminations or stampings varies from 0.35 mm to 0.5 mm. The thickness should not be made below 0.3 mm because in that case, the laminations become mechanically weak and tend to buckle.
As a rule, the cores of high-capacity power transformers (for more than 100 kva) are assembled of 0.5 mn steel sheets, since such construction is less labor-consuming than with 0.35 mm sheets.
The laminations are insulated from each other by a very thin coating of varnish or by using 0.03 min thick paper. Paper insulation is much cheaper than varnish, but its heat-resisting and heat-conducting properties and mechanical strength are worse.
Besides, paper insulation occupies too large a percentage of the stack cross-section. For this reason in high capacity power transformers, where these draw-backs are significant, varnish insulation
is used In all types of transformers, the core is constructed of transformer sheet steel laminations assem-
bled to provide a continuous magnetic path with a minimum of air-gap included.
2. Windings:
Transformer windings should comply to a number of requirements, the most important of which are: the winding should be economical both as regards initial cost, with a view to the market availability of copper, and the efficiency of the transformer in service; the heating conditions of the windings should meet standard requirements, since departure from these requirements towards allowing higher temperature will drastically shorten the service life of the transformer:winding should be mechanically stable in respect to the forces appearing when sudden short-circuits of the transformer occur; the winding should have the necessary electrical strength in respect to over voltages.
These requirements are often mutually contradictory. Thus, for example, with a larger current density in the winding less copper is required, but the copper losses increase and, therefore, the efficiency of the transformer decreases.
Larger temperature rises allowed in the winding result in reduction in overall dimensions of the transformer, but shorten its service life, etc. Therefore, the winding design of a modern transformer, especially in high-voltage transformer, is of utmost importance.
In a transformer, the high voltage and low voltage windings are so designed and are so chosen that the axial short-circuit force is reduced to minimum which allows a much simpler coil support.
Transformer windings are made of solid or stranded copper or aluminium strip conductors.
Heavy current capacity needs conductors of large cross-section. To reduce eddy current losses in the conductors, several small wires or parallel straps are preferred to one large strap.
This gives rise to unequal reactance of the components of the conductor which can be eliminated by transposition of conductors.
Instead of placing primary on one limb and secondary on the other limb. it is a usual practice to wind one-half of each winding on each limb.
This ensures tight coupling between the two windings Consequently leakage flux is considerably reduced.
The positioning of the hv and lv windings with respect to the core is also very important from the point of view of insulation requirement.
If the hv winding were placed next to the core, it would be necessary to insulate it from both the core and lv winding and two layers of hv insulation would be necessary. By placing the hv winding outside and around the lv winding only one layer of hv insulation that between the high-and low-voltage windings, is necessary.
In, practically, all transformers the coils are wound on a form, dipped in insulating varnish, baked into a rigid mass, and then are assembled with the core.
It is easiest to build the core with legs of rectangular cross-section, and so it is most economical to wind the coils on a rectangular form.
On small transformers rectangular concentric coils are practical but in a large unit the high repulsion forces generated between the primary and secondary coils under short-circuit conditions tend to "round out" the flat sides of the outer coil; if this happens, the resulting damage to the coil insulation will usually make the transformer unserviceable. Cylindrical concentric coils are the obvious answer to this problem of mechanical strength.
According to their arrangement high and low voltage windings can be either concentric (i.e., the windings, which in each cross-section are circles with a common centre) or sandwich, in which parts of
the hv and lv windings alternate along the height of the leg.
Concentric windings are employed in core type transformers as shown in fig.
whereas sandwiched windings are almost exclusively used in shell-type transformers as
shown in fig.
On account of the easier insulation facilities, the low voltage winding is placed nearer to the core in the case of concentric winding and on the outside positions in the case of sandwiched windings.
The insulation spaces between low and high-voltage coils also serve to facilitate cooling.
The concentric winding used for cover types of transformers can further be classified in many groups, the most important are
1. Cylindrical Windings
2. Helical Winding
3. Cross-Over windings
4. Continuous disc winding
1. Cylindrical Windings :
These windings are layered type and use either rectangular (or strip) or round conductors. If the cross-section of a turn does not exceed 8 to 10 mm2, the cylindrical winding is made multi-layer of round conductors; for a larger turn cross-section, the winding is made of rectangular (or strip) conductor, usually double layer.
Cylindrical winding with strip conductors is shown in fig.
The winding is made up of turns helically wound round the cylinder generatrix with the turns close to each other.
Hence, the height of the winding is equal to the height of the layer. The rectangular conductor can be wound either flatwise or edge wise.
In the former case the larger size of the conductor is arranged in the axial direction, in the latter case-in the radial direction of the winding. In case the turn crosssection exceeds 40-45 mm2 then a turn is made of several single conductors, which are placed close to each other along the height of the layer, so that they all occupy the same position in relation to the leakage field.
For improving the cooling of the winding layers ducts 5 to 8 mm wide are left between them (the greater figure relating to transformers with a higher capacity).
The cylindrical windings using circular conductors are mainly employed for hv windings with voltages 6.6,11 or 33 kv for ratings upto 600–1,000 kva whereas those using round conductors are mainly employed for low voltage windings upto 6.6 kv for ratings upto 600–750 kva.
2. Helical Windings:
Such windings are employed for lv coils of medium and high capacity transformers where the number of coil turns is small but the current is high (as high as 2,000 A).
Low voltage windings of medium and high capacity transformers, therefore, require the use of a conductor consisting of a row of parallel rectangular conductors arranged in one radial direction of the winding flatwise and close to each other.
For more uniform distribution of current between the parallel conductors, they are transposed so that each conductor may take each position shown in fig.
The number of parallel conductors in the turn of a helical winding usually varies from 6 to 20.
Basically, the helical winding is the same as the cylindrical winding except that in such windings between adjacent turns axial spacers are provided for duct formation to improve oil circulation for
better cooling.
The ducts are formed by spacers placed all the way round the periphery of the cylinder at regular intervals.
The continuous helical winding exhibits high axial mechanical strength and, therefore, finds wide application in lv windings of large size power transformers.
Helical windings are employed in power transformers of rating ranging from 150 kva to 30 Mva at voltages from 400 V to 11 kv and sometimes upto 33 kv.
3. Cross-over Windings:
Such windings are suitable for currents not exceeding 20 A and are very largely employed for hv windings of low rating transformers where a number of turns may be large but conductors are of small circular sections with double cotton covering or paper covering.
The whole winding is divided into a number of coils depending upon the voltage rating. Each coil is wound on a former, usually U pieces, with several layers of several turns per layer.
The coil ends, one from inside and one from outside, are joined to other similar coils in series, spaced with blocks of insulating material to allow free circulation of oil.
The axial length of each coil and the separation distance between each coil vary according to the voltage rating.
Cross-over windings are employed in the same range of ratings as cylindrical windings. The cross-over winding has a higher strength than the cylindrical winding under normal operating conditions.
However, this winding has a lower impulse strength than the cylindrical winding and also is more labour-consuming.
4. Continuous Disc Windings:
The disc coils, as their name suggests, consist of a number of flat coils or disc s connected in series or parallel.
The coils are formed with rectangular strips wound spirally from centre outwards in the radial direction, as illustrated in fig.
The conductor used is in such lengths as are sufficient for complete winding or section of winding between tappings.
The conductor can be a single strip or number of strips in parallel, wound on the flat side. This gives a robust construction for each of the discs.
The discs are wound on an insulating cylinder spaced from it by strips along the length of cylinder. The discs are separated from each other with press-board sectors attached to vertical strips.
The vertical and horizontal spacers provide radial and axial ducts for free circulation of oil which
comes in contact with every turn. If a winding turn contains several parallel conductors, transposition of
them is used as in the case of helical winding.
Continuous disc windings are reliable and strong and, therefore, they are widely employed both as
lv and hv windings in large rating transformers.
When the continuous winding is used as a hv winding, tappings are made for the regulation of the voltage ratio in the range of 5% or 2 (2.5%).
5. Sandwich Windings:
Such windings, as already mentioned, are most commonly employed in shell-type transformers and allow easy control over leakage reactance.
Leakage reactance can be reduced by subdividing the low and high-voltage windings into a large number of sections or coils and arranging alternately the hv and lv sections with the lv section nearer to the yoke, as illustrated in fig.
The two low voltage coils at the ends have half the turns of a normal low voltage coil. In order to balance the MMF of the adjacent sections, each normal section, whether hv or lv, carries the same number of ampere-turns.
The larger the number of sections (or coils), the smaller is the leakage reactance. The sectionalizing
of coils not only assists in controlling the leakage reactance but also assists in handling and improving cooling.
In comparison to concentric windings they have several drawbacks:
they are more labour-consuming in manufacture, less stable in respect to short-circuits and are more difficult to insulate from each other and from the yoke.
This is the reason why the core type transformers with concentric windings are more common.
3. Insulation:
No feature in the construction of a transformer is given more attention and care than the insulating materials because the life of the unit depends to a very large extent, upon the quality, durability, and handling of these materials.
All insulating materials, such as paper, pressboard, cloth, mica, asbestos, and impregnating compounds are selected on the basis of high quality and their ability to preserve this quality after many years of normal service.
The insulation used in a transformer may be classified into two major groups, viz, major insulation and
minor insulation.
The insulation provided between windings and earthed parts (insulation provided between lv winding and core and between hv winding and yoke) and the insulation between the windings (insulation between primary and secondary windings) falls into the category of major insulation.
For large core type units with concentric windings, major insulation is simply and effectively secured by employing special insulating cylinders and oil ducts extending the full length of the core.
In the interleaved construction the major insulation between windings usually consists of sheets of pressboard and oil ducts, while between windings and core the above-mentioned cylinders or press board sheets may be employed.
Minor insulation includes the insulation provided between the elements of a given winding such as
conductor insulation, insulation between turns, layers and coils.
The minor insulation between the coils of the same winding usually consists of press board sheets and oil ducts.
For very small units treated cloth or fibre may be used. Since the difference of pressure is small between the adjacent turns, the insulation need not be very thick.
It may consist of one or more wrapping of cotton or treated paper tape around each conductor. For small round conductors, enamel together with cotton may be used.
4. Insulating Oil:
The insulating oil has three functions. It provides additional insulation, protects the insulation from dirt and moisture and it carries away the heat generated in the cores and coils, The heat is produced in the metal of the transformer, passes through the insulation and raises the temperature of the oil, and is then conducted either through the walls of the tank to the surrounding air or to the water through water cooling tubes.
The oil which is extensively used is called transformer oil. It is obtained by fractional distillation of crude petroleum.
Vegetable and animal oils are not used in transformers as they form fatty acids that attack the fibrous insulating materials used Transformer oil has to fulfill certain specifications, including the following:
1. High dielectric strength.
As per ISS, the breakdown strength of new transformer oil when treated must be at least 50 kv.
RMS when measured with the help of two spherical electrodes of 12.5 mm diameter and with a gap spacing of 2.5 mm. However, the breakdown strength is greatly reduced due to the presence of impurities like moisture, gas bubbles, solid particles etc.
2. Low viscosity to provide good heat transfer.
A high viscosity is an obvious disadvantage because of the sluggish flow through the many small orifices in the windings.
3. Purity:
The oil must not contain impurities such as acid, alkali and sulphur or its compounds to prevent corrosion of metal parts and insulation. Sulphur compounds, if present accelerate the formation of sludge.
4. High flash point. The temperature at which oil vapour ignites spontaneously is called the flash point. The flash point of transformer oil should not be less than 135°C.
5. Free from sludging under normal operating conditions. Sludging means the slow formation of semi-solid hydrocarbon owing to heating and oxidation.
The sludge deposits itself on windings, tank walls, and in cooling ducts. Sludges, being a bad conductor of heat, greatly reduces the leat transfer from the windings to the oil and so increases the temperature of windings.
Moreover, sludges block the cooling tubes and cause a further increase in the temperature of the transformer and may make the transformer unserviceable because of overheating.
The chief cause of sludge formation is contact of oxygen in air with oil under hot conditions.
The contact of air with oil is minimized by using a conservator and breather.
Sometimes for preventing sludging certain chemicals called inhibitors are added to the transformer oil.
6. Good resistance to emulsion so that the oil may throw down any moisture entering the apparatus instead of holding it in suspense.
If the oil is found to contain moisture or suspended contaminants it should be filtered and if this treatment is considered to be inadequate, the oil should be replaced by a fresh one.
5. Tank:
Small capacity tanks are fabricated from welded sheet steel, while larger ones are assembled from plain boilerplates or cast-aluminium parts, usually mounted on a shallow fabricated steel base.
The lids of these transformer tanks can be of cast iron, a waterproof gasket being used at the joints.
For cooling purposes, cooling tubes are welded with the tank, but in the case of radiators, separate radiators are individually welded and then bolted onto the transformer tank after-wards.
A tank must be capable to withstand the stresses developed by jacking and lifting and shall be not larger than necessary to accommodate the core, windings and internal connections with appropriate electrical clearance.
6. Conservator or Expansion Tank:
The satisfactory operation of a transformer depends so largely on the condition of the oil and, therefore, devices and methods for keeping the oil clean and dry are of utmost importance.
The oil level of a transformer changes with changes in the temperature of oil which in turn depends upon the load on the transformer.
The oil expands with the increase in load and contracts when the load decreases.
Large power transformers are also liable to over-loads which may overheat the oil and consequently, there is a sludge formation if air is present.
Occasionally large power transformers also suffer short-circuits and temperature rise becomes excessively high. This causes vaporization of a part of the oil.
The oil vapors form an explosive mixture with air that ignites and may cause considerable damage.
For these reasons, it is necessary to prevent the oil from having contact with air as well as moisture For this purpose conservators are employed.
Small transformers are sometimes not provided with a conservator or expansion tank. So the oil level has to be kept some distance below the top cover to provide space for oil expansion under temperature
rise.
When the oil expands, the air is expelled out and the air is drawn inside under contraction of oil.
This process is termed breathing. Thus the oil is in contact with air. Exposure to air accelerates the deterioration of oil due to increased sludge and acid formation through moisture absorption and oxidation.
A conservator is a small auxiliary oil tank (an air tight cylindrical metal drum) that may be mounted above the transformer and connected to the main tank by a pipe. Its function is to keep the main tank of the transformer completely filled with oil in all circumstances despite expansion or contraction of oil with the changes in temperature conservator is always partly filled with oil and absorbs the expansion and contraction of oil and keeps the main tank full of oil.
It also reduces the rate of oxidation of oil, partly because less oil surface is exposed to air and partly because of the reduced temperature of the oil exposed to air.
Thus the sludge formation is considerably reduced and whatever sludge is formed settles to the bottom of the expansion chamber into a sludge pan from where it is periodically withdrawn by
means of a drain tap.
Normally the capacity of the conservator should be approximately 10-12% of the oil volume of the main tank.
The expansion tank is usually installed on the low-voltage side of the transformer tank above the level of the transformer cover on a supporting frame.
The position of the installation is governed by voltage clearance to leads and bushings.
A small pipe connection between the gas space in the expansion
tank and the cover of the transformer tank permit the gas above the oil in the transformer to pass into the expansion tank, so that the transformer main tank will be completely filled with oil.
7. Elbow Relief Device:
An elbow relief device is also provided on transformers equipped with expansion tanks.
This device consists of a large diameter steel pipe on the transformer cover or in the wall near the top.
It is usually at an angle to the vertical but may be vertical. The pipe, which has an elbow at its end is of sufficient length so that the oil can rise in it to the maximum level in the expansion tank without splitting over the elbow portion.
A thin glass relief diaphragm is placed at the top of the device, above the maximum oil level.
A fault in the transformer (say a breakdown or a short-circuit in the transformer winding) may produce considerable pressure inside the tank, and under these circumstances, the diaphragm should burst, thus protecting the tank.
A protective hood extends past the diaphragm and prevents moisture from entering the expansion tank in case the diaphragm is accidentally ruptured.
8. Temperature Gauge:
Every transformer is provided with a temperature gauge to indicate hot oil or hottest spot temperature.
It is a self-contained weatherproof unit made with alarm contacts. It is of dial type operated by Bourdon gauge connected to a thermometer bulb located in the region of hottest oil.
9. Oil Gange:
Every transformer is provided with an oil gauge to indicate the oil level so that it can be read by a person standing on the floor.
The oil gauge may be provided with the alarm contacts which
give an alarm to the switchboard when the oil level has dropped beyond permissible level due to oil leak, damage of cooler and due to any other reason.
10. Breather:
When the transformer becomes warm, the oil and gas expand. The gas at the top of the oil is expelled out.
When the transformer cools, the air is drawn into the transformer. Unless preventive measures are taken, moisture is drawn during this process, called breathing.
This moisture is readily absorbed by the oil, and the dielectric properties f the oil are correspondingly reduced.
The air entering the transformer is made moisture free by letting it pass through an apparatus called the breather.
A breather consists of a small container connected to the vent pipe and contains a dehydrating material like silica gel crystals impregnated with cobalt chloride.
The material is blue when dry and a front of whitish pink when damp.
The color can be observed through a glass window provided in front of the container.
11. Gas Operated Relay-Buchholz Relay:
It is a gas and oil actuated protective device and it is practically universally used on all oil-immersed transformers having rating more than 750 kva.
It is installed in the pipe joining the main tank of the transformer to the conservator and is used to give alarm in case of minor fault and to disconnect the transformer from supply mains in case of a severe
internal fault.
The use of such a relay is possible only with transformers having conservators.
The construction of the Buchholz relay is illustrated in fig. It consists of two elements mounted in a small chamber located in the pipe connection between the conservator and the transformer tank.
When a minor fault occurs, heat is produced due to current leakage, some of the oil in the transformer tank evaporates and some vapour collects in the top of the chamber while passing to the conservator.
When a predetermined amount of vapor accumulates in the top of the chamber, the oil level falls, the mercury type switch attached to the float is tilted and closes the alarm circuit to ring the bell.
A release cock is provided at the top of the chamber so that after the operation the pressure in the chamber can be released and gas emitted to allow the chamber to refill the oil.
When a severe fault occurs large volume of gas is evolved so that the lower element containing a mercury switch mounted on a hinged type flap is tilted and the trip coil is energized.
A test cock is provided at the bottom of the chamber to allow air to be pumped into the chamber for test purposes.
12. Leads and Terminals:
The connections to the windings are copper rods or bars, insulated
wholly or in part, and taken to the bus-bars directly in the case of air-cooled transformers, or to the insulator bushings mounted on the top of the transformer tank in case of oil-cooled transformers.
The shape and size of leads are very important in hv transformers, not due to current-carrying capacity, but due to dielectric stresses and corona, etc., at sharp bends and corners.
13. Bushings:
Transformers are connected to hv lines, and, therefore, care is to be taken to prevent flash-over from the high voltage connections to the earthed tank.
Connections from cables are made in cable boxes, but overhead connections are to be brought through bushings specially designed for different classes of voltages.
The bushing consists of a current-carrying element in the form of a conducting rod, and a porcelain cylinder installed in the hole of the cover of the transformer and employed to isolate the current-carrying
element.
Up to 33 kV, ordinary porcelain insulators can be used. Above this voltage, oil-filled or condenser-type bushings are used.
The oil-filled bushing consists of a hollow porcelain cylinder with a conductor, (usually a hollow tube) through its axis.
The space between the conductor and the inner surface of the porcelain is filled with oil.
Oil is filled into the bushing from the top, where there is a glass cylinder to indicate the oil level and to act as an expansion chamber to accommodate the variation of oil bulk due to variations in
operating temperature.
An hv oil-filled bushing is shown in fig.
The capacitor-type bushing is constructed of thick layers of backelized paper or synthetic resin bonded paper interleaved with thin graded layers of tin foils.
This results into a series of capacitors formed by the conductors and first tin foil layer, the first and second tin foil layers within synthetic
resin bonded paper cylinders in between and so on.
The lengths and thicknesses are so arranged as to give an approximate uniform electric stress distribution throughout the radial depth of the bushing.
For outdoor application, a bushing is covered by a porcelain rain shed, corrugated circumferentially to provide a longer and partially protected leakage path.
14. Tappings:
The transformers are usually provided with few tappings so that output voltage can be varied over a small range for constant input voltage, Most industrial transformers are provided with four tappings on the hv winding, two on each side of the 'normal' voltage, so that the tap changer has five positions.
In special cases, there may be tappings on the lv winding, usually in addition to those provided on hv winding.
Though the tappings can be changed while the transformer is supplying load by making some suitable arrangements but in most of the transformers, the tappings are to be changed after disconnecting them from the supply mains.