Transformer

Standard Transformer Fittings:

1) Standard Fittings

Rating and terminal marking plate.
Tap Changing arrangement
Off – circuit tap changing switch
Off – circuit tap changing link
On Load tap changer
Two earthing terminals
Lifting Lugs
Drain – cum filter valve
Pressure Relief Device
Silica gel dehydrating breather.
Oil Level Indicator.
Thermometer Pocket.
Conservator with drain plug and filling hole.
Air Release plug.
Jacking lugs (above 1600 KVA)
Filter valve (top tank)
Under base unidirectional flat rollers.

2) Terminal Arrangement:

Bare Bushings or Cable box.
Compound filled for PVC cables (up to 33000 Volts) or Air filled for PVC cable s (Up to 11000 Volts) or
Bus Duct (Bare bushing enclosed in housing up to 600 Volts)
Disconnection chamber between cable box and transformer tank.
Additional bare neutral terminal.

3) Optional Fittings:

These are optional fittings provided at an extra cost, if customer specifically orders them.
Winding temperature indicator
Oil temperature indicator
Gas and oil actuated (Buchholz) relay
Conservator drain valve
Shut off valve between conservator and tank.
Magnetic oil level gauge
Explosion vent
Filter valve (Bottom of tank)
Skid under base with haulage holes
Junction box.

Standard Transformer Accessories:

1) Thermometer Pockets:

This pocket is provided to measure temperature of the top oil in tank with a mercury in glass type thermometer. It is essential to fill the pocket with transformer oil before inserting the thermometer, to have uniform and correct reading. One additional pocket is provided for dial type thermometer (OTI) with contacts

2) Air release plug:

Air release plug is normally provided on the tank cover for transformer with conservator. Space is provided in the plug which allows air to be escaped without removing the plug fully from the seat. Plug should be unscrewed till air comes out from cross hole and as soon as oil flows out it should be closed. Air release plugs are also provided on radiator headers and outdoor bushings.

3) Winding temperature Indicator

The windings temperature indicator indicates ‘’ Hot spot’’ temperature of the winding. This is a ‘’Thermal Image type’’ indicator. This is basically an oil temperature indicator with a heater responsible to raise the temperature equal to the ‘’Hot spot’’ gradient between winding and oil over the oil temperature. Thus, this instrument indicates the ‘’Hot Spot’’ temperature of the windings. Heater coil is fed with a current proportional to the windings current through a current transformer mounted on the winding under measurement. Heater coil is either placed on the heater bulb enveloping the sensing element of the winding temperature indicator immersed in oil or in the instrument. The value of the current fed to the heater is such that it raises the temperature by an amount equal to the hot spot gradient of the winding, as described above. Thus temperature of winding is simulated on the dial of the instrument. Pointer is connected thought a mechanism to indicate the hot spot temperature on dial. WTI is provided with a temperature recording dial main pointer. Maximum pointer and re setting device and two sets of contacts for alarm and trip.

4) Oil Temperature Indicator

Oil temperature indicator provides local temperature of top oil. Instruments are provided with temperature sensing bulb, temperature recording dial with the pointer and maximum reading pointer and resetting device. Electrical contacts are provided to give alarm or trip at a required setting (on capillary tube type thermometer).

5) Conservator Tank:

It is an Expansion Vessel
It maintains oil in the Transformer above a Minimum Level
It has a Magnetic Oil Level Gage.
It can give an alarm if the oil level falls below the limit
A portion of the Tank is separated for use with OLTC.
This usually has oil level indicators
Main Conservator Tank can have a Bellow
It has an oil filling provision
It has an oil drain valve
Provision is there for connecting a Breather

6) Silica Gel Breather:

Prevents Moisture Ingress.
Connected to Conservator Tank
Silica Gel is Blue when Dry; Pink when moist
Oil Seal provides a Trap for Moisture before passing thro Silica Gel

7) Cooling:

ONAN .. Oil Natural Air Natural
ONAF .. Oil Natural Air Forced
OFWF .. Oil Forced Water Forced
ODWF .. Oil directed Water Forced.
By Forced Cooling, the Transformer capacity can be increased by more than 50%

8) Bushing:

Insulators and Bushings are built with the best quality Porcelain shells manufactured by wet process.
For manufacture of electro porcelain, high quality indigenous raw materials viz, China Clay, Ball Clay, Quartz and Feldspar is used Quartz and feldspar are ground to required finesses and then intimately mixed with ball and china clay in high speed blungers. They are then passed through electromagnetic separators, which remove iron and other magnetic impurities. The slip produced is passed to a filter press where extra water is removed under pressure and the resulting clay cakes are aged over a period. The aged cakes are extruded to required form viz., cylinders, on high vacuum de-airing pug mill. The extruded blanks or cylinders are given shapes of Insulators / Bushings which are conditioned and are shaped on copying lathes as the case may be.
Testing, Assembly & packing:
All insulators & bushings undergo routine electrical and mechanical tests. The tests before and after assembly are carried out according to IS Specifications, to ensure their suitability for actual conditions of use. Porosity tests are also carried out regularly on samples from every batch, to ensure that the insulators are completely vitrified. These insulators are then visually checked and sorted, before they are packed in sea worthy packing, to withstand transit conditions.
Types of Insulators & Bushings:
Bushing Insulators: Hollow Porcelain Bushings up to 33 KV
Application : Transformers, Capacitors, Circuit Breakers
Solid Core Insulators:
Line Post
Long Rod
Support
Special Type Insulators
C.T. up to 66 KV
P.T. up to 33 KV
Weather Casing
L.T. Insulators
Shackel Type
Spool Type
Pin Type
Guy strain
H.V. Bushings (IS:3347)

Pin Insulators: Up to 33 KV
Post type Insulators: Post type insulators, complete with metal fittings, generally IS Specifications and other International Standards up to 33 KV

12 to17.5 KV / 250 amps	24 KV / 1000 amps
12 to 17.5 KV / 630 amps	24 KV / 2000 to 3150 amps
12 to 17.5 KV / 1000 amps	36 KV / 250 amps
12 to 17.5 KV / 2000 to 3150 amps	36 KV / 630 amps
24 KV / 250 amps	36 KV / 1000 amps
24 KV / 630 amps	36 KV / 2000 to 3150 amps

L.V. Bushings (IS:3347)

11 KV / 250 amps	1 KV / 2000 amps
1 KV / 630 amps	1 KV / 3150 amps
1 KV / 1000 amps

H.V. Bushings (IS:8603)

12 KV / 250 amps	36 KV / 250 amps
12 / 630 amps	36 KV / 630 amps
12 KV / 1000 amps	6 KV / 1000 amps
12 KV / 2000 to 3150 amps	36 KV 3150 amps

C.T. Bushings (IS:5612)

11 KV	1 KV / 2000 amps
1 KV / 630 amps	1 KV / 3150 amps
1 KV / 1000 amps

Epoxy Bushing:
All Epoxy Resin Cast Components are made from hot setting reins cured with anhydrides; hence these provide class-F Insulation to the system. In an oxidizing atmosphere, certain amine cured Epoxy Resins can start to degrade at 150ºC whereas the anhydride cured systems are stable at 200ºC therefore our epoxy components are cured with anhydrides which gives them a longer life.

9) Buchholz Relay:

The purpose of such devices is to disconnect faulty apparatus before large scale damage caused by a fault to the apparatus or to other connected apparatus. Such devices generally respond to a change in the current or pressure arising from the faults and are used for either signaling or tripping the circuits.
Considering liquid immersed transformer, a near ideal protective device is available in the form of gas and oil operated relay described here. The relay operates on the well known fact that almost every type of electric fault in a liquid immersed transformer gives rise to a gas. This gas is collected in the body of the relay and is used in some way or the other to cause the alarm or the tripping circuit to operate.
In the event of fault in an oil filled transformer gas is generated, due to which buchholz relay gives warning of developing fault. Buchholz relay is provided with two elements one for minor faults (gives alarm) and other for major faults (tripping). The alarm elements operate after a specific volume gets accumulated in the relay. Examples of incipient faults which will generate gas in oil are:- Buchholz Relay
i) Failure of core bolt insulation.
ii) Shorting of lamination and core clamp.
iii) Bad Electrical contact or connections.
iv) Excessive hot spots in winding.
The alarm element will also operated in the event of oil leakage. The trip element operates due to sudden oil surge in the event of more serious fault such as: –
i) Earth fault due to insulation failure from winding to earth.
ii) Winding short circuit inter turn, interlayer, inter coil etc.
iii) Short circuit between phases.
iv) Puncture of bushing.
The trip element will also operate if rapid loss of oil occurs. During the operation of transformer, if there is an alarm transformer should be isolated from lines and possible reasons, listed above for the operation of relay should be checked starting with simple reason such as loss of oil due to leaks, air accumulation in relay chamber which may be the absorbed air released by oil due to change in temperature etc. Rating of contacts: – 0.5 Amps. At 230 Volts AC or 220 Volts. DC.

Pre commissioning Inspection of Transformer:

Sample of oil taken from the transformer and subjected to electric test (break down value) of 50KV (RMS) as specified in IS : 335.
Release trapped air through air release plugs and valve fitted for the purpose on various fittings like radiators, bushing caps, tank cover, Bushing turrets etc.
The float lever of the magnetic oil level indicator (if provided) should be moved up and down between the end position to check that the mechanism does not stick at any point. If the indicator has signaling contact they should be checked at the same time for correct operation. Checking the gauge by draining oil is a more positive test.
Check whether gas operated really (if provided) is mounted at angle by placing a spirit level on the top of the relay. See that the conservator is filled upto the filling oil level marked on plain oil gauge side and corresponding to the pointer reading in MOG side. Check the operation of the alarm and trip contacts of the relay independently by injecting air through the top cocks using a dry air bottle. The air should be released after the tests. Make sure that transformer oil runs through pert cock of Buchholz relay.
Check alarm and trip contacts of WTIs, Dial type thermometer, magnetic oil gauge etc. (if provided).
Ensure that off circuit switch handle is locked at the desired tap position with padlock.
Make sure that all valves except drain, filter and sampling valves are opened (such as radiator valves, valves on the buchholz relay pipe line if Provided).
Check the condition of silicagel in the breather to ensure that silicagel in the breather is active and colour is blue. Also check that the transformer oil is filled in the silicagel breather upto the level indicated.
Check tightness of external electrical connections to bushings.
Give a physical check on all bushing for any crack or any breakage of porcelain. Bushing with cracks or any other defects should be immediately replaced.
Check the neutral earthing if specified.
Make sure that neutrals of HV / LV are effectively earthed.
Tank should be effectively earthed at two points.
Check that the thermometer pockets on tank cover are filled with oil.
If the oil temperature indicator is not working satisfactorily, loosen and remove the thermometer bulb from the pocket on the cover and place it with a standard thermometer in a suitable vessel filled with transformer oil. Warm the oil slowly while string it and take reading of the thermometers if an adjustment of the transformer thermometer is necessary the same many be done. Also check signaling contacts and set for the desired temperature.
CT secondary terminals must be shorted and earthed if not in use.
Check relief vent diaphragm for breakage. See that the Bakelite diaphragm at bottom and glass diaphragm at top are not ruptured.
Check all the gasket joints to ensure that there is no leakage of transformer oil at any point.
Clear off extraneous material like tools earthling rods, pieces of clothes, waste etc.
Lock the rollers for accidental movement on rails.
Touching of paint may be done after erection.

Parts of Transformer:

1) Transformer Oil

Oil is used as coolant and dielectric in the transformer and keeping it in good condition will assist in preventing deterioration of the insulation, which is immersed in oil. Transformer oil is always exposed to the air to some extent therefore in the course of time it may oxidize and form sludge if the breather is defective, oil may also absorb moisture from air thus reducing dielectric strength.

2) Transformer Winding:

The primary and secondary windings in a core type transformer are of the concentric type only, while in case of shell type transformer these could be of sand-witched type as well. The concentric windings are normally constructed in any of the following types depending on the size and application of the transformer.
(1)Cross over Type.

(2) Helical Type.
(3) Continuous Disc Type.
Distributed.
Spiral.
Interleaved Disc.
Shielded Layer

a) Distributed Winding :

Used for HV windings of small Distribution Transformers where the current does not exceed 20 amps using circular cross section conductor .

b) Spiral:

Used up to 33 kv for low currents using strip conductor. Wound closely on Bakelite or press board cylinders generally without cooling ducts. However, multi layer windings are provided with cooling ducts between layers. No Transposition is necessary.

c) Interleaved Disc:

Used for voltages above 145 kv . Interleaving enables the winding withstand higher impulse voltages.

d) Shielded Layer :

Used up to 132 kv in star connected windings with graded insulation. Comprises of a number of concentric spiral coils arranged in layers grading the layers.
The longest at the Neutral and the shortest at the Line Terminal. The layers are separated by cooling ducts. This type of construction ensures uniform distributed voltages.

e) Cross-over type winding:

It is normally employed where rated currents are up-to about 20 Amperes or so.
In this type of winding, each coil consists of number of layers having number of turns per layer. The conductor being a round wire or strip insulated with a paper covering.

It is normal practice to provide one or two extra lavers of paper insulation between lavers. Further, the insulation between lavers is wrapped round the end turns of the lavers there by assisting to keep the whole coil compact.
The inside end of a coil is connected to the outside end of adjacent coil. Insulation blocks are provided between adjacent coils to ensure free circulation of oil.

f) Helical winding:

Used for Low Voltage and high currents .The turns comprising of a number of conductors are wound axially. Could be single, double or multi layer winding. Since each conductor is not of the same length, does not embrace the same flux and of different impedances, and hence circulating currents, the winding is Transposed.
The coil consists of a number of rectangular strips wound in parallel racially such that each separate turn occupies the total radial depth of the winding.
Each turn is wound on a number of key spacers which form the vertical oil duct and each turn or group of turns is spaced by radial keys sectors.
This ensures free circulation of oil in horizontal and vertical direction.
This type of coil construction is normally adopted for low voltage windings where the magnitude of current is comparatively large.
Helical Disc winding:
This type of winding is also termed “interleaved disk winding.”
Since conductors 1 – 4 and conductors 9 – 12 assume a shape similar to a wound capacitor, it is known that these conductors have very large capacitance. This capacitance acts as series capacitance of the winding to highly improve the voltage distribution for surge.
Unlike cylindrical windings, Helical disk winding requires no shield on the winding outermost side, resulting in smaller coil outside diameter and thus reducing Transformer dimension. Comparatively small in winding width and large in space between windings, the construction of this type of winding is appropriate for the winding, which faces to an inner winding of relatively high voltage.
Thus, general EHV or UHV substation Transformers employ Helical disk winding to utilize its features mentioned above.

g) Continuous disc type of windings:

Used for 33kv and 132 kv for medium currents. The coil comprises of a number of sections axially. Cooling ducts are provided between each section.
IT is consists of number of Discs wound from a single wire or number of strips in parallel. Each disc consists of number of turns, wound radically, over one another.
Arrangement of layers
The conductor passing uninterruptedly from one disc to another. With ultiple-strip conductor. Transpositions are made at regular intervals to ensure uniform resistance and length of conductor. The discs are wound on an insulating cylinder spaced from it by strips running the whole length of the cylinder and separated from one another by hard pressboard sectors keyed to the vertical strips.
This ensures free circulation of oil in horizontal and vertical direction and provides efficient heat dissipation from windings to the oil.
The whole coil structure is mechanically sound and capable of resisting the most enormous short circuit forces.
This is the most general type applicable to windings of a wide range of voltage and current
Rectangular wire is used where current is relatively small, while transposed cable Fig. (12) is applied to large current. When voltage is relatively low, a Transformer of 100MVA or more capacity handles a large current exceeding 1000A. In this case, the advantage of transposed cable may be fully utilized
Since the number of turns is reduced, even conventional continuous disk construction is satisfactory in voltage distribution, thereby ensuring adequate dielectric characteristics. Also, whenever necessary, potential distribution is improved by inserting a shield between turns.
According to the number of layers used the paper is applied as follows.
Two layers: =Where there are two layers both of them are wound in opposite directions.
More than two layers: =Where there are more than two layers all the layers are applied in the same direction, all, except the outermost layer is butt wound, and the outermost layer is overlap wound. Within each group of papers the position of the butt joints of any layer relative to the layer below is progressively displaced by approximately 30 percent of the paper width.
Note: Overlapping can also be done as per customer requirements.
Grade of paper
The paper, before application, is ensured to be free from metallic and other injurious inclusions and have no deleterious effect on insulating oil.
The thickness of paper used is between 0.025 mm to 0.075 mm.
Enameled Conductor
Apart from paper covered conductors, we have all the facilities of producing enameled conductors as per customer specified requirements.
Copper – Usually in 8 – 16mm rods is drawn to the required sizes and then insulated with paper etc..
Annealing is done for softening and stress relieving in electrically heated annealing plant under vacuum upto 400-500ºC. After 48hrs when the temperature reaches ambient, the vacuum is slowly released and the material is transferred to Insulation section.
Conductors are one of the principal materials used in manufacturing of transformers. Best quality of copper rods are procured from indigenous as well as foreign sources. Normally 8 mm & 11 mm rods are procured. For each supply of input, test certificate from suppliers is obtained and at times.
After the wires & strips are drawn as per clients requirements they are moved on to paper covering process.
To prevent the inclusion of copper dust or other extraneous matter under paper covering the conductor is fully cleaned by felt pads or other suitable means before entering the paper covering machine. As per the customers requirements DPC, TPC & MPC conductors are produced. It is ensured that each layer of paper is continuous, firmly applied and substantially free from creases.
No bonding or adhesive material is used except to anchor the ends of paper. Any such bonding materials used to anchor the ends do not have deleterious effect on transformer oil, insulating paper or the electric strength of the covering. It is ensured that the overlapping percentage is not less than 25% of the paper width.
The rectangular paper-covered copper conductor is the most commonly used conductor for the windings of medium and large power transformers.
These conductors can be individual strip conductors, bunched conductors or continuously transposed cable (CTC) conductors.
In low voltage side of a distribution transformer, where much fewer turns are involved, the use of copper or aluminum foils may find preference.
To enhance the short circuit withstand capability, the work hardened copper is commonly used instead of soft annealed copper, particularly for higher rating transformers
In the case of a generator transformer having high current rating, the CTC conductor is mostly used which gives better space factor and reduced eddy losses in windings. When the CTC conductor is used in transformers, it is usually of epoxy bonded type to enhance its short circuit strength.

3) Transformer Core:

Purpose of the core:
To reduce the magnetizing current. (For topologies such as Forward, Bridge etc we need the magnetizing current to be as small as possible. For fly-back topology, though the magnetizing current is used to transfer energy, the size of the transformer will be very large to get the required inductance if a core is not used.)
To improve the linkage of the flux within windings if the windings are separated spatially.
To contain the magnetic flux within a given volume
In magnetic amplifier applications a saturable core is used as a switch.
Core Material:
Different types of material used for cores
Iron-Silicon Steel- Nickel-Iron-Iron-Cobalt-Ferrite-Molybdenum-Met-glass
Salient characteristics of a core material:
Permeability, Saturation flux density, Coercive force, Remnant flux, Losses due to Hysteresis & Eddy Current.
The power loss is a function of frequency and the ac flux swing and is given by the equation P = K1 * (frequency)K2 * (Flux Density)K3
Every transformer has a core, which is surrounded by windings. The core is made out of special cold rolled grain oriented silicon sheet steel laminations. The special silicon steel ensures low hysteretisis losses. The silicon steel laminations also ensure high resistively of core material which result in low eddy currents. In order to reduce eddy current losses, the laminations are kept as thin as possible. The thickness of the laminations is usually around 0.27 to 0.35 mm.
Transformer cores construction is of two types, viz, core type and shell type. In core type transformers, the windings are wound around the core, while in shell type transformers, the core is constructed around the windings. The shell type transformers provide a low reactance path for the magnetic flux, while the core type transformer has a high leakage flux and hence higher reactance.
The limb laminations in small transformers are held together by stout webbing tape or by suitably spaced glass fiber bends. The use of insulated bolts passing through the limb laminations has been discontinued due to number of instances of core bolt failures. The top and bottom mitered yokes are interleaved with the limbs and are clamped by steel sections held together by insulated yoke bolts. The steel frames clamping the top and bottom yokes are held together by vertical tie bolts.
Grain Oriented steel sheets namely ORIENTCORE, ORIENTCORE H1-B & ORIENTCORE HI-B.LS are some of the finest quality of core.
ORENTCORE.HI-B is a breakthrough in that it offers higher magnetic flux density, lower core loss and lower magnetostriction than any conventional grain-oriented electrical steel sheet.
ORIENT.HI-B.LS is a novel type with marked lower core losses, produced by laser irradiation of the surface of ORIENTCORE.HI-B sheets.
Annealing of stacked electrical sheets
Annealing is to be done at 760 to 845ºC to
Reduce mechanical stress
Prevent contamination
Enhance insulation of lamination coating
Though ORIENTCORE and ORIENTCORE.HI-B are grain orient steel sheets with excellent magnetic properties, mechanical stress during such operations as cutting, punching and bending affect their magnetic properties adversely. When these stress are excessive, stress relief annealing is necessary.
Following method is observed for stress relief annealing
Available Grades:

Stacked electrical steel sheets are heated thoroughly in the edge-to-edge direction rather than in the face-to-face direction, because heat transfer is far faster in side heating.
A cover is put over sheets stacked on a flat plate. Because ORIENTCORE and ORIENTCORE.HI-B have extremely low carbon content and very easily decarburized at annealing temperatures, the base, cover and other accessories used are of very low carbon content .
To prevent oxidation so as to protect the coating on the sheets, a no oxidizing atmosphere free from carbon sources is used having less than 2%hydrogen or high-purity nitrogen gas. Due point of the atmosphere is maintained at 0ºC or less.
Care is taken to the flatness of annealing base, because an uneven base distorts cores, leading to possible distortion during assembly.
Annealing temperature ranging from 780ºC to 820ºC is maintained for more than 2 hours or more. Cooling is done upto 350ºC in about 15 hours or more.

ORIENTCORE :M1, M2, M3, M4, M5 & M6
ORIENTCORE.HI-B :23ZH90, 23ZH95, 27ZH95, 27ZH100, 30ZH100,M-0H, M-1H, M-2H, M-3H
ORIENTCORE.HI-B.LS: 23ZDKH90, 27ZDKH95
Non-oriented silicon steel, hot rolled grain oriented silicon steel,cold rolled grain oriented (CRGO) silicon steel, Hi-B, laser scribed and mechanically scribed. The last three materials are improved versions of CRGO.
Saturation flux density has remained more or less constant around 2.0 Tesla for CRGO; but there is a continuous improvement in watts/kg and volt-amperes/kg characteristics in the rolling direction.
The core building technology has improved from the non-mitred to mitred and then to the step-lap construction
The better grades of core steel not only reduce the core loss but they also help in reducing the noise level by few decibels
Use of amorphous steel for transformer cores results in substantial core loss reduction (loss is about one-third that of CRGO silicon steel). Since the manufacturing technology of handling this brittle material is difficult, its use in transformers is not widespread
In the early days of transformer manufacturing, inferior grades of laminated steel (as per today’s standards) were used with inherent high losses and magnetizing volt-amperes. Later on it was found that the addition of silicon content of about 4 to 5% improves the performance characteristics significantly, due to a marked reduction in eddy losses (on account of the increase in material resistivity) and increase in permeability. Hysteresis loss is also lower due to a narrower hysteresis loop. The addition of silicon also helps to reduce the aging effects.
Although silicon makes the material brittle, it is well within limits and does not pose problems during the process of core building.
The cold rolled manufacturing technology in which the grains are oriented in the direc tion of rolling gave a new direction to material development for many decades, and even today newer materials are centered around the basic grain orientation process.
Important stages of core material development are: non-oriented, hot rolled grain oriented (HRGO), cold rolled grain oriented (CRGO), high permeability cold rolled grain oriented (Hi-B), laser scribed and mechanically scribed.
Laminations with lower thickness are manufactured and used to take advantage of lower eddy losses. Currently the lowest thickness available is 0.23 mm, and the popular thickness range is 0.23 mm to 0.35 mm for power transformers.
Maximum thickness of lamination used in small transformers can be as high as 0.50 mm.
Inorganic coating (generally glass film and phosphate layer) having thickness of 0.002 to 0.003 mm is provided on both the surfaces of laminations, which is sufficient to withstand eddy voltages (of the order of a few volts).
Since the core is in the vicinity of high voltage windings, it is grounded to drain out the statically induced voltages. While designing the grounding system, due care must be taken to avoid multiple grounding, which otherwise results into circulating currents and subsequent failure of transformers.

4) Transformer Core:

a) Core Type Construction: (Mostly Used):

Generally in India, Core type of construction with Two/Three/Five limbed cores are used. Generally five limbed cores are used where the dimensions of the Transformer is to be limited due to Transportation difficulties. In three limbed core the cross section of the Limb and the Yoke are the same where as in five Limbed core, the cross section of the Yoke and the Flux return path Limbs are ver y less (58% and 45% of the principal Limb).
Limb:which is surrounded by windings, is called a limb or leg?
York: Remaining part of the core, which is not surrounded by windings, but is essential for completing the path of flux, is called as yoke.
Advantage:
Construction is simpler, cooling is better and repair is easy.
The yoke and end limb area should be only 50% of the main limb area for the same operating flux density.
Zero-sequence impedance is equal to positive-sequence impedance for this construction (in a bank of single-phase transformers).
Sometimes in a single-phase transformer windings are split into two parts and placed around two limbs as shown in figure (b). This construction is sometimes adopted for very large ratings. Magnitude of short-circuit forces are lower because of the fact that ampere-turns/height are reduced. The area of limbs and yokes is the same. Similar to the single-phase three-limb transformer.
The most commonly used construction, for small and medium rating transformers, is three-phase three-limb construction as shown in figure (d).For each phase, the limb flux returns through yokes and other two limbs (the same amount of peak flux flows in limbs and yokes).
limbs and yokes usually have the same area. Sometimes the yokes are provided with a 5% additional area as compared to the limbs for reducing no-load losses.
It is to be noted that the increase in yoke area of 5% reduces flux density in the yoke by 5%, reduces watts/kg by more than 5% (due to non-linear characteristics) but the yoke weight increases by 5%. Also, there may be additional loss due to cross-fluxing since there may not be perfect matching between lamination steps of limb and yoke at the joint. Hence, the reduction in losses may not be very significant.
In large power transformers, in order to reduce the height for transportability, three-phase five-limb construction depicted in figure (e) is used. The magnetic length represented by the end yoke and end limb has a higher reluctance as compared to that represented by the main yoke. Hence, as the flux starts rising, it first takes the path of low reluctance of the main yoke. Since the main yoke is not large enough to carry all the flux from the limb, it saturates and forces the remaining flux into the end limb. Since the spilling over of flux to the end limb occurs near the flux peak and also due to the fact that the ratio of reluctances of these two paths varies due to non-linear properties of the core.
Fluxes in both main yoke and end yoke/end limb paths are non-sinusoidal even though the main limb flux is varying sinusoidal [2,4]. Extra losses occur in the yokes and end limbs due to the flux harmonics. In order to compensate these extra losses, it is a normal practice to keep the main yoke area 60% and end yoke/end limb area 50% of the main limb area.
The zero-sequence impedance is much higher for the three-phase five-limb core than the three-limb core due to low reluctance path (of yokes and end limbs) available to the in-phase zero-sequence fluxes, and its value is close to but less than the positive-sequence impedance value.

b) Shell-type construction:

Cross section of windings in the plane of core is surrounded by limbs and yokes, is also used.
Shell type of construction of the core is widely used in USA.
Advantage:
One can use sandwich construction of LV and HV windings to get very low impedance, if desired, which is not easily possible in the core-type construction.
Analysis of overlapping joints and building factor:
While building a core, the laminations are placed in such a way that the gaps between the laminations at the joint of
limb and yoke are overlapped by the laminations in the next layer.
This is done so that there is no continuous gap at the joint when the laminations are stacked one above the other (figure). The overlap distance is kept around 15 to 20 mm.
There are two types of joints most widely used in transformers: non-mitred and mitred joints.
Non-mitered joints:
In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-oriented material was used
Non-mitered joints:
In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-oriented material was used
Mitered joints:
The joint where these laminations meet could be Butt or Mitred. In CRGO, the Mitred Joint is preferred as it reduces the Reluctance of the Flux path and reduces the No Load Losses and the No Load current (by about 12% & 25% respectively).
The Limb and the Yoke are made of a number of Laminations in Steps. Each step comprises of some number of laminations of equal width. The width of the central strip is Maximum and that at the circumference is Minimum. The cross section of the Yoke and the Limb are nearly Circular. Mitred joint could be at 35/45/55 degrees but the 45 one reduces wastage.
The angle of overlap (a) is of the order of 30° to 60°, the most commonly used angle is 45°. The flux crosses from limb to yoke along the grain orientation in mitred joints minimizing losses in them. For airgaps of equal length, the excitation requirement of cores with mitred joints is sin a times that with non-mitred joints.
Better grades of core material (Hi-B, scribed, etc.) having specific loss (watts/kg) 15 to 20% lower than conventional CRGO material (termed hereafter as CGO grade, e.g., M4) are regularly used. However, it has been observed that the use of these better materials may not give the expected loss reduction if a proper value of building factor is not used in loss calculations
The building factor generally increases as grade of the material improves from CGO to Hi-B to scribed (domain refined). This is a logical fact because at the corner joints the flux is not along the grain orientation, and the increase in watts/kg due to deviation from direction of grain orientation is higher for a better grade material.
The factor is also a function of operating flux density; it deteriorates more for better grade materials with the increase in operating flux density. Hence, cores built with better grade material may not give the expected benefit in line with Epstein measurements done on individual lamination. Therefore, appropriate building factors should be taken for better grade materials using experimental/test data.
Also the loss contribution due to the corner weight is higher in case of 90° joints as compared to 45° joints since there is over-crowding of flux at the inner edge and flux is not along the grain orientation while passing from limb to yoke in the former case. Smaller the overlapping length better is the core performance; but the improvement may not be noticeable.
The gap at the core joint has significant impact on the no-load loss and current. As compared to 0 mm gap, the increase in loss is 1 to 2% for 1.5 mm gap, 3 to 4% for 2.0 mm gap and 8 to 12% for 3 mm gap. These figures highlight the need for maintaining minimum gap at the core joints.
Lesser the laminations per lay, lower is the core loss. The experience shows that from 4 laminations per lay to 2 laminations per lay, there is an advantage in loss of about 3 to 4%. There is further advantage of 2 to 3% in 1 lamination per lay. As the number of laminations per lay reduces, the manufacturing time for core building increases and hence most of the manufacturers have standardized the core building with 2 laminations per lay.
Joints of limbs and yokes contribute significantly to the core loss due to cross-fluxing and crowding of flux lines in them. Hence, the higher the corner area and weight, the higher is the core loss.
The corner area in single-phase three-limb cores, single-phase four-limb cores and three-phase five-limb cores is less due to smaller core diameter at the corners, reducing the loss contribution due to the corners. However, this reduction is more than compensated by increase in loss because of higher overall weight (due to additional end limbs and yokes).
Building factor is usually in the range of 1.1 to 1.25 for three-phase three-limb cores with mitred joints. Higher the ratio of window height to window width, lower is the contribution of corners to the loss and hence the building factor is lower.
Step-lap joint :
It is used by many manufacturers due to its excellent performance figures. It consists of a group of laminations (commonly 5 to 7) stacked with a staggered joint as shown in figure.
Its superior performance as compared to the conventional mitred construction.
It is shown that, for a operating flux density of 1.7 T, the flux density in the mitred joint in the core sheet area shunting the air gap rises to 2.7 T (heavy saturation), while in the gap the flux density is about 0.7 T. Contrary to this, in the step-lap joint of 6 steps, the flux totally avoids the gap with flux density of just 0.04 T, and gets redistributed almost equally in laminations of other five steps with a flux density close to 2.0 T. This explains why the no-load performance figures (current, loss and noise) show a marked improvement for the step-lap joints.
The assembled core has to be clamped tightly not only to provide a rigid mechanical structure but also required magnetic characteristic. Top and Bottom Yokes are clamped by steel sections using Yoke Studs. These studs do not pass through the core but held between steel sections. Of late Fiber Glass Band tapes are wound round the Limbs tightly upto the desired tension and heat treated. These laminations , due to elongation and contraction lead to magnetostriction, generally called Humming which can be reduced by using higher silicon content in steel but this makes the laminations become very brittle.
The choice of operating flux density of a core has a very significant impact on the overall size, material cost and performance of a transformer.
For the currently available various grades of CRGO material, although losses and magnetizing volt-amperes are lower for better grades, viz. Hi-B material (M0H, M1H, M2H), laser scribed, mechanical scribed, etc., as compared to CGO material (M2, M3, M4,M5, M6, etc.), the saturation flux density has remained same (about 2.0 T).
The peak operating flux density (Bmp ) gets limited by the over-excitation conditions specified by users.
The slope of B-H curve of CRGO material significantly worsens after about 1.9 T (for a small increase in flux density, relatively much higher magnetizing current is drawn). Hence, the point corresponding to 1.9 T can be termed as knee-point of the B-H curve.
It has been seen in example 1.1 that the simultaneous over-voltage and under-frequency conditions increase the flux density in the core. Hence, for an over-excitation condition (over-voltage and under-frequency).
When a transformer is subjected to an over-excitation, core contains an amount of flux sufficient to saturate it. The remaining flux spills out of the core. The over-excitation must be extreme and of a long duration to produce damaging effect in the core laminations
The laminations can easily withstand temperatures in the region of 800°C (they are annealed at this temperature during their manufacture), but insulation in the vicinity of core laminations, viz. press-board insulation (class A: 105°C) and core bolt insulation (class B: 130°C) may get damaged. Since the flux flows in air (outside core) only during the part of a cycle when core gets saturated, the air flux and exciting current are in the form of pulses having high harmonic content which increases the eddy losses and temperature rise in windings and structural parts.

Winding Insulation in Transformer:

Requirement of Insulating Oil:
1.0 lit / kva for Trs from 400 – 1600 Kva
0.6 lit / kva for Trs from 1600 – 80,000 kva
0.5 lit / Kva for Trs above 80,000 Kva.
In Transformers, the insulating oil provides an insulation medium as well as a heat transferring medium that carries away heat produced in the windings and iron core. Since the electric strength and the life of a Transformer depend chiefly upon the quality of the insulating oil, it is very important to use a high quality insulating oil
Provide a high electric strength.
Permit good transfer of heat.
Have low specific gravity-In oil of low specific gravity particles which have become suspended in the oil will settle down on the bottom of the tank more readily and at a faster rate, a property aiding the oil in retaining its homogeneity.
Have a low viscosity- Oil with low viscosity, i.e., having greater fluidity, will cool Transformers at a much better rate.
Have low pour point- Oil with low pour point will cease to flow only at low temperatures.
Have a high flash point. The flash point characterizes its tendency to evaporate. The lower the flash point the greater the oil will tend to vaporize When oil vaporizes, it loses in volume, its viscosity rises, and an explosive mixture may be formed with the air above the oil
The Core Insulation is:
SRBP- Synthetic Resin Bonded Paper
OIP – Oil Impregnated Paper
RIP – Resin Impregnated Pape
Resin Coated Paper/ Kraft Paper/ Crepe Kraft Paper are used for making core for the above It is Hermetically Sealed.
Pre-compressed pressboard is used in windings as opposed to the softer materials used in earlier days. The major insulation (between windings, between winding and yoke, etc.)
Mineral oil has traditionally been the most commonly used electrical insulating medium and coolant in transformers. Studies have proved that oil-barrier insulation system can be used at the rated voltages greater than 1000 Kv.
A high dielectric strength of oil-impregnated paper and pressboard is the main reason for using oil as the most important constituent of the transformer insulation system.
Manufacturers have used silicon-based liquid for insulation and cooling. Due to non-toxic dielectric and self-extinguishing properties, it is selected as a replacement of Askarel. High cost of silicon is an inhibiting factor for its widespread use.
Super-biodegradable vegetable seed based oils are also available for use in environmentally sensitive locations.
SF6 gas has excellent dielectric strength and is non-flammable. Hence, SF6 transformers find their application in the areas where fire-hazard prevention is of paramount importance.
Due to lower specific gravity of SF6 gas, the gas insulated transformer is usually lighter than the oil insulated transformer. The dielectric strength of SF6 gas is a function of the operating pressure; the higher the pressure, the higher the dielectric strength.
However, the heat capacity and thermal time constant of SF6 gas are smaller than that of oil, resulting in reduced overload capacity of SF6 transformers as compared to oil-immersed transformers. Environmental concerns, sealing problems, lower cooling capability and present high cost of manufacture are the challenges.
Dry-type resin cast and resin impregnated transformers use class F or C insulation. High cost of resins and lower heat dissipation capability limit the use of these transformers to small ratings.
The dry-type transformers are primarily used for the indoor application in order to minimize fire hazards. Nomex paper insulation, which has temperature withstand capacity of 220°C, is widely used for dry-type transformers. The initial cost of a dry-type transformer may be 60 to 70% higher than that of an oil-cooled transformer at current prices, but its overall cost at the present level of energy rate can be very much comparable to that of the oil-cooled transformer.

Transformer Noise:

Transformers located near a residential area should have sound level as low as possible.
Levels specified are 10 to 15 dB lower than the prevailing levels mentioned in the international standards.
Core, windings and cooling equipment are the three main sources of noise.
The core is the most important and significant source of the transformer noise.
The core vibrates due to magnetic and magnetostrictive forces. Magnetic forces appear due to non-magnetic gaps at the corner joints of limbs and yokes
These magnetic forces depend upon the kind of interlacing between the limb and yoke; these are highest when there is no overlapping (continuous air gap).
The magnetic forces are smaller for 90° overlapping, which further reduce for 45°overlapping. These are the least for the step-lap joint due to reduction in the value of flux density in the overlapping region at the joint.
The forces produced by the magnetostriction phenomenon are much higher than the magnetic forces in transformers.
Magnetostriction is a change in configuration of magnetizable material in a magnetic field, which leads to periodic changes in the length of material. An alternating field sets the core in vibration.
This vibration is transmitted, after some attenuation, through the oil and tank structure to the surrounding air. This finally results in a characteristic hum.
The magnetostriction force varies with time and contains even harmonics of the power frequency (120, 240, 360, —Hz for 60 Hz power frequency). Therefore, the noise also contains all harmonics of 120 Hz.
The amplitude of core vibration and noise increase manifold if the fundamental mechanical natural frequency of the core is close to 120 Hz.
The value of the magnetostriction can be positive or negative, depending on the type of the magnetic material, and the mechanical and thermal treatments.
Magnetostriction is generally positive (increase in length by a few microns with increase in flux density) for CRGO material at annealing temperatures below 800°C, and as the annealing temperature is increased (=800°C), it can be displaced to negative values.The mechanical stressing may change it to positive values
Magnetostriction is minimum along the rolling direction and maximum along the 90° direction.
Most of the noise transmitted from a core comes principally from the yoke region because the noise from the limb is effectively damped by windings (copper and insulation material) around the limb.
The quality of yoke clamping has a significant influence on the noise level.
Apart from the yoke flux density, other factors which decide the noise level are: limb flux density, type of core material, leg center (distance between the centers of two adjacent phases), core weight, frequency, etc.
The higher the flux density, leg centers, core weight and frequency of operation, the higher is the noise level.
The noise level is closely related to the operating peak flux density and core weight.
If core weight is assumed to change with flux density approximately in inverse proportion, for a flux density change from 1.6 T to 1.7 T, the increase in noise level is 1.7 dB
Hence, one of the ways of reducing noise is by designing transformer at lower operating flux density. For a flux density reduction of 0.1 T, the noise level reduction of about 2 dB is obtained. This method results into an increase of material content and it may be justified economically if the user has specified a lower no-load loss, in which case the natural choice is to use a lower flux density.
Use of step-lap joint gives much better noise reduction (4 to 5 dB).
Some manufacturers also use yoke reinforcement (leading to reduction in yoke flux density); the method has the advantage that copper content does not go up since the winding mean diameters do not increase. Bonding of laminations by adhesives and placing of anti-vibration/damping elements between the core and tank can give further reduction in the noise level.
The use of Hi-B/scribed material can also give a reduction of 2 to 3 dB. When a noise level reduction of the order of 15 to 20 dB is required, some of these methods are necessary but not sufficient.

Transformer Protection:

Internal Protection:

(1) Bucholtz Relay:

This Gas operated relay is a protection for minor and major faults that may develop inside a Transformer and produce Gases.
This relay is located in between the conservator tank and the Main Transformer tank in the pie line which is mounted at an inclination of 3 to 7 degrees.
A shut off valve is located in between the Bucholtz relay and the Conservator.
The relay comprises of a cast housing which contains two pivoted Buckets counter balanced weights.
The relay also contains two mercury y switches which will send alarm or trip signal to the breakers controlling the Transformer. In healthy condition, this relay will be full of oil and the buckets will also be full of oil and is counter balanced by the weights.
In the event of a fault inside the transformer, the gases flow up to the conservator via the relay and pushes the oil in the relay down. Once the oil level falls below the bottom level of the buckets, the bucket due to the weight of oil inside tilts and closes the mercury switch and causes the Alarm or trip to be actuated and isolate the transformer from the system.

(2) Oil Surge / Bucholtz Relay for OLTC:

This relay operating on gas produced slowly or in a surge due to faults inside the Diverter Switch of OLTC protects the Transformer and isolates it from the system.

(3) Pressure Relief Valve for Large Transformers:

In case of a serious fault inside the Transformer, Gas is rapidly produced.
This gaseous pressure must be relieved immediately otherwise it will damage the Tank and cause damage to neighboring equipment.
This relay is mounted on the top cover or on the side walls of the Transformer. The valve has a corresponding port which will be sealed by a stain less steel diaphragm .
The diaphragm rests on a O ring and is kept pressed by two heavy springs. If a high pressure is developed inside, this diaphragm lifts up and releases the excessive gas.
The movement of the diaphragm lifts the spring and causes a micro switch to close its contacts to give a trip signal to the HV and LV circuit breakers and isolate the transformer.
A visual indication can also be seen on the top of the relay. For smaller capacity transformer, an Explosion vent is used to relieve the excess pressure but it cannot isolate the Transformer.

(4) Explosion Vent Low & Medium Transformers :

For smaller capacity Transformers, the excessive pressures inside a Transformer due to major faults inside the transformer can be relieved by Explosion vents. But this cannot isolate the Transformer.

(5) Winding /Oil Temperature Protection :

These precision instruments operate on the principle of liquid expansion.
These record the hour to hour temperatures and also record the Maximum temperature over a period of time by a resettable pointer.
These in conjunction with mercury switches provide signals for excessive temperature alarm annunciation and also isolate the Transformer for very excessive temperatures.
These also switch on the cooler fans and cooler pumps if the temperature exceeds the set values. Normally two separate instruments are used for oil and winding temperatures.
In some cases additional instruments are provided separately for HV,LV and Tertiary winding temperatures.
The indicator is provided with a sensing bulb placed in an oil pocket located on the top cover of the Transformer tank. The Bulb is connected to the instrument housing by means of flexible connecting tubes consisting of two capillary tubes.
One capillary tube is connected to an operating Bellow in the instrument. The other is connected to a compensating Bellow .
The tube follows the same path as the one with the Bulb but the other end, it does not end in a Bulb and left sealed. This compensates for variations in Ambient Temperatures.
As the temperature varies, the volume of the liquid in the operating system also varies and operates the operating Bellows transmitting its movements to the pointer and also the switching disc. This disc is mounted with mercury float switches which when made provides signals to alarm/trip/cooler controls.
Oil and winding temperature indicators work on the same principles except that the WTI is provided with an additional bellows heating element. This heating element is fed by a current transformer with a current proportional to the load in the winding whose temperature is to be measured/monitored. The tem premature increase of the heating element is proportional to the temperature rise of winding over top oil temperature.
The operating bellow gets an additional movement simulating the increase of winding temperature over top oil temperature and represents the Winding Hot Spot. This is called Thermal Imaging process.

(6) Conservator Magnetic Oil Level Protection :

Inside the conservator tank, a float is used to sense the levels of oil and move. This is transmitted to a switch mechanism by means of magnetic coupling. The Float and the Magnetic mechanism are totally sealed. The pointer connected to the magnetic mechanism moves indicating the correct oil level and also provision is m ade for Low oil level alarm by switch.

(7) Silica gel Breather:

This is a means to preserve the dielectric strength of insulating oil and prevent absorption of moisture, dust etc. The breather is connected to the Main conservator tank. It is provided with an Oil seal. The breathed in air is passed through the oil seal to retain moisture before the air passes through the silica gel cr ystals which absorbs moisture before breathing into the conservator tank. In latest large transformers, Rubber Diaphragm or Air cells are used to reduce contamination of oil.

(8) Transformer Earthing :

For Distribution Transformers, normally Dy11 vector Group, the LT Neutral is Earthed by a separate Conductor section of at least half the section of the conductor used for phase wire and connected to a Separate Earth whose Earth Resistance must be less than 1 ohm.
The Body of the Tank has two different earth connections, which should be connected to two distinct earth electrodes by GI flat of suitable section.
For Large Power Transformers, Neutral and Body Connections are made separately but all the Earth Pits are connected in parallel so that the combined Earth Resistance is always maintained below 0.1 ohm.
The individual and combined earth resistance is measured periodically and the Earth Pits maintained regularly and electrodes replaced if required.

External Protection:

Lightning Arrestors on HV & LV for Surge Protection
HV / LV Over Current Protection(Instantaneous /IDMT- Back up)
Earth Fault Protection ( Y connected side)
REF (HV & LV) ( For internal fault protection)
Differential Protection (for internal fault protection)
Over Fluxing Protection (against system Kv & HZ variations)
HG Fuse Protection for Small Capacity Transformers.
Normally Each Power Transformers will have a LV Circuit Breaker. For a Group of Transformers up to 5 MVA in a substation, a Group control Circuit Breaker is provided. Each Transformer of 8 MVA and above will have a Circuit Breaker on the HV side.

Transformer Cooling:

The Heat in a transformer is produced due to I square R in the windings and in the core due to Eddy Current and Hysteresis Loss.
In Dry type Transformer the Heat is directly dissipated into the atmosphere but in Oil filled Transformer, the Heat is dissipated by Thermosyphon and transmitted to the top and dissipated into the atmosphere through Radiators naturally or by forced cooling fans or by Oil pumps or through Water Coolers.
The following Standard symbols are adopted to denote the Type of Cooling:
A =Air Cooling
N =Natural Cooling by Convection
B= Cooling by Air Blast Fans
O=Oil (mineral) immersed cooling
W= Water Cooled
F =Forced Oil Circulation by Oil Pumps
S=Synthetic Liquid used instead of Oil
G =Gas Cooled (SF6 or N2)
D=Forced (Oil directed)
ONAF=Oil immersed Transformer with natural oil circulation and forced air external cooling is designated.
ONAN= Oil Immersed Natural cooled
ONAF= Oil Immersed Air Blast
ONWN=Oil Immersed Water Cooled
OFAF=Forced Oil Air Blast Cooled
OFAN=Forced Oil Natural Air Cooled
OFWF=Forced Oil Water Cooled
ODAF=Forced Directed Oil and Forced Air Cooling.

Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three systems e.g., ONAN/ONAF/ OFAF