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 amps24 KV / 1000 amps            
12 to 17.5 KV / 630 amps24 KV / 2000 to 3150 amps
12 to 17.5 KV / 1000 amps    36 KV / 250 amps              
12 to 17.5 KV / 2000 to 3150 amps36 KV / 630 amps
24 KV / 250 amps              36 KV / 1000 amps     
24 KV / 630 amps36 KV / 2000 to 3150 amps
  • L.V. Bushings (IS:3347) 
11 KV / 250 amps1 KV / 2000 amps
1 KV / 630 amps1 KV / 3150 amps
1 KV / 1000 amps 
  • H.V. Bushings (IS:8603)    
12 KV / 250 amps36 KV / 250 amps              
12 / 630 amps36 KV / 630 amps
12 KV / 1000 amps    6 KV / 1000 amps            
12 KV / 2000 to 3150 amps36 KV 3150 amps
  • C.T. Bushings (IS:5612)   
11 KV1 KV / 2000 amps
1 KV / 630 amps1 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:
  1.  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.
  2.  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 .
  3. 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.
  4. Care  is  taken  to  the  flatness  of  annealing  base,  because  an  uneven  base distorts cores, leading to possible  distortion during assembly.
  5.  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
  •  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