Oil-Immersed Power Transformers Design Requirements

This article is about requirements for design, manufacture, testing  of three-phase, two-winding, oil-immersed, air-cooled power transformers with a rating of 500 kVA and above with a nominal voltage rating of 230 kV and below. Main keywords for this article are Oil-Immersed Power Transformers Design Requirements, Manual Tap Changer, On Load Tap Changer, Basic Impulse Insulation Level, Oil-Immersed Transformer Tests.

Oil-Immersed Power Transformers References

American National Standards Association (ANSI)
C57.12.00 Standard General Requirements for Liquid Immersed Distribution, Power,  and Regulating Transformers
C57.12.10 Safety Requirements 230 kV and Below, 833/958 Through 8333/10000, 417  kVA Single-Phase, and 750/862 Through 60,000/80,000/100,000 kVA,
Three-Phase Without Load Tap Changing; and 3750/4687 Through 60,000/80,000/100,000 kVA With Load Tap changing
C57.12.80 Standard Terminology for Power Distribution Transformers
C57.12.90 Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers; and Guide for Short-Circuit Testing of Distribution and Power Transformers
C57.13 Standard Requirements for Instrument Transformers
C57.104 Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers
American Society for Testing and Materials (ASTM)
B 117 Test Method of Salt Spray (Fog) Testing
D 714 Standard Method for Evaluating Degree of Blistering of Paint
D 1535 Method for Specifying Color by the Munsell System
D 1654 Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments
D 3487 Specification for Mineral Insulating Oil Used in Electrical Apparatus
D1275-06 Standard Test Method for Corrosive Sulfur in Electrical Insulating Oils

National Electrical Manufacturer’s Association (NEMA)
MG1 Motors and Generators

International Electrotechnlcal Commission (IEC)
IEC 62535 Insulating liquids – Test Method for Detection of Potentially Corrosive Sulphur  in Used and Unused Insulating Oil

Oil-Immersed Power Transformers Design Requirements

• Primary and secondary windings shall be made of electrical grade copper with a minimum conductivity of 98 percent International Annealed Copper Standard (IACS) for copper used in electrical conductors. The conductors shall be machine spirally wound with several overlapping layers of insulation paper. This process shall detect and eliminate conductor burrs that might affect the dielectric integrity of the winding.
• Permanent current carrying joints, except threaded connections and compression joints, shall be welded or soldered with silver solder or silver alloy having a melting point of 590 °C or above. Bolted or clamp type connections may be used at bushings, tap changers and terminal boards providing that suitable locking devices are used to prevent the connections from becoming loose.
• Leads from the windings to the terminal boards and bushings shall be rigidly supported to prevent damage from vibration. Guide tubes shall be used wherever possible.
• Coil windings shall be given not more than one dip treatment of varnish which shall be applied for mechanical protection and not for improvement in dielectric properties. However, in no case shall varnish or other adhesive be used which will seal the coil and prevent evacuation of air and moisture, and impregnation by oil.
• Transformer and accessories shall be designed to provide the specified self cooled (ONAN) rating, with a winding temperature rise by resistance of not more than 55 °C at nominal kVA rating.
• Transformer and accessories shall be able to operate continuously with an average winding temperature rise of 55 °C above ambient when operated at forced air-cooled (ONAF) rating.
• Forced air-cooled (ONAF) kVA rating of transformer shall conform to Table 12 of ANSI C57.12.10. kVA rating shall be continuous at a winding temperature rise of 55 °C and shall be based on not exceeding the specified average winding temperature.
• The guidelines provided in Table 12 of ANSI C57.12.10 for ONAF rating are given below:
  500-2000 kVA : 1.15 x ONAN rating at specific winding temperature rise Above ambient
  2,500-10,000 kVA : 1.25 x ONAN rating at specific winding temperature rise Above ambient
• For transformers using two stages of forced air cooling, that is ONAN/ONAF/ONAF the increase in rating may vary between manufacturers. Transformers above 10,000 kVA shall be built in accordance with ANSI C57.12.10, where 1 represents the ONAN rating at 55 °C rise.
• Average winding temperature rise above ambient shall not exceed 55 °C when measured by resistance and the winding hottest spot temperature shall not exceed 70 °C above ambient. Temperature rise shall be determined in accordance with ANSI C57.12.00, section 5.11.2. If temperature detectors are specified, temperature rise shall be measured by winding temperature detectors.
• Temperature rise of the insulating liquid at Class ONAN rating and measured near the top of the tank shall not exceed the average winding temperature rise above ambient.
• Temperature of metallic parts in contact with the insulation of current carrying conductors shall not exceed the winding hottest spot temperature rise.
• Temperature gradient from the winding hottest spot to the top of the insulating liquid shall be provided with the test data.
• Standard percent impedance voltages shall conform to ANSI C57.12.10, Table 10.
• Impedance voltage shall be referred to at temperature 75 °C. This is the sum of the rated average winding temperature rise by resistance (55 °C) plus 20 °C.
• All switches and devices, except tap changers, required to be operated or tested shall be located at a maximum height of 1.5 m above the transformer base. Devices mounted above this height shall be legible from grade.
• Primary winding shall be delta connected and secondary winding shall be wye-connected. The angular displacement between the primary and secondary phase voltages shall be 30° with the secondary lagging the primary. Neutral shall be brought out through a bushing. The angular displacement shall be in accordance with ANSI C57.12.00, Section 5.7.2.

Core and Coil Construction

• The core shall be of high quality grain oriented, non-aging electrical silicon steel having low hysteresis loss and high permeability. The core legs shall be securely held by means of through bolts insulated with not less than Class B insulation, or glass cloth tape, with adequate padding adjacent to the iron, or be encased in a strong fiber tube and securely wedged within the tube. The use of organic tape or pressed board
  wrapping to hold the laminations is not acceptable.
• Industry designations for electrical grade core steel are as follows:
  Grade H1 – Laser scribed super grain oriented steel. This grade is used where core losses are at a premium and is often used by electrical utilities.
  b. Grade M6 – Straight grain oriented steel. This grade has higher core losses than H1 and is normally used in distribution service at industrial locations.
• The core laminations shall be free of burrs and sharp projections. Each sheet shall have an insulating coating resistant to the action of hot oil.
• The completed core and coil assembly shall be assembled and braced in a way that the center line and the outside surface of the coil stack shall lie in a vertical plane. The deviation of this plane from the vertical shall be kept to a minimum.
• The core shall be tightly clamped, the coils shall be securely fixed to the core and the core and coils shall be rigidly fixed in the tank to prevent movement under short circuit stresses and while handling during shipment. The structure shall be securely grounded to prevent electrostatic potentials. This ground shall be brought to the outside through an insulating bushing, to be easily accessible for checking the core in the field with a megger type meter for unintentional grounds. The ground point shall be clearly and permanently labeled.
• Electrical conductors shall be electrolytic copper, conductivity 98 percent IACS.

Oil-Immersed Power Transformers Bushings

• Bushings located in air-filled terminal chambers shall be made wet process glazed with threaded stud porcelain. Bushings for use above 15 kV shall be capacitor tap for testing power factor values.
• Bushings mounted on the transformer cover or in oil filled terminal chambers shall be made of porcelain.
• Bushings shall have bolted construction. Gaskets shall be retained by positive means, for example recessed spaces for gaskets, to avoid deformation when bushing is installed. Integrity of gaskets shall be tested by
  pressurizing the tanks to a minimum of 6 psi for 2 hours and checking for leaks.
• Bushings shall have tin-plated copper pads with NEMA standard four-hole drillings (14.3 mm in diameter: spaced 44.5 mm apart center to center) for the specified quantity and type of connections.
• Neutral bushing shall be identical to phase bushings, that is, fully rated to minimize spare parts.
• Neutral bushing shall be located in the terminal chamber or on the cover as indicated on the data sheet.
• The bushing with the same voltage rating shall not be interchangeable with the bushing that have different design with much higher current value.

Oil-Immersed Power Transformers Tank and Enclosure

• Transformer tank shall be designed to eliminate movement of the coils’ bushing terminals over the full range of positive 10 psi and negative 8 psi pressures.
• Assembled transformer shall be able to withstand, without permanent deformation, a pressure 25 percent greater than the maximum operating pressure.
• The inside of the tank shall be clean and free of slag and foreign particles after welding of the tank cover.
• Tank cover shall be either continuous welded or bolted-on. In case of a continuous welded cover, there shall either be one manhole or two (2) hand holes.
• Manhole and hand holes shall have gasketed and bolted covers.
• If manhole is round, it shall have a minimum diameter of 610 mm. If manhole  is rectangular it shall have a minimum dimension of 610 mm.
• Manhole cover shall have a minimum of two handles for lifting.
• A sealed tank construction shall be achieved by a formed flange, a flame and heat resistant gasket in a retainer around the manhole and a main cover.
• Hand holes shall be located and sized to permit change-out of bushings.
• Transformer shall have two stainless steel grounding pads in accordance  with ANSI C57.12.10, Section 5.5.
• Grounding pads shall be welded on the base or on the tank wall near the base and located to not interfere with jacking facilities.
• One grounding pad shall be located near the neutral bushing and the other  shall be located on the opposite side. Each pad shall be drilled and tapped for 13 mm thread at a minimum depth of 13 mm. Two threaded holes spaced to suit a standard two-hole non-insulated compression lug shall be provided on each pad. Plastic, flanged cup type thread protectors shall be inserted in each threaded hole prior to shipping.
• Lifting eyes or lugs shall be provided for lifting the cover, the core and coil assembly and the complete transformer. The bearing surface of the lifting lugs shall be free of sharp edges and shall have a minimum opening of 30 mm for guying purposes.
• Welded steel pads shall be located at the extreme or corners of the transformer tank to provide a bearing surface for the jacks. The jack pads shall be accessible for the insertion of jacks when the transformer is completely assembled, and the underside of the pads to the base of the transformer shall be in line.
• Tank bottom plate shall be raised from the normal foundation surface to prevent rusting of the tank. Bottom plate shall be braced in a way that it does not sag. Tank bottom and the underside of supports shall be coated with a rubberized asphalt compound.
• Primary and secondary transformer interface shall permit removal of the transformer without disturbing the adjacent switchgear. Disassembly of the transition section is acceptable.
• The liquid preservation system shall meet the requirements of ANSI C57.12.10 clauses 5.7 and 9.13 as applicable. Conservator (expansion tank) for non-sealed tank construction shall be provided with bladder (air bag).

Oil-Immersed Power Transformers Cooling

• Radiators shall be detachable panel type.
• Panel type radiators shall be made of galvanized steel.
• Cooling fans shall be provided and sized to allow continuous operation of the transformer at ONAF rating without exceeding the specified winding temperature rise.
• Fans shall be supported on brackets attached to the main tank and not to the radiators.
• Fans shall be fitted with fan guards. Maximum size of opening on guards shall be 12.5 mm.
• Operation of fans shall be controlled with a three-position selector maintained switch – automatic/off/manual. In the automatic mode, fan operation shall be controlled.
• For transformers 2000 kVA and below, fan operation shall be controlled by the temperature of the insulating liquid in accordance with Section 5.9.1.1 of ANSI C57.12.10.
• For transformers above 2000 kVA, fan operation shall be controlled by the high temperature of the winding in accordance with Section 5.9.1.2 of ANSI C57.12.10.
• Provisions shall be made for remote maintained position contact to run the fans. Controller for the fan shall have two (2) normally open auxiliary contacts wired to a terminal box to indicate fan operation.
• Fan motors shall be 480 V, 3 phase, 60 Hz (ANSI/NEMA), totally-enclosed, squirrel cage, induction type, rated for a 50 °C ambient. They shall conform to the requirements of NEMA MG1.

Terminal Chambers, External Clearances, and Space Heaters

• Transformers shall have throat supported air terminal chambers (ATC) for high and low voltage power termination for primary voltages up to 34.5 kV unless specified otherwise on the data sheet. Transformer throat shall be drilled and gasketed. Gaskets shall be one piece cork neoprene material.
• For primary voltages above 115 kV the terminal chamber shall be oil filled from the main tank. Fill, drain and vent valves shall be provided. Valves shall be stainless steel with positive shut off. Piping and piping fittings shall be hot dipped galvanized steel.
• Terminal chambers shall have a front cover. Cover shall be gasketed with stainless steel hinges and three points latching with pad locking provision.
• Terminal chambers shall have a removable aluminum bottom plate, 6.35 mm thick minimum, to accept multiple single conductor cables per phase. For single or multiple three conductor cables the plate may be steel. Plate shall be bolted and gasketed.
• There shall be a minimum space of 838 mm between the cable entry point and the bottom of the bushing connection pad.
• Terminal chambers shall have the required cable supports spaced at a maximum distance of 305 mm.
• When specified on the data sheet, connection shall consist of a drilled and gasketed flange on the end wall of the tank, for close coupling of an off load primary disconnect switch.
• Air filled terminal chambers shall have two drains at their lowest points. Drains shall have screens made of 300 series stainless steel. Maximum size of screen opening shall be 3.17 mm.
• Terminal chambers shall have a drip shield over all the horizontal gasketed surfaces.
• External electrical clearances for exposed conducting parts shall be based on voltage and basic impulse insulation level (BIL) levels specified. Certified testing proving adequate clearances shall be performed on transformers prior to shipping.
• Space heaters rated at 240 V single phase shall be provided inside each air filled terminal chamber.
• Space heaters shall:
  Be operated at 120 V ac
  b. Have an expanded metal mesh cover to prevent accidental contact with  hot surfaces
  c. Be controlled by an adjustable thermostat with a bypass switch for  manual operation
  d. Be individually fused and wired to a separate terminal strip in control cabinet.
  e. Be mounted on suitable standoff insulators to prevent hot spots on the outside of the terminal chambers
  f. Have an ammeter to continually monitor space heater operation
• The high voltage terminal chamber shall include grounding terminals for the grounding of cable armor, shield and stress relieving devices.

Oil-Immersed Power Transformers Accessories

57. a) Monitoring and measuring instruments shall be mounted at grade  level. View of the instruments shall not be obstructed when viewing from the grade level. Oil sampling valve shall not be located under or above the radiators.
    b) Transformers shall have a dial type temperature indicator to indicate temperature of the insulating liquid. The indicator shall have two hermetically sealed normally closed contacts that are adjustable from 55 °C to 120 °C. The indicator shall be provided with a floating pointer to show the maximum indication since the last setting and shall be capable of being reset without the use of tools. Resetting by means of a permanent magnet permanently
    attached to the indicator by a chain or otherwise will be acceptable.
    c) Transformers shall have the following accessories:
    Standard ANSI accessories as listed in ANSI C57.12.10 Table  11 under Power BIL column
    b. Gas sampling valve (as applicable based on type of transformer)
    c. Buchholz Relay (as applicable based on type of transformer)
    d. Automatic resetting tank pressure relief device with indication of  operation, and hermetically sealed normally closed alarm contact
    e. Upper filter valves
    f. Nitrogen purge nozzle and shut-off valve
    g. Drain, filter and oil sampling valve. These valves shall not be  located under or above the radiators.
    d) Transformers above 2000 kVA shall have a dial type winding  temperature indicator located at a maximum of 1.7 m above the transformer base on the high voltage side. It shall be provided with two single pole double throw hermetically sealed contacts adjustable from 65 °C to 125 °C, and a floating pointer to show the maximum temperature indication since the last setting. The floating pointer shall be capable of being reset without tools.
    Resetting by means of a permanent magnet permanently attached to the indicator by a chain or otherwise will be acceptable. The indicator shall include the necessary current transformers and heaters to indicate the hottest spot of the windings under all specified loading conditions.
    e) Transformers shall have a magnetic type level indicator for the insulating liquid. Indicator shall have a 138 mm dial and nonadjustable alarm contacts set to close at the minimum safe operating level of the insulating liquid. Indicator shall also have permanent markings that indicate the insulating liquid level at 25 °C. The indicator shall be readable from the transformer base level.
    f) Contacts shall be wired with color coded conductors and have permanent labeling (wire markers) at the control box end in conformance with ANSI C57.12.10, Section 9.7.2.  
    g) Transformer shall have a stainless steel nameplate mounted on the tank at approximately eye-level. This shall include the following information, and also manufacturer standard items not covered herein:
    a. Serial number
    b. Class (ONAN, OFAF/ONAF, etc.)
    c. Number of phases
    d. Frequency
    e. kVA (or MVA) rating
    f. Voltage ratings
    g. Tap voltages
    h. Temperature rise, °C
    i. Vector group / Angular displacement
    j. Phasor diagram (polyphase transformers)
    k. Percent impedance
    l. Basic lightning impulse insulation levels (BIL)
    m. Approximate masses in kg
    n. Connection diagram
    o. Name of manufacturer
    p. Installation and operating instructions reference
    q. The word transformer or autotransformer
    r. Step-up operation suitability
    s. Tank, pressure, and liquid data
    t. Type of insulating liquid (generic name preferred)
    u. Conductor material (of each winding) 

• Current Transformers
  a) High voltage winding current transformers (CT’s) shall be multi- ratio with taps as specified in ANSI C57.13.
  b) CT’s located in air-filled terminal chambers shall be constructed of  oil resistant insulating materials.

57. c) Accuracy class (full winding) shall conform to Table 6 in  ANSI C57.13.
    d) Secondary leads shall be run to shorting type terminal blocks located in a control junction box on the side of the transformer tank. Terminal blocks shall be clearly marked designating the CT phase and tap number in accordance with ANSI C57.13.
    e) Insulated, brazed-seam ring-type lugs shall be used for CT circuits.
    f) The secondary lead wire shall be 10 AWG, wire routed in rigid metal conduit from the junction box on the transformer.

• Current Monitoring System for Cooling Fans
  a) System shall have minimum two (2) normally open (NO) and two (2) normally closed (NC) alarm contacts for customer use, to indicate nonfunctional fans with fan control switch in the manual mode, or in the automatic mode when the fan operation is required.
  b) Manufacturer shall provide a complete description of the system with the quotation.
• Sudden Pressure Relay
  a) transformers 1000 kVA and  above shall have a sudden pressure relay with alarm and shutdown contact. The relay circuit and its contacts shall operate at 125 V dc from the station battery.
• On-line Continuous Monitoring and Warning Devices
  a) If specified on the data sheet, transformer shall be provided with  the following on-line continuous monitoring and warning devices-
  a) Concentration of fault gases CO & H2
  b) Moisture in the insulating oil system
  c) Acetylene gas, in addition, a combined incipient fault gas reading of hydrogen, CO, acetylene and ethylene
  d) Partial discharge

The above devices shall be provided with but not limited to the  following features-
a) Expanded history logging of data and events at adjustable rates.
b) Adjustable alarms on inputs
c) Communication / HMI facility
d) Self testing facility

e) Remote or local configuration and program upgrading facility
f) Full stand-alone capabilities when host computer is not required
g) Sensor measurement update times are under 10 minutes
h) No additional oil lines for installation
i) No pumps or moving parts are used for maximum long term reliability.
j) Five dry contacts are available for indication and alarm outputs.

Oil-Immersed Power Transformers Hardware

• Hardware not under oil, for example bolts, screws, fasteners, and plugs, shall be made of 300 series stainless steel.
• Bolts shall have captive hexagon heads.
• Bolt connections to the transformer enclosure shall be made using threaded  openings on the enclosure or nuts welded inside the enclosure.
• Hexagonal nuts shall be used for welded studs.

Alarm and Control Devices

• Alarm and control devices shall have single-pole, double-throw contacts. Contacts shall be rated in accordance with ANSI C57.12.10, Section 9.7.1.
• Alarm and control devices shall:
  Be clearly identified on the drawings
  b. Have identification plates nearby. Plates shall be mounted with stainless steel screws or drive pins
• Identification plates shall be made of laminated material and shall be white  with black engraved lettering.

Wiring and Control Cabinet

• Control, alarm, and power wiring shall:
  Contain no splices
  b. Be run in rigid galvanized steel conduit and short lengths of liquid-tight  flexible conduit
  c. Be minimum 2.5 mm2 stranded copper for control and alarm circuits
  d. Be minimum 4 mm2 stranded copper for power circuits
  e. Wiring shall be 90° XHHW as defined by NEC Table 310-13
• Wires shall be identified at both ends with permanent wire markers. Stick-on wire markers and wire supports are not acceptable.
• Wiring, except CT wiring, shall be terminated with locking-fork, insulated, brazed-seam type connectors (see 5.9.2.5 for CT wiring).
• Wiring shall be terminated in terminal strips in the control cabinet. Terminal blocks shall be of solid one-piece design with an inter-phase barrier and hinged plastic covers.
• Auxiliary contacts shall be wired to terminals in the control cabinet.
• Control cabinet shall:
  Be NEMA 4X
  b. Be made of 300 series stainless steel or painted heavy gage sheet steel  with all stainless steel internal and external hardware. The coating shall follow requirements of section 5.14.
  c. Have a continuously hinged door that can be padlocked
  d. Not be mounted on radiators or on the high or low voltage terminal chambers
• Wiring entrances to the control cabinet shall be made through the sides or  bottom.
• Control cabinet shall have a drain with stainless steel screen at its lowest  point.
• Control cabinet shall have separate terminals for power and control  wiring. These terminals shall be identified as such. Terminal blocks shall be one-piece, phenolic barrier type, rated at 20A, 600 V minimum with prenumbered marking strips, suitable for two terminations per point. A maximum of two wires per point is permitted.
• Control cabinet shall not have any sharp edges or corners inside or outside.
• Disconnect devices shall be identified with engraved nameplates and shall  have provisions for padlocking in the ‘off’ position. Relays, switches, terminal blocks and other devices shall be identified by engraved nameplates. Nameplates on exposed surfaces shall be securely attached without the use of adhesives. Adhesives may be used on nameplates attached to internal surfaces of control panels and similar enclosures. 
• Terminals for wiring 50 V and higher shall be guarded.
• All energy isolating devices shall be lockable in the OFF or disconnect  position.
• Control cabinets shall be provided with a 240 V rated space heater with  thermostat to be operated at 120 V, a 120 V convenience outlet and a switched 120 V overhead light. A door-mounted ammeter shall be provided for indication of space heater operation. A wiring diagram indicating the control cabinet circuits and fuse or circuit breaker operation shall be affixed inside the cabinet door.

Oil-Immersed Power Transformers Coatings

1535. a) Transformer assembly (including tank, radiators, throats and  terminal chambers) containing external ferrous parts shall be cleaned to near white metal and sealed with a moisture resistant, long-life coating.
      b) Finish coat shall have a nonmetallic pigment surface. Color of finish coat shall be light gray number 61 in accordance with ASTM D 1535. (Munsell Notation 8.3 G6 10/0.54) and covered with an epoxy coat for protection against chalking and fading due to environmental conditions.
      c) A litre of touch up paint with a Material Safety Data Sheet (MSDS) sheet for the finish coat shall be provided.
      d) Base of the tank and channels shall be coated with an asphalt based paint.

Coating System Qualification

1. a) Coating system shall be qualified by testing a minimum of four  panels.
2. b) Panels shall be of standard thickness ferrous metal equivalent to that used for the transformer enclosure.
   c) Test panels shall be coated with the same coating to be used on the transformer enclosure and shall be allowed to age for a minimum of seven days before being tested.
   d) Test apparatus shall conform to ASTM B 117. 
   e) Two of the test panels shall be scribed.
   f) Panels shall be tested in accordance with ASTM D 1654. Panels  shall be tested in a salt spray chamber for a continuous period of 200 hours except for the interruptions required to inspect the panels or replenish the solution in the reservoir.
   g) After completion of the time period in the salt spray chamber, the test panels shall be processed in accordance with ASTM D 1654.
   h) Evaluation of the coating performance shall be conducted on the scribed panels and shall be judged to be acceptable if their rating number is 5 or higher as determined by ASTM D 1654.
   i) Coating on unscribed panels shall be judged to be acceptable if their blistering size is 6 or higher, and their frequency designation is F or M, as determined by ASTM D 714.

Manual Tap Changer

• High voltage winding shall have a manual tap changer, in accordance with Section 1.1 of ANSI C57.12.10, for de-energized operation. The tap changer shall be capable of all voltage requirements and handling the maximum 65 °C current in the winding.
• Tap changer handle shall be able to be padlocked, and shall have visible indication of the tap position without unlocking.
• The mechanism shall be of a type that it will be impossible to leave a winding open or short circuited when the operating handle is placed in a locked position.
• For a given winding, the number ‘1’ or the letter ‘A’ shall be assigned to the tap that has the greatest number of effective turns.
  15.5 Unless otherwise specified on the data sheet, tap changer shall:
  a. Be five-position 
  b. Have four (4) 2 and 2 /1 percent full capacity taps, two above and two below rated primary voltage
  c. Be located on the side of the transformer tank
• There shall be a minimum of 1 square inch mating surface between tapchanger contacts.

On Load Tap Changer

• Transformers with primary voltage of 115 kV and above shall be provided with an automatic On Load Tap Changer (OLTC).
• The preferred method of tap changing is that using the high-speed transition, resistor principle, but methods using a reactor or auto transformer are acceptable. See Figure 1 for the circuit and description of operation of the preferred method.
• The resistor principle of on line tap changing has the advantage of relatively small sized resistors being required, but requires a transformer tap for each voltage required. The reactor or auto transformer principle uses a tap bridging position for operating voltage, thus requiring half the number of transformer taps. This method can also be used for resistors to increase the life of diverter switches.
• The number of taps and voltage tap range shall be as specified in the engineered equipment specification, but shall not be less than 16 and ±10 percent respectively.
• The arrangement of selector switch and diverter switch shall be the manufacturer’s standard, but the minimum requirement is that the diverter switch shall be mounted in a separate oil tank mounted inside the transformer main tank. The diverter switch oil tank shall be accessible from the top cover of the transformer main tank enabling the switch to be removed from its tank for maintenance if required. It shall be fitted with a pressure relief diaphragm with single pole, double throw alarm contacts, local position switch indicator and provision for analog or digital remote indication of the actual OLTC position. Operation of the tap changer shall be by single or three phase totally enclosed induction motor. Manual emergency crank, motor protection devices and mechanical and electrical overrun protection shall be provided.
• Voltage control shall be fully automatic by means of a solid state voltage controller mounted locally or, if provided, at the transformer supervisory panel.
• Transformers with three windings shall be provided with an on-load tap changer on the secondary windings and on the tertiary winding. Each on load tap changer shall operate independently.

Oil-Immersed Power Transformers Short Circuit

The transformer shall be capable of withstanding without damage, the mechanical and thermal stresses caused by short circuits on the external terminals of any winding or windings in accordance with ANSI C57.12.90. In determining the maximum short circuit current, only the impedance of the transformer shall be considered.