50 MVA 138KV AUTOTRANSFORMER SPECIFICATION SAMPLE

Type:    Outdoor use, Oil-immersed, OA/FA/FA, 3 Windings with rubber diaphragm
             conservator vented via silica gel dehydrating breather, On-load-tap changer,  
             manufactured according to ANSI C57.12.00 Std., All Copper Windings,                                                        
            For use as a Step-down transformer in an electric utility transmission substation.
           Complete with standard accesories.

Rating:
              HV    -     30/40/50 MVA
              LV    -      30/40/50 MVA
             TV    -      12/16/20 MVA

Cooling Method:  OA/FA1/FA2

Rated Voltage:      HV -  138KV
                            LV -    69KV
                            TV -   13.2KV

Tap Voltage:   HV Side OLTC: 138 KV + 8, - 12 x 1.0%,  21 Taps


OLTC: ABB type UZFRT 550/300, 138,000 Volts,
             3 Phase, 60 Hz, 21 positions,

            With Motor Drive Mechanism type BUF 3,
            Motor : 460 Volts, 3 Phase, 60 Hz
            Contactors: 230 Volts AC,
            Position Transmitter: 230 Volts AC
            Heating Element      : 230 Volts AC
            With Manual/Automatic Change over-switch,  Raise & Lower pushbuttons.
           
BIL:
      Winding             :    HV      –  650 KV
                                  LV       -  350 KV
                                  Neutral – 150 KV
                                TV       -  110 KV

     Bushing               : HV        -  650 KV
                              LV        -  350 KV
                                Neutral   - 150 KV
                                   TV          - 150 KV

Frequency : 60 Hz
Connection:
            HV  -  Star with Neutral (Auto-Star) brought out to a bushing
            LV  -   Star with Neutral (Auto-Star) brought out to a bushing
            TV  -   Delta

Vector Group : Yyna0d1

Guaranted Losses at rated voltage, frequency, unity pf & @ 85 deg C (50 MVA):        
          No -Load Loss:               18.6 KW
          Load Loss @ 50 MVA:   157.6 KW
         Efficiency :                       99.65% @ 50 MVA( Without Auxiliary Loss)

Temperature Rise Limits:
             Oil         -  65 deg C
             Winding – 65 deg C

% Impedances  @ 85 deg
                  HV – LV@ 50 MVA        HV - TV@ 20 MVA       LV - TV@ 20 MVA
         8L    -   149,040 V   -   10.28            -       10.45
         N     -   138,000 V   -   10.50            -       10.31              -            5.6          
       12R   -   121,440 V   -   11.09            -       10.52
     
           
Audible Sound Level @ 50 MVA with all fans running: 72dB

Service Condition:
                Maximum ambient air temperature:                                             40 deg C
                Average ambient air temperature for any 24h period:                  30 deg C
                Maximum altitude above sea level:                                             1000 meters
               Maximum ambient relative humidity:                                            88%
                Mean annual rainfall:                                                                  2400 mm
               Maximum wind velocity:                                                             220 km/hr
               Maximum seismic factor:                                                            0.45g

CONSTRUCTIONS
     
a) Core:
The core of the transformer will be constructed of the highest quality, non-aging high permeability, cold-rolled gain-oriented  silicon steel sheet especially suitable for the purpose. Every care will be taken during slitting and cutting process to avoid burrs. Both sides of each sheet will be special glass film insulated on to minimize eddy current losses. The cores will be carefully assembled and rigidly clamped to ensure adequate mechanical strength to support the windings and also reduced vibration to minimum under operating conditions.
      b) Windings:
                       The winding of the transformer shall be made of  high tensile strength electrolytic copper of a high conductivity (Class A, in accordance with ANSI)
                       and insulation material of high quality shall be used. The windings shall be  free from burrs, scales and splinters.
 
The insulation material of windings and connections shall not shrink, soften or collapse during service. Thermally upgraded paper shall be used for conductor insulation. The design, construction and treatment of windings shall give proper consideration to all service factors, such as high dielectric and mechanical strenght of insulation, coil characteristics, uniform electrostatic flux distribution, prevention of corona formation, and minimum restriction to oil flow.

Moreover, under any load condition, none of the material used shall disintegrate, carbonizer or become brittle under the action of hot oil.

The coils must be capable of withstanding movement and distortion caused by abnormal operating conditions. Adequate barriers shall be provided between windings and core as well as between high voltage and low voltage windings. All leads or bars from the windings to the terminal boxes and bushings shall be rigidly supported. Stresses on coils and connections must be avoided.

Due to very unfavorable short-circuit conditions and numerous short-circuits in the network, special measures have to be taken to increase the capability of the winding to withstand short-circuit currents. Winding and arrangement of coils shall be designed so as to unify the initial potential distribution caused by impulsive traveling waves, as much as possible, to avoid potential oscillation and in order to withstand abnormal high voltage due to switching.

To increase the capability of the transformer windings to witstand electromagnetic forces under short circuit conditions, modern technology in design and construction shall be applied. (e.g. low current density, provision of pressure limiting devices and spring elements, use of perfectly dried pre-compressed pressboard, maintaining a balance of ampere-turns between windings, ets.)

Measures against coil displacement as generated by the radial and longitudinal forces shall be considered. Computation of strength against these forces including the description of the method being applied shall be submitted in detail.

The tank, conservation, coolers and bushings shall be adequately braced to withstand ocean shipment, and earthquake with seismic coeffecient of 0.45 g (horizontal)

c)   Short Circuit Withstand Capability
The transformer shall withstand the combined effects of thermal, mechanical and electromagnetic stresses arising under short-circuit conditions based on the maximum durations of fault:

Primary  Winding: 2 seconds

Secondary  Winding: 2 seconds

Tertiary    Winding                    2          seconds

The maximum sustrained short-circuit current in each windings shall be stated by the manufacturers. The maximum temperatures of the windings shall not exceed 250 deg C within the seconds duration of fault. All transformer accessories, parts, components (CT's, bushings, tap-changer, etc.) shall be capable of withstanding the cumulative effects of repeated mechanical and thermal over-stressing as produced by short-circuits and loads above the nameplate rating.

For design purposes, the following network data shall be take into consideration. The available system fault currents as as follows (in rms):

138 KV: Ik"  60 KA         69 KV : Ik" = 50 kA 13.2 KV : Ik" = 40 kA

The transformer shall be capable of withstanding the resulting successive short-circuits, without cooling to normal operating temperature between successive occurence of the short-circuit, provided the accumulated duration of short-circuit does not exceed the maximum duration permitted for single short-circuit defined above.

The upper limits of the symmetrical overcurrent due to such short-circuits as a multiple of rated current shall also be specified by the manufacturer.

      d)  Overload Capability
The short-time overload rating and operation of the transformer shall be in accordance with ANSI C57.92 or IEC 354. All other auxiliary equipment (bushings, CT's, etc) affected shall be rated to match the transformer overload rating.

e) Transformer Tanks:
The tank should have sufficient strength to withstand full vacuum and internal pressure of 1.0 kg/cm2, with cooling equipment & conservator connected. The tank cover will be clamped with bolts and nuts, and will be provided with handhole or manholes of suitable size. All seams and jointwill be oil tight. Guides within the tank will be furnished to facilitate tanking and untanking, and to prevent movement of the core and coil assembly, in transit. The casing will be provided with suitable lugs for lifting the completely assembled transformer filled with oil. All gaskets will be synthetic rubber bonded cork.

f) Radiators:
The transformer will be provided with a number of sufficient radiators for self-cooled (OA) operation. The radiator will be installed on the tank via radiator valves, so that each radiator can be detached from the tank independently of the oil in the main tank. The radiator valves will have the open and close positions clearly marked. Radiators will be equipped with provisions for draining. Radiators shall be made of galvanized steel.

g) Forced-air-cooling system:
For forced-air-cooled (FA) operation, the transformer will be provided with automatically controlled three phase motor-fans actuated from winding temperature. Fan motor, weather proofed, three phase, Hz, and will be thermal protected. The cooling-fans will be mounted on the radiators and the control box will be mounted on the wall of the tank. Motor Voltage: 460 VAC, 3 phase, 60 Hz.

h) On-load tap-changer:
The following tap-changer will be equipped on H.V. side for the regulation of voltage under loading conditions.
Type Type UZFRT 550/300
                        3 phase,60 Hz, 21 positions
Number of tap positions           21 taps positions
                                                                                   (138KV + 8 X 1380 V, - 12 X 1380 V)
                                   Manufacturer ABB

                                    Motor Drive Mechanism:
                                    Type:                                        ABB type BUF 3
Motor Voltage: 460 Volts, 3 Phase, 60 Hz
                                    Contactors Voltage:                  230 VAC
                                    Position Transmitter:                 230 VAC
                                    Heating Element:                       230 VAC
                             
                            Motor-Drive Mechanism Accessories:
                                   1. Standard Accessories
                                   2. Phase Failure Relay
                                   3.Circuit Breakers for Control & Auxiliary circuits
                                   4. Accessories for paralleling with 2 transformers using MASTER-FOLLOWER method.

                          OLTC Accessories:
                                   1.  Oil Conservator
                                   2.  Oil Level Indicator with contacts for Alarm
                                   3.  Dehydrating Breather
                                   4.  Pressure Relief Valve/Device with contacts for tripping
                                   5.  Pressure Relay with contacts for tripping
                                   6. Oil Flow Controlled Relay with contacts for alarm
                                   6. Thermoswitch Housing
                                   7. Valve for oil filtration mounted on the top
                                   8. Valve for oil filling, draining & filtration
                                   9. Earthing terminal
                                  10. Prepared for on - line oil filter unit



i) Oil preservation system:
Conservator system with sealed diaphragm will be used. Conservator with low-profile design having a moisture-proof barrier made with an oil-resisting diaphragm will be applied and placed at the level slightly higher than the transformer tank.

j) Bushings:
Primary:          ABB type GOB 650-1250-0.3 Brown, Cat # 123 193-K
                                              1250 Amps, Nominal Voltage: 170 KV rms,
                                               Phase to Earth Voltage:145 KV rms, BIL: 650KV,
                                               Creepage Distance: 4080 mm
                                               Porcelain Color: Brown
                                               Short end shield

Secondary:       ABB type GOB 380-800-0.3 Brown, Cat # 123-185-K
           800 Amps, Nominal Voltage: 100 KV rms,
                                                Phase to Earth Voltage:72.5 KV rms, BIL: 380 KV
                                                Creepage Distance:2210 mm
                                                Porcelain Color: Brown
                                                Short end shield

                       Tertiary:           CEDASPE s.p.a. Italy type Dt 30 Nf 1000
                                              1000 Amps, Nominal Voltage: 36 KV,
                                               Maximum Voltage to Ground: 30 KV, BIL:170 KV,
                                               Creepage Distance: 640 mm
                                               Porcelain Color: Brown
                                               Threaded Extended Rod

                      Neutral:            CEDASPE s.p.a. Italy type Dt 52 Nf 1000
                                              1000 Amps, Nominal Voltage: 52 KV
                                              Maximum Voltage to Gound: 52 KV, BIL 250 KV,
                                              Creepage Distance: 1080 mm
                                              Porcelain color: Brown
                                             Threaded Standard Rod


Complete with the following accessories:

31.  One (1) Buchholz Relay with 2 contacts for alarm & tripping
32.  Two (2) Dial type Oil Level Indicators for Main Tank & OLTC with contacts for alarm.
33.  One (1) Oil Temperature Indicator & Relay type AKM OTI series 34 for alarm.
34.  Three ( 3) Winding Temperature Indicators & Relays for HV, LV & TV windings with
        3 contacts each for alarm, tripping & fan control, AKM type WTI series 35.
35.  Qualitrol type self resetting mechanical Pressure Relief Device with contacts for tripping
36.  Conservator for main Tank - Sealed Diaphragm constant pressure type.
38.  Breather type conservator for OLTC.
39.  Annunciators (Marshalling Kiosk)
40.  Bushing Current Transformers
         HV:     300/200/100:5A;    0.6 - B 0.5
         LV & Neutral:  600/500/400/300/200/100:5A;   C-400
         TV :    1200/1000/900/800/600/500/400/300/200/100:5A;  C-400
       

41.     Galvanized Steel Radiators

42.     Bushing Terminals

         HV- Universal 4 hole NEMA Flat Terminals
         LV- Universal 4 hole NEMA Flat Terminals
         TV – Universal Multi -hole NEMA Flat Terminals

43.   Sets of Surge Arresters mounted nearest to the HV, LV & TV transformer bushings,
       with Surge Counters & 4/0 AWG THW Copper conductors connected
       to grounding terminals.                                                                                                                                                                              


HV     : 120 KV Voltage Rating, 98 KV MCOV, Station Class, Polymer housing,      
      Metal Oxide, Line Discharge Class 4 per IEC, 12 KJ/KV Energy capability
      65 KA Pressure Relief Capability, Grey Silicone Insulator
      ABB type PEXLIM-P
      Complete with top clamps to hold a 336.4 MCM Aluminum Conductor
and 4/0 AWG THW Copper green wire ground conductor connected to ground terminal.

       LV     : 60 KV voltage Rating, 48 KV MCOV, Station Class, Polymer housing, Metal
      Oxide, Line Discharge Class4 per IEC, 12 KJ/KV Energy Capability
      65 KA Pressure relief Capability, Grey Silicone Insulator,
      ABB type PEXLIM-P
      Complete with top clamps to hold a 795 MCM Aluminum Conductors
      and  a 4/0 AWG THW Copper green wire ground conductor connected
      to ground terminal.

       TV     : 18 KV Voltage Rating, 15 KV MCOV Station Class, Polymer housing,
      Metal Oxide, Line Discharge Class 3 per IEC, 9.0 KJ/KV energy capability
      65 KA pressure relief capability, Grey Silicone Insulator,
      ABB type POLIM-S 15N
                   Complete with top clamps to hold a 795 MCM Aluminum Conductors
      and a 4/0 AWG THW Copper green wire ground conductor connected to
      Ground terminal.

44.    Neutral Conductor: 4/0 AWG THW Copper wire colored green connected to
         ground pad.

45.   Insulating Oil – Shell Diala B or equivalent

46.  With provision for Built-in OLTC Insulating Oil Filter Machine, such as mounting
        brackets, connecting flange, connecting valves, etc.

47.  Cooling Fans must be 3 Phase, 460 VAC, 60 Hz, Winding Temperature Controlled
        for Automatic Operation, with automatic/manual change over switch.
       With Circuit breakers for motor overload & short circuit protection.

48.  Grounding Pads for HV Arresters, LV Arresters, TV Arresters & Neutral Cables.

49.  Steel Ladder with caution marking

50.  All External Power & Control cables must be flexible, multicore,  PVC insulated
       & enclosed in conduit pipes & flexible hoses.

51.  All power & control circuits must be protected by circuit breakers.

52. Welded Tank Cover.

53. All wiring connections & terminations must be ANSI standard using crimp type
       terminal lugs with insulator caps.

54.  All wirings must be color-coded.

55.  With replacement gaskets

56.  Anchor Bolts

57. One (1) Spare bushing each for HV, LV & TV

58. One (1) Spare OLTC Tap Position Indicator for remote use

59. With provisions for parallel operation of existing power transformer using
      Master - Follower method of OLTC Control.
       
60. TESTS:
      The following tests shall be carried out at the factory with the presence
      of user representative and records of testing will be submitted.
         a. Winding  Resistance Tests
         b. Turns Ratio Tests
         c. Polarity & Phase Relation Tests on rated voltage
        d. Measurement of no-load losses and excitation current @ 90%, 100%
           & 110% of rated secondary voltage @ rated frequency
           rated voltage connection.
        e. Measurement of impedance voltage and load loss tests at rated current
           and rated frequency.
        f. Low frequency tests (Applied Voltage & Induced Voltage) including
           partial discharge measurement in terms of RIV.
        g. Leak Test.
        h. Routine test certificate for the bushings, current transformers and surge
            arresters shall be submitted.
        i. Temperature Rise Test at OA, FA1 & FA2 ratings on the tapping
          with maximum losses.
       j. Lightning Impulse Test on HV, LV , Neutral & Tertiary terminals.
        (Full wave, Chopped wave & reduced full wave)
      k. Audible Sound level Test at no-load and rated frequency and
         with all fans operating.
            l.  Measurement of zero-sequence impedance.
           m. Insulation Power factor
           n.  Insulation Resistance Tests at ambient temperature.
           o. Vacuum test on transformer tank, conservator & radiators; and pressure
               test on tank and oil-filled compartments.
           p. Determination of Capacitances (windings to ground & between windings)
          q.  Tests on auxiliary equipment & accessories ( functional tests
               including cooling control)
           r.  Voltage regulation
          s. Measurement of the power taken by the fan and oil pumps motors.
          t. Functional tests on Tap Changer
          u. Test on Current Transformers ( Check on polarity, ratio & wiring)
          v. Mechanical inspection, ( check of layout, dimensions, nameplate
             data, clearances, etc)
         w. Oil tests
         x. Efficiency at principal tap and full load for unity & 0.8 power factor.
       
61.  SPECIAL TEST
       Certificate of Short Circuit Test on power transformers of similar rating
       shall be submitted.
   
62.  Other Accessories, Tools

    a. Pressure gauge with nitrogen tube and automatic filling device which
         fill the transformert through the tube in case of any leakage shall
         be supplied.
    b. Three-dimensional impact recorder with time period recording chart of
        at least 3 months for use during transport of the transformers.
    c. Silica -gel breathers for main and OLTC conservators.
 
63.  Painting
      Special attention should be given to the protection of all iron-work.
      The methods propised and the means adopted should be fully described
      in the offer.

     All surfaces shall be thoroughly cleaned of rust, scale, grease and dirt and
    other foreign matters and all imperfections shall be removed by means
    of approved methods.

     The following treatments shall be applied:

     a.  External surfaces

           All steel surfaces shall be sand-blasted in accordance with SIS 055900,
          and shall then be painted in the following sequence:
       
            1. two (2) primer coats:                      2 x 35 um (micrometer)
           
            Binder  : epoxy resin hardened with polyamide              
                  Pigment: titanium dioxide, zinc oxide, zinc phosphate, tinting additives

                  2. one (1) intermediate coat:               35 um
 
                 Binder:  epoxy resin hardened with polyamide
                 Pigment: titanium dioxide, micaceous iron oxide, tinting additives

                 3. one (1) top coat (polyurethane base):   35 um

                 Binder:  epoxy resin hardened with isocyanat
                 Pigment: titanium dioxide, micaceous iron oxide, tinting additives

                 Coating thickness:                   Total        140um

                 The color code shall be Munsell Gray No. N7.0

        b. Internal surfaces
   
                Inside the transformers vessel, sand-blasting shall be performed in
               accordance with SIS 055900. After that solvent-free,
               oil-resistant coating shall be applied.

               The minimum dry film thickness shall be 40 um.

25 KVA DISTRIBUTION TRANSFORMER SPECIFICATION SAMPLE TUTORIALS

25 KVA DISTRIBUTION TRANSFORMER SPECIFICATION SAMPLE
Specification Sample of 25 KVA Distribution Transformer


Pole Mounted Amorphous Transformer Specification

Brand:
Rating: 25 kVA, Single Phase
Primary Voltage: 7620/13,200V-Y
Secondary Voltage: 120/240 Volts
BIL: 95kV
Frequency: 60 hz
Cooling Class: ONAN
Temperature Rise: 65 deg. Centigrade
No Load Loss: at most 72 watts
Load Loss: at most 411 watts
Tap Changer: +/- 2 - 2.5% above and below nominal voltage The tap changer switch shall be an externally operated
through a rotating switch.Externally accessible.
Conductor: All Aluminum conductor
Core: Amorphous Alloy
Insulation: Oil Immersed (Mineral)
Primary Connection: Eye Bolt Clamp or Plug Type
Secondary COnnection: Eye Bolt Clamp or Plug Type
Other Features: (1) Double Bushing on the Primary side, H1 & H2
(2) Secondary Bushing shall be X1,X2,X3 configuration
(3) Polarity: Additive
(4) Lifting Lugs
(5) Support Lugs
(6) Collor: Gray



TESTED & BUILT IN ACCORDANCE WITH ALL APPLICABLE ANSI STANDARDS

69 kV VOLTAGE TRANSFORMER SPECIFICATION EXAMPLE TUTORIALS

69 kV VOLTAGE TRANSFORMER SPECIFICATION EXAMPLE 
Specification Example of 69 kV Voltage Transformer


VOLTAGE TRANSFORMER

Brand:
Rated Voltage (kV): 72.5kV
Rated Frequency (Hz): 60
Type:         EMFC 84
Design:         Outdoor
Basic Insulation Level (kV) : 350kV
Rated primary voltage and ratio 69,000 Grd Y/40,250
Ratio: 350/600 & 350/600:1
Accuracy Class at Standard Burden: 0.3
Thermal Burden Rating: 200 VA @ 300 ambient

15 kV VOLTAGE TRANSFORMER SPECIFICATION SAMPLE TUTORIALS

15 kV VOLTAGE TRANSFORMER SPECIFICATION SAMPLE 
Specification Sample of 15 kV Voltage Transformer



VOLTAGE TRANSFORMER

Brand:
Rated Voltage (kV): 15kV 15kV
Rated Frequency (Hz): 60 60
Type: VEF 24-03 (Single Bushing) VRM-24
Design: Outdoor Outdoor
Basic Insulation Level (kV) : 125 kV 125 kV
Rated primary voltage and ratio 8,400 for 14,400Y (8,400:120) 8,400 for 14,400Y (8,400:120)
Ratio: 70:1 70:1
Accuracy Class at Standard Burden: 0.3 W, X 0.3 W, X
Thermal Burden Rating: 690 VA @ 300 ambient Temp. 690 VA @ 300 ambient Temp.

DISTRIBUTION TRANSFORMER TECHNICAL SPECIFICATION EXAMPLE DOWNLOAD LINK

TECHNICAL SPECIFICATIONS OF DISTRIBUTION TRANSFORMER LINKS
Technical Specifications Distribution Transformer

DISTRIBUTION TRANSFORMER SAMPLE SPECS FROM INDIA


2.1 All equipment and material shall be designed manufactured and tested in accordance with the  latest applicable Indian Standard. IEC standard and CBIP manuals except where modified  and / or supplemented this specification.

2.2. Equipment and material confirming to any other standard, which ensures equal or better  quality, may be accepted. In such case copies of English version of the standard adopted  shall be submitted.

2.3. The Transformer offered shall in general comply with the latest issues including amendments  of the following Indian standards but not restricted to it.

3. System Description:
The distribution transformers shall be installed at outdoor/indoor location along 11 kV  distribution networks, which consists of both underground and overhead network. The HV  winding would be connected to SFU/OCB/VCB of the ring main unit through 11 kV (E)  XLPE/PILC cable. LV winding would be connected to switch type Fuse section pillar through 1.1  kV 1c, 400 sq mm XLPE cable.

4. Design Criteria:
4.1. The transformers shall be installed in hot, humid tropical atmosphere. All equipment  accessories and wiring shall be provided with tropical finish to prevent fungus growth..

4.2. The transformers shall be capable of continuous operation of rated output under the operating conditions of voltage and frequency variations as per statutory limits governed by relevant  Indian Standard and Electricity Act-2003 and its amendments in force.

4.3. The transformer shall conform to best engineering practice.

4.4. The design material construction shall be such that to secure reliability, economy, safe and  convenient operation and shall include   all specified or unspecified incidental items  necessary for similar equipment for convenient working in every respect.

4.5. The transformers shall be capable of withstanding the short circuit stresses due to terminal  fault between phase to phase and phase to ground on one winding with full voltage  maintained on the other windings for a minimum period of three seconds.

4.6. The transformers shall be free from annoying hum or vibration. The design shall be such as  not to cause any undesirable interference with radio or communication circuits.

4.7. Atmospheric Condition: The equipment offered shall be suitable for continuous satisfactory  operation in following prevailing climatic conditions.

DOWNLOAD THE COMPLETER DOCUMENT HERE!!!

POWER TRANSFORMERS COOLING CLASSES BASICS AND TUTORIALS

COOLING CLASSES OF POWER TRANSFORMERS BASIC INFORMATION
What Are The Cooling Classes of Power Transformers?


Since no transformer is truly an “ideal” transformer, each will incur a certain amount of energy loss, mainly that which is converted to heat. Methods of removing this heat can depend on the application, the size of the unit, and the amount of heat that needs to be dissipated.

The insulating medium inside a transformer, usually oil, serves multiple purposes, first to act as an insulator, and second to provide a good medium through which to remove the heat.

The windings and core are the primary sources of heat, although internal metallic structures can act as a heat source as well. It is imperative to have proper cooling ducts and passages in the proximity of the heat sources through which the cooling medium can flow so that the heat can be effectively removed from the transformer.

The natural circulation of oil through a transformer through convection has been referred to as a “thermosiphon” effect. The heat is carried by the insulating medium until it is transferred through the transformer tank wall to the external environment.

Radiators, typically detachable, provide an increase in the surface area available for heat transfer by convection without increasing the size of the tank. In smaller transformers, integral tubular sides or fins are used to provide this increase in surface area.

Fans can be installed to increase the volume of air moving across the cooling surfaces, thus increasing the rate of heat dissipation. Larger transformers that cannot be effectively cooled using radiators and fans rely on pumps that circulate oil through the transformer and through external heat exchangers, or coolers, which can use air or water as a secondary cooling medium.

Allowing liquid to flow through the transformer windings by natural convection is identified as “nondirected flow.” In cases where pumps are used, and even some instances where only fans and radiators are being used, the liquid is often guided into and through some or all of the windings. This is called “directed flow” in that there is some degree of control of the flow of the liquid through the windings.

The use of auxiliary equipment such as fans and pumps with coolers, called forced circulation, increases the cooling and thereby the rating of the transformer without increasing the unit’s physical size. Ratings are determined based on the temperature of the unit as it coordinates with the cooling equipment that
is operating.

Usually, a transformer will have multiple ratings corresponding to multiple stages of cooling, as the supplemental cooling equipment can be set to run only at increased loads.

Methods of cooling for liquid-immersed transformers have been arranged into cooling classes identified
by a four-letter designation as follows:

Table 2.1.2 lists the code letters that are used to make up the four-letter designation.

This system of identification has come about through standardization between different international standards organizations and represents a change from what has traditionally been used in the U.S. Where OA classified a transformer as liquid-immersed self-cooled in the past, it is now designated by the new

system as ONAN. 

Similarly, the previous FA classification is now identified as ONAF. FOA could be OFAF or ODAF, depending on whether directed oil flow is employed or not. In some cases, there are transformers with directed flow in windings without forced circulation through cooling equipment.

An example of multiple ratings would be ONAN/ONAF/ONAF, where the transformer has a base rating where it is cooled by natural convection and two supplemental ratings where groups of fans are turned on to provide additional cooling so that the transformer will be capable of supplying additional kVA. This rating would have been designated OA/FA/FA per past standards.

POWER TRANSFORMERS RATING BASICS AND TUTORIALS

RATING OF POWER TRANSFORMERS
What Are The Basic Rating of Power Transformers?


Power Transformer Rating

In the U.S., transformers are rated based on the power output they are capable of delivering continuously at a specified rated voltage and frequency under “usual” operating conditions without exceeding prescribed internal temperature limitations.

Insulation is known to deteriorate with increases in temperature, so the insulation chosen for use in transformers is based on how long it can be expected to last by limiting the operating temperature.

The temperature that insulation is allowed to reach under operating conditions essentially determines the output rating of the transformer, called the kVA rating. Standardization has led to temperatures within a transformer being expressed in terms of the rise above ambient temperature, since the ambient temperature can vary under operating or test conditions.

Transformers are designed to limit the temperature based on the desired load, including the average temperature rise of a winding, the hottest-spot temperature rise of a winding, and, in the case of liquid-filled units, the top liquid temperature rise.

To obtain absolute temperatures from these values, simply add the ambient temperature. Standard temperature limits for liquid-immersed power transformers are listed in Table 2.1.1.

The normal life expectancy of a power transformer is generally assumed to be about 30 years of service when operated within its rating. However, under certain conditions, it may be overloaded and operated beyond its rating, with moderately predictable “loss of life.”

Situations that might involve operation beyond rating include emergency rerouting of load or through-faults prior to clearing of the fault condition.

Outside the U.S., the transformer rating may have a slightly different meaning. Based on some standards, the kVA rating can refer to the power that can be input to a transformer, the rated output being equal to the input minus the transformer losses.

Power transformers have been loosely grouped into three market segments based on size ranges. These
three segments are:

1. Small power transformers: 500 to 7500 kVA
2. Medium power transformers: 7500 to 100 MVA
3. Large power transformers: 100 MVA and above

Note that the upper range of small power and the lower range of medium power can vary between 2,500 and 10,000 kVA throughout the industry.

It was noted that the transformer rating is based on “usual” service conditions, as prescribed by standards. Unusual service conditions may be identified by those specifying a transformer so that the desired performance will correspond to the actual operating conditions.

Unusual service conditions include, but are not limited to, the following: high (above 40˚C) or low (below –20˚C) ambient temperatures, altitudes above 1000 m above sea level, seismic conditions, and loads with total harmonic distortion above 0.05 per unit.