Transformer Class
≤500 kVADistribution

Distribution Transformers

≤500 kVA

Step-down transformers for final voltage conversion to utilization voltage (400/230V). Pole-mounted or pad-mounted. Serve residential, commercial, and light industrial loads. Typically oil-filled or dry-type.

Standard
IEC 60076 / ANSI C57
Ratings
11 sizes

Standard kVA Ratings

IEC 60076
Rating (kVA)Primary VoltageSecondary VoltagePhaseNotes
1611/22/33 kV400/230VThree-phaseSmall commercial, rural
2511/22/33 kV400/230VThree-phaseSmall commercial
5011/22/33 kV400/230VThree-phaseCommercial, small industrial
6311/22/33 kV400/230VThree-phaseStandard distribution (common)
10011/22/33 kV400/230VThree-phaseStandard distribution (very common)
16011/22/33 kV400/230VThree-phaseLarge commercial (common)
20011/22/33 kV400/230VThree-phaseIndustrial, large commercial
25011/22/33 kV400/230VThree-phaseStandard industrial
31511/22/33 kV400/230VThree-phaseVery common distribution
40011/22/33 kV400/230VThree-phaseLarge distribution
50011/22/33 kV400/230VThree-phaseMaximum distribution size

Technical Notes

  • • R10 series: Ratings follow IEC 60076 preferred numbers (16, 25, 40, 63, 100...)
  • • Impedance: Typically 4-6% for distribution transformers
  • • Vector group: Dyn11 most common (delta primary, wye secondary, 30 ° lag)
  • • Cooling: ONAN (oil natural, air natural) for most; ONAF with fans for overload
Transformer Class
630-5MVAPower Medium

Power Transformers (Medium)

630-5,000 kVA

Substation transformers for medium voltage distribution networks. Step-down from transmission (66-150kV) to distribution voltage (11-33kV). Oil-immersed with tap changers. Critical grid infrastructure.

Standard
IEC 60076
Ratings
10 sizes

Standard kVA Ratings

IEC 60076
Rating (kVA)Primary VoltageSecondary VoltagePhaseNotes
63033/66/132 kV11/22/33 kVThree-phaseSmall substation
80033/66/132 kV11/22/33 kVThree-phaseDistribution substation
1,00033/66/132 kV11/22/33 kVThree-phaseStandard substation (common)
1,25033/66/132 kV11/22/33 kVThree-phaseDistribution substation
1,60033/66/132 kV11/22/33 kVThree-phaseStandard substation
2,00033/66/132 kV11/22/33 kVThree-phaseLarge substation (common)
2,50033/66/132 kV11/22/33 kVThree-phaseTransmission substation
3,15033/66/132 kV11/22/33 kVThree-phaseMajor substation
4,00033/66/132 kV11/22/33 kVThree-phaseLarge transmission
5,00033/66/132 kV11/22/33 kVThree-phaseMaximum medium power

Technical Notes

  • • Tap changer: OLTC (on-load tap changer) for voltage regulation ±10%
  • • Impedance: 8-12% typical; higher for fault current limitation
  • • Protection: Buchholz relay, pressure relief, temperature indicators
  • • Losses: No-load (core) and load (copper) losses per IEC 60076-10
Transformer Class
6.3-500MVAPower Large

Power Transformers (Large)

6,300-500,000 kVA

High-voltage transmission transformers for grid interconnection. Step-up at generating stations or step-down at transmission substations. Three-phase banks or single-phase units. Critical national infrastructure.

Standard
IEC 60076
Ratings
14 sizes

Standard kVA Ratings

IEC 60076
Rating (kVA)Primary VoltageSecondary VoltagePhaseNotes
6,300132/220/400 kV33/66/132 kVThree-phaseTransmission substation
10,000132/220/400 kV33/66/132 kVThree-phaseLarge transmission
16,000132/220/400 kV33/66/132 kVThree-phaseGrid interconnection
20,000132/220/400 kV33/66/132 kVThree-phaseMajor substation
25,000132/220/400 kV33/66/132 kVThree-phaseRegional grid
31,500132/220/400 kV33/66/132 kVThree-phaseTransmission hub
40,000132/220/400 kV33/66/132 kVThree-phaseLarge transmission
50,000132/220/400 kV33/66/132 kVThree-phaseMajor grid connection
63,000220/400/765 kV132/220 kVThree-phaseEHV transmission
100,000220/400/765 kV132/220 kVThree-phaseUltra-high voltage
160,000400/765 kV220/400 kVThree-phaseGenerator step-up
250,000400/765 kV220/400 kVThree-phaseLarge power plant
400,000400/765 kV220/400 kVThree-phaseMajor generating station
500,000765 kV400 kVThree-phaseMaximum standard

Technical Notes

  • • Generator step-up (GSU): Typically 20-500 MVA; connects generators to grid
  • • Autotransformers: Used for EHV (400/765kV); more efficient but less isolation
  • • Transport: Large units may exceed road/rail limits; shipped in sections
  • • Cooling: OFAF (oil forced, air forced) or OFWF (water cooled) for large units
Transformer Class
Air CooledDry-Type

Dry-Type Transformers

Indoor Applications

Air-cooled transformers for indoor installations where fire safety is critical. No oil, reduced fire risk. Used in buildings, hospitals, data centers, and hazardous locations. Lower overload capacity than oil-filled.

Standard
IEC 60076-11
Ratings
13 sizes

Standard kVA Ratings

IEC 60076
Rating (kVA)Primary VoltageSecondary VoltagePhaseNotes
25400/230V400/230VThree-phaseIsolation, small loads
5011/22 kV400/230VThree-phaseBuilding services
6311/22 kV400/230VThree-phaseCommercial buildings
10011/22 kV400/230VThree-phaseStandard dry-type (common)
16011/22 kV400/230VThree-phaseLarge buildings
20011/22 kV400/230VThree-phaseIndustrial indoor
25011/22 kV400/230VThree-phaseStandard size (common)
31511/22 kV400/230VThree-phaseLarge commercial
40011/22 kV400/230VThree-phaseData centers
50011/22 kV400/230VThree-phaseMaximum common dry-type
63011/22/33 kV11/22 kVThree-phaseLarge indoor
80011/22/33 kV11/22 kVThree-phaseSpecial applications
1,00011/22/33 kV11/22 kVThree-phaseMaximum standard dry-type

Technical Notes

  • • Insulation class: Class F (155 °C) or H (180 °C) for windings
  • • Cooling: AN (air natural) or AF (forced air with fans for overload)
  • • Fire safety: No oil = reduced fire risk; suitable for indoor, hospitals, high-rises
  • • Efficiency: Slightly lower than oil-filled; higher no-load losses