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Thermal limits

When modelling the hot-spot and top-oil temperature, it is also important to know what temperatures are considered acceptable. Below, a brief description can be found of different temperatures prescribed by the loading guide.

Static loadability

In the standards (IEC 60076-1 paragraph 4.2 & IEC 60076-2 Table 1), temperature limits are described for the environment, oil, and windings that must not be exceeded under continuous load with the assigned power.

  • Ambient temperature:
  • Always between -25°C and +40°C;
  • The average temperature in the hottest month is less than +30°C;
  • The annual average does not exceed +20°C.
  • Transformer temperature:
  • The top-oil temperature rise is less than 60 Kelvin;
  • The average winding temperature rise is less than 65 Kelvin;
  • The hot-spot temperature is less than:
    • Normal paper: 98°C (78 Kelvin temperature rise);
    • Thermally Upgraded Paper (TUP): 110°C (90 Kelvin temperature rise).

Cyclic loadability

In the loading guide, guidelines are given regarding cyclic loadability (IEC 60076-7, chapter 7, table 2). A distinction is made between:

  • Normal cyclic loadability: a loadability that can always be maintained with a cyclic profile.

  • Limit top-oil temperature: 105 °C

  • Limit hot-spot temperature: 120 °C

  • Limit current load: 150% (< 100MVA), 130% (> 100 MVA)

  • Long-term emergency load: a loadability that can be maintained for several days with a cyclic profile.

  • Limit top-oil temperature: 115 °C
  • Limit hot-spot temperature: 140 °C

  • Limit current load: 180% (< 10MVA), 150% (10 - 100 MVA) and 130% (> 100 MVA)

  • Short-term emergency load: a loadability that can be maintained for several hours with a cyclic profile.

  • Limit top-oil temperature: 115 °C
  • Limit hot-spot temperature: 160 °C
  • Limit current load: 200% (< 10MVA), 180% (10 - 100 MVA) and 150% (> 100 MVA)