• Distribution transformers: Only hot spot temperature in the windings and
thermal deterioration are considered.
• Medium power transformers: Hot spot temperature in the windings, thermal
deterioration and variations in the cooling modes are considered.
• Large power transformer: Hot spot temperature in the windings, thermal
deterioration, variations in the cooling modes and effects of stray leakage flux are
considered.
Conductors on top of the winding experience the maximum leakage field and the
highest transformer oil temperature. It would be natural to consider that conductors at
the top have the hottest spot. However, measurements have shown that the hottest spot
might be moved to conductors in the lower part of the winding. Therefore, direct hot
spot temperature measurement is difficult. Hence, it should be calculated using the
empirical formulae given by relevant standards. The hot spot temperature shall be
monitored continuously so that it will not exceed the transformer oil flashover value.
Figure 315 shows the complex transformer temperature distribution. The assumptions
made are:
• Oil temperature increases linearly from bottom to top irrespective of the cooling
system.
• Winding temperature rise is parallel to the oil temperature rise with constant
difference ‘g’ (average winding to average oil temperature gradient).
• The hot spot temperature rise is higher than the top winding temperature rise with
the factor called Hot Spot Factor (H).
Top oil
Hot spot
Top of winding
Average winding
Temperature
Bottom oil
Bottom of
winding
Height of transformer
Average oil
g
H g
x
Top of
winding
IEC15000440-1-en.vsdx
IEC15000440 V1 EN
Figure 315: Thermal diagram
Section 17 1MRK 502 071-UEN -
Monitoring
654 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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