ABB REC670 Applications Manual page 514

Section 17
Monitoring
508
• 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 of
winding
Average oil
Bottom oil
Bottom of
winding
IEC15000440 V1 EN-US
Figure 215: Thermal diagram
Winding hot spot temperature depends on the oil temperature inside the winding,
load losses in the winding, cooling type and ambient temperature. For most
transformers in service, oil temperature inside a winding is difficult to measure. On
the other hand, top oil temperature at the top of the tank is well known, either by
measurement or calculation.
In addition to loading of the transformer, oil temperature rise depends also on
reduced oil flow inside the winding and malfunctioning/failure of the cooling
system (water or air circulation). Therefore, hot spot temperature can be measured
by sensing top oil temperature without separately considering the effects of oil flow
blockage and malfunction of cooler groups.
Normal life expected of the transformer is a conventional reference based on the
designed operating condition and ambient temperature. If the transformer load
exceeds its rated condition, ageing will accelerate. Consequences of excessive
transformer loading leads to unacceptable temperature rise in windings, leads,
insulation and oil. When temperature changes, moisture and gas content in the
insulation and oil will change.
Top oil
g
1MRK 511 401-UEN A
H g
x
Hot spot
Top of winding
Average winding
Temperature
IEC15000440-1-en.vsdx
Bay control REC670 2.2 IEC
Application manual