ABB REC670 Applications Manual page 553

1MRK 511 401-UUS A
Bay control REC670 2.2 ANSI
Application manual
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 215 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 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
Top oil
g
Section 17
Monitoring
H g x
Hot spot
Top of winding
Average winding
Temperature
IEC15000440-1-en.vsdx
547