ABB RELION RET670, Applications Manual

Page: 907 / 1016

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 411 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
Heig
ht
of transformer
Average oil
g
H g
x
Top of
winding
IEC15000440-1-en.vsdx
IEC15000440 V1 EN-US
Figure 411: 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
1MRK 504 163-UUS A
Section 18
Monitoring
Transformer protection RET670 2.2 ANSI 901
Application manual