ABB Relion 670 Series Applications Manual page 507
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- Technical manual (1432 pages) ,
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1MRK 511 401-UEN Rev. K
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
249
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 '
(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 (
Top of
winding
Bottom of
winding
IEC15000440 V1 EN-US
Figure 249: 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.
IEEE C57.91-1995 standard has developed four different loading conditions beyond nameplate
to explain the risk involved in the higher operating temperatures, see Figure 250. The four
types of loading are:
Bay control REC670
Application manual
shows the complex transformer temperature distribution. The assumptions made
Top oil
Average oil
Bottom oil
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H ).
H g
x
g
Hot spot
Top of winding
Average winding
Temperature
IEC15000440-1-en.vsdx
Section 18
Monitoring
g '
501
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