ABB Relion 670 Series Applications Manual page 480

Section 17
Monitoring
Top of
winding
Bottom of
winding
IEC15000440 V1 EN-US
Figure 220: 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 221. The four types of
loading are:
• Normal life expectancy
•
• Sacrifice of life expectancy
474
Top oil
Average oil
Bottom oil
Normal life expectancy loading: The transformer loading is continuous at rated output
when operated under usual conditions.
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H g
x
Top of winding
Average winding
g
Temperature
IEC15000440-1-en.vsdx
1MRK 511 401-UUS F
Hot spot
Bay control REC670
Application manual