ABB REC670 Applications Manual page 554

Section 17
Monitoring
548
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
216. The four types of loading are:
• Normal life expectancy
• Normal life expectancy loading: The transformer loading is continuous at
rated output when operated under usual conditions.
• Sacrifice of life expectancy
• Planned loading beyond nameplate: Restricted to transformers that do not
carry a continuous steady load and it is a normal, planned repetitive load.
• Long time emergency loading: Loading results from the prolonged outage of
some system element. This is not a normal operating condition, but may
persist for some time.
• Short time emergency loading: Unusually heavy loading for short time due
to occurrence of one or more unwanted events that disturb the normal system
loading seriously.
1MRK 511 401-UUS A
Bay control REC670 2.2 ANSI
Application manual