ABB REG670 Applications Manual page 661

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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
316. 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.
Section 17
Monitoring
655