ABB RELION 620 Series Technical Manual page 336

Section 4
Protection functions
330
During stressed situations in power systems, it is required to overload the transformers
for a limited time without any risks. The thermal overload protection provides
information and makes temporary overloading of transformers possible.
The permissible load level of a power transformer is highly dependent on the
transformer cooling system. The two main principles are:
• ONAN: The air is naturally circulated to the coolers without fans, and the oil is
naturally circulated without pumps.
• OFAF: The coolers have fans to force air for cooling, and pumps to force the
circulation of the transformer oil.
The protection has several parameter sets located in the setting groups, for example
one for a non-forced cooling and one for a forced cooling situation. Both the
permissive steady-state loading level as well as the thermal time constant are
influenced by the transformer cooling system. The active setting group can be
changed by a parameter, or through a binary input if the binary input is enabled for it.
This feature can be used for transformers where forced cooling is taken out of
operation or extra cooling is switched on. The parameters can also be changed when
a fan or pump fails to operate.
The thermal overload protection continuously estimates the internal heat content, that
is, the temperature of the transformer. This estimation is made by using a thermal
model of the transformer which is based on the current measurement.
If the heat content of the protected transformer reaches the set alarm level, a signal is
given to the operator. This enables the action that needs to be taken in the power
systems before the temperature reaches a high value. If the temperature continues to
rise to the trip value, the protection initiates the trip of the protected transformer.
After the trip, the transformer needs to cool down to a temperature level where the
transformer can be taken into service again. T2PTTR continues to estimate the heat
content of the transformer during this cooling period using a set cooling time constant.
The energizing of the transformer is blocked until the heat content is reduced to the set
level.
The thermal curve of two time constants is typical for a transformer. The thermal time
constants of the protected transformer are given in seconds with the Short time
constant and Long time constant settings. If the manufacturer does not state any other
value, the Long time constant can be set to 4920 s (82 minutes) for a distribution
transformer and 7260 s (121 minutes) for a supply transformer. The corresponding
Short time constants are 306 s (5.1 minutes) and 456 s (7.6 minutes).
If the manufacturer of the power transformer has stated only one, that is, a single time
constant, it can be converted to two time constants. The single time constant is also
used by itself if the p-factor Weighting factor p setting is set to zero and the time
constant value is set to the value of the Long time constant setting. The thermal image
corresponds to the one time constant model in that case.
1MRS757644 F
620 series
Technical Manual