ABB 615 series Technical Manual page 287

1MAC059074-MB A
615 series ANSI
Technical Manual
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. 49T 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.
Section 4
Protection functions
281