ABB RELION REX640 Technical Manual page 394

Protection functions
transformer manufacturers. The actual alarm, operating and lockout temperatures
for T2PTTR are given as a percentage value of the
When the transformer temperature reaches the alarm level defined with the
Alarm temperature setting, the ALARM output signal is set. When the transformer
temperature reaches the trip level value defined with the
setting, the OPERATE output is activated. The OPERATE output is deactivated when
the value of the measured current falls below 10 percent of the
value or the calculated temperature value falls below
There is also a calculation of the present time to operation with the present current.
T_OPERATE is only calculated if the final temperature is calculated to be above the
operation temperature. The value is available in the monitored data view.
After operating, there can be a lockout to reconnect the tripped circuit due
to the thermal overload protection function. The BLK_CLOSE lockout output is
activated when the device temperature is above the
release temperature setting value. The time to lockout release T_ENA_CLOSE is also
calculated. The value is available in the monitored data view.
4.1.6.6 Application
The transformers in a power system are constructed for a certain maximum load
current level. If the current exceeds this level, the losses are higher than expected.
This results in a rise in transformer temperature. If the temperature rise is too high,
the equipment is damaged:
• Insulation within the transformer ages faster, which in turn increases the risk of
internal phase-to-phase or phase-to-earth faults.
• Possible hotspots forming within the transformer degrade the quality of the
transformer oil.
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
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1MRS759142 F
Max temperature setting.
Operate temperature
Current Reference
Operate temperature .
Reclose temperature lockout
Technical Manual
REX640