ABB 615 Series ANSI, Technical Manual

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Section 4
1MAC050144-MB C
Protection functions
302 615 series ANSI
Technical Manual
below 10 percent of the Current Reference value or the calculated temperature value falls
below Operate temperature .
There is also a calculation of the present time to operation with the present current.
T_TRIP
is only calculated if the final temperature is calculated to be above the operation
temperature. The value is available through the Monitored data view.
After operating, due to the thermal overload protection function, there can be a lockout to
reconnect the tripped circuit. The
BLK_CLOSE
lockout output is activated when the
device temperature is above the Reclose temperature lock-out release temperature setting
value. The time to lock-out release
T_ENA_CLOSE
is also calculated. The value is
available through the Monitored data view.
4.5.2.5 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-ground 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 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.