Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.1.5.6
REX640
Technical Manual
menu. The temperature is stored in a nonvolatile memory and restored if the protection
relay is restarted.
The Max temperature setting defines the maximum temperature of the transformer in
degrees Celsius (°C). The value of the Max temperature setting is usually given by
transformer manufacturers. The actual alarm, operating and lockout temperatures for
T2PTTR are given as a percentage value of the Max temperature setting.
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 Operate temperature setting,
the OPERATE output is activated. The OPERATE output is deactivated when the value
of the measured current falls 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_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 Reclose temperature lockout release
temperature setting value. The time to lockout release T_ENA_CLOSE is also
calculated. The value is available in the monitored data view.

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.
Section 4
Protection functions
371