Application - ABB 615 series Technical Manual

Section 4
Protection functions
4.1.5.5
280
The temperature calculation is initiated from the value defined with the Initial
temperature and Max temperature setting parameters. The initial value is a percentage of
Max temperature defined by Initial temperature. This is done when the protection relay
is powered up or the function is disabled and then enabled or reset through the Clear 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, trip and lockout temperatures for 49T 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 Trip temperature setting, the TRIP output is
activated. The TRIP 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 Trip temperature.
There is also a calculation of the present time to trip with the present current. T_TRIP
calculation is only monitored if the final temperature is calculated to be above the trip
temperature. The value is available in the monitored data view.
After tripping, 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-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.
1MAC059074-MB A
615 series ANSI
Technical Manual