input. The
Env temperature Set
setting is also used when the ambient temperature
measurement connected to T2PTTR is set to “Not in use” in the X130 (RTD) function.
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 turned off and back on 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, 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.
4.1.7.5 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-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
1MRS757644 H
Protection functions
620 series
Technical Manual
347
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