The analysis of the RMS values of the currents over a broad frequency band also includes the harmonic
components. These harmonic components contribute to the temperature rise of the equipment.
Current Influence
The thermal replica based on the single-body model applies only with limitations to high overcurrent situa-
tions (short circuits, motor startup currents). To avoid an overfunction of the overload protection, the thermal
replica must be influenced in case of overcurrents (exceeding I
threshold
). You can select between 2 strategies for
this:
•
Freezing of the thermal memory
•
Limitation of the input current for the thermal replica to the set current. The temperature rise is thereby
retarded at high currents.
Warning Thresholds
The thermal warning threshold issues a warning indication before the tripping threshold (tripping tempera-
ture) is reached. In this way, for example, a load can be reduced in sufficient time and a disconnection can be
avoided. If the first setting value of the user-defined characteristic curve is set to 1.1 of the rated current, a
thermal memory value of 83 % is set to the continuously flowing rated current.
Apart from the thermal warning threshold, the overload protection also has a current-warning threshold. This
current-warning threshold can signal an overload current in sufficient time before the overtemperature value
has reached the warning or tripping threshold.
Dropout of Tripping
Once the thermal memory has fallen below the setting value of the Dropout threshold operate, the trip
command is cancelled upon tripping. In contrast, the current-warning threshold and the thermal warning
threshold are reduced at a fixed dropout threshold (see Technical Data).
Behavior in the Event of Auxiliary-Voltage Failure
The behavior of the thermal replica can be controlled upon auxiliary-voltage failure via the setting parameter
Storage of thermal replica. You can save the thermal state for a time of 500 min. Once the supply
voltage returns, the thermal replica continues to function with the saved thermal state.
If the thermal replica is not saved, it will be reset to 0 upon failure of the auxiliary voltage.
Resetting the Thermal Memory
You can reset the thermal memory via the binary input indication >Reset thermal replica. The thermal
memory will then have a 0 value.
Emergency Start
Depending on the operating conditions, tripping can be blocked or closure enabled despite the permissible
thermal limits being exceeded. Upon activation of the binary input signal >Emergency start, tripping is
blocked and closure enabled. This does not affect the state of the thermal memory. After the input >Emer-
gency start disappears, the blocking remains in effect for the set Emerg. start T overtravel.
Blocking the Function
Blocking will cause a picked up function to be reset. The tripping function can be blocked externally or inter-
nally by the binary input signal >Block stage.
All indications then drop out and the thermal memory is not calculated.
Blocking Closure
The signal Block close can be used to prevent closure of the protected object at a high thermal load. The
signal is set if the tripping overtemperature is exceeded, and cancelled if the temperature falls below the
dropout threshold.
Protection and Automation Functions
6.20 Thermal Overload Protection, User-Defined Characteristic Curve
SIPROTEC 5, Overcurrent Protection, Manual 669
C53000-G5040-C017-8, Edition 07.2017
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