Functions
2.9 Thermal Overload Protection 49
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
152
Example: Cable and current transformer with the following data:
Permissible Continuous Current I
max
= 500 A at Θ
u
= 40 °C
Maximum Current for 1 s I
1s
= 45 · I
max
= 22.5 kA
Current Transformer 600 A / 1 A
Example: Cable and current transformer with the following data:
Thus results:
The settings are: 49 K-FACTOR = 0.83; TIME CONSTANT = 33.7 min
Warning Elements
By setting the thermal warning element 49 Θ ALARM (adress 4204), a warning message can be issued before
reaching the tripping temperature. Tripping can thus be avoided by initiating early load reduction measures.
This warning element simultaneously represents the dropout level for the trip signal. Only when this threshold
is undershot, will the tripping command be reset and the protected equipment can be switched on again.
The thermal element level is given in % of the tripping overtemperature.
A current warning level is also available (parameter 4205 I ALARM). The setting is in secondary amperes and
should be equal to or slightly less than the permissible current k I
N sec
.. It can be used instead of the thermal
warning element by setting the thermal warning element to 100 % thus virtually disabling it
Extension of Time Constants
TIME CONSTANT set in address 4203 is valid for a running motor. When a motor without external cooling is
running down or at standstill, the motor cools down more slowly. This behavior can be modeled by increasing
the time constant by factor Kτ-FACTOR, set at address 4207. Motor stop is detected if the current falls below
the threshold value BkrClosed I MIN of the current flow monitoring (see side title "Current Flow Monitoring"
in Subsection 2.1.3.2). This understands that the motor idle current is greater than this threshold. The pickup
threshold BkrClosed I MIN affects also the following protection functions: voltage protection and restart
inhibit for motors.
If no differentiation of the time constants is necessary (e.g. externally-cooled motors, cables, lines, etc.) the Kτ-
FACTOR is set at 1 (default setting value).
Dropout Time after Emergency Starting
The dropout time to be entered at address 4208 T EMERGENCY must ensure that after an emergency startup
and after dropout of the binary input „>EmergencyStart“ the trip command is blocked until the thermal
replica is below the dropout threshold again.
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