k This factor indicates the maximum continuous permissible phase current. The factor
refers to the rated current of the protected object (k = I
max
/I
rated, obj
)
I
rated, obj
Rated current of the protected object
At the same time, I
rated, obj
is the rated current of the assigned protected object side:
•
In the case of transformers, the rated current of the winding to be protected, which the device calculates
from the set rated apparent power and rated voltage, is decisive.
•
The uncontrolled winding forms the basis in the case of transformers with voltage control
•
In the case of generators, motors and reactors, the rated current, which the device calculates from the
set rated apparent power and rated voltage, is decisive.
•
In the case of lines, nodes and busbars, the rated current of the protected object is set directly
In a steady state, the solution to the thermal differential equation is an e-function whose asymptote repre-
sents the final overtemperature Θ
end
. The time constant τ
th
determines the rise. After reaching an initial adjust-
able overtemperature threshold Θ
warn
( Threshold thermal warn. ), a warning indication is given.
If the overtemperature limit Θ
Off
(tripping overtemperature) is exceeded, an operate indication is immediately
issued and the equipment disconnected from the power supply. This threshold is specified at 100 % and corre-
sponds to the final temperature set at a flowing permissible continuous current (I
max
).
Figure 7-190 shows the temperature rise at different overload currents and the supervision thresholds.
[dwtempve-030211-01.tif, 1, en_US]
Figure 7-190 Temperature History for Different Overload Currents ( K-factor = 1.1)
The overtemperature is calculated separately for each phase. The current overtemperature can be obtained
from the operational measured values. It is shown in percent. An indication of 100 % means that the thermal
threshold has been reached. The maximum overtemperature of the phases is regarded as the tripping temper-
ature. This means that the highest of the 3 phase currents is always assumed.
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.
If the flowing current falls below an adjustable minimum current Imin cooling , the Cooling time
constant is activated.
Protection and Automation Functions
7.26 Thermal Overload Protection, 3-Phase - Advanced
SIPROTEC 5, High-Voltage Bay Controller, Manual 883
C53000-G5040-C015-9, Edition 11.2017
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