Siemens SIPROTEC 7UT613 series Manual page 210

2 Functions
Thermal Characteris-
tic
Cool Down, Drop-
Out
Resulting Character-
istic
210
The thermal characteristic allows an approximate calculation of the thermal loading of
the electrical motor rotor by load imbalance in the stator. This follows the simplified
equation:
with:
t Tripping time
O Asymmetry factor
I2 Negative sequence current
I Rated current of the protective object
N Obj
The asymmetry factor K designates how long a negative sequence current may flow
at nominal machine current. It is therefore the distinctive number of the object to be
protected.
If the constantly permissible unbalanced load „I2 ADM" is exceeded, the summation
of the warming negative sequence system power commences. In this context, the
current-time-area is calculated constantly to ensure a correct consideration of chang-
ing load cases. As soon as the current-time-area (I
asymmetry factor, the thermal characteristic is tripped.
The model of the heating of the object to be protected is limited to 200 % of the thermal
tripping limit.
The „pickup" of the unbalanced load protection falls back, when the allowable unbal-
anced load „I2 adm" is undershot. The thermal image maintains its state and an ad-
justable cool down time „T COOL DOWN" is started. In this context, this cool down
time is defined as the time required by the thermal replica to cool down from 100 % to
0 %. In synchronous machines this depends on the construction, especially the
damper winding. If there is again an asymmetrical loading during the cool-down
phase, the previous history is considered. The tripping time would then decrease con-
siderably.
As the thermal replica only works after exceeding the permissible continuous negative
sequence current „I2 adm", this value is the lower limit for resulting tripping character-
istic Fig. 2-95). The area of the thermal replica connects to it with increasing negative
sequence current. High negative sequence currents can only be caused by a phase-
to-phase short circuit, which must be covered in accordance with the grading coordi-
nation chart. The thermal characteristics are therefore cut off by the definite time I
stage (see above under „Definite time stage (DT)"). The trigger time of the thermal
replica does not fall below the trigger time of the I
2
/I ) · t) has exceeded the K
2 NObj
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7UT613/63x Manual
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