2.21 Thermal Overload Protection
373
7SD5 Manual
C53000-G1176-C169-1
2.21 Thermal Overload Protection
The thermal overload protection prevents damage to the protected object caused by
thermal overloading, particularly in case of transformers, rotating machines, power re-
actors and cables. It is in general not necessary for overhead lines, since no meaning-
ful overtemperature can be calculated because of the great variations in the environ-
mental conditions (temperature, wind). In this case, however, a current-dependent
alarm stage can signal an imminent overload.
2.21.1 Method of Operation
The unit computes the overtemperature according to a thermal single-body model as
per the following thermal differential equation
with
The solution of this equation is in steady-state operation is an e-function whose as-
ymptote represents the final temperature
Θ
End
. When the overtemperature reaches
the first settable temperature threshold
Θ
alarm
, which is below the overtemperature, a
warning alarm is given in order to allow a preventive load reduction. When the second
temperature threshold, i.e. the final overtemperature = tripping temperature, is
reached, the protected object is disconnected from the network. The overload protec-
tion can, however, also be set to
Alarm Only
. If this option is set, the device only
outputs an alarm, even if the end temperature is reached.
The temperature rises are calculated separately for each phase in a thermal replica
from the square of the associated phase current. This guarantees a true RMS value
measurement and also considers the effect of harmonic content. A choice can be
made whether the maximum calculated overtemperature of the three phases, the
average overtemperature, or the overtemperature calculated from the phase with
maximum current should be decisive for evaluation of the thresholds.
The maximum permissible continuous thermal overload current
I
max
is described as a
multiple of the nominal current
I
N
:
I
max
= k·
I
N
In addition to the k-factor, the time constant
τ
th
as well as the alarm temperature
Θ
alarm
must be entered as settings of the protection.
Overload protection also features a current warning element
I
alarm
in addition to the
temperature warning stage. It reports an overload current prematurely, even if the cal-
Θ
– Actual operating temperature expressed in per cent of the oper-
ating temperature corresponding to the maximum permissible
operating current k·
I
N
τ
th
– Thermal time constant for the heating
I
– Present rms current
k
– k–factor indicating the maximum permissible constant current
referred to the nominal current of the current transformers
I
N
– Nominal current of the current transformers
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