Functions
6-2657SA6 Manual
C53000-G1176-C156-2
6.19 Thermal Overload Protection
6.19.1 Method of Operation
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.
The unit computes the overtemperature according to a thermal single-body model as
per the following thermal differential equation
with Θ – instantaneous overtemperature referred to the final temperature
rise for the maximum permissible line current
k·I
N
τ
th
– thermal time constant for heating
k–k-factor which states the maximum permissible continuous
current referred to the rated current of the current transformers
I – currently measured r.m.s. current
I
N
– rated current of current transformers
The solution of this equation under steady-state conditions is an e-function whose as-
ymptote shows the final overtemperature
Θ
end
. When the overtemperature reaches
the first settable temperature threshold
Θ
alarm
, which is below the final overtempera-
ture, a warning alarm is given in order to allow a timely load reduction. When the sec-
ond temperature threshold, i.e. the final overtemperature or tripping temperature, is
reached, the protected object is disconnected from the network. The overload protec-
tion can, however, also be set on
Alarm Only. In this case only an alarm is output
when the final overtemperature is reached.
The overtemperature is calculated separately for each phase with a thermal replica
from the square of the associated phase current. This guarantees a true r.m.s. value
measurement and also includes the influence of harmonic content. A choice can be
made whether the maximum calculated overtemperature of the three phases, the av-
erage overtemperature, or the overtemperature calculated from the phase with maxi-
mum 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 rated current
I
N
:
I
max
=k·I
N
In addition to the k-factor, the time constant τ
th
as wellas the alarm temperatureΘ
alarm
must be entered in the protection.
Apart from the thermal alarm stage, the overload protection also includes a current
overload alarm stage
I
alarm
, which can output an early warning that an overload cur-
rent is present, even if the overtemperature has not yet reached the alarm or trip over-
temperature values.
The overload protection can be blocked via a binary input. In doing so, the thermal im-
ages are also reset to zero.
dΘ
dt
--------
1
τ
th
-------
Θ⋅+
1
τ
th
-------
I
k I⋅
N
-------------
èø
æö
2
⋅=
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