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Functions
2.7 Thermal Overload Protection 49
2.7
Thermal Overload Protection 49
The thermal overload protection prevents damage to the protected object caused by thermal overloading,
particularly in case of transformers, rotating machines, power reactors and cables. It is in general not neces-
sary for overhead lines, since no meaningful overtemperature can be calculated because of the great varia-
tions in the environmental conditions (temperature, wind). In this case, however, a current-dependent alarm
element is able to warn of an imminent overload.
Functional Description
2.7.1
The unit computes the overtemperature according to a thermal single-body model as per the following
thermal differential equation
[formel-therm-diffgl-wlk-010802, 1, en_US]
with
Θ
τ
th
Ι
k
Ι
N
The solution of this equation is an e-function in steady-state operation whose asymptote represents the final
overtemperature Θ
which is below the final overtemperature, an alarm is generated in order to allow a preventive load reduction.
When the second overtemperature threshold, i.e. the final overtemperature (= tripping temperature), is
reached, the protected object is disconnected from the network. The overload protection can, however, also
be set to Alarm Only. In this case, only an alarm is output when the final temperature is reached.
The overtemperatures 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 Ι
current Ι
:
Nom
= k·Ι
Ι
max
Nom
In addition to the k-factor, the time constant τ
settings of the protection.
Overload protection also features a current warning element Θ
element which can output an early warning that an overload current is imminent, even when the temperature
rise has not yet reached the alarm or trip temperature rise values.
The overload protection can be blocked via a binary input. In doing so, the thermal images are also reset to
zero.
104
– Current overtemperature, referred to the final overtemperature at
maximum permissible phase current k·Ι
– thermal time constant for the heating
– present rms current
– k–factor indicating the maximum permissible constant current
referred
– rated current of the device
. When the overtemperature reaches the first settable temperature threshold Θ
end
Nom
is described as a multiple of the rated
max
as well as the alarm temperature Θ
th
in addition to the temperature warning
alarm
must be entered as
alarm
SIPROTEC 4, 7SD80, Manual
E50417-G1100-C474-A2, Edition 02.2018
,
alarm
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