Functions
2.7 Thermal Overload Protection 49
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
112
2.7 Thermal Overload Protection 49
The thermal overload protection prevents damage to the protected object caused by thermal overloading, par-
ticularly in case of transformers, rotating machines, power reactors and cables. It is in general not necessary
for overhead lines, since no meaningful overtemperature can be calculated because of the great variations in
the environmental conditions (temperature, wind). In this case, however, a current-dependent alarm element
is able to warn of an imminent overload.
2.7.1 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 an e-function in steady-state operation whose asymptote represents the final
overtemperature Θ
End
. When the overtemperature reaches the first settable temperature threshold Θ
alarm
,
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 as-
sociated phase current. This guarantees a true RMS value measurement and also considers the effect of har-
monic 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 rated
current I
Nom
:
I
max
= k·I
N
In addition to the k-factor, the time constant Ï„
th
as well as the alarm temperature Θ
alarm
must be entered as set-
tings of the protection.
Overload protection also features a current warning element I
alarm
in addition to the temperature warning
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.
Θ – Current overtemperature, referred to the final overtemperature at
maximum permissible phase current k·I
N
Ï„
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
Nom
– Rated current of the device
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