Functions
2.23 Thermal Overload Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
414
2.23
Thermal Overload Protection
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 stage can
signal an imminent overload.
2.23.1
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
temperature
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 pro-
tected object is disconnected from the network. The overload protection can, however, also be set to
Alarm
Only
. If this option is set, the device only generates an alarm, even if the end 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 includes 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 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 set-
tings of the protection.
In addition to the temperature warning stage, the overload protection also features a current warning element
I
alarm
. It reports an overload current prematurely, even if the calculated overtemperature has not yet attained
the warning or tripping temperature levels.
The overload protection can be blocked via a binary input. In doing so, the thermal images are also reset to
zero.
Θ
– Current overtemperature in per cent of the final overtemperature at the
maximum permissible phase 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
– Rated current of the device
