Thermal Overload Protection 49; Method Of Operation - Siemens SIPROTEC 7SD80 Manual

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, 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
Θ
– Current overtemperature, referred to the final overtemperature at
τ
– thermal time constant for the heating
th
– present rms current
I
k – k–factor indicating the maximum permissible constant current referred
– Rated current of the device
I
Nom
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 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
current I :
Nom
I = k·I
max N
In addition to the k-factor, the time constant τ
tings of the protection.
Overload protection also features a current warning element I
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.
112
maximum permissible phase current k·I
to the nominal current of the current transformers
. When the overtemperature reaches the first settable temperature threshold Θ
End
as well as the alarm temperature Θ
th
N
is described as a multiple of the rated
max
must be entered as set-
alarm
in addition to the temperature warning
alarm
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
,
alarm