Siemens siprotec 7SA6 User Manual

Functions
6-244 7SA6 Manual
C53000-G1176-C133-1
6.17 Thermal Overload Protection
6.17.1 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
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.
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
asymptote shows the final overtemperature Θ
end
. When the overtemperature reaches
the first settable temperature threshold Θ
alarm
, which is below the final
overtemperature, a warning alarm is given in order to allow a timely load reduction.
When the second temperature threshold, i.e. the final overtemperature or tripping
temperature, is reached, the protected object is disconnected from the network. The
overload protection can, however, also be set on $ODUP2 QO\. 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
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
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 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
current is present, even if the overtemperature has not yet reached the alarm or trip
overtemperature values.
The overload protection can be blocked via a binary input. In doing so, the thermal
images are also reset to zero.
d Θ
dt
--------
1
τ
th
-------
Θ⋅
+
1
τ
th
-------
I
k I⋅
N
-------------


2
⋅
=

Siemens siprotec 7SA6 Specifications

General

Brand	Siemens
Model	siprotec 7SA6
Category	Relays
Language	English