Thermal Overload Protection; Method Of Operation - Siemens siprotec 7SA6 User Manual

Functions
6.17

Thermal Overload Protection

6.17.1 Method of Operation

6-244
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
d Θ

1 1
------ - Θ ⋅ ⋅
------- - ------ -
+ =

τ τ
dt
th th
with Θ
instantaneous overtemperature referred to the final temperature
–
rise for the maximum permissible line current
τ
thermal time constant for heating
–
th
k-factor which states the maximum permissible continuous
k –
current referred to the rated current of the current transformers
currently measured r.m.s. current
I –
rated current of current transformers
–
I
N
The solution of this equation under steady-state conditions is an e-function whose
asymptote shows the final overtemperature Θ
the first settable temperature threshold Θ
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 2QO\. 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
multiple of the rated current I
= k · I
I
max N
In addition to the k-factor, the time constant τ
must be entered in the protection.
Apart from the thermal alarm stage, the overload protection also includes a current
overload alarm stage I
alarm
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.
2

I
-------------

k I ⋅
N
end
, which is below the final
alarm
:
N
as well as the alarm temperature Θ
th
, which can output an early warning that an overload
k · I
N
. When the overtemperature reaches
is described as a
max
C53000-G1176-C133-1
alarm
7SA6 Manual