Table of Contents
Advertisement
6.19
Thermal Overload Protection
6.19.1 Method of Operation
7SA6 Manual
C53000-G1176-C156-2
The thermal overload protection prevents damage to the protected object caused by
thermal overloading, particularly in case of transformers, rotating machines, power re-
actors and cables. It is in general not necessary for overhead lines, since no meaning-
ful overtemperature can be calculated because of the great variations in the environ-
mental 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
dΘ
1
1
æ
------ - Θ
⋅
⋅
------- -
------ -
+
=
è
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 as-
ymptote shows the final overtemperature Θ
the first settable temperature threshold Θ
ture, a warning alarm is given in order to allow a timely load reduction. When the sec-
ond temperature threshold, i.e. the final overtemperature or tripping temperature, is
reached, the protected object is disconnected from the network. The overload protec-
tion can, however, also be set on Alarm Only . 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 av-
erage overtemperature, or the overtemperature calculated from the phase with maxi-
mum 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
rent is present, even if the overtemperature has not yet reached the alarm or trip over-
temperature values.
The overload protection can be blocked via a binary input. In doing so, the thermal im-
ages are also reset to zero.
2
I
ö
-------------
ø
k I ⋅
N
end
, which is below the final overtempera-
alarm
:
N
as well as the alarm temperature Θ
th
, which can output an early warning that an overload cur-
k · I
N
. When the overtemperature reaches
is described as a
max
Functions
alarm
6-265
Advertisement
Table of Contents
