Siemens SIPROTEC 7ST6, Руководство пользователя, Страница: 94 / 356

2 Functions
94
7ST6 Manual
E50417-G1176-C251-A3
2.7 Thermal Overload Protection
The thermal overload protection prevents damage to the overhead contact line caused
by thermal overloading. It is based on the principle of creating a thermal model of the
overhead contact line on the basis of the current load and the ambient temperature.
Where no ambient temperature sensing is provided, a fixed value can be used instead
of an ambient temperature measurement.
2.7.1 Thermal Overload Protection
General The thermal overload protection can be switched on or off by parameter settings, or
via binary inputs. The figure below shows the control logic:
Figure 2-22 Control logic of the overload protection
When overload protection is disabled, the thermal replica will also be reset.
The device calculates the overtemperature in accordance with a single-body thermal
model, based on the following differential equation:
with
TEMP – Current overtemperature (referred to the ambient
temperature)
τ
– Thermal time constant of the contact line
TRise – Final overtemperature at current ITRise
ITRise – Current leading to the final temperature TRise
CHANGE TO CAT – Parameter for catenary changeover; used for modi-
fying the characteristic where there is more than one
contact line
I – Present r.m.s. current
The solution of this equation is in steady-state operation an e-function whose asymp-
tote represents the final temperature
T Rise
. The overtemperature is proportional to
the square of the contact line current. When the overtemperature reaches the first set-
table temperature threshold
TEMP ALARM
, which is below the overtemperature = trip-
ping overtemperature, a warning alarm is given in order to allow a preventive load re-
duction. When the second temperature threshold, i.e. the final overtemperature or
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