Siemens Siprotec 7UM611, Руководство пользователя

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2.9 Thermal Overload Protection (ANSI 49)
59
7UM61 Manual
C53000-G1176-C127-3
2.9 Thermal Overload Protection (ANSI 49)
The thermal overload protection prevents thermal overloading of the stator windings
of the machine being protected.
2.9.1 Functional Description
Thermal Profile The device calculates the excessive temperatures in accordance with a single-body
thermal model, based on the following differential equation:
with
Θ
– Current overtemperature as a percent of final overtemperature at
the maximum admissible phase current k ·
I
N
Θ
K
– Coolant temperature as a difference to the 40
°
C reference temper-
ature
τ
– Thermal time constant for heatup of the equipment being protected
I – Current rms phase current as a percentage of maximum admissible
phase current I
max
= k · I
N
The protection function models a thermal profile of the equipment being protected
(overload protection with memory capability). Both the previous history of an overload
and the heat loss to the environment are taken into account.
The solution of this equation is in steady-state operation an exponential function
whose asymptote represents the final temperature
Θ
End
. After an initial settable over-
temperature threshold´is reached, an alarm is issued for e.g. load reduction mea-
sures. If the second overtemperature threshold, i.e. final overtemperature = trip tem-
perature, is reached, the protected equipment is disconnected from the network. It is
also possible, however, to set the overload protection to
Alarm Only
. In this case
only an indication is issued when the final temperature is reached.
The overtemperature is calculated from the largest of the three phase currents. Since
the calculation is based on rms values of currents, harmonics which contribute to a
temperature rise of the stator winding are also considered.
The maximum thermally admissible continuous current I
max
is described as a multiple
of the nominal current I
N
:
I
max
= k · I
N
Apart from the k factor (parameter
K-FACTOR
) the
TIME CONSTANT
τ
and the alarm
temperature
Θ
ALARM
(as a percentage of the trip temperature
Θ
TRIP
) are to be en-
tered.
Overload protection also has a current alarm feature (
I ALARM
) in addition to the tem-
perature alarm stage. This may report an overload current prematurely (before I
max
is
exceeded), even if the overtemperature has not yet attained the alarm or tripping
levels.
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