Functions
2.11 Thermal Overload Protection (ANSI 49)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
81
2.11 Thermal Overload Protection (ANSI 49)
The thermal overload protection prevents thermal overloading of the stator windings of the machine being pro-
tected.
2.11.1 Functional Description
Thermal Profile
The device calculates the overtemperature in accordance with a single-body thermal model, based on the fol-
lowing differential equation:
with
Θ Actual operating temperature expressed in percent of the operating temperature correspond-
ing to the maximum permissible operating current k·I
N
Θ
K
Coolant temperature or ambient temperature as a difference to the 40 °C reference temper-
ature
τ Thermal time constant for the heating of the equipment being protected
I Operating current expressed in percent of the maximum permissible operating 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 in steady-state operation is an e-function whose asymptote represents the final
temperature Θ
End
. After an initial settable overtemperature threshold is reached, an alarm is issued, e.g. for
timely prompting of load reduction. If the second overtemperature threshold, i.e. final overtemperature = trip
temperature, is reached, the protected equipment is disconnected from the network. The overload protection
can, however, also be set to Alarm Only. In this case only an alarm is issued even if 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 con-
sidered.
The maximum thermally permissible continuous current I
max
is described as a multiple of the nominal current
I
N
of the protected object:
I
max
= k·I
N
In addition to the k factor (parameter K-FACTOR), the TIME CONSTANT τ and the alarm temperature Θ ALARM
(in percent of the trip temperature Θ
TRIP
) must be specified.
Overload protection also has a current alarm feature (I ALARM) in addition to the temperature alarm stage.
The current warning element may report an overload current prematurely (when I
max
is exceeded), even if the
calculated operating temperature has not yet attained the warning or tripping levels.
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