Siemens siprotec SJ62, User Manual

Page: 177 / 715

2 Functions
175
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
2.10 Thermal Overload Protection 49
The thermal overload protection is designed to prevent thermal overloads from dam-
aging the protected equipment. The protection function models a thermal profile of the
object being protected (overload protection with memory capability). Both the history
of an overload and the heat loss to the environment are taken into account.
Applications • In particular, the thermal overload protection allows the thermal status of motors,
generators and transformers to be monitored.
• If an additional thermal input is available, the thermal profile may take the actual
ambient or coolant temperature into consideration.
2.10.1 Description
Thermal Profile The device calculates the overtemperatures in accordance with a single-body thermal
model, based on the following differential equation:
with
Θ
Present overtemperature related to the final overtem-
perature at maximum allowed phase current k ·
I
Nom Obj
τ
th
Thermal time constant of the protected object's heating
I
Present rms value of phase current
k k–factor indicating the maximum permissible constant
phase current referred to the nominal current of the
protected object
I
Nom Obj.
Nominal current of protected object
with
Θ
u
Measured ambient temperature or coolant tempera-
ture
Θ
Nom
Temperature at object nominal current
If the ambient or coolant temperature is not measured, a constant value of
Θ
u
= 40
°
C
or 104
°
F is assumed so that
Θ
u
’= 0.
The protection feature models a thermal profile of the equipment being protected
(overload protection with memory capability). Both the history of an overload and the
heat loss to the environment are taken into account.
When the calculated overtemperature reaches the first settable threshold
49
Θ
ALARM
, an alarm annunciation is issued, e.g. to allow time for the load reduction mea-
sures to take place. When the calculated overtemperature reaches the second thresh-