Thermal Overload Protection 49; Description - Siemens siprotec SJ62 User Manual

2 Functions
2.10

Thermal Overload Protection 49

Applications

2.10.1 Description

Thermal Profile
175
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.
• 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.
The device calculates the overtemperatures in accordance with a single-body thermal
model, based on the following differential equation:
with
Θ
τ
th
I
k
I
Nom Obj.
with
Θ
u
Θ
Nom
If the ambient or coolant temperature is not measured, a constant value of Θ
or 104°F is assumed so that Θ
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-
Present overtemperature related to the final overtem-
perature at maximum allowed phase current k · I
Thermal time constant of the protected object's heating
Present rms value of phase current
k–factor indicating the maximum permissible constant
phase current referred to the nominal current of the
protected object
Nominal current of protected object
Measured ambient temperature or coolant tempera-
ture
Temperature at object nominal current
'= 0.
u
C53000-G1140-C147-A, Edition 07.2015
u
SIPROTEC 4, 7SJ62/63/64 Handbuch
Nom Obj
= 40 °C