Thermal Overload Protection 49; Description - Siemens SIPROTEC 7SJ62 Manual

2.10

Thermal Overload Protection 49

The thermal overload protection is designed to prevent thermal overloads from damaging the protected equip-
ment. The protection function represents a thermal replica of the equipment to be protected (overload protec-
tion with memory capability). Both the previous 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 transform-
ers to be monitored.
• If an additional thermal input is available, the thermal replica may take the actual ambient or coolant tem-
perature into account.
2.10.1

Description

Thermal Replica
The device calculates the overtemperature in accordance with a single-body thermal replica, based on the fol-
lowing 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
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
Present overtemperature related to the final overtemperature at maximum allowed phase
current k · I
Nom Obj
Thermal time constant of the protected object's heating
Present true r.m.s 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 temperature
Temperature at object nominal current
Functions
2.10 Thermal Overload Protection 49
= 40°C is assumed so that Θ
u
' = 0.
u
187