Functions
2.10 Thermal Overload Protection 49
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
187
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
Present overtemperature related to the final overtemperature at maximum allowed phase
current k ·
I
Nom Obj
τ
th
Thermal time constant of the protected object's heating
I
Present true r.m.s 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 temperature
Θ
Nom
Temperature at object nominal current
If the ambient or coolant temperature is not measured, a constant value of
Θ
u
= 40°C is assumed so that
Θ
u
' = 0.
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