Functions
2.7 Thermal Overload Protection 49
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
124
2.7 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.
2.7.1 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.
The protection feature therefore represents a thermal replica of the equipment to be protected (overload pro-
tection with memory capability). Both the previous history of an overload and the heat loss to the environment
are taken into account.
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