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Functions
2.9 Thermal Overload Protection 49
2.9
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.
2.9.1
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
The protection function 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.
154
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
= 40°C is assumed so that Θ
u
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
' = 0.
u
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