Functions
2.11 Thermal Overload Protection (ANSI 49)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
85
2.11.2 Setting Notes
General
Overload protection is only effective and accessible if address 116 Therm.Overload is set to Enabled
during configuration. If the function is not required, it is set to Disabled.
Transformers and generators are especially prone to damage by extended overloads. These overloads cannot
and should not be detected by short-circuit protection. Time overcurrent protection should be set so high that
it only detects faults, since short-circuit protection only permits short time delays. Short time delays, however,
do not allow measures for unburdening the overloaded equipment nor do they permit advantage to be taken of
its (limited) overload capacity.
The 7UM62 protective relay features an overload protective function with thermal tripping characteristic adapt-
able to the overload capability of the equipment being protected.
At address 1601 Ther. OVER LOAD the thermal overload protection ON or OFF can be set, the trip command
blocked (Block relay) or the protection function set to Alarm Only. In the latter case no fault record is
created should an overload occur. If overload protection is switched ON, tripping is also possible.
K-Factor
The overload protection is set with quantities per unit. The nominal current I
N, Mach
of the object to be protected
(generator, motor, transformer) is typically used as base current for overload detection. The thermally permis-
sible continuous current I
max prim
can be used to calculate a factor k
prim
:
The thermally admissible continuous current for the equipment being protected is generally obtainable from
manufacturer's specifications. If no specifications are available, a value of 1.1 times the nominal current rating
is assumed.
The K-FACTOR to be set at the 7UM62 (address 1602) refers to the secondary nominal current (= device cur-
rent). The following applies for the conversion:
with
I
max prim
thermally continuously permissible primary current of the machine
I
N Mach
Nominal Current of the Machine
I
NCT prim
Nominal primary CT current
Example: Generator and current transformer with the following data:
Permissible Continuous Current I
max prim
= 1.15 · I
N, Mach
Generator Nominal Current I
N Mach
= 483 A
Current Transformer 500 A / 1 A
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