Functions
2.10 Thermal Overload Protection 49
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
191
2.10.2 Setting Notes
General
The overload protection is only in effect and accessible if address 142 49 = No ambient temp or = With
amb. temp. during configuration. If the function is not required Disabled is set.
Transformers and cable are prone to damage by overloads that last for an extended period of time. Overloads
cannot and should not be detected by fault protection. Time overcurrent protection should be set high enough
to only detect faults since these must be cleared in a short time. Short time delays, however, do neither allow
measures to discharge overloaded equipment nor do they permit to take advantage of its (limited) overload ca-
pacity.
The protective relays 7SJ62/64 feature a thermal overload protective function with a thermal tripping curve
which may be adapted to the overload tolerance of the equipment being protected (overload protection with
memory capability).
Overload protection can be switched ON or OFF or set to Alarm Only at address 4201 FCT 49. If overload
protection is ON, tripping, trip log and fault recording is possible.
When setting Alarm Only no trip command is given, no trip log is initiated and no spontaneous fault annun-
ciation is shown on the display.
Note
Changing the function parameters resets the thermal replica. The thermal model is frozen (kept constant), as
soon as the current exceeds the setting value 1107 I MOTOR START.
Overload Parameter k-factor
The overload protection is set in reference values. The nominal current I
Nom Obj.
of the protected object (motor,
transformer, cable) is used as the basic current for overload protection. By means of the thermal consistently
permissible current I
max
, a factor k
prim
can be calculated:
The thermally-permissible continuous current for the equipment being protected is known from the manufac-
turers specifications. This function is normally not applicable to overhead lines since the current capability of
overhead lines is generally not specified. For cables, the permissible continuous current is dependent on the
cross-section, insulating material, design, and the cable routing, among other things. It may be taken from per-
tinent tables, or is specified by the cable manufacturer. If no specifications are available, a value of 1.1 times
the nominal current rating may be assumed.
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