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Functions
2.9 Thermal Overload Protection 49
2.9.2
Setting Notes
General
Overload protection is only effective and accessible if address 142 49 is set to No ambient temp or to With
amb. temp.. If this function is not required, then 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 7SK80 devices feature an overload protection function with thermal tripping characteristic adaptable to the
overload capability of the equipment being protected (overload protection with memory function).
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
transformer, cable) is used as the basic current for overload protection. By means of the thermal consistently
permissible current I
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.
158
, a factor k
can be calculated:
max
prim
of the protected object (motor,
Nom Obj.
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
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