Siemens SIPROTEC 4 7SJ61 Manual page 116

Functions
2.7 Thermal Overload Protection 49
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 capacity.
The 7SJ61 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
i
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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 Ι
transformer, cable) is used as the basic current for overload protection. By means of the thermal consistently
permissible current Ι
[formel-kfaktor-260602-kn, 1, en_US]
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
pertinent 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.
For the 49 K-FACTOR to be set in the device the following applies (address 4202)
[fo_einstellwert-kfaktor-allg, 1, en_US]
with
Ι
max prim
Ι
Nom Obj.
Ι
NomCT prim
NOTE
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The setting for 49 K-FACTOR (address 4202) has always to be made as a secondary value.
Example: Motor and current transformer with the following data:
Permissible Continuous Current
Nominal Motor Current
Current Transformer
Setting value FACTOR K:
116
, a factor k can be calculated:
max prim
Permissible thermal primary current of the motor
Nominal current of the protected object
Nominal primary CT current
Nom Obj.
= 1.2 · Ι
Ι
max prim Nom Obj.
= 1100 A
Ι
Nom Obj.
1200 A/1 A
C53000-G1140-C210-6, Edition 05.2016
of the protected object (motor,
SIPROTEC 4, 7SJ61, Manual