4.1.4.2 Function block
49F
I_A
I_B
I_C
ENA_MULT
BLK_OPR
AMB_TEMP
TRIP
PICKUP
ALARM
BLK_CLOSE
GUID-8E72B316-4C05-468D-B691-D3A654CAE834 V1 EN
Figure 130: Function block
4.1.4.3 Functionality
The increased utilization of power systems closer to the thermal limits has generated a
need for a thermal overload function for power lines as well.
A thermal overload is in some cases not detected by other protection functions, and the
introduction of the three-phase thermal protection for feeders, cables and distribution
transformers function 49F allows the protected circuit to operate closer to the thermal
limits.
An alarm level gives an early warning to allow operators to take action before the line
trips. The early warning is based on the three-phase current measuring function using a
thermal model with first order thermal loss with the settable time constant. If the
temperature rise continues the function operates based on the thermal model of the line.
Re-energizing of the line after the thermal overload operation can be inhibited during the
time the cooling of the line is in progress. The cooling of the line is estimated by the
thermal model.
4.1.4.4 Operation principle
The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are “Enable” and “Disable”.
The operation of 49F can be described using a module diagram. All the modules in the
diagram are explained in the next sections.
The function uses ambient temperature which can be measured locally or remotely. Local
measurement is done by the protection relay. Remote measurement uses analog GOOSE
to connect AMB_TEMP input.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 269
Technical Manual
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