4.1.5.2 Function block
A070691 V2 EN
Figure 115: Function block
4.1.5.3 Functionality
The increased utilization of power systems closer to the thermal limits has
generated a need for a thermal overload function also for power lines.
A thermal overload is in some cases not detected by other protection functions, and
the introduction of the thermal overload function T1PTTR 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 will operate 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.
4.1.5.4 Operation principle
The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of the three-phase thermal protection for feeders, cables and
distribution transformers can be described using a module diagram. All the
modules in the diagram are explained in the next sections.
Section 4 1YHT530004D05 D
Protection functions
226 615 series
Technical Manual
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