4.1.6.2 Function block
Figure 168: Function block
4.1.6.3 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 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 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.
4.1.6.4 Operation principle
The function can be enabled and disabled with the
Operation
setting. The
corresponding parameter values are "On" and "Off".
The operation of T1PTTR 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.
If the quality of remotely measured temperature is invalid or
communication channel fails the function uses ambient temperature set
in
Env temperature Set
.
1MRS757644 H Protection functions
620 series
Technical Manual
337
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