Application - ABB REF615 Applications Manual

Section 5
Protection functions
5.1.3.3
144
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.

Application

The lines and cables in the power system are constructed for a certain maximum load
current level. If the current exceeds this level, the losses will be higher than expected.
As a consequence, the temperature of the conductors will increase. If the temperature
of the lines and cables reaches too high values, it can cause a risk of damages by, for
example, the following ways:
• The sag of overhead lines can reach an unacceptable value.
• If the temperature of conductors, for example aluminium conductors, gets too
high, the material will be destroyed.
• In cables the insulation can be damaged as a consequence of overtemperature,
and therefore phase-to-phase or phase-to-earth faults can occur.
In stressed situations in the power system, the lines and cables may be required to be
overloaded for a limited time. This should be done without any risk for the above-
mentioned risks.
The thermal overload protection provides information that makes temporary
overloading of cables and lines possible. The thermal overload protection estimates
the conductor temperature continuously. This estimation is made by using a thermal
model of the line/cable that is based on the current measurement.
If the temperature of the protected object reaches a set warning level, a signal is given
to the operator. This enables actions in the power system to be done before dangerous
temperatures are reached. If the temperature continues to increase to the maximum
allowed temperature value, the protection initiates a trip of the protected line.
1MRS756378 D
REF615
Application Manual