Section 4
1MAC050144-MB C
Protection functions
296
615 series ANSI
Technical Manual
Current multiplier
parameter to the number of parallel lines (cables), the actual current on
one line is used in the protection algorithm. To activate this option, the
ENA_MULT
input
must be activated.
The
Env temperature Set
setting is used to define the ambient temperature.
The temperature calculation is initiated from the value defined with the
Initial temperature
setting parameter. This is done in case the IED is powered up, the function is "disable" and
"enable" or reset through the Clear menu. The temperature is also stored in the nonvolatile
memory and restored in case the IED is restarted.
The thermal time constant of the protected circuit is given in minutes with the
Time
constant
setting. Please see cable manufacturers manuals for further details.
4.5.1.5
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 becomes too high it can cause damage. For example:
• The sag of overhead lines can reach an unacceptable value.
• An aluminum conductor will be destroyed if the temperature becomes too high
• Overheating can damage the insulation on cables which in turn increase the risk of
phase-to-phase or phase-to-ground faults.
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.
The Sensor available setting should not be used. This setting is not
supported in Ver.1.1.
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