ABB RELION REX640 Technical Manual page 387

Protection and control

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1MRS759142 F

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, becomes

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.

T1PTTR Input signals

Table 434: T1PTTR Input signals

Technical Manual

Protection functions

Three-phase currents

Block signal for oper-

Enable Current multi-

The ambient temper-

ature used in the cal-