6.6.2 ApplicationLines and cables in the power system are designed for a certain maximum load currentlevel. If the current exceeds this level the losses will be higher than expected. As aconsequence the temperature of the conductors will increase. If the temperature of thelines and cables reaches too high values the equipment might be damaged:• The sag of overhead lines can reach unacceptable value.• If the temperature of conductors, for example aluminium conductors, get too high thematerial will be destroyed.• In cables the insulation can be damaged as a consequence of the overtemperature. Asa consequence of this phase to phase or phase to ground faults can occurIn stressed situations in the power system it can be required to overload lines and cablesfor a limited time. This should be done without risks.The thermal overload protection provides information that makes a temporaryoverloading of cables and lines possible. The thermal overload protection estimates theconductor temperature continuously, in Celsius or Fahrenheit depending on whether,LFPTTR or LCPTTR (26) is chosen. This estimation is made by using a thermal model ofthe line/cable based on the current measurement.If the temperature of the protected object reaches a set warning level AlarmTemp, a signalALARM can be given to the operator. This enables actions in the power system to be takenbefore dangerous temperatures are reached. If the temperature continues to increase to thetrip value TripTemp, the protection initiates trip of the protected line.6.6.3 Setting guidelinesThe parameters for the Thermal overload protection one time constant, Fahrenheit/Celsius LFPTTR/LCPTTR (26) are set via the local HMI or PCM600.The following settings can be done for the thermal overload protection.GlobalBaseSel: Selects the global base value group used by the function to define (IBase),(VBase) and (SBase).Operation: Disabled/EnabledIRef: Reference, steady state current, given in % of IBase that will give a steady state (end)temperature rise TRef. It is suggested to set this current to the maximum steady statecurrent allowed for the line/cable under emergency operation (a few hours per year).TRef: Reference temperature rise (end temperature) corresponding to the steady statecurrent IRef. From cable manuals current values with corresponding conductortemperature are often given. These values are given for conditions such as ground1MRK 511 286-UUS A Section 6Current protection129Application manual
