4.1.4.3 FunctionalityThe increased utilization of power systems closer to the thermal limits has generateda need for a thermal overload function also for power lines.A thermal overload is in some cases not detected by other protection functions, and theintroduction of the three-phase thermal protection for feeders, cables and distributiontransformers function T1PTTR allows the protected circuit to operate closer to thethermal limits.An alarm level gives an early warning to allow operators to take action before the linetrips. The early warning is based on the three-phase current measuring function usinga thermal model with first order thermal loss with the settable time constant. If thetemperature rise continues the function will operate based on the thermal model of theline.Re-energizing of the line after the thermal overload operation can be inhibited duringthe time the cooling of the line is in progress. The cooling of the line is estimated bythe thermal model.4.1.4.4 Analog channel configurationT1PTTR has one analog group input which must be properly configured.Table 398:Analog inputsInput DescriptionI3P Three-phase currentsSee the preprocessing function blocks in this document for thepossible signal sources.Improper analog channel configuration causes a validation error if the analogchannels are not completely configured or they do not match with certain settings. Fortroubleshooting, check the contents of this chapter and also the preprocessing blocksin this document. The configuration can be written to the protection relay once themismatch is corrected.4.1.4.5 Operation principleThe function can be enabled and disabled with the Operation setting. Thecorresponding parameter values are "On" and "Off".The operation of T1PTTR can be described using a module diagram. All the modulesin the diagram are explained in the next sections.Section 4 1MRS759142 CProtection functions360 REX640Technical Manual
