7.5 Thermal overload protection, two time constantsTRPTTR IP14513-1 v47.5.1 IdentificationM14877-1 v2Function description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberThermal overload protection, two timeconstantsTRPTTRSYMBOL-A V1 EN-US497.5.2 Application M15341-3 v5Transformers in the power system are designed for a certain maximum load current(power) level. If the current exceeds this level the losses will be higher thanexpected. As a consequence the temperature of the transformer will increase. If thetemperature of the transformer reaches too high a value, the equipment might bedamaged;• The insulation within the transformer experiences forced ageing. As aconsequence of this, the risk of internal phase-to-phase or phase-to-earth faultsincreases.• There might be hot spots within the transformer, which degrades the paperinsulation. It might also cause bubbling in the transformer oil.In stressed situations in the power system it can be required to overloadtransformers for a limited time. This should be done without the above mentionedrisks. The thermal overload protection provides information and makes temporaryoverloading of transformers possible.The permissible load level of a power transformer is highly dependent on thecooling system of the transformer. There are two main principles:• OA: The air is naturally circulated to the coolers without fans and the oil isnaturally circulated without pumps.• FOA: The coolers have fans to force air for cooling and pumps to force thecirculation of the transformer oil.The protection can have two sets of parameters, one for non-forced cooling and onefor forced cooling. Both the permissive steady state loading level as well as thethermal time constant is influenced by the cooling system of the transformer. Thetwo parameters sets can be activated by the binary input signal COOLING. This1MRK 504 169-UEN A Section 7Current protectionTransformer protection RET650 2.2 IEC 133Application manual
