7.5.1 IdentificationFunction description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberThermal overload protection, two timeconstantsTRPTTRSYMBOL-A V1 EN497.5.2 ApplicationTransformers 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 than expected. Asa consequence the temperature of the transformer will increase. If the temperature of thetransformer reaches too high a value, the equipment might be damaged;• The insulation within the transformer experiences forced ageing. As a consequenceof this, the risk of internal phase-to-phase or phase-to-ground faults increases.• There might be hot spots within the transformer, which degrades the paper insulation.It might also cause bubbling in the transformer oil.In stressed situations in the power system it can be required to overload transformers fora limited time. This should be done without the above mentioned risks. The thermaloverload protection provides information and makes temporary overloading oftransformers possible.The permissible load level of a power transformer is highly dependent on the coolingsystem of the transformer. There are two main principles:• OA: The air is naturally circulated to the coolers without fans and the oil is naturallycirculated without pumps.• FOA: The coolers have fans to force air for cooling and pumps to force the circulationof the transformer oil.The protection can have two sets of parameters, one for non-forced cooling and one forforced cooling. Both the permissive steady state loading level as well as the thermal timeconstant is influenced by the cooling system of the transformer. The two parameters setscan be activated by the binary input signal COOLING. This can be used for transformerswhere forced cooling can be taken out of operation, for example at fan or pump faults.The thermal overload protection estimates the internal heat content of the transformer(temperature) continuously. This estimation is made by using a thermal model of thetransformer which is based on current measurement.Section 7 1MRK 505 337-UUS -Current protection198Application manual
