7.6.1 IdentificationM14877-1 v2Function description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberThermal overload protection, twotime constantsTRPTTRSYMBOL-A V1 EN-US497.6.2 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 than expected. As a consequencethe temperature of the transformer will increase. If the temperature of the transformerreaches too high a value, the equipment might be damaged;• The insulation within the transformer experiences forced ageing. As a consequence ofthis, the risk of internal phase-to-phase or phase-to-earth faults increases.• There might be hot spots within the transformer, which degrades the paper insulation. Itmight also cause bubbling in the transformer oil.In stressed situations in the power system it can be required to overload transformers for alimited time. This should be done without the above mentioned risks. The thermal overloadprotection provides information and makes temporary overloading of transformers possible.The permissible load level of a power transformer is highly dependent on the cooling systemof 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 circulation ofthe transformer oil.The protection can have two sets of parameters, one for non-forced cooling and one for forcedcooling. Both the permissive steady state loading level as well as the thermal time constant isinfluenced by the cooling system of the transformer. The two parameters sets can beactivated by the binary input signal COOLING. This can be used for transformers where forcedcooling 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.If the heat content of the protected transformer reaches a set alarm level a signal can be givento the operator. Two alarm levels are available. This enables preventive actions in the powersystem to be taken before dangerous temperatures are reached. If the temperature continuesto increase to the trip value, the protection initiates a trip of the protected transformer.After tripping by the thermal overload protection, the transformer will cool down over time.There will be a time gap before the heat content (temperature) reaches such a level so that thetransformer can be taken into service again. Therefore, the function will continue to estimatethe heat content using a set cooling time constant. Energizing of the transformer can beblocked until the heat content has reached a set level.Section 7 1MRK 504 158-UEN ACurrent protection120Application manual
