time constant is influenced by the cooling system of the transformer. The twoparameters sets can be activated by the binary input signal COOLING. This can beused for transformers where forced cooling can be taken out of operation, for exampleat 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 begiven to the operator. Two alarm levels are available. This enables preventive actionsin the power system to be taken before dangerous temperatures are reached. If thetemperature continues to increase to the trip value, the protection initiates a trip of theprotected transformer.After tripping by the thermal overload protection, the transformer will cool down overtime. There will be a time gap before the heat content (temperature) reaches such alevel so that the transformer can be taken into service again. Therefore, the functionwill continue to estimate the heat content using a set cooling time constant. Energizingof the transformer can be blocked until the heat content has reached a set level.7.5.3 Setting guideline M13250-3 v8The parameters for the thermal overload protection, two time constants (TRPTTR, 49)are set via the local HMI or Protection and Control IED Manager (PCM600).The following settings can be done for the thermal overload protection:Operation: Disabled/EnabledOperation: Sets the mode of operation. Disabled switches off the complete function.GlobalBaseSel: Selects the global base value group used by the function to define(IBase), (UBase) and (SBase).IRef: Reference level of the current given in % of IBase. When the current is equal toIRef the final (steady state) heat content is equal to 1. It is suggested to give a settingcorresponding to the rated current of the transformer winding.IRefMult: If a binary input ENMULT is activated the reference current value can bemultiplied by the factor IRefMult. The activation could be used in case of deviatingambient temperature from the reference value. In the standard for loading of atransformer an ambient temperature of 20°C is used. For lower ambient temperaturesthe load ability is increased and vice versa. IRefMult can be set within a range: 0.01 -10.00.Section 7 1MRK 505 370-UUS ACurrent protection212 Busbar protection REB670 2.2 ANSIApplication manual
