7.6.3 Setting guideline M13250-3 v8The parameters for the thermal overload protection, two time constants (TRPTTR) are set viathe local HMI or Protection and Control IED Manager (PCM600).The following settings can be done for the thermal overload protection:Operation:Off/OnOperation: Sets the mode of operation.Off 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 % ofIBase. When the current is equal toIRef thefinal (steady state) heat content is equal to 1. It is suggested to give a setting correspondingto the rated current of the transformer winding.IRefMult: If a binary input ENMULT is activated the reference current value can be multiplied bythe factorIRefMult. The activation could be used in case of deviating ambient temperaturefrom the reference value. In the standard for loading of a transformer an ambient temperatureof 20°C is used. For lower ambient temperatures the load ability is increased and vice versa.IRefMult can be set within a range: 0.01 - 10.00.IBase1: Base current for setting given as percentage ofIBase. This setting shall be related tothe status with no COOLING input. It is suggested to give a setting corresponding to the ratedcurrent of the transformer with natural cooling (OA).IBase2: Base current for setting given as percentage ofIBase. This setting shall be related tothe status with activated COOLING input. It is suggested to give a setting corresponding tothe rated current of the transformer with forced cooling (FOA). If the transformer has noforced coolingIBase2 can be set equal toIBase1.Tau1: The thermal time constant of the protected transformer, related toIBase1 (no cooling)given in minutes.Tau2: The thermal time constant of the protected transformer, related toIBase2 (with cooling)given in minutes.The thermal time constant should be obtained from the transformer manufacturers manuals.The thermal time constant is dependent on the cooling and the amount of oil. Normal timeconstants for medium and large transformers (according to IEC 60076-7) are about 2.5 hoursfor naturally cooled transformers and 1.5 hours for forced cooled transformers.The time constant can be estimated from measurements of the oil temperature during acooling sequence (described in IEC 60076-7). It is assumed that the transformer is operated ata certain load level with a constant oil temperature (steady state operation). The oiltemperature above the ambient temperature is DQo0. Then the transformer is disconnectedfrom the grid (no load). After a time t of at least 30 minutes the temperature of the oil ismeasured again. Now the oil temperature above the ambient temperature is DQot. The thermaltime constant can now be estimated as:0ln lno ottt = DQ - DQEQUATION1180 V1 EN-US (Equation 50)If the transformer has forced cooling (FOA) the measurement should be made both with andwithout the forced cooling in operation, givingTau2 andTau1.1MRK 504 158-UEN A Section 7Current protection121Application manual
