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 actions inthe 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 a levelso that the transformer can be taken into service again. Therefore, the function willcontinue to estimate the heat content using a set cooling time constant. Energizing of thetransformer can be blocked until the heat content has reached a set level.7.5.3 Setting guidelineThe parameters for the thermal overload protection, two time constants (TRPTTR, 49) areset 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 to IRefthe 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 a transformeran ambient temperature of 20°C is used. For lower ambient temperatures the load abilityis increased and vice versa. IRefMult can be set within a range: 0.01 - 10.00.IBase1: Base current for setting given as percentage of IBase. This setting shall be relatedto the status with no COOLING input. It is suggested to give a setting corresponding to therated current of the transformer with natural cooling (OA).IBase2: Base current for setting given as percentage of IBase. This setting shall be relatedto the status with activated COOLING input. It is suggested to give a settingcorresponding to the rated current of the transformer with forced cooling (FOA). If thetransformer has no forced cooling IBase2 can be set equal to IBase1.1MRK 505 337-UUS - Section 7Current protection199Application manual
