tPRCrvx, tTRCrvx, tCRCrvx: Parameters for user programmable of inverse reset timecharacteristic curve. Further description can be found in the technical referencemanual.9.4.3.6 Transformer application exampleTwo main cases are of interest when residual overcurrent protection is used for apower transformer, namely if residual current can be fed from the protectedtransformer winding or not.The protected winding will feed earth-fault (residual) current to earth faults in theconnected power system. The residual current fed from the transformer at externalphase-to-earth faults is highly dependent on the total positive and zero-sequencesource impedances. It is also dependent on the residual current distribution betweenthe network zero-sequence impedance and the transformer zero-sequence impedance.An example of this application is shown in Figure 183.IN>altThree phase CTsummatedSingle CTYN/D or YN/Ytransformeren05000490.vsdIEC05000490 V1 ENFigure 183: Residual overcurrent protection application on a directly earthedtransformer windingIn this case the protection has two different tasks:• Detection of earth faults on the transformer winding.• Detection of earth faults in the power system.It can be suitable to use a residual overcurrent protection with at least two steps. Step1 shall have a short definite time delay and a relatively high current setting, in orderto detect and clear high current earth faults in the transformer winding or in the powersystem close to the transformer. Step 2 shall have a longer time delay (definite orinverse time delay) and a lower current operation level. Step 2 shall detect and clear1MRK 502 071-UEN - Section 9Current protectionGenerator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 347Application manual
