ActivationSOTF: This setting will select the signal to activate SOTF function;CB positionopen/CB position closed/CB close command.tSOTF: Time delay for operation of the SOTF function. The setting range is 0.000 - 60.000 s instep of 0.001 s. The default setting is 0.100 sStepForSOTF: If this parameter is set on the step 3 start signal will be used as current set level.If set off step 2 start signal will be used as current set level.t4U: Time interval when the SOTF function is active after breaker closing. The setting range is0.000 - 60.000 s in step of 0.001 s. The default setting is 1.000 s.ActUnderTime: Describes the mode to activate the sensitive undertime function. The functioncan be activated by Circuit breaker position (change) or Circuit breaker command.tUnderTime: Time delay for operation of the sensitive undertime function. The setting range is0.000 - 60.000 s in step of 0.001 s. The default setting is 0.300 s7.4.3.6 Transformer application example SEMOD55591-4 v7Two main cases are of interest when residual overcurrent protection is used for a powertransformer, namely if residual current can be fed from the protected transformer winding ornot.The protected winding will feed earth-fault (residual) current to earth faults in the connectedpower system. The residual current fed from the transformer at external phase-to-earth faultsis highly dependent on the total positive and zero-sequence source impedances. It is alsodependent on the residual current distribution between the network zero-sequenceimpedance and the transformer zero-sequence impedance. An example of this application isshown in Figure 50.IN>altThree phase CTsummatedSingle CTYN/D or YN/Ytransformeren05000490.vsdIEC05000490 V1 EN-USFigure 50: Residual overcurrent protection application on a directly earthed transformerwindingIn this case the protection has two different tasks:• Detection of earth faults on the transformer winding.• Detection of earth faults in the power system.Section 7 1MRK 504 158-UEN ACurrent protection108Application manual
