It can be suitable to use a residual overcurrent protection with at least two steps. Step 1 shallhave a short definite time delay and a relatively high current setting, in order to detect andclear high current earth faults in the transformer winding or in the power system close to thetransformer. Step 2 shall have a longer time delay (definite or inverse time delay) and a lowercurrent operation level. Step 2 shall detect and clear transformer winding earth faults with lowearth-fault current, that is, faults close to the transformer winding neutral point. If the currentsetting gap between step 1 and step 2 is large another step can be introduced with a currentand time delay setting between the two described steps.The transformer inrush current will have a large residual current component. To preventunwanted function of the earth-fault overcurrent protection, the 2nd harmonic restrainblocking should be used, at least for the sensitive step 2.If the protected winding will not feed earth-fault (residual) current to earth faults in theconnected power system the application is as shown in Figure 51.IN>Three phase CTsummatedY/Y, Y/D or D/Ytransformeren05000491.vsdIEC05000491 V1 EN-USFigure 51: Residual overcurrent protection application on an isolated transformerwindingIn the calculation of the fault current fed to the protection, at different earth faults, are highlydependent on the positive and zero sequence source impedances, as well as the division ofresidual current in the network. Earth-fault current calculations are necessary for the setting.Setting of step 1 SEMOD55591-35 v4One requirement is that earth faults at the busbar, where the transformer winding isconnected, shall be detected. Therefore a fault calculation as shown in figure 52 is made.1MRK 504 158-UEN A Section 7Current protection109Application manual
