ABB REG670 Applications Manual page 354

Section 9
Current protection
348
transformer winding earth faults with low earth-fault current, that is, faults close to the
transformer winding neutral point. If the current setting gap between step 1 and step
2 is large another step can be introduced with a current and time delay setting between
the two described steps.
The transformer inrush current will have a large residual current component. To
prevent unwanted function of the earth-fault overcurrent protection, the 2nd harmonic
restrain blocking 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 the
connected power system, the application is as shown in Figure 184.
IEC05000491 V1 EN
Figure 184: Residual overcurrent protection application on an isolated
transformer winding
In the calculation of the fault current fed to the protection, at different earth faults, are
highly dependent on the positive and zero sequence source impedances, as well as the
division of residual current in the network. Earth-fault current calculations are
necessary for the setting.
Setting of step 1
One requirement is that earth faults at the busbar, where the transformer winding is
connected, shall be detected. Therefore a fault calculation as shown in Figure
made.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Y/Y, Y/D or D/Y
transformer
Three phase CT
summated
IN>
en05000491.vsd
1MRK 502 071-UEN -
185 is
Application manual