ABB RET670 Applications Manual page 438

Section 3
IED application
432
The transformer inrush current will have a large residual current component. To
prevent unwanted function of the ground-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 ground-fault (residual) current to ground faults in
the connected power system the application is as shown in figure 195.
ANSI05000491 V1 EN
Figure 195: Residual overcurrent protection application on an isolated transformer
winding
In the calculation of the fault current fed to the protection, at different ground faults,
are highly dependent on the positive and zero sequence source impedances, as well as
the division of residual current in the network. Ground-fault current calculations are
necessary for the setting.
Setting of step 1
One requirement is that ground faults at the busbar, where the transformer winding is
connected, shall be detected. Therefore a fault calculation as shown in figure
WYE/DELTA, DELTA/WYE or
WYE/WYE transformer
Three phase CT
summated
51N
en05000491_ansi.vsd
1MRK504116-UUS C
196 is made.
Application manual