ABB RELION RET670 Applications Manual page 552

Section 9
Current protection
546
ANSI05000491 V1 EN-US
Figure 272: 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
made.
WYE/DELTA, DELTA/WYE or
WYE/WYE transformer
Three phase CT
summated
51N
en05000491_ansi.vsd
Transformer protection RET670 2.2 ANSI
1MRK 504 163-UUS A
SEMOD55591-35 v5
273 is
Application manual