The protected winding will feed ground-fault (residual) current to ground faults in the
connected power system. The residual current fed from the transformer at external phase-
to-ground faults, is highly dependent of the total positive and zero-sequence source
impedances as well as the residual current distribution between the network zero-
sequence impedance and the transformer zero-sequence impedance. An example of this
application is shown in figure
194.
51N
alt
Three phase CT
summated
Single CT
WYE/DELTA or WYE/WYE
transformer
en05000490_ansi.vsd
ANSI05000490 V1 EN
Figure 194: Residual overcurrent protection application on a directly grounded
transformer winding
In this case the protection has two different tasks:
• Detection of ground faults on the transformer winding, to which the protection is
connected.
• Detection of ground faults in the power system, to which the protected winding is
connected.
It can be suitable to use a residual overcurrent protection with at least two steps. Step 1
shall have a short definite time delay and a relatively high current setting, in order to
detect and clear high current ground faults in the transformer winding or in the power
system close to the transformer. Step 2 shall have a longer time delay (definite or
inverse time delay) and a lower current operation level. Step 2 shall detect and clear
transformer winding ground faults with low ground-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.
1MRK504116-UUS C Section 3
IED application
431
Application manual
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