51N
alt
Three phase CT
summated
Single CT
WYE/DELTA or WYE/WYE
transformer
en05000490_ansi.vsd
ANSI05000490 V1 EN-US
Figure 271: 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.
• Detection of ground faults in the power system.
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.
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
272.
1MRK 504 163-UUS A Section 9
Current protection
Transformer protection RET670 2.2 ANSI 545
Application manual
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