ABB RELION 670 Series Applications Manual page 419

1MRK 504 152-UEN B
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 earth 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 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 255.
IEC05000491 V1 EN-US
Figure 255: Residual overcurrent protection application on an isolated transformer
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
Application manual
Three phase CT
summated
IN>
winding
Y/Y, Y/D or D/Y
transformer
en05000491.vsd
Section 8
Current protection
SEMOD55591-35 v4
256 is made.
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