ABB RET650 Applications Manual page 136

Section 7
Current protection
130
• Detection of earth faults on the transformer winding.
• Detection of earth 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 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 55.
IEC05000491 V1 EN-US
Figure 55: Residual overcurrent protection application on an isolated
transformer winding
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.
Y/Y, Y/D or D/Y
transformer
Three phase CT
summated
IN>
en05000491.vsd
Transformer protection RET650 2.2 IEC
1MRK 504 169-UEN A
Application manual