ABB RET670 Applications Manual page 161

1MRK504116-UUS C
Application manual
Where 200mA is the current drawn by the IED circuit and 50mA is the current drawn
by each CT just at pickup. The magnetizing current is taken from the magnetizing
curve of the current transformer cores, which should be available. The value at
TripPickup is taken.
Restricted earth fault protection REFPDIF (87N)
For solidly grounded systems a restricted earth fault protection REFPDIF (87N) is
often provided as a complement to the normal transformer differential IED. The
advantage with the restricted ground fault IEDs is their high sensitivity. Sensitivities of
2-8% can be achieved whereas the normal differential IED will have sensitivities of
20-40%. The level for high impedance restricted ground fault function is dependent of
the current transformers magnetizing currents.
Restricted ground fault IEDs have very fast response time due to the simple measuring
principle and the measurement of one winding only.
The connection of a restricted earth fault IED is shown in figure 44. It is connected
across each directly or low ohmic grounded transformer winding in figure 44.
It is quite common to connect the restricted earth fault IED in the same current circuit
as the transformer differential IED. Due to the difference of measuring principle, the
detection of ground faults may be somewhat limited. Such faults are then only detected
by REFPDIF (87N) function. The mixed connection using the 1Ph High impedance
differential protection HZPDIF (87) function should be avoided and the low impedance
scheme should be used instead.
Section 3
IED application
155