ABB RELION REX640 - Directional function for earth faults

Directional function for earth faults
In low-impedance earthed networks, when the System grounding GFC setting is set
to "Low impedance", the earth-fault detection function can be optionally supervised
by a directional function which can be enabled with the Dir mode EF GFC setting.
If Dir mode EF GFC is set to "Forward" or "Reverse", the earth-fault direction
is also detected. The calculated direction for the earth fault is indicated in the
DIR_E_FLT monitored data ("unknown", "forward", "backward" (reverse), "both").
Direction criteria do not directly affect earth-fault detection, but the earth-fault
direction information is used for zone starting, when only those directional zones
that are set to the indicated earth-fault direction can be started.
The operation characteristic of the directional function is defined by three settings:
Chr angle GFC , Min phase angle GFC and Max phase angle GFC .
Min phase angle GFC
Min phase angle GFC
Max phase angle GF
C
Max phase angle GFC
maximum torque line
Chr angle GFC = +45 deg.
FORWARD
UNKNOWN
UNKNOWN
REVERSE
Figure 527: Characteristic of the directional function for earth faults
The Chr angle GFC setting, also known as relay characteristic angle or basic angle, is
used to turn the directional characteristic. It has a positive value when the operating
quantity lags the polarizing quantity and a negative value when the operating
quantity leads the polarizing quantity. For low-impedance earthed networks, Chr
angle GFC equals the angle of the fault loop impedance and is typically positive.
The Min phase angle GFC and Max phase angle GFC settings define the width of
the operating sector. These settings are mirror-symmetric to the origin for reverse
(backward).
The directional function for earth faults must operate at fault current values below
the magnitude of the load currents. The use of sequence quantities is preferred
for polarizing quantity as phase quantities are affected by the load currents. Four
possibilities are available and defined by the Pol quantity GFC setting.
1MRS759142 F
Protection functions
REX640
Technical Manual
947