ABB RELION REX640 Technical Manual page 878

Section 4
Protection functions
872
Faulted phase selection, the underimpedance method
The "Under impedance" and "OC AND Und impedance" methods use fault loop
impedance for identifying the faulted phases. The advantage of the underimpedance
criterion is that the sensitivity of the fault detection is independent of the source
impedance. The underimpedance function has six impedance measuring elements.
The phase-to-earth measuring elements are calculated:
U
Z A
=
A
I
A
GUID-8BC4AA18-8C97-4DF4-B37C-3764ECD7CBF4 V1 EN
U
Z B
=
B
I
B
GUID-CCB3A2D5-2ACF-44BA-A1E7-8106463E92F5 V1 EN
U
Z C
=
C
I
C
GUID-8683695B-2801-419A-80F7-12BB075992CC V1 EN
The phase-to-phase measuring elements are based on the formula:
U
Z AB
=
AB
I
AB
GUID-68CD4469-2405-4270-92A7-9EEDA93F17FA V1 EN
U
Z BC
=
BC
I
BC
GUID-7DCE31EC-D965-4B3B-A3F6-F928059AF77F V1 EN
U
Z CA
=
CA
I
CA
GUID-DF0CF850-561E-416D-86F5-5CF22B495B1C V1 EN
The faulted phase identification is done by comparing the measured impedance to the
operating characteristics in the impedance domain. If a measured fault loop
impedance is inside the characteristics, the corresponding phases are identified as
faulty.
The shape of the characteristic is selected with the Z Chr Mod Ph Sel GFC setting.
Two shapes are available.
• "Mho (circular)", Non-directional mho
• "Quadrilateral", Non-directional quadrilateral
The two shapes can be flexibly configured. It is possible to set the reactive reaches
independently in forward and in reverse direction and for phase-to-earth (Gnd) and
phase-to-phase (PP) measurement elements, for example, the X Gnd Fwd reach GFC,
X Gnd Rv reach GFC, X PP Fwd reach GFC and X PP Rv reach GFC settings. Also,
the resistive reach in case of quadrilateral characteristic can be set both forward and
reverse independently of reactive reaches, for example, the Ris Gnd Fwd Rch GFC,
1MRS759142 C
(Equation 215)
(Equation 216)
(Equation 217)
(Equation 218)
(Equation 219)
(Equation 220)
REX640
Technical Manual