R.1.27
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Fault Type Identification Selection Logic
Fault Type Identification Selection Logic
This logic identifies the faulted phase(s) for all faults involving ground by
comparing the angle between I
0
and I
2
.
For cases where only zero-sequence current flows through the relay terminal
(that is, no negative-sequence current and no positive-sequence current), the
Fault Type Identification Selection (FIDS) Logic uses single-phase
undervoltage elements for faulted phase selection.
The FIDS logic is not active during an SPO (single pole open) condition (i.e.,
when SPO equals logical 1).
Ground Directional Element
The SEL-421 offers a choice of three independent directional elements to
supervise the ground distance elements and directional residual ground
overcurrent elements (67Gn, where n equals 1 through 4) during ground
faults. You can also use the ground directional element for torque control.
Internal logic selects the best choice automatically. Table 1.26 lists the
directional elements the relay uses to provide ground directional decisions.
The negative-sequence voltage polarized directional element 32QG listed in
Tab le 1.26 supervises the ground distance elements and residual ground
directional overcurrent elements. The negative-sequence voltage polarized
directional element 32Q illustrated in Figure 1.26 only supervises the phase
distance elements.
The relay internal logic selects the best choice for directional supervision
according to prevailing power system conditions during the ground fault. The
logic determines the best choice for the ground directional element (32G)
from among the negative-sequence voltage-polarized directional element
(32QG), zero-sequence voltage-polarized directional element (32V), or the
zero-sequence current-polarized directional element (32I). The ground
directional element also supervises the quadrilateral ground distance elements.
Table 1.25 FIDS Relay Word Bits
Name Description
FIDEN FIDS logic enabled
FSA A-phase-to-ground fault or B-phase to C-phase-to-ground fault selected
FSB B-phase-to-ground fault or C-phase to A-phase-to-ground fault selected
FSC C-phase-to-ground fault or A-phase to B-phase-to-ground fault selected
Table 1.26 Directional Elements Supervising Ground Elements
Directional
Elements
Description
Forward
Output
Reverse
Output
32QG Negative-sequence voltage polarized for
ground faults
F32QG R32QG
32V Zero-sequence voltage polarized F32V R32V
32I Zero-sequence current polarized F32I R32I
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