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I.L. 40-385.7
NOTE:
IOM is used to supervise all ground units and IM is used to supervise all phase units,
including Zone 1,2,3 and pilot for tripping.
3.2.1
Single-Phase-to-Ground
Single-phase-to-ground fault detection (see Figure 3-2, page 3-28) is accomplished by three
self-polarized phase-ground units (ph-A, ph-B, ph-C). Equations (1) and (2) below are for Op-
erate and Reference quantities, respectively. The unit will produce output when the Operate
quantity leads the Reference quantity in phase angle. The Operate quantity is essentially iden-
tical to that in REL300 Version 2.2 and earlier relays. The Reference quantity is changed from
earlier versions, in order to produce a fixed mho characteristic which will have better perfor-
mance when applied to resistance grounded systems. The "forward" reach applies to the oper-
ate (trip) direction of the unit, while the "reverse" reach applies to the restraint direction and is
used primarily to define the overall size of the characteristic and the amount of reach along the
R-axis. (In the case of the reversible Zone-3, the "forward" reach actually is in the reverse line
direction, since that is the direction where tripping takes place; the "reverse reach is then in the
forward line direction.)
V
XG
(
j V
XG
Where V
Z
, Z
0L
1L
Z
, Z
CGF
As in the standard REL300, these units are directionally supervised by FDOG (RDOG for re-
verse zone three). Because of internal clipping of unfaulted phase voltages during phase-
ground faults, use of only ZSEQ or DUAL polarization of DIRU is recommended when ap-
plied to resistance grounded systems; calculation of V2 magnitude and angle is likely to be in-
correct in this application, causing NSEQ polarization to give false directional sense (see
Section 3.4.11).
3-2
[
]Z
–
I
+
k
I
X
O
O
CGF
Z
–
Z
OL
IL
k
=
----------------------- -
=
o
Z
IL
1
(
I
=
-- - I
+
I
+
O
A
B
3
[
]Z
+
I
+
k
I
X
O
O
CGR
,
=
V
V
XG
AG
BG
,
,
I
=
I
I
or I
X
A
B
C
=
zero and positive sequence line impedance in relay ohms
=
zone forward and reverse reach settings in secondary ohms of Z1L, for ph-
CGR
G fault (both are in angle PANG)
∠
(
Z
GANG PANG
–
R
)
I
C
)
,
or V
CG
) 1
–
(1)
(2)
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