Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
Figure 9-3 and Table 9-4 show the phase-to-phase fault Mho characteristics. These figures have been
drawn for a forward fault current. In the case of a reverse fault current, the characteristic layout would
be different, and the origin would be outside the characteristic as shown in Figure 9-2.
Z
SL
(I
ph-ph
-I
load
)
I
ph-ph
Z
set
F
I
Fph-ph
R
F
/2
V
ph-ph
1M
Figure 9-3. Phase-Phase Mho Characteristic (I)
Figure 9-3 shows a characteristic at the instant of a fault. Using the voltage memory feature, the
positive sequence voltage prior to the fault is used to polarize the characteristic. The characteristic
diameter is increased by the impedance
(
)
LS
load
Z
I
I
ΦΦ
−
When the buffer for the voltage memory has expired during a sustained fault, the source impedance
vector becomes:
1
(
)
LS
Z
I
I
ΦΦ
ΦΦ
−
where
Table 9-4. Phase-Phase Mho Characteristic Definitions
Z
LS
Local source positive sequence impedance
I
ph-ph
Phase to phase current (I
ab
, I
bc
, I
ca
)
I
ph-ph1
Positive sequence phase to phase current
I
load
Phase to phase load current
V
ph-ph
Phase to phase voltage
V
ph-ph
1M
Polarizing voltage. Positive sequence voltage for the
corresponding phases, directly or from memory buffer
August 1, 2009
9-5
973.334.3100
