Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
The figure below shows the zero sequence network for a forward fault.
Figure 9-53. Zero Sequence Network for a Forward Fault
It can be deducted that V0 = ZA0
⋅
(-I0) where ZA0 is the zero sequence impedance of the local
source. Consequently, the angle between –V0 and I0 will be the angle of ZA0. For this reason, the 67G
characteristic angle should be set to this value; ANG_67G =
∠
ZA0.
The relative phase difference between –V0 and I0 will determine the direction. However, a Zero
sequence compensation factor may be used to increase the polarization phasor magnitude.
When the zero sequence impedance of the local source is small, the V0 voltage may be below the 67G
minimum voltage setting. In order to increase this voltage, a phase shifted compensation term, I0 x
COMP is added to V0.
The compensation value should be restricted so that misoperation does not occur for reverse faults. For
faults in the reverse direction, V0 = (ZL0 +ZB0)
⋅
I0, where ZL0 is the line zero sequence impedance
and ZB0 the zero sequence impedance of the remote source. If the angle of (ZL0 + ZB0) is close to
ANG_67G, V0 and I0
⋅
COMP will be out of phase, and may even reverse its direction.
9.12.2.2
CURRENT POLARIZED GROUND DIRECTIONAL UNIT
When current polarization is used, the characteristic angle used is 0
°
. In forward direction, the
operating current I0 is rotated 180
°
with respect to the current flowing through the grounding and as
the relay shifts the polarizing current with the same amount, I0 and IPOL are in phase for a forward
fault. 67G_ANG should be set to 0
°
.
The following table shows the operating criterion:
Table 9-16. Directional Ground Element (Current Element)
Directional ground element (current polarization)
Operating phasor
Polarizing phasor
Criteria
I0 -Ipol
-90
°≤
[arg(OP)-arg(POL)]
≤
90
°
August 1, 2009
9-59
973.334.3100
