Schweitzer Engineering Laboratories SEL-421-4 - Table 6.25 Tilt Resulting from Nonhomogeneity; Figure 6.23 Apparent Fault Impedance Resulting from Nonhomogeneity

6.98
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Table 6.25 provides the results of Equation 6.59 and Equation 6.60 for both the
negative-sequence and zero-sequence networks. Remember that T0 depends on
the return path of the ground fault; i.e., sheath, ground, or a parallel combination
of both. The distance to fault is assumed to be 100 percent (m equals 1).
The negative-sequence network is more homogeneous than the zero-sequence
network when compared with two of the three corresponding cable zero-
sequence impedances. Choose negative-sequence current for polarizing the quad-
rilateral ground-distance protection.
XGPOL := I2. Quadrilateral Ground Polarizing Quantity (I2, IG)
Selection I2 indicates that the negative-sequence current flowing in the cable is
the polarizing quantity for the reactance line.
Figure 6.23 Apparent Fault Impedance Resulting From Nonhomogeneity
Table 6.25 Tilt Resulting From Nonhomogeneity
Angle T
2
T
0
(sheath)
T
0
(ground)
T
0
(ground and sheath)
Negative-Sequence Network –2.2°
Zero-Sequence Network –5.8° –1.1° –5.6°
X
R
S
R
XAG
+T
—T