Schweitzer Engineering Laboratories SEL-421-4 - Figure 6.27 Reverse Unbalanced Fault on Cable Circuit (Shunt Admittance)

6.109
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
The technical paper Underground/Submarine Cable Protection Using a Sequence
Directional Comparison Scheme (see selinc.com for a copy of this paper) pro-
vides an equation that allows you to express the apparent negative-sequence
impedance at the relay terminal for a reverse unbalanced fault when accounting
for charging capacitance:
Equation 6.68
The SEL-421 uses Equation 6.69 to calculate the apparent negative-sequence
impedance during unbalanced faults:
Equation 6.69
Equation 6.69 yields a more conservative result for the negative-sequence imped-
ance when the parallel cable is out of service:
|Z2S| = 2.97 
The result of Equation 6.69 is greater than the default setting for Z2R;
Z2R = (Z2F + 1/(2 • I
NOM
). (See Ground Directional Elements on page 1.18 for
more information.)
Figure 6.27 Reverse Unbalanced Fault on Cable Circuit (Shunt Admittance)
Z
1L1
Z
1R
Z
1S
SR
Z
1L1
R
Z
1R
Z
1S
I
2
I
2
I
2S
I
2R
I
2
V
2
S
Y
1L1
2
Y
1L1
2
Impedance Diagram
Negative-Sequence Network
SEL-421
Z
2S
V
2S
–
I
2S
-------------
=
4Z
1L1
2+ Y
1L1
Z
1L1
Z
1R
4+ Z
1R
• • • • •
44Y
1L1
Z
1R
2+ Y
1L1
Z
1L1
Y
2
1L1
+ Z
1L1
Z
1R
• • • • • • +
----------------------------------------------------------------------------------------------------------------------------------------------------------=
3.86 =
z2
Re V
2
I
2
Z1ANG• •
*

I
2
2
----------------------------------------------------------------------=