6.87
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
EHV Parallel 230 kV Underground Cables Example
This application example presents an underground cable system with double-
ended 230 kV parallel cables (see Figure 6.19). SEL-421 Relays protect each end
of the first circuit. This example explains settings calculations for the SEL-421 at
Station S that protects Cable 1 between Station S and Station R.
The SEL-421 uses communications-assisted high-speed tripping to provide pro-
tection for faults along the 230 kV underground cable.
The two 230 kV underground cables run from Station S to Station R. Each circuit
consists of three single-phase cables, each having an oil-filled copper conductor
(hollow core). The cables are insulated with impregnated paper and have a lead
sheath to prevent intrusion of moisture and to withstand fluid pressure. The
cables are also grounded at both ends. Depending on the nature of a ground fault,
ground fault current can return via the sheath, the ground, or both the sheath and
ground.
Power System Data
Table 6.22 lists the power system data for this application example. Substitute the
values and parameters that correspond to your system when you set the relay,
using this example as a guide.
Figure 6.19 230 kV Parallel Underground Cables
R (Grangeville)
SEL-421
Z
1S
, Z
0S
Z
1R
, Z
0R
BK1
IW IW
VYVY
Z
1L1
, Z
0L1
BK2
BK3
Z
1L2
, Z
0L2
BK4
S (Lewiston)
Cable 1
Cable 2
230 kV
SEL-421
230 kV
Table 6.22 System Data—230 kV Parallel Underground Cables (Sheet 1 of 2)
Parameter Value
Nominal System Line-to-Line Voltage 230 kV
Nominal Relay Current 5 A secondary
Nominal Frequency 60 Hz
Cable Length 25 miles
Cable Impedances:
Z
1L1
= Z
1L2
Z
0L1
(sheath return) = Z
0L2
(sheath return)
Z
0L1
(ground return) = Z
0L2
(ground return)
Z
0L1
(sheath and ground return) = Z
0L2
(sheath and ground return)
4.78 42.5° primary
9.45 17.4° primary
91.4 84.9° primary
9.58 21.7° primary
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