Copyright SEL 2006
Protection of High-VoltageAC Cables
Demetrios A. Tziouvaras
Schweitzer Engineering Laboratories, Inc.
Overview
z Cable typesz Cable-sheath grounding methodsz Electrical characteristics of cablesz Short-circuit protection of cablesz Pilot-channel applicationsz Cable protection application examples
Short-Circuit Protection of UG Cables
z Protect UG cables from excessive heating caused by faults
z Reduce down time and cost of cable repairs by limiting cable damage to only a small portion
z Prevent pipe ruptures in pipe-type cables Prevent insulating fluid spills Protect environment
z Improve power system transient stability high-speed protection is a must
UG Cable Types
z HPFF pipe-type cables three conductors in steel pipe
z Self-contained fluid-filled cables single-phase cables with pressurized dielectric fluid
z Solid dielectric XLPE cables Single-phase or three-phase cables Recent developments for up to 500 kV
Solid Dielectric Cable
OuterSheath
InsulationScreen
Insulation
Conductor
ConductorScreen
MetallicSheath
Cable-Sheath GroundingGrounded in at Least One Point
z Safety reasonsz Limit sheath voltagesz Reduce sheath losses to a minimumz Maintain a continuous sheath circuit for
fault-current return
Sheath-Bonding Methods
z Single-point bondingz Solid bondingz Cross bonding
Single-Point Bonding
Ground Continuity Conductor
SheathVoltageLimiters
Joints With Sheath Interrupts
Cross Bonding
SheathVoltageLimiters
Ground Continuity Conductor
Joints WithSheath Interrupts
Major SectionMinor
Section
Electrical Characteristics of Cables
z Cable design features affect electrical characteristics Use of solid dielectric Sheath, and in some cables the armor Close spacing of the conductors
z Overall results Low series inductance (3050% lower than
OH lines) High charging currents (3040 times higher
than OH lines)
Cable Versus OH Line Characteristics
0.50.23 + j 1.600.06 + j 0.47230-kV OH Line
18.00.45 + j 0.40
at 5000 Amp 3I0
0.03 + j 0.15230-kV HPOF Pipe-Type Cable
9.40.17 + j 0.080.04 + j 0.13230-kV SC Cable
Charging Current in
A/km
Z0 in /kmZ1 and Z2 in /km
Circuit Type
Zero-Sequence Impedance of UG Cables
z Difficult to determine accuratelyz During unbalanced faults the ground current
can return through: Ground only Sheath only Ground and sheath in parallel
Ground and sheath of adjacent cables
Zero-Sequence Impedance of UG Cables
Return path made complex by presence of:
z Gas and water pipesz Railwaysz Other parallel cables
Zero-Sequence Return Currents and Equivalent Circuit of SC Cables
0gI
0sI
I0
0m0sZ - Z
0mZ0gI
I0s
Z0m
0Z0c
0s
I
ZSheath
Conductor
0cZ 0m Z
Zero-Sequence Impedance of SC Cables With Three Different
Return Pathsz Current return in the sheath only
Z0 = Z0c + Z0s 2 Z0m
z Current return in the ground onlyZ0 = Z0c Z0m + Z0m = Z0c
z Current return in the sheath and ground in parallel
s0
2m0
c0 ZZZ0Z =
Zero-Sequence Impedances for SC Cable
0.17 + j 0.07Sheath Only
0.17 + j 0.08Ground and Sheath in Parallel
0.20 + j 2.17Ground Only
Z0 in Ground Return Current Path
Variation of Zero-Sequence Resistance and Reactance in Pipe-Type Cables
Single-Point Bonded Cable
S R
Ground continuity conductor
S-End Compensated Loop ReactanceConductor to Sheath Faults
0 0.2 0.4 0.6 0.80
0.01
0.02
0.03
0.04
0.05
1.0
S-End Compensated loop X in Ohms
Distance from the S-End in per unit
C
o
m
p
e
n
s
a
t
e
d
l
o
o
p
X
i
n
O
h
m
s
0.135 Core-to-ground fault at R-End
Core-to-sheath fault at R-End
R-End Compensated Loop ReactanceConductor to Sheath Faults
R-End Compensated Loop X and R Conductor to Sheath Faults
Solid-Bonded SC Cable
S R
Ground continuity conductor
Ground Current Return Path inSolid-Bonded SC Cables
Ground Conductor
Source at S-End Only
S-End Compensated Loop Z in OhmsSolid-Bonded SC Cable
Compensated Loop ReactanceSolid-Bonded SC Cable
Different Zero-Sequence Current Compensation Factors
S-End Compensated Loop R in OhmsSolid-Bonded Cable
S-End Compensated Loop Z in OhmsCross-Bonded SC Cable
End of 1st minor section
End of 2nd minor section
S-End compensated loop Z in Ohms
End of 3rd minor section
Compensated loop R in Ohms k0
0
= 0.0057-j 0.3498
Re(ZrS1)
Im(ZrS1)
C
o
m
p
e
n
s
a
t
e
d
l
o
o
p
X
i
n
O
h
m
s 0.
0.
0.
0.
-0.02 0.140.02 0.040 0.06 0.08 0.1 0.12
4
2
3
1
0
S-End Compensated Loop X in OhmsCross-Bonded SC Cable
End of 1st minor section
End of 2nd minor section
S-End compensated loop X in Ohms
Distance from the S-End in p.u.
End of 3rd minor section
X
i
n
O
h
m
s
w
i
t
h
k
0
=
0
.
0
0
5
7
-
j
0
.
3
4
9
0.4
0.2
0.3
0.1
00.2 0.60.4 0.8 10
Distance Element Considerations
z Majority of faults in UG cables involve the sheath and ground
z In overhead lines the Z0L is proportional to distance
z In SC cables the Z0 may be nonlinear with respect to distance
z Important to look at impedances seen by ground distance elements
z Sheath bonding plays an important role in compensated loop impedance
Distance Element Considerations
z Zero-sequence compensation factor influences reach and performance of ground distance elements
z Choose a K0 that provides a constant or increasing slope of the compensated loop reactance for faults at the cable end
z Consider the network topology Parallel cables Adjacent sections (overhead versus
underground)
High-Speed Cable Protection
z Current differentialz Phase comparisonz Directional comparison
Current Differential Protection Advantages
z Requires current information onlyz Less dependent on cable electrical
characteristics
z Easy to setz Immune to power swingsz Immune to current reversals
Conventional Current Differential Protection Disadvantages
z Includes no backup protectionz Requires current information from all
terminals
z Depends on channel performancez Requires added security for CT saturationz Provides limited RF coverage
Phase Comparison Protection Advantages
z Very popular in the past because of minimal communications requirements
z More secure than current differential scheme for external faults with CT saturation
Directional Comparison Scheme Advantages
z Provides main and backup protectionz Loss-of-channel does not disable local and
remote backup protection
z POTT schemes operate with FSK communications
z Negative-sequence directional elements provide excellent RF coverage
Directional Comparison Scheme Disadvantages
z Requires voltage and current informationz Can be affected by power swings and
current reversals
z More dependent on cable electrical characteristics
Digital Pilot Channels
z Dedicated fiberz Multiplexed fiberz Multiplexed microwavez Digital telephone circuits
Ground potential rise concerns Noise coupled from the faulted power system
could cause bit errors or a complete loss of signal
Cable Protection Applications
z Circuit consisting of a cable onlyz Cable circuit terminated into a transformerz Mixed overhead and underground cable
circuits Cable at the beginning of the line Cable in the middle of the line
Three terminal OH line with cable
Mixed Overhead and Cable CircuitProtection System A Shown Only
M
87T
Transfer trip keying fortransformer faults
87L-1 87L-1
Cable
Digital communications channels
OH line
87L-187L-3
87L-3
Blockreclosing
Conclusions
z Apply modern relays that have integrated full distance, line differential, and digital relay-to-relay communications
z Be careful when applying ground distance elements for cable protection Select the proper K0 Compensated loop impedance is affected by
cable grounding and bonding methods
Cable Z0 is not linearly related to distance
Conclusions
z Apply high-speed relay systems for cable protection
z Use relay-to-relay communications to create new schemes and to combine traditional protection schemes to: Reduce costs Increase reliability
Enhance performance of cable protection
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