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