UTILITY APPLICATION OF FIBER OPTIC CABLES

12/22/2000 State of Art Fiber Optic 1

UTILITY APPLICATION OF FIBER OPTIC CABLES

George G. KaradyArizona State University

2000

© Arizona State University. All rights reserved.



Utilities are installing fiber optic cables on high voltage transmission lines. Three basic designs employed are: • 1) OPGW (optical ground wire)• 2) Wrap-type• 3) ADSS (all dielectric self supporting


FIBER OPTIC CABLES



OPGW (optical ground wire) which replaces shield wires• Provides lightning protection• Provides communication• Lightning short circuit damage • Installation requires long tem outage• Expensive• Superior performance



Wrap-type which is wound around shield wires and, in some instances, around energized conductors • Hot-line installation is difficult• Cost more than ADSS, but less than OPGW• Need a shield wire• No operation problem is observed



ADSS (all dielectric self supporting) which is mounted at various locations, typically 3 to 10 meters below the phase conductors.• ADSS costs less than OPGW• Higher fiber count than Wrap type.• Can be installed on towers not designed for

shield wires.• Suitable for hot line installation



• Loose tubes around central FRP• Super absorbent tape• Inner sheath: PE• Para-aramide yarns• Cable jacket



ADSS DisadvantagesHigh electric field caused cable failure has been reported by utilities• Corona problem• Dry-band arcing in polluted area



ADSS INSTALLATIONTECHNIQUE



Corona Location Corona Location Corona Location

Corona Location

CoronaLocation

CoronaLocation



Fiber optic cableassembly



ADSS CORONA CAUSED DAMAGE

Analysis and mitigation techniques



15 kV20 kV

25 kV

Conductor

Fiber Optic Cable

Grid Size =1.2 x 1.2 m

10 kV



Table 1. Space potential and rod tip surface gradients.

StructureEvenendedrods

2.5 cmExtension

SpacePotential(2D)

Tower/Pole kV/cm kV/cm kV230 kV Wood Pole 12.8 25.9 22.60

230kVSteel lattice 10.1 20.2 25.00345 kV Wood Pole 13.8 26.6 28.70500 kV Steel delta 0.9 1.4 3.00



High Electric field generates corona dischargeThe long-term effect of this discharge is the deterioration of the cable jacket, which may result in puncture and failure.


Corona Caused Aging of Fiber Optic Cable


Corona Caused Aging of Fiber Optic Cable





Location SpacePotential

Rod TipGradients

PDIntensity

A 33 kV 60 kV/cm heavyB 25 kV 35 kV/cm mediumC 19 kV 25 kV/cm light



Exposure 1000 hours 5500 hoursArmor Rod Core Full Core FullA 1 2.3 14.7 24.2 84.8A 2 0 0 12.7 43.2A 3 0 95.4 33.9 163Average A 0.76 36.7 23.6 97B 1 0 0 1.9 15.2B 2 4.2 37.1 10.9 96.6B 3 9.41 51.3 18.5 195.3Average B 4.53 29.46 10.4 102.2





ADSS CORONA CAUSED DAMAGE

CABLE TESTING TECHNIQUES



Five different cables were tested The cables were energized to generate a maximum field of 48.3 kV/mmThe field was calculated by a 3D field plotting programThe test duration was 3052 hours







The discharge caused two types of deterioration:

–Surface damage: deterioration spread along the surface at a shallow depth.

–Localized damage: deterioration evidenced by deep, localized erosion craters on the surface.



CableNumber

Max. depth ofdeterioration,

µm

Rank

A 210 3B 371 4C 132 2D 45 1E 411 5

Table 2: Maximum depth of deterioration



ADSS DRY-BAND ARCING CAUSED

DAMAGE



Dry-band arcing occurs when:• The cable is polluted and wet• The longitudinal electric field is sufficiently

large to flashover the dry-band• The current in the dry band is around 0.5-5

mA• The deterioration of the jacket is caused by

the heating of the arc root.



The high voltage conductor induces a space potential on the fiber optic cable.The voltage difference between the grounded armor rod assembly and cable

space potential generates a longitudinal field along the cable jacket.This voltage drives a surface current along the cable if the jacket is covered by a

conductive layer.Pollution and simultaneous wetting produces a conductive layer.

HV conductor

Fiber optic cable



The current is the maximum at the armor rod assembly.The current drays the wet layer on the cable and produces a small dry band near the armor rod.High voltage will appearacross the dry-band.This high voltage produces an arc, which destroys the cable.

E

Fiber Optic Cable

Thin Conductive LayerArmor Rod Assembly

E

Dry-Band Arcing



Dry-band arcing caused damage ranges from erosion to fire on the cable jacket

Fire extinguished Fire did not extinguished



ADSS DRY-BEND ARCING CAUSED

DAMAGEAnalysis and mitigation techniques


Experimental Study of Dry-band Arcing

Test circuit

Current limitingresistor

Shunt

HV transformer

Fiber –optic cable

ElectrodeDischarge

Water spray

Two electrodes were placed on the cable. The distance between the electrodes was 150mm, the applied voltage was : 2.5-10 kV.The current was limited by a capacitor (470pF, 2000pF, 40nF) or a resistance (1.5, 5, 10 Mohm).

The cable was wetted by water, consisting of wetting agent and salt.


Typical Experimental Results

After wetting the cable, the system was energized and the dry-band formation was monitored.

After energization a sinusoidal leakage current started to flow.



Dry-band development



The high voltage flashed over the dry-band, which modified the current shape. The arc current is shown below:



The dry band arcs over.



The arc intensity increases.



The dry-band expands and the current become irregular.



The further extension of the dry-band stops arcing.The current rapidly decreases to the dry cable current.The voltage increases to the open-circuit voltage.After about 300 second the current and voltage become constant.The current flows through the dry cable surface. The layer resistance increases to 5-6 Mohm/cm.


Experimental Study of Dry-band Arcing

ConclusionDry band arcing needs a current more than 0.8-2 mA.Dry-band arcing needs a gap voltage more than 7-8kV.Capacitance and resistance together determine the character of arcing.The obtain values depend on the source impedance (resistance or capacitance).


Spark Discharge

The cables were energized to 10 kV.

The cables were sprayed by tap water, light pollution.The water formed droplets on the hydrophobic cable surface.

Spark discharge developed at different points on the cable.The discharge contained short, bluish arcs.


Spark Discharge

Low energy bluish spark discharge


Artificial Dry-band

Sparking produces bluish discharge


Artificial Dry-band

Sparking DischargeThe current contains high amplitude, fast rising short duration current pulses.The high current pulse generates sudden collapse of the voltage.The voltage and current oscillation are shown in the next slide:


Artificial Dry-band

Sparking generated current pulse and voltage


Artificial Dry-band

Arcing discharge• The discharge 60 Hz component appears on

the discharge current, supply voltage 8 kV.


Artificial Dry-band

Transition from spark to arc:• Change of the color of the discharge.• Both pulses and 60Hz component appear.

• When 60 Hz current flows, the voltage decreases to the arc voltage.

• When pulse current flows, high frequency disturbance occurs on the voltage wave.


Artificial Dry-band

The results clearly demonstrate the presence of a low type of pollution discharge:• Low energy spark discharge, which produces

short duration high current pulses and bluish discharge on a hydrophobic surface.

• Dry- band arcing produces 60 Hz current and reddish, purple, flame-like discharge close to the electrodes on a non hydrophobic surface.



ADSS DRY-BEND ARCING CAUSED

DAMAGEFailure prediction method

12/22/2000 State of Art Fiber Optic 5212/96 4EPRI_Fiberoptic

Model Development

The purpose of this study is the computer simulation of dry band arcing. The polluted and wet fiber optic cable is covered by a conducting layer. This can be simulated by resistances. The unit length resistance varies between 1-10 MΩΩΩΩ/m.The phase voltages of the high voltage conductors drive currents through the capacitors between the fiber optic cable and conductors.


Model Development

R0j/2 R0j/2

C0g

C02 C03C01

1 2 3

•The capacitor values are calculated using the Maxwell potential coefficients.•The inversion of the potential coefficient's matrix gives the partial capacitance matrix.

•The figure shows the equivalent circuit of a unit length section.


Model Development

R0j/2 R0j/2

C0g

C02 C03C01

1 2 3•The ground capacitance is the sum of the elements of the capacitance matrix in row 0.

•C0g = C00 + C01 + C02 + C03.

•The capacitance between the cable and the phase conductors is the positive values of C01, C02, and C03 in the capacitance matrix.


Model Development

Rs3

Rseries

Rs1

Rseries

C0g_1C0g

C20C20_1

C10C10_1

Rs2

Rseries

C30

C30_1

C10_2C10

C20_2C20 C30_2

C30

C0g_2C0g

C10_3C10

C20_3C20 C30_3

C30

C10_4C10

C20_4C20 C30_4

C30

C10_5C10

C20_5C20 C30_5

C30

C0g_3C0g

C0g_4C0g

C0g_5C0g

V2

V3

V1

X13ph_n

Res

R1

Res

R2

Res

R3

Res

R4

Res

R5 Rend

Res

0

0 0 0 0 0

0 0

V

54321

•The circuits representing the sections are connected in series. •The number of sections used for simulations is 250.•Two feet per section.


Model Development

The circuit was analyzed using the SPICE program.The calculation assumes:• Cable is covered by a thin conductive

pollution layer.• Fully wetted by drizzle, fog or light rain.• Pollution levels: Light, Medium or

Heavy.• Heavy rain cleans the cable, washes off

the pollution.


Results of Computer Analyses

The voltage and current distribution was calculated.Open circuit voltage was also calculated.

The effect of span length was analyzed.



Criteria for dry-band arcing:• Wetted conductive layer on the cable

surface.

• More than 1 mA short circuit current.

• More than 8-10 kV open circuit voltage.



Voltage Distribution along Fiber Optic CableSpan = 500 Feet; Wooden Towers

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600

Distance from Tower (ft)

Peak

Vol

tage

(kV

)

Light Medium Heavy

· Fiber optic cable on 220 kV, 3 phase line· Calculated space potential = 31.11 kV· 250 sections at approx. 2 ft per section· Light Pollution = 3.0 MΩ/m· Medium Pollution = 1.0 MΩ/m· Heavy Pollution = 0.1 MΩ/m



Current Distribution along Fiber Optic CableSpan = 500 Feet; 230 kV Wood Towers

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 100 200 300 400 500 600

Distance from Tower (ft)

Peak

Cur

rent

(mA

)

Light Medium Heavy

· Fiber optic cable on 230 kV, 3 phase line· Calculated space potential = 31.94 kV· 250 sections at approx. 2 ft per section· Light Pollution = 3.0 MΩ/m· Medium Pollution = 1.0 MΩ/m· Heavy Pollution = 0.1 MΩ/m



Effect of Span on Voltage Distribution

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Normalized Span

Peak

Vol

tage

(kV

)

1000 500 250 100



CONCLUSIONS:• The mathematical analyses calculates

the open circuit voltage and short circuit current

• This permit the prediction of dry band arcing.

• Based on the results the utility changes the location of the cable to eliminate arcing

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UTILITY APPLICATION OF FIBER OPTIC CABLES

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