Ageing of PILC Cables

8/9/2019 Ageing of PILC Cables

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Ageing of Paper Insulated Lead-Covered

(PILC) Cables

Farhan Hameed [email protected]


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Agenda:

PILC Cables

Ageing

Factors Affecting The Ageing of PILC Cables

ec s on e per ormance o a es ueto ageing

Monitoring of PILC Cables

Conclusion & Future Work

Discussion


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Paper Insulated Lead-Covered(PILC) Cables

Internal Structure of PILC Cables::

on uc or Conductor Shield Paper Insulation Insulation Shield Paper and Copper Tape Core Binder Lead-Alloy Sheath PVC Outer Jacket


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AgeingBitter reality of universe!!!

Its the matter of timeAccumulation of changes over time

Changes = (time)

Time

Operational

Effects

ConstructionalDeficiencies

Environmental

Effects


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AgeingPerformance

Age


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Factors Affecting The Ageing ofPILC Cables:

Environmental Effects Moisture Soil Thermal Resistivity Vibrations

Thermal Degradation

Electrical Stress

Mechanical Damage

Excessive Conductor Current

PD and Overvoltage


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Environmental Effects

(Moisture)

Installation

Sheath damage

Moisture Penetrates

~ Tens of years

~ Months

(from dig up or animals)

The moisture content of theatmosphere does not play a significant

role unless the protective lead sheet isdamaged mechanically

nsu a on

Low Level PDsOccur

Sporadic Arcs

Intermittent Arcs

Continuous Arcs

~ Weeks

~ Days

~ Days

~ Hours

(current reaches ~10 mA)

(current reaches ~100 mA)

Failure!


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Environmental Effects(Moisture)

Current through full cable insulation with time for penetrated sheath immersed in tap water, and cores heated to 75c

The max. arc conductance is plotted against the max. Current during each successive half cycle for seq. of arcs


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Soil Thermal Resistivity:

May cause inappropriate heat transfer


Temperature rise and boost up further degradation processes


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Environmental Effects(Electrochemical Reactions)

Due to high dielectric strength of cellulose it isemployed as Paper Insulation in PILC Cables

Degradation occurs due:

Hydrolysis Oxidation

Decarboxylation Depolymerization

Tensile Strength of Paper:

Ts = (1/Fb + 1/Fs) -1

Where, Ts is tensile strength, Fb is fiber-fiber boding strength and Fs is fiber strength

Fb increases with ageing due to cross-linking reactions

Fs decreases due to depolymerization reactions


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(Electrochemical Reactions)

Wet-tensile strength after ageing EHV 5.5 mil kraft paper under 200 psig


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Thermal DegradationPaper is mostly degraded by thermal effects!

OverloadingTransient Overcurrents Continuous Overcurernts

Overheating

Breakdown of PolymersDeformation of Insulation

Layer

Inappropriate heat transfer

Soil Thermal Resistivity Increased Heat


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Thermal Degradation


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Electrical Stress(Partial Discharge (PD))

A discharge which doesnt bridge the electrodes

Major Causes of PD in PILC Cables:

Due to small mechanical damage of insulation

Or Insulators have voids due to manufacturing faults Once the voids withstand voltage is exceeded, partial discharge

occurs

This produces a rapid rate of change of current

Positive ions and electrons released by the discharge collide withthe void wall causing slow erosion

PD is a symptom of insulation weakness

This phenomenon occurs in PILC Cables but is generally less destructive to them as they

exhibit a unique self-healing property due to the mobile impregnated compound


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Electrical Stress(PD , tan () and ageing)


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Any voltage level above the rated voltage

Over voltages are dangerous for the insulation material

Some are momentary whereas others are sustained

Electrical Stress(Overvoltage)

ncrease t e rate o n t e nsu at on Increases dielectric losses

May become the cause of complete failure of the cablein the long run as every transient phenomenon in the

Power System leave some permanent effects


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Mechanical Damage Damage is usually attributed to activities during or after installation

Due to some accident

Other causes may be insect and rodent attack, vibration, soilerosion etc

Most severe type of damage and may cause immediate failure ofca e

One of the major causes that support other breakdown mechanismswhich are slow (e:g PD, corrosion, moisture etc)

Vibrations:

Contraction and Expansion

Conductor & Insulation displace from their original position

May also cause some severe damage to the outer insulation


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Effects on the performance ofPILC Cables due to ageing

The electrical losses are directly responsible for theheating of the insulation & also affect the residual life ofPILC Cables:

Conductor Losses

Dielectric Losses

Additional Losses due to Harmonic Content


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Monitoring of PILC Cables


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Intelligence is being introduced in the cable connection itself

A self health check

Prevent outages and thereby increasing the reliability of the (smart)grid

Monitoring of PILC Cables(Partial Discharge Online System)

Have constant insight in the risk on failure of each cable connectionand thereby insight in the derived operational risks

Have better insight in the remaining life of the cable connections andthereby determine optimal time for replacement

System can be applied to newly installed cable connection as amethod of performing a prolonged field acceptance test


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Conclusion & Future Work

Ageing is a natural process and cant be stoppedcompletely but rate of ageing can be slowed down withproper techniques

Cable

Some formulation should be made for estimating ageingrate and the remaining life span of PILC Cables by

considering all the major affecting constituentssimultaneously


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Questions and Discussion

an ou

Ageing of PILC Cables

Documents

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