cesc cable joints

7/28/2019 Cesc Cable Joints

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Analysis of HT cable

and joint failures

and associateddesign modifications

Authors:

K. Rana,

Manager (Jointing), CESC Ltd.

B. Dasgupta,

Mains Engineer,CESC Ltd

CESC Ltd , in existence since 1897,

generates and distributes electricity to the

twin cities of Calcutta and Howrah on either

side of river Hooghly spanning over an area

of 567 sq. km, serving a demand of 1657

MW. It is now a RP-Sanjiv Goenka group

company.

Both the cities being highly congested, need

was felt from the early days for

underground transmission and distribution

on preference to cheaper overhead option

because of greater way leave requirement

of overhead lines. We presently have 4950

ckt kms of 6/11KV cable network with a

consumer base of 1673 HT and 2.49 million

LT consumers.

This paper discusses the cable and joint

fault analysis which is regularly conducted

in our system consequent to faults. All new

joint failures and XLPE Cable failures in run

are analyzed in stages to identify the root

cause of such failure. The observations

regarding failure and statistical analysis of

trends are used to as a tool to development

of our cable construction and joint design.

Why cables fail?Power cables are manufactured in factories

under controlled environment and

sophisticated online monitoring. The

completed cables are further tested

according to standard testing guidelines

before acceptance for use.

However, the cables laid at site may notdeliver the required performance due to

adverse installation conditions and

unintentional damage during cable laying.

Jointing is required to be done at site of

installation where the trench is often

infested with dust, moisture, vibrations etc.

these unavoidable factors can be

detrimental for a high tension cable joint

which requires a clean environment for

manufacture. Human factors are also

present in jointing as the job is done

manually. Though joints are always done by

trained and experienced jointers of

sufficient reliability, human error can creep

in which is unavoidable.

Furthermore, a joint is a weakest part of an

underground cable system owing to the 3

types of stresses which are predominant in

joint. These are the thermal stress (caused

mainly by the externally applied insulationbuild up and joint encapsulation), electrical

stress (caused mainly due to termination of

cable screen in high tension cables) and

mechanical stress (as the conductor jointing

region is more prone to stress and strain

during normal cable loading and

development of transient overcurrent


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during fault conditions superimposed on

daily and annual temperature variations).

Types of failuresMost of the failures that occur in cable

system have a cause that is well known. For

instance, failures due to digging activities of

other utilities which damages our cable or

due to ageing of older

components(eg.Insulation or metallic

sheath) at their end of life. If we study the

type of failures keeping in mind the basiccable construction, then we categorize the

failure along following line:

a) In conductor : Most found in jointsat the conductor connection points

b) In insulation : In joint or cable andmostly related to ageing

c) In sheath (metallic or non-metallic):Mostly in cable and generally it is

not the ultimate cause but always

the incipient one.

Failures in insulation of cables and

accessories are mostly related to ageing and

typical basic ageing processes are:

Thermal breakdown Partial discharge Electrical treeing Water treeing

Thermal breakdown:

It is a very common form of breakdown in

cable insulation. Generally a thermal

breakdown is recognized by:

The breakdown channelis radial.

There is typical burningsmell from the

breakdown zone.

Thermal breakdown occurs when rate of

energy and heat transfer to insulation

material as a result of electric field exceeds

the rate of heat dissipation and absorption.

This type of breakdown is therefore not

common in XLPE cables but if the properties

of insulating material are quite inferior then

there is always possibility of such failure.

Electrical breakdown:

Electrical breakdown in polymeric cablescan occur due to treeing. Treeing is a

phenomenon occurring in polymer

insulated cables in 2 forms:

Electrical treeing:

In high tension cables

(11KV and above), the

voltage stress appearing

across cable insulation is

considerably high. Every

precaution is taken in cable

factories so that the

polymeric insulation is free

from voids, impurities,


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semicon protrusions etc.

However, in real life, it is not

possible to design a 100%

void and impurity free cable

insulation. The voids and

impurities are region oflocalized discharge and

heating which ultimately

develops into a carbonized

path in the insulation.

Formation of carbonized

pockets cause the effective

insulation thickness to

reduce and develop a

carbonized conducting

tracking path which

ultimately results in

dielectricbreakdown(Fig : 1).

Water treeing:

Polymeric insulations are

hygroscopic to some extent. The

seepage of moisture through the

cable sheath can percolate through

the insulation(Fig : 2). The watermolecules get charged as the

conductor acts as cathode and the

screen as anode. The charged water

molecules travel through the

insulation from the earthed screen

towards the live conductor via

insulation by a process called

electrophoresis.

(Fig : 2)

(Fig : 1)

Water causes conducting path

through the insulation resulting indielectric failure in the long run

unlike electrical treeing, water

treeing is a very slow process which

develops over a long period of time.

Different seamless water barrier

tapes and extruded metallic sheaths

are used in cable to provide water

tightness to the cable.

Ageing:

The term ageing is used for old PILC cables

in service for more than 50 years. Ageing is

the natural degradation of the cable

insulation caused by various reasons. The

major reason of ageing in PILC cables is the

cyclic overloading. Overloading heats up the

paper insulation causing the impregnation

oil to dry up. Dry paper insulation greatlyhampers the dielectric strength of the same

which is often sufficient enough to cause

breakdown.

The quantity of paper degrades over a

period of time as it is made up of cellulose.


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This phenomenon along with drying of

impregnating oil can cause failure. The Lead

sheath of the cable reacts with the

chemicals present in the soil and gets

corroded over a period of time. This can

cause seepage of moisture into the paperinsulationand subsequent breakdown of the

cable dielectric.

Our experiences

Now we will focus on our experiencesregarding cable faults. Our primary high

tension network comprises of 6 and11KV

and Sub transmission voltage of 33KV. We

are recently installing 3 core XLPE insulated

cables in our primary distribution network

and single core XLPE cables for 33 KV

networks.

We have 3 types of 33KV Grade cables

existing in our system:

PILC Cables (3 core H Cables

and Single core HSL type cables

XLPE CABLES (Single core

only)

Gas Filled PILC Cables

6/11KV Grade cables:

PILC belted cables (bothAluminum and Copperconductors)

XLPE cables (3 corealuminum conductors)

We are using only XLPE cables in all voltage

grades now but we still have a large part of

our existing network comprising of PILC

cables and few Gas filled cables (33KV). Due

to the above reason, we often require to

join our and existing PILC cables to maintain

network continuity mainly during cable

breakdowns. It is our observation that these

transitionjoints is more prone to fault. Thejoint is designed to suit our requirements.

6/11KV Joint

failures

The major areas of fault as observed in our

system in a 6/11KV straight through joint

and terminations are discussed below:

The continuity of the lead sleeve

with the PILC cable lead sheath:

In our transition joint design we have a lead

sleeve prepared at site by beating up a

rectangular flat lead sheet to size, to

encapsulate the joint. The earth continuity

of the joint is ensured by a tinned copper

braid of suitable size, connected to the

armour wire of XLPE cable with jubilee clips

and solder tacked on to lead sleeve of the

joint. The lead sleeve is plumbed on to the

sheath of PILC cable and provides actually a

hermetic sealing on the PILC side of the

joint which is most vulnerable to moisture

ingress(Fig : 3).

(Fig : 3)


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The region of contact of the lead sleeve

with the PILC lead sheath is of crucial

importance. Insufficient application of

plumbing metal or inappropriate

workmanship during plumbing can pose

high resistance to the earth fault currentdue to any fault in downstream network.

Repetitiveoccurrence can melt the plumb

and allow subsoil water to enter the belt

paper insulation beneath and cause

dielectric failure (Fig : 4 and 5). Most of our

failures in transition joint have been

attributed to the failure of paper insulation

near the crutch region due to moisture

ingress.

The above pie chart shows the percentage of failure

of transition joints for various reasons in our system.

(Fig : 4)

(Fig : 5)

The above 3 pics show failure of XLPE-PILC

transition joints from the plumb region.

Crutch region of the PILC Cable

PILC Cables existing in our system are

mostly aged and as a result the strength

and durability of the paper insulation has

degraded over the period of long service.

The oil impregnation of the paper can also

get partially dried making the paper brittle.

During breakdown repair jobs, we need to

join these old PILC Cables with new XLPE

Cables.

(Fig : 6)


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Handling of the old PILC Cables during

aligning for jointing is therefore of an

extremely skillful task to avoid cracking of

paper insulation at the crutch region which

is most mechanically stressed. Bad cross in

PILC cores and damage to paper during corehandling can cause phase-to phase short at

the crutch region which is in turn most

electrically stressed also(Fig : 6).

Conductor jointing region:

The next most fault prone portion of our

joint is the ferrule zone. In Al-Al conductor

jointing in 6/11KV, we employ crimping

technique using ratchet type crimping tool.Failure from crimping area has been mostly

due to unacceptable gap between

successive crimps and incorrect crimping

sequence resulting in inadequate cold flow

of the metal inside the ferrule (Fig : 7). On

dissecting such poorly crimped ferrules, we

have found voids inside the ferrule and

consequent radial failure during heating

under load cycle(Fig : 9).

(Fig : 7)

(Fig : 9)

(Fig : 8)

However, some of our existing PILC Cables

have Copper Conductors. We employ solder

basting technology using weak back Copper

ferrule for jointing the same with Aluminum

conductors. It is necessary to ensure that

the conductor jointing region has low

resistivity in order to allow smooth flow of

current across it.

Insulation build up:

In our 6KV and 33KV joints, failure at

conductor joint region was also observed in

our design in hand applied polymeric

insulating tape. Analysis of faulty joints

reveled that failure in all cases have

occurred from the edge of the ferrule. The

root cause behind the failure was improper


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insulation build up profile. The insulation

build up on ferrule was done by hand

applied self-amalgamating insulating tape

over the conductor jointing region. The

varying tension of hand applied insulating

tape can cause insufficient thickness ofbuild up at some places over the ferrule(Fig

: 10). Dielectric breakdown occurs from the

region of minimum insulation build up

which is usually at the edge of ferrule(Fig :

11).

Correct procedure of tape buildup: ~

1.6 times the insulation thickness on

cable

Wrongprocedure of tape build up

(Fig : 11)

(Fig : 10)

A typical failure due to inadequate tape

build up thickness at the edge of the

ferrule

Improper core disposition:

Fault can also develop in transition joints

where the cores can be in contact with the

metallic lead sleeve at earth potential dueto improper disposition of the cores as

shown in the diagram below:


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XLPE Insulation and Insulation

screen cut region

The extruded XLPE insulation screen isrequired to be removed up to a

predetermined distance away from the

insulation cut as specified by the joint

manufacturer in order to provide the

necessary safe creepage distance. The

screen cut region is a region of high electric

stressand improper termination of screen

and inadequate stress control can cause

high partial discharge inside the joint.

Void filling high permittivity mastics applied

at the screen cut point

Accidental nick on exposed portion of XLPE

insulation or semicon screen can cause

concentration of high electrical stress at thenick point and resultant dielectric failure

may occur within a short period of time due

to electrical treeing. The picture below

depicts such a breakdown which was

probably caused for the above reason (Fig :

12 a and Fig : 12 b).

(Fig : 12 a)

(Fig : 12 b)

A typical fault due to inadequate

stress control at screen cut region

Failure at termination:

The major fault prone areas of a XLPE and

PILC Termination are:

The conductor Jointing region Semicon screen cut region


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The contact region of the lugwith the dropper or stud

Earthing region

(Fig : 13) Failure at semicon screen cut

region

The reason of failure due to improper

conductor jointing is the same as explained

above in straight through joints. In

termination joints, it is important to ensuresufficient surface area of contact between

the palm of the lug and the surface onto

which it is connected. In our system, it is

often required to fit the lug made of

Aluminum with Copper droppers or studs.

In that case, use of bimetallic washer is

absolutely essential.

33KV Joint failures

In our 33KV system, we have observed fault

at the following regions of joint:

Region of armour connection

In an XLPE XLPE single corestraight through joint, the subsoil

water can enter the cable sheath

due to improper joint encapsulation.

The water can easily travel throughthe Copper Earth braid inside the

joint used for armour continuity by

capillary action. The water readily

oxidizes the Aluminum armour wires

and the Poly Al sheath. This

phenomenon is accelerated due to

the cumulative heating of the screen

wires resulting from the continuous

flow of circulating current as both

ends of the cable screen are solidly

earthed.

Due to oxidation of the Al screenwires, the effective electrical cross

sectional area of the armour gets

reduced causing continuous over

heating of the same which in turn

facilitates further oxidation.

Moreover, use of spring band of

different metal at this region to

make armour and Poly Al

connectivity has an inherent heatingdue to bimetallic contact of the two.

This cumulative heating results in

the thermal breakdown of insulation

at this region (Fig : 14 a and Fig 14

b).


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(Fig : 14 a)

(Fig : 14 b)

It was also seen in some casesthat the spring band has lost its

constant pressure property

over a period of time during

breathing of the cable under

varying load cycle. This has

loosened the compression and

causes heating at the point of

current transfer between

aluminum armour wires and the

copper braid.

Failure at heat shrink PILC

terminations

The impregnation of oil of thepaper insulation often oozes outdue to the effect of gravity and

breathing of the cable at

terminations. This causes drying

of the paper insulation which in

turn greatly hampers the

dielectric property of the same.

This is specifically pronounced inheat shrinkable outdoor type

terminations in PILC cables.

Failures in cable

run

Apart from joint failures, fault also occurs in

cable run for the following reasons:

Direct spiking: -

In a metropolitan city like ours, the route of

underground power cables are often shared

with other utilities like telecommunication,

municipal sewage or water supply works,

civil construction works like road widening,

pillar erection for flyover etc. This can cause

accidental damage to the power cables laid

undergrounddue to direct spiking during

excavation. This problem is specifically

pronounced in a city distribution network as

in our case. Direct spiking by pickaxe, JCB or

other metallic digging instruments is

therefore a common occurrence which is

beyond anybodys control.(Fig : 15 a and

Fig:15 b)show typical direct spike cases.


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(Fig : 15 a)

(Fig : 15 b)

Typical cases of direct spiking on XLPE

cables

After effect of spiking

Direct spiking can damage the cable but

often that does not cause feeder tripping

immediately if the penetration is not that

serious. However, the damage meted out to

the cable outer sheath and armour in case

of PILC Cables can cause corrosion of Lead

Sheath and provide path for moisture

seepage into the paper insulation causing

dielectric breakdown of the cable. XLPE

cable can also fail in case of damage toscreen or due to damage in armour as XLPE

insulation is to some extent

hygroscopic.(Fig : 16)

(Fig : 16)

Improper cable laying

Deviation from standard installation

procedures can occur in some places due to

hindrances on cable route like previouslyexisting concrete construction,

communication cables etc. The inadequate

depth of laying can increase the chance of

damage to the cable due to spiking or heavy

vehicular movement above the cable.

Ageing of cable

This is applicable in case of PILC Cables

which are in service for more than 50 years

in our system. Apart from the aboveexternal reasons, failure of cable in run can

also occur due to natural ageing. Ageing can

be accelerated due to adverse installation

conditionsand cyclic overloading. The

dielectric strength of paper insulation can

degrade due to irregular load pattern,

frequent overloading with cables installed

in soil having high thermal resistivity and

bending the cable beyond the safe radius.

Corrosion of Lead sheath can also occur due

to presence of strong chemicals in soil

which can puncture the Lead sheath and

allow subsoil water to enter the paper

insulation.


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Manufacturing defects

The cables manufactured in factory in

controlled environment are always tested

prior to acceptance according to Indian

Standards for Acceptance Testing. However,

defects can persist in cable like

discontinuity of screen, damaged insulation

etc. These defects can generate into fault

when the cable is placed in cyclic load.

____________________________________

Failure Analysis

The analysis of failures is done in stages to

arrive at probable root cause of the failure.

We do the following routine analysis to all

our failures.

Information from fault site: the fault site is

visited after occurrence of the fault. This

visit is aimed to obtain relevant information

which may guide us to identify the cause offault. The type of information includes:

Installation conditions: The cableinstalled at improper depth,

inadequate bend etc. can give rise to

fault. The condition of soil is tested

to estimate subsoil water level

which may have entered inside the

cable or joint during fault.

Availability of tiles and side blocks:The positioning of protective tiles

which is usually placed on the laid

up cable give us an indication of any

underground excavation job which

may have been carried out by some

other agency in near pastand

ourcable could have been damaged

in the process. Scattered tiles at the

fault region indicate that some other

agency has exposed our cable which

strengthens the probability of cable

damage by spiking.

Local reports of faultsite:Information gathered from local

residents near the fault zone often

provide us important clues in fault

analysis. They apprise us of any

digging activity along the cable route

in near past, frequency of

occurrence of fault at the region etc.

Load of traffic: In an urbandistribution network like ours, the

heavy traffic frequency above the

road often causes joints to vibrate

under vehicular movement and can

weaken the sophisticated areas of a

high voltage joint.

Equipped with the information gathered

from fault site, we investigate the following

records pertaining to the faulty feeder:

o Load pattern of the cabledaily and annual

o Tripping history of the cablesection over a period of time

o Statistics of nature of fault(fault in joint or in cable run

etc)

At final stage, we conduct stage by stagedissection of the faulty piece at our

materials laboratory which is equipped with

various testing equipmentand tools for this

type of analysis. The different parts of the

faulty piece are exposed with utmost care

so as to preserve the proof of reason of

failure.


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The following pictures show a stage by

stage post mortem of a PILC cable fault in

run. The individual cable components are

separated and observed for clues of failure.

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

We arrive at probable cause of failure after

summing up all the relevant information

and prepare the final report for archiving

and necessary actions.

Inference

Consequent to the above failures, we have

come to the conclusion that failure of our

high tension cables and joints are mainly

occurring for the following reasons:

Uneven thickness of hand appliedinsulation build up on ferrule

Ageing of old PILC cables Poor mechanical strength of outer

jacketing of the cable.

Inadequate conductor connectiondue to improper crimping.

Ingress of moisture inside the cableand joints due to elevated subsoil

water level

Inadequate path for circulatingcurrent flow in both ends bonded33KV system.

Migration of impregnatingcompound in 33KV PILC

terminations.


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Remedial

Measures

The root causes of failures that were

detected from our analysis of cable and

joint failures guided us in making the

following course corrections in our cable

and joint design:

We have phased out the taped typeXLPE joint design and incorporated

heat shrink technology in all ourhigh tension jointing system.

Crimping is to some extentdependent on manual skill where

equal spacing requires to be kept.

Therefore in order to arrest failures

at ferrule region due to bad

crimping, we have introduced

shearing bolt connector at 33KV

level.

We have ceased to use Poly Alsheathed cables in our 33KV system

and proportionately increased the

number and cross sectional area of

the armour wires to compensate

reduction in area of Poly Al. In our

modified joint design, the armour

continuity is being done using

aluminum connector which has not

only eliminated the bimetallic effect

of the spring band but also reduced

the number of junctions in the path

of the flow of the circulating current.

In the process we have done away

with Copper braid which was

responsible for ingress of moisture

inside the joint when the

encapsulation was not proper. We

have also selectively gone for single

end bonding with the other end

earthed through SVL for long circuits

lengths where the sheath circulatingcurrent are considerably high. For

short circuit lengths where the

sheath induced voltage is within

limits we have gone for single end

bonding.

We have identified the repetitivespike prone cable routes and

replaced the same as far as

practicable to minimize the numberof cable breakdowns caused due to

the presence of innumerable joints

in between a small cable length and

thereby reducing chances of failure.

Our 11KV grade cables hadpreviously PVC as outer sheath

material which was subsequently

changed to HDPE owing to its tough

and rigid property in view to protect

the cable components from spiking.It is also stressed upon to adhere to

the installation protocols of HT

cables regarding depth and

protection by tiles to minimize

chance of direct spiking.


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Conclusion

Post mortem of fault is always a probableassumption of the cause of failure as the

direct evidence for the failure is often

disappeared in the flashover occurred

during the fault. This makes confident

reconstruction of failure difficult as most

important clues are often lost. In this paper

we have tried to explain how failure

analysis of joints and terminations help us

to establish the most probable cause of

failure and subsequently give us a directionin which future modifications of cable and

joint designs should be carried out to

prevent such failures.

However, a more futuristic method would

be an analysis which can detect a failure

before It actually occurs, thereby ensuring

that all the evidences of the root cause

which is responsible for the imminent

failure are still alive. We in CESC are nowtrying to preempt failures using state of the

art Condition Monitoring equipments which

detect abnormal hot spots, partial

discharges occurring in the joints and

terminations while they are in service. Any

abnormalities detected in any joint or

termination is further analyzed after

arranging a planned shutdown to ascertain

the root cause and take necessary

corrective measures. This technique not

only helps us to avert possible shutdown or

blackouts associated with joint or cable

breakdown but also helps us to pinpoint the

root cause more accurately.

cesc cable joints

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