Analysis of HT cable and joint failures and associated design 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 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 not deliver 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 thermal stress (caused mainly by the externally applied insulation build up and joi nt 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
during fault conditions superimposed on daily and annual temperature variations).
Types of failures
Thermal breakdown: It is a very common form of breakdown in cable insulation. Generally a thermal breakdown is recognized by:
Most 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 basic cable construction, then we categorize the failure along following line: a) In conductor : Most found in joints at the conductor connection points b) In insulation : In joint or cable and mostly 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
The breakdown channel is radial. There is typical burning smell 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 cables can 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,
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 of localized 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 water molecules 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 in dielectric 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 greatly hampers 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.
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 paper insulationand subsequent breakdown of the cable dielectric.
Our experiences Now we will focus on our experiences regarding 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:
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 ‘transition joints’ is more prone to fault. The joint 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).
PILC belted cables (both Aluminum and Copper conductors) XLPE cables (3 core aluminum 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
(Fig : 3)
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 current due 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.
(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.
The above pie chart shows the percentage of fa ilure of transition joints for various reasons in our system.
(Fig : 6)
(Fig : 4)
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 core handling 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 : 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.
(Fig : 7)
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
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 of build 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
(Fig : 11)
(Fig : 10)
Wrongprocedure of tape build up
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 due to improper disposition of the cores as shown in the diagram below:
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 the nick 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).
XLPE Insulation and Insulation screen cut region The extruded XLPE insulation screen is required 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.
(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
•
•
The contact region of the lug with the dropper or stud Earthing region
33KV Joint failures In our 33KV system, we have observed fault at the following regions of joint:
Region of armour connection
(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 ensure sufficient 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.
In an XLPE – XLPE single core straight through joint, the subsoil water can enter the cable sheath due to improper joint encapsulation. The water can easily travel through the 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 screen wires, 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 heating due 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).
Failure at heat shrink PILC terminations
(Fig : 14 a)
The impregnation of oil of the paper insulation often oozes out due 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 in heat 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: (Fig : 14 b)
Direct spiking:
It was also seen in some cases that 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.
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 anybody’s control.(Fig : 15 a and Fig:15 b)show typical direct spike cases.
(Fig : 15 a)
(Fig : 16)
Improper cable laying
(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 to screen or due to damage in armour as XLPE insulation is to some extent hygroscopic.(Fig : 16)
Deviation from standard installation procedures can occur in some places due to hindrances on cable route like previously existing 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 above external 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.
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.
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.
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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 of fault. The type of information includes:
Local reports of fault site: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 urban distribution 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
Installation conditions : The cable installed 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
o
o
Load pattern of the cable daily and annual Tripping history of the cable section over a period of time Statistics of nature of fault (fault in joint or in cable run etc)
At final stage, we conduct stage by stage dissection 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.
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.
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
Stage 1
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:
Stage 2
Stage 3
Stage 4
Stage 5
Uneven thickness of hand applied insulation build up on ferrule Ageing of old PILC cables Poor mechanical strength of outer jacketing of the cable. Inadequate conductor connection due to improper crimping. Ingress of moisture inside the cable and joints due to elevated subsoil water level Inadequate path for circulating current flow in both ends bonded 33KV system. Migration of impregnating compound in 33KV PILC terminations.
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 circulating current are considerably high. For short circuit lengths where the sheath induced voltage is within limits we have gone for single end bonding.
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 type XLPE joint design and incorporated heat shrink technology in all our high tension jointing system. Crimping is to some extent dependent 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 Al sheathed 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
We have identified the repetitive spike prone cable routes and replaced the same as far as practicable to minimize the number of 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 had previously 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.
Conclusion Post mortem of fault is always a probable assumption 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 direction in 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 now trying 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.
