RELIABILITY ASPECTS OF POWER CABLES AND ACCESSORIES

BY

ALI HIRJI DINESH PATRAWALAGENERAL MANAGER - TECHNICAL GENERAL MANAGER, EPDRAYCHEM RPG LTD. RAYCHEM RPG LTD.

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MEDIUM VOLTAGE (MV) CABLE CONSTRUCTIONS

MV cable designs vary widely to meet the diverse requirements of the users, but there are certain components which are common to all constructions of cables. All types of electric cables consist essentially of a low resistance conductor to carry the current and insulation to isolate the conductors from each other and from their surroundings. Other main components may include screening to obtain a radial electrostatic field, a metal sheath to keep out moisture or to retain a pressurising medium (oil impregnant in case of paper insulated cables), armouring for mechanical protection, corrosion protection for the metallic components and a variety of additions extending, for example to internal and external pipes to remove the heat generated in the cable.

The choice of insulation materials is based on factors such as dielectric losses, thermal stability and cost. The 90 Deg.C continuous operation of XLPE insulation, very low dielectric losses, coupled with the benefits of ease of laying and jointing the cable reliably has encouraged the adoption of XLPE insulated cables all over the world.

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FAILURES IN XLPE CABLES

Statistical data indicates that overall average underground distribution system failures with XLPE cables are due to:

a) Physical damage during laying and burying the cablesand also during excavations by other utilities 43%

b) Treeing in the XLPE insulation 20%

c) Corrosion and environment related causes 4%

d) Faulty cable terminations and joints 19%

e) Other causes (overloading, short circuit effects, surge voltages) 14%

As compared to similar studies about 10 years ago, the failures due to physical damage particularly during excavation by other utilities has shown a decreasing trend due to proper co-ordination between the various utilities. The failures due to physical damage has decreased from 70% to 43%.

However, there has been a dramatic increase in failures due to water treeing (1/2% to 20%) as more experience has been gained with XLPE cables and the treeing effects show up only gradually and in failures due to faulty cable terminations and joints (5% - 19%) as material and design defects as well as some installation defects result in failures over a period of time.

Several problems related to spillage of hydrocarbons in the oil refining and petrochemical industry have been reported. These problems are due to the fact that solvents are able to penetrate through the extruded insulation and sheath materials. Though PVC oversheaths are comparitively more resistant to attack by petrol or similar hydrocarbons and suffer only from leaching out of some of the pasticiser due to occasional spillages with corresponding hardening, long duration contact with hydrocarbons can result in the diffusion of these through the PVC and filling up of the interstices in the cable by these hydrocarbons. They can then travel along the cable to leak out at joints and terminations and this creates a fire hazard as well as failures of the cable, joints and terminations. The hydrocarbons can affect the semiconducting properties of the cablescreen and make it insulating as well as lead to loss of air and moisture tightness of the cable accessories as a result of the dissolving action on the adhesives.

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It is sometimes necessary therefore that the cables used in refineries and petrochemical industries be protected with a metallic sheath over the inner sheath. In the U.K., a lead sheath is used with steel wire armour over it. In North America, an aluminium sheath with PVC oversheath and no armour is preferred. This reduces cost but the construction is more vulnerable to the effects of corrosion.

The use of a metal sheath also protects the XLPE insulation from coming in contact with sub-soil water and prevents consequent water tree formation in the XLPE insulation.

CABLE FAILURES DUE TO WATER AND ELECTRIC TREEING

Water treeing is now known to be one of the prime causes of XLPE cable failure. Presently, water is considered to attack XLPE insulation in at least three major ways; repetitive hydraulic impulses under AC electric field; boring into mechanical weaknesses and; electrical breakdown at the tip of the advancing "tree" dendrites and slow saturation of the XLPE. Dissolved chemical compounds in the water, such as copper / steel earth screen corrosion by products aggravate the situation. The following measures in the construction, manufacture and installation of XLPE cables to prevent water-tree failures have been adopted:

a) Minimisation of fault locations in insulation and at the boundaries of the insulation and conducting layers through optimisation of the purity of the insulating and semiconducting compounds and cleanliness of the co-extrusion manufacturing process.

b) Reduction of water content and prevention of ingress of moisture;

i) Into the conductors and the screen region in the manufacturing stage and with the use of heat shrinkable, adhesive coated caps during storage, transport installation and in service.

c) Use of water swellable powders and tapes to ensure radial and axial water-tightness.

d) Use of absolutely reliably sealed cable terminations.

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In addition to the above, intensive development is in progress to increase the resistance of XLPE insulation compounds to water treeing by means of additives.

FAILURES IN XLPE CABLE TERMINATIONS AND JOINTS

A.1 FAILURES RELATED TO IMPROPER CABLE PREPARATION AND CONDUCTOR CONNECTION

The most common mistake the jointer makes is radially nicking into the insulation at the point of semi-conducting screen cut-back, with a sharp knife. This is a region of high electrical stress and electrical discharges in the voids caused by nicking will result in a failure through a pin-hole in the XLPE insulation caused by the discharges.

Failures due to this reason can only be prevented if the jointer is retrained to use a small diameter round file to abrade through the screen until the XLPE insulation just becomes visible.

In case the insulation screen is of the easy strip type, a scoring tool with a guarded blade should be used.

In case the insulation screen is of the firmly bonded type, use a stripping tool as illustrated. Remove any remaining conductive particles with aluminium oxide tape.

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USE OF ROUND FILE

STRIPPING TOOL

A.2. IMPROPER SELECTION OF LUGS / FERRULES AND CRIMPING TOOL / DIES

XLPE cables need specially developed lugs and ferrules in view of the facts listed below:

a) More compacted conductor

b) XLPE cable has a higher current rating than PILC cable of the same size

A lug or ferrule which is suitable for PILC cable when used with XLPE cables will result in a high temperature at the connector especially under full load conditions. The excessive temperature results in premature thermal ageing of the joint insulation and failure.

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A.3. UNEQUAL CURRENT SHARING BETWEEN SINGLE CORE CABLES CONNECTED IN PARALLEL PER PHASE

Unequal current sharing leads to some cables being overloaded and some underloaded. To ensure equal current load sharing between the cables, it is necessary that the inductance of each of the parallel cable should be equal. This inductance is most unbalanced when cables of one phase are grouped laid side by side next to one another. A preferred arrangement is for the cables of different phases to be grouped into systems such that the spacing of cables within one system is less than the spacing between systems. The clearance between two systems should be approximately twice the axial spacing of individual cables in a system. In addition, the sequence of phases within a system is important. The following phase relationship is recommended:

RYB BYR RYB BYR ETC.

On racks or cable trays the arrangements should be as shown below:

RYB BYR

RYB BYR, etc. with a distance of 300 mm between the racks.

For tefoil grouping do not arrange the systems above one another. Instead use the following arrangement:

Y Y Y YR B B R R B B R ETC.

B. FAILURES IN CABLE TERMINATIONS

A failure of a cable termination is due to one or combination of the following causes:

i) Inadequate and / or improper stress control applied at the semi-conducting screen cut back area

ii) Tracking, erosion and / or weathering of the external leakageinsulation between lug and ground

iii) Ingress of moisture

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i) INADEQUATE AND / OR IMPROPER STRESS CONTROL

Stress control is provided in high voltage cable terminations for one primary purpose; that is to control the stress which exists at a point where the screen is terminated.

Failures in different types of cable terminations due to inadequate / improper stress control can be listed as under:

a) TAPE SYSTEM USING STRESS GRADING PADS

The stress grading pad is folded over the screen cut back and is taped with self-amalgamating tape to apply pressure on it to provide a void filling function at the semi-conducting screen cut back step.

Analysis of several failed terminations reveal that the stress grading pad has not flowed and fully filled the step resulting in discharges. The two causes for this are

- inadequate tensioning of the self-amalgamating tape resulting in insufficient pressure on the pad

- erosion of the tape through surface leakage currents, dry banding and surface scintillations.

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b) PREMOULDED PUSH-ON SYSTEM

Inadequate tensioning of the tape applied over the semi conducting pad connecting the screen of the cable to the semi conducting portion of the push-on cone results in void formation in the screen cut back step and failure through discharge activity.

c) HEAT SHRINK SYSTEM

Failure investigations reveal:

- The material properties of the stress control tubing were incorrect, viz. the specific a.c. impedance was not between 1 x 10 ohm-cm - 7 x 10 ohm-cm. this could be due to the formulation not being proper or as a result of thermal ageing influence on the properties of the tubing.

- Insufficient overlap of the stress control tubing over the XLPE insulation; the jointer having maintained a longer overlap on the screen.

- The cable jointer not having used semi-conducting paint because it had dried up in an insufficiently sealed bottle.

In addition to the above causes peculiar to each system one common cause of failure of indoor terminations is the inadequate spacing between cores especially when the jointer has to cross the cores in a confined space of a cable box. The air between the cores breaks down due to electrical stressing and discharge activity erodes through the cable insulation. To prevent failures, the minimum spacing between the cores even at the point of crossing as per the manufacturers recommendations should be followed.

ii) TRACKING, EROSION AND WEATHERING OF THE EXTERNALLEAKAGE INSULATION BETWEEN THE LUG AND GROUND

To prevent failures the user must insist that the external leakage insulation material must meet the requirements of Track and Erosion Resistance as per ASTM D 2303 'Inclined Plane Track & Erosion Test'.

In addition the material must have been evaluated for its weathering resistance and product testing including salt fog testing and several years of field experience.

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iii) INADEQUATE SEALING

Moisture ingress into the cable through a cable termination can cause failures of the cable through water treeing over a few years. However, if water seeped into the strands of the cable, then failures of joints through hydrogen gas pressure build up in the strands as a result of the reaction between the water and aluminium conductors, can take place. The hydrogen gas vents out through the interface between the cable insulation and the joint insulation resulting in breakdown through surface discharge.

C. FAILURES OF CABLE JOINTS

The causes of failures peculiar to each system of jointing XLPE cables are listed below:

TAPE AND RESIN SYSTEM

The joint reinsulation is built with several layers of tape. The taping process results in the inclusion of air voids.

Hand taping often leads to eccentric joint insulation as a result of the jointer applying more tension when the tape is moved towards the jointer than when the tape is moved away from the jointer. This results in an uneven insulation thickness. The jointer being trained to simply measure the diameter of the tape wrapping is unable to detect the less than adequate thickness caused by the uneven wall thickness.

A further problem is created by the shrink back of XLPE insulation ends through thermal cycling and the resulting void creation as explained in the sketch below:

PREMOULD JOINTS:

The most common causes of failures are:

i) Improper positioning leading to discharge activity at the screen cut back area.

ii) Moisture ingress through the intrface between the core and the push-on body.

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HEAT SHRINK JOINTS:

Besides employment of poor quality stress grading materials, the main causes of failues of heat shrink joints are:

- inadequate insulation rebuild whereas a good joint should provide at least 33% more thickness of insulation than the cable insulation. Many of the commercially available heat shrink joints provide less than the cable insulation as the total rebuilt insulation. This is clearly inadequate and is shown in the photograph of a cut section of a low priced commercially available 11kV heat shrink joint using two thin walled red tubings and a black conductive screen tubing.

- moisture ingress through use of inferior quality mastics and sealants.

Users can prevent failures by including in their specifications that the cable joint must provide a minimum rebuild of insulation which will be 33% (min.). More than the cable insulation thickness, the semi conductive screen should be firmly bonded and co-extruded with the cable insulation (just like the cable) and the material properties of all components must strictly conform to ESI-09-13.

CONCLUSIONS:

Users of cables and accessories can prevent failures in their underground systems by ensuring:

The material properties of the components of the cable meet or exceed the requirements of IS 7098 - Part II for cables and ESI-09-13 for accessories.

The cable meets the requirements of electrical and mechanical tests of IS 7098 Part II and the accessories meet the performance requirements of VDE 0278. It is important that the user has adequate knowledge of the manufacturing capability of the supplier and of the material properties of the components used for testing and those which the supplier is offering against a specific tender.

The supplier furnishes adequate proof of satisfactory long term performance.

The supplier is able to support the sale through training and supervision of the installation by the cable jointers of the users.

OVERALL AVERAGE UNDERGROUNDDISTRIBUTION SYSTEM FAILURES

WITH XLPE CABLES

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1. PHYSICAL DAMAGE 43%

2. WATER TREEING 20%

3. ENVIRONMENT 4%

4. FAULTY CABLE TERMINATIONS& OTHER JOINTS 19%

5. OTHER CAUSES 14%

FAULTS IN CABLE ACCESSORIES

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1. CABLE PREPARATION

2. CONDUCTOR CONNECTION

3. MATERIAL SHORT COMING

4. DESIGN DEFECT

5. INSTALLATION DEFECT

FAILURES IN CABLE TERMINATIONS

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1. INADEQUATE / IMPROPER STRESS CONTROL

2. TRACKING / EROSION / WEATHERINGOF EXTERNAL LEAKAGE INSULATION

3. INGRESS OF MOISTURE

FAILURES IN CABLE JOINTS

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1. VOIDS IN INSULATION

2. INADEQUATE INSULATION

3. IMPROPER STRESS CONTROL

4. MOISTURE INGRESS