buckling performance of cable systems 9a

8/4/2019 Buckling Performance of Cable Systems 9a

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Buckling Performance of 66kV - 132kV Oil Filled Cable Systemspast and present

Prepared byBob Dean, ERA Technology Ltd

Bob Rosevear, Pirelli Cables, LtdTas Scott, Orion

Presented byMark Coates, ERA Technology Ltd

Tas Scott, Orion

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Abstract

As the temperature of a conductor increases, due to load current, the conductor will expand. Theforces this expansion generates are termed thermomechanical forces. If the cable is restrained

the expansion will tend to push into the joints and terminations. Test work to measure the effectof this on cable joints has been carried out at various times since the 1940s.

Thermomechanical testing has traditionally involved the use of purpose built test rigs to measurethe forces developed by cables and measure the friction characteristics of the cables.Appropriate loads can then be applied to complete joints and the joint deflection characteristicsobserved.

The cable failures in Auckland in 1998 highlighted the need for a means of determining whetherexisting installed equipment, or new cables and joints, are likely to suffer fromthermomechanical damage. A method of estimating the likely thermomechanical performance of

cable joints, without the need for full scale testing, has been used to assess the performance ofthe Pirelli strengthened joint when used with Orions 66kV cable. The method utilised existingdata from tests on a range of cables and data from compression tests on cable cores in a standardtensile test machine.

1 General history of thermomechanical buckling in oil filled cables

The temperature rise due to current loading in a cable causes the conductors to expand. If thecable is buried directly in the ground the conductors will tend to expand into the joints or cableterminations. This expansion produces compressive forces within the joint or termination. Suchforces are normally termed as thermomechanical forces.

In a multicore cable joint the cores are splayed out between the cable crutch and the ferrules toenable the joint geometry to be accommodated. The effect of this is that the stiffness of the coreswithin the joint is less than that of the cores within the cable. If the cores within the joint are notsufficiently stiff, or well supported, they will buckle under compressive thermomechanical loads.In addition if permanent compression of the conductors occurs they can go into tension when thecable cools back to ambient temperature and this can cause the conductors to pull out of theferrules.

Since at least the early 1940s cable engineers have been aware of the need to design cablesystems so that they will withstand thermomechanical forces. ERA undertook thermomechanical

tests on 33kV PILC cables in 1944 and on 1.1kV and 11kV PILC cables in 1956 but the firstrecord of a thermomechanical failure that the author is aware of is a report on 33kV 3-core gaspressure cable joints which failed in 1963.

One of the joints is shown in Fig. 1. The scanned image is a little unclear but it can be seen thatthe cores have buckled and twisted under the compressive load caused by the expansion of thecable conductors. It is also apparent that the cores within the joint were only supported withloose spider type separators and tape bindings.

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Figure 1: 33kV 3-core gas pressure cable joint from 1963

In the 1970s tests were developed for measuring the thermomechanical properties of 33kV oil-filled 3-core cables and determining the degree of conductor movement which could be permitted within the joint. The results of these tests were then used to assess the likelyperformance of 33kV oil-filled 3-core cable joints.

Fig. 2 shows a meeting of the UKs Super Tension Cables Group where the results of this workwere being presented. The test rig on the right was used to test the cables. The rig on the leftwas used to test the joints.

Figure 2: Thermomechanical tests in the 1970s

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This work was conducted in order to address a specific problem that had been identified on 33kVcables. The findings were not applied to higher voltage level systems where no specific issueshad been experienced.

In the 1980s ERA carried out thermomechanical tests on a wide range of XLPE cables using test

rigs similar to those shown in Fig. 2. At that time one concern was the effect of the 250

o

C short-circuit temperature on the thermomechanical forces generated by the cables. However modern polymeric cables tend to be single-core constructions rather than three-core designs. As aconsequence accessories have a relatively rigid construction and less problems can be expected.There have however been some thermo-mechanical problems in the UK on a three corearmoured XLPE insulated cable installed in air, where rotation of the cable occurred as aconsequence of thermal excursions which in turn necessitated some modifications to theinstallation design.

In 1985 a failure occurred on a 90kV oil-filled 3-core copper conductor cable linking the islandof Jersey to France. A straight joint failed due to the cores buckling and making contact with the

metallic joint shell. This fault occurred within one year of commissioning. The cable had beensubjected to cyclic loads around its maximum rated current.

A test conducted on the same joint design in 1988 showed that the cable cores were notadequately supported within the joint. The cores twisted and buckled as can be seen in Fig. 3.The design was stronger than that shown in Fig. 1 in that the spider separator had T shapedextensions designed to keep the spider correctly aligned within the joint shell. However thedesign still relied on taped bindings to hold the cores rigid.

Figure 3: Buckling of unstrengthened 90kV design

To avoid a repetition of this failure both of the major transmission voltage cable manufacturersin the UK produced strengthened 66kV and 132kV 3 core oil filled cable joint designs, Fig. 4and 5. ERA conducted an extensive series of tests on 90kV and 132kV cables, breeches pieces,straight joints and stop joints from 1992 to 1995 to determine whether the strengthened designswould withstand any thermomechanical forces likely to occur in service.

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Figure 4: Cable glove and spreader on strengthened 630 mm2

joint design

Figure 5: Overall view of strengthened 150mm2

straight joint

Thermomechanical failures again made the headlines in 1998. Two 110kV gas filled cablefailures and one of the two 110kV oil-filled cable failures on the supply to Auckland CBD wereattributed to long-term cyclical thermomechanical movement.

The original Ministerial Report into the failures concluded that the cables were installed withincorrect assumptions being made about the thermal resistivity of the cable backfill and as aresult the cables were running above their rated temperatures. This conclusion has beensubsequently refuted by Vector, who believe that a 90 degree temperature was not exceeded bythe cables that failed. However a very hot summer caused high ground temperatures and soildrying which contributed to the thermomechanical failures. Telltale signs such as numerous gas

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leaks from the wiped plumbs between the cable sheath and the joint shells were not followed upto find the cause of the problem.

The Auckland incident heightened awareness of thermomechanical effects on oil and gas filledcable systems. In checks conducted on Orions oil filled cables it was found that

thermomechanical buckling had occurred on some oil filled cable joints.

The first two 66kV oil filled cable joints investigated in detail by Orion were located on the twoAddington grid exit points to Armagh cable circuits which supply almost one half of theChristchurch CBD load. These cables consist of two 3 phase SCOF 300mm2 aluminiumconductor, aluminium sheathed PVC over-sheathed cables laid in a common trench withextensive weak mix concrete thermal back-fill present.

The joint bay initially investigated contained two joints, one on each circuit. One of these jointshad a history of leaking which had been previously repaired.

The results of the investigation were initially a little surprising as one joint showed clear signsof buckling while the other one contained perfectly straight cores under considerable tension.

The buckled joint was very similar in design to the Vector 110kV joint which failed in Auckland.

Figure 6: Vector/Orion cable joint comparison, Vector 110 kV joint

Figure 7: Vector/Orion cable joint comparison, Orion 66 kV joint

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The cores are twisted in both cases due to thermo-mechanical expansion of the cable cores intothe joint space.

These findings, along with a comprehensive review of the soundness of thermal back-fill

treatment along the cable route, lead to Orion de-rating the overall circuit capacity by 30% toavoid further serious buckling of the joints.

Thermocouples have been fitted at suspect hot spots to enable on line SCADA monitoring ofcable sheath temperatures.

The two cable joints investigated were replaced with a new reinforced Pirelli design accessorywhich was tested and evaluated as described in section two of this paper.

Orion has subsequently instituted a $6M joint replacement programme which is intended totackle all substandard 66kV joint designs. Orion commissioned ERA to carry out bench top

studies on all existing in-service joint designs which concluded that there will be a difference inthermo-mechanical performance. These studies can only give an indication of the likely performance and may be confirmed by carrying out full testing in line with the methodologyoutlined in this paper.

Over 126 joints and 58km of oil filled 66kV cable are currently in service in the Orion network,representing a major concern for long term reliability of supply if a remedial programme is notcompleted as soon as practicable. The alternative to joint replacement is additional 66kV cablereinforcement at upwards of $1M per kilometre. With upwards of 30km of the cable withsuspect joints to be replaced, the choice between $6M for joint replacement or $30M forcomplete replacement seems obvious.

The rest of the 66kV oil filled cable installed 20 to 30 years ago is in very good condition withno electrical failures (so far!) and an ongoing sheath testing programme in place.

2 Thermomechanical testing of oil filled cables and joints

The tests developed in the 1970s are still in use today. To fully test an oil-filled cable joint in thelaboratory a programme of four tests is required:

1. Measurement of cable conductor force coefficient

2. Measurement of cable frictional constants3. Determination of maximum acceptable conductor movement within joint4. Full scale test on cable joint

To determine the cable conductor force coefficient a representative length of cable, taking intoaccount the cable core lay length, is rigidly supported to prevent lateral movement. The cable isthen held at a constant length, loaded with current to achieve the rated conductor temperature rise

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and the compressive load developed in the conductors measured. A typical graph obtained fromsuch a test is shown in Fig. 8:

0

10

20

30

40

50

60

70

0 20 40 60 80 1

Conductor temperature degrees C

ThrustkN

00

Figure 8: Typical cable thrust temperature characteristics

To determine the cable friction coefficients the cable is heated to its normal operatingtemperature and measurements are made of the force required to push the conductors through thetest rig at different compressive loads. The friction coefficients are calculated using the formula

Friction per unit length = kP + K

Where P is the total force applied to the cableand k and K are the friction coefficients.

To determine the maximum core movement a number of half joints are made up and installedas shown in Fig. 9 and 10. The joint shells are fixed to the test rig and the cable cores areattached to a lever that moves the cores backwards and forwards within the joint shells. Thecores are attached to the lever at different points so that several samples can be tested at the sametime with different degrees of movement.

In a typical test the samples would be subjected to 5000 cycles at about 3 cycles per hour. Hotcable oil is circulated through the samples during the cycle. The test samples are inspectedwithout dismantling every 1000 cycles to determine the degree of damage where the cores passthough the glove box. At the end of the test the samples are fully dismantled and inspected.

From the core movement tests the maximum amount of cyclic conductor movement that can beaccepted is determined.

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Figure 9: Core movement test rig with 3 half joints installed

Figure 10: Schematic view of core movement test rig

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The aim of the full scale test is to determine the amount of movement into the joint that can beexpected from the force applied by the cable conductors. As the conductors expand into the jointthen the force exerted by the conductors will reduce.

Initially the force/movement characteristics of the cable are plotted for two temperature risevalues, typically 65 and 80o C. The higher value represents the unrelaxed condition where anewly installed cable is taken up to its maximum rated conductor temperature. The lower valuerepresents normal operating conditions after relaxation of the conductors has occurred.

The cable characteristics are adjusted for thermal expansion of the conductor within the joint andare calculated using the following formula:

( ) ( )

+

+=

o

o

okPACk

KkP

KACkACk

ACkx

ln

2

Where xo = movement of cable conductors into one end of joint = mm

= thermal coefficient of linear expansion of copper = 17 x 10-6 C-1k = frictional constant = mm-1A = total conductor cross-sectional area in cable = mm2C = force coefficient of the cable conductor = kN mm

-2C

-1

= conductor temperature rise = 80CK = frictional constant = 0 N mm

-1

Po = total compressive force in conductors at joint = kNXo

= total conductor movement into straight joint = 2xoand

= joint expansion = LWhere

= conductor temperature rise = C = thermal coefficient of linear expansion of copper = C-1L = conductor length within the joint = mm

The test joint is prepared with observation holes in the joint shell so that any core movement can be observed. The joint is installed in a rigid framework and heated to the rated conductortemperature by passing current through the conductors. When the temperature has stabilised anincremental compressive load is applied and the amount of core movement into the jointrecorded. The load on the joint is increased at a steady rate until the force/movement plot for thejoint crosses the unrelaxed cable curve, assuming that the cores do not buckle beforehand. Theload is then reduced to zero and the joint examined. The total conductor movement at the pointwhen the force/movement plot for the joint crosses the relaxed cable curve is used to determinethe amount of cyclic core movement likely to occur in service.

A typical result for a full joint test is shown in Fig. 11.

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0

5

10

15

20

25

30

0 5 10 15

Total conductor movement mm

Force

kN 80C unrelaxed

65C relaxed

Joint

Figure 11: Plot obtained from a full joint test

To date most of the thermomechanical testing on oil-filled cables has been conducted on copperconductor cables. Results for 66 to 132kV oil-filled cables are shown in Table 1 together withone result for a 33kV aluminium conductor cable.

Table 1: Measured thermomechanical characteristics for oil-filled cables

Frictional ConstantsCable Stranded

conductor

Force Coefficient C

kN mm-2

oC

-1K kN mm

-1K mm

-1

66kV 129mm2

Shaped Al 1.38 x 10-3

- -

90kV 150mm2

Circular Cu 0.79 x 10-3

0 25 x 10-3

132kV 150mm2

Circular Cu 0.79 x 10-3

0.044 x 10-3

25 x 10-6

132kV 630mm2 Circular Cu 0.45 x 10-3 0.14 x 10-3 13 x 10-6

132kV 630mm2 Circular Cu 0.73 x 10-3 1.11 x 10-3 4.5 x 10-6

33kV 400mm2 Oval Al 0.634 x 10-3 0.04 x 10-3 118 x 10-6

3 Study of Pirelli 66kV straight joint with Orions aluminium conductor cable

In 2002 ERA conducted a study of the thermomechanical buckling performance of PirelliCables strengthened joint design when used with Orions 66kV 3 core 300mm2 aluminiumconductor oil filled cable. It was not feasible to undertake a full thermomechanical test programme in the laboratory because the necessary cable and cable accessories were not

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available. A limited test programme was devised which required only short lengths of cable andmade use of previous thermomechanical test results.

Samples of core from Orions cable were prepared to simulate sections of core within Pirellisjoint design. The longer lengths were 378mm long straight sections to simulate the core profile

between the spider support and the position where the reinforcing paper is applied. The shorterlengths were 206mm long and had a 19mm offset to simulate the core profile between the glove box and the spider support in a strengthened Pirelli cable joint. The core samples werecompressed in a tensile test machine, Fig. 12. The change in length was monitored againstcompressive force and the force required to cause each sample to buckle recorded.

The 378mm straight 300mm2

aluminium cores buckled at an average load of 21.2kN with aminimum of 18.9kN. The 19mm offset 206mm lengths buckled at an average load of 26.1kNwith a minimum of 24.1kN.

The total displacement at both ends of a strengthened Pirelli 66kV 300 mm2 aluminium cable

joint can be obtained by adding together the displacements for the straight aluminium coresample and the off-set aluminium core sample at each increment of load and then multiplying theresult by two.

Figure 12: Buckling test on core from Orion cable

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In Fig. 13 the total displacement of the joint at each increment of load is plotted against the loaddisplacement characteristics of the cable following a temperature rise of 80 C and 65 C.

80 C is the rated maximum operating temperature rise of the cable. The 80 C loaddisplacement curve is used to ascertain the maximum load, which will be applied to the joint.

The 65 C load displacement curve represents the relaxed condition after the cable hasundergone a number of load cycles.

0

10

20

30

40

50

60

0 2 4 6 8

total conductor movement mm Xo (=2xo)

Load

kN 66kV 300 Al 80 deg C

66kV 300 Al 65 deg C

Orion joint

Figure 13: Load displacement curves for strengthened Pirelli joint and 300mm2 aluminiumcable

The points of interest are the intersections between the load displacement curve for the joint andthe 65C and 80C curves for the cable. From the intersection between the joint loaddisplacement curve and the 80C curve the maximum load to be expected due tothermomechanical forces is 40kN. The results of the compression tests on the cores indicatedthat the minimum load required to buckle one core was 18.7kN. Thus the load required tobuckle 3 cores simultaneously can be taken as 56.7kN. This is well above the 40kN that is themaximum load that the cable is expected to exert.

The maximum amount of axial core displacement which will occur under load cycling conditionsis determined from the intersection of the load displacement curve for the joint and the 65o

Ccurve for the cable. The 65

oC curve represents the condition where the cables is fully relaxed

having already undergone a number of load cycles.

For the strengthened Pirelli joint Fig. 13 indicates that the maximum displacement under loadcycling conditions is approximately 3.5mm, i.e. 1.75mm at each end of the joint. This is well below 4.5mm, the amount of axial cyclic movement that has been found by long-term cyclic

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testing at ERA to cause damage to the cable core papers where the cores exit from cable glovebox.

It was therefore concluded that, based on this analysis, the strengthened Pirelli cable joint wouldnot be adversely affected by thermomechanical forces and cyclic core movement due to thermal

expansion of the aluminium conductors.

Conclusions

Several observations can be made from a review of the history of multi-core cable systems andconclusions drawn from the past service history:

Service experience is based mainly on circuits that have been operated under conditionsof modest loads with only a few circuits being fully loaded,

With the exceptions of a very small number of three-core cable systems, the majority of

such systems have excellent service records, Working cables harder may present some risk. It is important to understand the condition

of the cables before loads are increased.

Designs of both cables and accessories have been improved over the years, leading to anoverall enhanced performance.

Systems are now available to monitor temperatures of underground cables usingdistributed temperature (fibre optic) measurement. This technology allows the operatorto safely use the cable circuits up to their maximum potential.

Risks associated with inadequate system engineering still exist and are greater wherecable systems are installed on a non-turn-key basis.

buckling performance of cable systems 9a

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