Revisiting 66-kV to 132-kV Fluid-Filled Cable Joints : Aug 1, 2005 12:00 PM

By Bob Dean, ERA Technology, and Tasman Scott, Orion New Zealand Ltd.


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»More from this section Cable failures in Auckland, New Zealand, in 1998 highlighted the need for a means to determine whether existing and new underground cables and joints are likely to suffer from thermomechanical damage. A previous failure in the United Kingdom prompted two major transmission voltage cable manufacturers to produce strengthened 66-kV and 132-kV, three-core, fluid-filled cable joint designs. Subsequently, ERA Technology (Leatherhead, Surrey, U.K.) conducted an extensive series of tests on 90-kV and 132-kV cables, breeches pieces, straight joints and stop joints from 1992 to 1995 to determine whether the strengthened designs could withstand thermomechanical forces likely to occur in service.



Auckland's Cable Failures

Thermomechanical failures on the circuits supplying to Auckland's central business district (CBD) made headlines in 1998. Two 110-kV, gas-filled cable failures and one of two 110-kV, fluid-filled cable failures were attributed to long-term cyclical thermomechanical movement.



The original ministerial report on these failures concluded that the cables were installed on the basis of incorrect assumptions on the thermal resistivity of the cable backfill, and as a result, the cables were being operated above their rated conductor temperatures. However, Vector Electricity of Auckland refuted this conclusion, considering the possibility that the 90°C (194°F) conductor temperature was not exceeded but that a very hot summer caused high ground temperatures and soil drying, and contributed to the thermomechanical failures. Further significant signs, including numerous gas leaks from the wiped plumbs between the cable sheath and the joint shells, were not properly examined to identify the cause of the problem.



Orion's Fluid-Filled Cable Survey

The Auckland incident heightened awareness of thermomechanical effects on fluid- and gas-filled cable systems. In subsequent checks conducted by Orion (Christchurch City, Canterbury, New Zealand), the company found that thermomechanical buckling had occurred on some fluid-filled cable joints.



The first two 66-kV, oil-filled cable joints investigated by Orion were located on the two Addington Grid to Armagh cable circuits, which supply almost half of Christchurch's CBD load. The cables consist of two three-phase, single-core fluid-filled, 300-mm2 aluminum conductor, aluminum sheathed PVC over-sheathed cables laid in a common trench with extensive weak-mix concrete thermal backfill present.



The joint bay that was initially investigated contained two joints, one on each circuit. One of the joints had a history of leaking and had been repaired previously.



Initially, the results of the investigation were somewhat surprising, as one joint showed clear signs of buckling while the other one contained perfectly straight cores under considerable tension. The buckled joint was similar in design to the 110-kV Vector joint that failed in Auckland.



In both cases, the cores were “twisted” from thermomechanical expansion of the cable cores into the joint space. These findings, along with a comprehensive review of the soundness of thermal backfill treatment along the cable route, led Orion to de-rate the overall circuit capacity by 30% to avoid further serious buckling of the joints.



Thermocouples have been fitted at suspect hot spots to enable on-line SCADA monitoring of cable sheath temperatures. The two cable joints investigated were replaced with a new reinforced Pirelli design accessory, which was tested and evaluated as detailed later in this article.



Subsequently, Orion has instituted a US$11 million joint replacement program intended to tackle all substandard 66-kV joint designs. Orion commissioned ERA Technology with carrying out bench-top studies on all existing in-service joint designs, which concluded that there are indeed differences in thermomechanical performance. These studies could only give an indication of the likely performance and could possibly be confirmed by carrying out full testing in line with the methodology summarized in this article.



More than 126 thru joints, 32 trificating joints and 58 km (36 miles) of fluid-filled, 66-kV cable are currently in service in the Orion network, representing a major concern for the long-term reliability of supply if a remedial program is not completed as soon as practicable. The alternative to joint replacement is 66-kV cable reinforcement costing in excess of $700,000 per kilometer. With 58 km of the cable with suspect joints to be replaced, the decision between $11 million for joint replacement or $40 million for complete replacement led Orion to opt for joint replacement.



The rest of the 66-kV, oil-filled cable installed 20 to 30 years ago is in very good condition with no electrical failures and an ongoing sheath-testing program in place.



Thermomechanical Testing of Oil-Filled Cables and Joints

The tests developed in the 1970s are still in use today, and the comprehensive testing of a fluid-filled cable joint in the laboratory is as follows:





Measurement of cable conductor force coefficient



Measurement of cable frictional constants



Determination of maximum acceptable conductor movement within joint



Full-scale test on cable joint.





To determine the cable conductor force coefficient, a sample length of cable is held at a constant length, loaded with current to achieve the rated conductor temperature rise, and the compressive load developed in the conductors measured.



In a typical test, samples are subjected to 5000 cycles at approximately three cycles per hour. Hot cable oil is circulated through the samples during each cycle. The test samples are inspected without dismantling every 1000 cycles to determine the degree of damage where the cores pass through the glove box. At the end of the test, the samples are fully dismantled and inspected. Based on the core movement tests, the maximum amount of cyclic conductor movement that can be accepted is determined.



The aim of the full-scale test is to determine the amount of movement into the joint that can be expected from the force applied by the cable conductors. As the conductors expand into the joint, then the force exerted by the conductors will decrease.



Initially, the force/movement characteristics of the cable are plotted for two temperature rise values, typically 65°C (149°F) and 80°C (176°F). The higher value represents the unrelaxed condition where a newly installed cable is taken up to its maximum-rated conductor temperature. The lower value represents normal operating conditions after relaxation of the conductors has occurred. A typical result for a full joint test is shown in the figure above.



Study of Pirelli's 66-kV Straight Joint with Orion's Aluminum Conductor Cable

In 2002, ERA conducted a study of the thermomechanical buckling performance of Pirelli Cable's (Milan, Italy) strengthened joint design when used with Orion's 66-kV three-core, 300-mm2 aluminum conductor fluid-filled cable. It was not feasible to perform a full thermomechanical test in the laboratory because the necessary cable and cable accessories were unavailable. A limited test program was devised that required only short lengths of cable and made use of previous thermomechanical test results.



Samples of core from Orion's cable were prepared to simulate sections of core within Pirelli's joint design. The core samples were compressed in a tensile test machine. The change in length was monitored against compressive force, and the force required to cause each sample to buckle was recorded.



The straight 300-mm2 aluminum cores buckled at an average load of 21.2 kN, with a minimum of 18.9 kN. The sample lengths with a 19-mm offset to simulate the core profile buckled at an average load of 26.1 kN, with a minimum of 24.1 kN.



The total displacement at both ends of a strengthened Pirelli 66-kV 300-mm2 aluminum cable joint can be obtained by adding together the displacements for the straight aluminum core sample and the offset aluminum core sample at each increment of load, and then multiplying the result by two.



In the figure on page 54, the total displacement of the joint at each increment of load is plotted against the load-displacement characteristics of the cable following temperature rises of 80°C and 65°C. The rated maximum operating temperature rise of the cable is 80°C. The 80°C load-displacement 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 has undergone a number of load cycles.



The points of interest are the intersections between the load-displacement curve for the joint and the 65°C and 80°C curves for the cable. From the intersection between the joint load-displacement curve and the 80°C curve, the maximum load to be expected from thermomechanical forces is 40 kN. The results of the compression tests on the cores indicated that the minimum load required to buckle one core was 18.7 kN. Thus, the load required to buckle three cores simultaneously can be taken as 56.7 kN, which is well above the 40 kN that is the maximum load the cable is expected to exert.



The maximum amount of axial core displacement that will occur under load cycling conditions is determined from the intersection of the load-displacement curve for the joint and the 65°C curve for the cable. The 65°C curve represents the condition where the cable is fully relaxed having already undergone a number of load cycles.



For the strengthened Pirelli joint, the maximum displacement under load cycling conditions is approximately 3.5 mm (1.75 mm at each end of the joint). This is well below 4.5 mm, the amount of axial cyclic movement that has been found through long-term cyclic testing by ERA to cause damage to the cable core papers where the cores exit from cable glove box.



Therefore, it was concluded that, based on this analysis, the strengthened Pirelli cable joint would not be adversely affected by thermomechanical forces and cyclic core movement due to thermal expansion of the aluminum conductors.



Summary

A number of observations can be observed by reviewing the history of multicore cable systems and past service history:





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



With the exception of a small number of three-core cable systems, the majority of such systems have excellent service records.



Loading cables to higher levels may present some risk, so it is important to understand the condition of the cables before loads are increased.



The designs of both cables and accessories have improved over the years, leading to an overall enhanced performance.



Systems are now available to monitor temperatures of underground cables using distributed temperature (fiber-optic) measurement. This technology allows the operator to safely use the cable circuits up to their maximum potential.



Risks associated with inadequate system engineering still exist and are greater where cable systems are installed on a “non-turnkey” basis.





Acknowledgment

The authors wish to thank Bob Rosevear of Pirelli Cables for his assistance in developing the thermomechanical assessment technique and for helping with this article.



Bob Dean started his career as a development engineer at BICC Cables Ltd. He joined ERA Technology Ltd. in 1983, specializing in the nondestructive testing of power cable systems and the investigation of cable systems. He currently heads Forensic Engineering and Expert Witness Services at ERA. A member of the Institutions of Mechanical and Electrical Engineering, Dean has served as a director of the British Approvals Service for Cables (BASEC), on the British Standards Committee on Cable Joints and Terminations, and on the IEE Professional Committee P8 (Power Cables). bob.dean@era.co.uk



Tasman “Tas” L. Scott is a professional electrical engineer who has held various engineering positions for Canterbury Distribution Utilities in the last 34 years. He is currently general manager network development for Orion New Zealand Ltd., which is a distribution utility that supplies 175,000 customers. tas.scott@oriongroup.co.nz



Review of Thermomechanical Buckling in Fluid-Filled Cables

The temperature rise due to current loading in a cable causes the conductors to expand, and if the cable is buried directly in the ground, the conductors tend to expand into the joints or cable terminations. The expansion produces compressive forces within joints or terminations. These forces are normally termed “thermomechanical forces.”



In a multicore cable joint, the cable cores are splayed out between the cable crutch and the ferrules to enable the joint geometry to be accommodated. The effect of this practice is that the stiffness of the cores introduced within the joint is less than the cores within the cable. If the cores within the joint are not sufficiently stiff or well supported, they will buckle under compressive thermomechanical loads. Additionally, if permanent compression of the conductors occurs, they can go into tension when the cable cools back to ambient temperature, and this can cause the conductors to pull out of the ferrules.

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