Using overhead covered conductors at high voltages

20 July 1998

A prototype high voltage line with covered conductors is in operation in Finland and similar conductors are now increasingly being considered for high voltage applications elsewhere, for example in Slovenia, Hungary, the UK and Poland.

Among the numerous potential advantages deriving from using covered conductors in high voltage transmission applications are the following:

  • Greater compactness, due to reduced phase spacings. The spacings can be less because the conductors are able to touch each other during adverse weather without disturbing power transmission. Increased compactness is beneficial for new installations, eg in terms of reduced environmental impact, but also means that older transmission lines can be upgraded to higher voltages and still use existing wayleaves.

  • Towers can be shorter and less bulky – again because the conductors can be closer together and the installation more compact.

  • Reduced electromagnetic fields (due to less space between conductors). Field reductions can be as much as 30 per cent.

  • Lessening of outage rate (eg due to conductor clashing.

    LMF SAX is a covered conductor for high voltage overhead lines that has been developed by NK Cables and IVO, the LMF standing for low magnetic field.

    While covered conductors for medium voltages are usually made of aluminium alloy and covered with a 2 mm layer of black weather resistant XLPE, this single layer is not sufficient at higher voltages, so triple extruded XLPE layers are used.

    Work on developing overhead covered conductors goes back to the late 1970s, with 20 kV lines. When this proved successful, development started on 110 kV lines. In 1990, NK Cables and IVO joined forces on more intensive research, with Fingrid getting involved later. The first test line was erected in 1993, feeding Helsinki airport, while the continuing debate about magnetic fields and their health effects gave added impetus to the development project. In spring 1996 a 110 kV prototype line was installed between Mätäkivi and Sula and this has yielded very encouraging results. Currently a 132 kV installation is underway in Poland, due to be completed this year, and discussions on 132 kV applications continue in the UK.

    Development programme

    LMF SAX conductors have been subjected to a long programme of tests over the development period, both in laboratories and in the field. Among the issues addressed were the following:

  • Behaviour of the conductors when clashing together, eg in high winds. In the tests, conductors with a voltage of 120 kV between them were subjected to clashes at a rate of about 20 times/min. The test was stopped after 540 000 clashes with no damage to the covering of the conductors.

  • Behaviour when conductors are in continuous contact with each other. The test voltage was again 120 kV. The test was stopped after 17 days with no puncturing being encountered. The outer layer of the covering suffered only minor damage from electrical discharges.

  • Effect of trees leaning on conductors. Various trees were taken from a forest and "planted" on the earthed laboratory floor, with trunks or branches touching the conductor. The voltage between conductor and ground was 71 kV. Noise caused by corona and radio interference levels was very high in these tests. In some tests, the trees started to dry out and some burning was seen.

  • Electrical water trees. These are treelike channels which can grow slowly in the XLPE insulation if moisture gets inside the conductor. This phenomenon can destroy the electrical integrity of the insulation. A variety of alternative conductor structures and materials were subjected to long term water tree tests. As a result, the high voltage covered conductor was waterblocked to avoid water progress inside the cable and special water-tree-retardant XLPE material was chosen as the covering plastic. The cables are insulated using the CDCC (Completely Dry Curing and Cooling) method developed by NK Cables.

  • Ageing of the outer layer of the conductor, which consists of black UV and weather resistant XLPE compound containing 2-3 per cent carbon black. The aim of the testing was to demonstrate long term operation in various climatic conditions.

  • Properties of the triple extruded XLPE layer. These have been established in a range of tests, including those described in IEC publications 502 and 811.

    Electrical design

    In terms of electrical design, the covered conductor is different from the normal overhead line in a number of respects.

    For example, in normal overhead line design the phase-to-phase clearance at the tower is determined by the need to maintain an acceptable level of mid-span clearance under specified conditions. This means that the phase-to-phase clearance at the towers of a conventional overhead line is significantly overdimensioned from the purely electrical point of view. With covered conductors the situation is totally different, although at the tower the insulation does not play much role because the armour rods of the suspension clamp are at full line voltage potential.

    Another difference is in arcing behaviour. When flashover occurs between the phases of a normal overhead line the arc starts to move immediately, but if the flashover and puncture occurs with covered conductors the arc will remain in the extinction point and is very likely to melt the conductors. This must be prevented and effective shielding against lightning strikes must be provided. Furthermore it has been assumed that flashover may happen at the tower and arcing horns are needed. The live end arcing horns must also have galvanic contact with the conductor.

    The Mätäkivi-Sula prototype line

    In 1995, Tuusulanjarven Energia, a utility located near Helsinki, decided to build a new 110 kV transmission line to feed the industrial area of Sula.

    The line, a little less than 6km long, was located in a growing urban area and a large part of it was visible. Consequently, environmental issues played a big role from the very beginning of the project. For example, special attention had to be paid to the types of towers to be used, partly because of aesthetics and partly because of the narrow wayleave.

    During the contract negotiations, the utility was informed of the opportunity to use covered conductors on the line. Eventually it was decided to use normal bare conductors for half the line and covered LMF SAX conductors for the other half. NK Cables supplied the conductors and IVO Power Engineering did the construction work.

    For the LMF SAX line in the Mätäkivi–Sula installation a compacted aluminium alloy conductor of 355 mm2 cross section is used, with triple extruded XLPE layers. Designated LMF SAX 355, the structure of this conductor is as follows:

  • Stranded and compacted aluminium alloy conductor waterblocked with swellable powder

  • Semiconducting swellable tape wrapped around the conductor

  • Extruded semiconducting screen, with a thickness of 1.5 mm

  • Extruded water tree resistant XLPE insulation compound, thickness 5.0 mm, with 1.5 mm thick outer layer of weather and track resistant black XLPE insulation compound.

    For LMF SAX 355, the DC resistance (at +20°C) is 0.0949 ohm/km, breaking load is 108 kN, mass is 1730 kg/km and diameter is 39 mm.

    The line was taken into service in April 1996. So far it has performed very well without any disturbances in power delivery.

    Towers: The basic tower type in the test line is an inverted U-shaped tubular tower with vertical phase configuration.

    In spite of the fact that the height of the lowest LMF SAX conductor is 1 m higher due to lower conductor tension (40 N/mm2 versus 45 N/mm2 for the bare conductor) the overall height of the LMF SAX tower is less.

    Insulator sets and accessories: To minimise the extra costs arising from the use of covered conductors, the design concept envisaged use of conventional material whenever possible. That is why conventional insulator sets were chosen. The insulator units are of normal U 70 BL toughened glass. The narrow wayleave called for V-sets. The only change from conventional insulator sets was the design of new arcing horns. This was necessitated by the small conductor spacing. The horns were not allowed to unduly reduce the air gap between the phases. Also the smaller spacing increases the voltage gradient on arcing devices, leading to somewhat more sturdy horns.

    One major departure from conventional design was the use of armour grip suspension (AGS) clamps on suspension towers, as commonly used in optical fibre cable installations. The reason for this decision was that by using AGS clamps the stress on the covered conductor is minimised. Also the clamp can be easily fitted on the conductor without the need to peel off the covering. However, since it was considered unreasonable to let the clamp float at a potential between the conductor and ground, a galvanic path was established from the AGS clamp rods to the conductor.

    The tension clamps of the conductors are of the conventional wedge-type design, requiring peeling of the conductor. Although it has been verified that the covering adheres very tightly to the conductor and one can pull the conductor using the covering, there was no evidence that a preformed type tension clamp used on unstripped conductor would be suitable in the long run. This was also confirmed by preliminary tests.

    No vibration dampers have been installed on this prototype line because initial estimations did not call for their use. Also, measurements done on the line indicated that no extra damping was needed. However, if dampers were deemed necessary it would be straightforward to install them at the end of the AGS clamp armour rods.

    Installation procedure

    The installation procedure for covered conductors basically does not differ from that for stringing of conventional bare conductors, although some extra care is needed. In the Mätäkivi–Sula installation optical cable stringing equipment was used.

    Special attention must be paid to the grounding of the conductor before and during stringing especially when working in the vicinity of other energised lines. For example because the covering prevents the conduction of induced voltages to ground special care must be taken to keep the conductor free end on the reel constantly grounded and during stringing it is advisable to have the conductor ends always in contact with ground.

    Costs compared

    Cost comparison between a normal 110 kV transmission line with bare conductors and one with covered conductors is not an easy task. Lines with covered conductors are meant for urban areas where the costs are high whatever line type is to be used. The line length and spans tend to be short and there are many line angles. Therefore cost comparison should only be made between "urban" lines. Since both bare and covered conductors were used on the Mätäkivi–Sula prototype line, it provided an excellent opportunity to compare costs in a meaningful way.

    The costs of towers and foundations were similar in both cases, but the installation work for the covered conductor was about 20 per cent more expensive than that for bare conductors, mainly due to the use of AGS clamps.

    Overall, the biggest contributors to increased cost were the extra material costs of the AGS-clamps and of the covered conductor itself, the price of which was more than double that of an equivalent bare conductor.

    All this led to a total line cost increase per kilometre of the order of 25 per cent. However, this is not unreasonable and must be seen against the benefits of having a line which is on the whole probably more acceptable to the public, in terms of appearance and reduced electromagnetic fields. The cost of underground cable is about 5-10 times as much as an overhead covered conductor installation.

    Measuring vibrations

    From the very beginning it was considered important to study the vibration behaviour of the new conductor. Therefore conductor vibrations were studied theoretically and also measured.

    The basic types of vibration are aeolian, galloping and wake-induced. In the case of a single covered conductor the only possible types are aeolian vibration and galloping. While galloping is a rather rare phenomenon, aeolian vibration is almost always present. Consequently the main emphasis was put on aeolian vibration.

    Vibration measurement on the Mätäkivi–Sula line started in March 1996 and ended in June 1997. Four spans were monitored and always the lowest phase. The total sampling time was close to 455 hours. One of the vibration recorders also measured wind speed perpendicular to the span and ambient temperature.

    The estimated minimum lifetime based on the measurements was calculated to be about 455 years. It was also estimated that the conductor tension could be increased by about 25 per cent while still achieving a life of 50 years.

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