Power cables and submarine power cables

Power cables have been in use in underground and submarine transmission for over a hundred years. With the increased interconnection between lands and integration with power generation from offshore renewable energy, the demand for submarine power cables is getting stronger. 

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Aug 21, 2017
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Author(s): Dr Chuan Zhang

Abstract

This article discusses the differences between power cables used for land and submarine application and also investigates the impact of AC (alternating current) and DC (direct current) operation on power cable designs. Specific technical issues with submarine power cables in both AC and DC applications are highlighted and addressed. Unless otherwise indicated, the discussion in this article focuses on the transmission level, that is, 110 kV (in the mainland Europe) or 132 kV/275 kV (in the UK) and above for both AC and DC applications. However, the content of this article should largely be valid for lower voltage power cables, such as inter-array cables linking offshore wind turbines to offshore substations or distribution links across rivers or linking islands located closely to the mainland. Owing to the length limitation on text, this article has not discussed issues in relation to the technical and practical deployment of high voltage submarine power cables.

Principal features of power cables

A basic in-land alternating current (AC) power cable and its components are shown in Fig 1.

Fig 1: Illustration of a basic in-land single core AC power cable and its components [1]

1 – conductor, 2 – conductor screen, 3 – insulation, 4 – insulation screen, 5 – metallic sheath (and basic water barriers) and 6 – outer sheath

In principle, power cables consist of two functioning components: conductor and insulation.

  • Conductor: Suitable materials for conductors on a commercial scale are copper and aluminium. Cables using copper have a smaller cross section diameter than those using aluminium as copper has higher conductivity and requires a smaller cross section area for the same current carrying capability (sometimes called ampacity). Copper also has better corrosion resistance than aluminium. The majority of land cables use aluminium conductor whereas the majority of submarine power cables use copper conductor, although some submarine projects have used aluminium conductors for cost reason. It is also possible to use copper for some parts of the route, and aluminium for others, for example, to use aluminium conductor for the deeper cold part of the submarine cable route and copper conductors for the warmer parts buried close to the beach. Jointing between copper and aluminium conductors is feasible. Superconducting materials, such as carbon nano-tubes are being researched.
  • Insulation: The insulation is required to bring the voltage potential of the conductor down to close to zero at the outside surface of the cable (connectable to the earth). Two main types of insulation that are used on a commercial scale are oil impregnated paper and polymeric materials, among which the latter is relatively newer and came to use in 1980s at a transmission voltage.

All other components of power cables are for supporting purposes but also essential. These components include:

  • Screens: Normally there are two sheets of screens, both are made of semiconductors. One is between the conductor and the insulation to equalise the electric field and the other screen sheet is immediately outside of the insulation to seal off the electric field directly generated by the cable and prevent it from leaking to outside of the cable.
  • Sheath: This consists of layers made of lead alloy, aluminium, copper and polymeric material. It provides protection for the insulation and the entire cable. The sheath along with the metallic screen and armour wires (when available) also carries fault current. Some basic water barrier layers are normally installed just inside the sheath. The sheath also has a function to prevent corrosion.

Principal features of submarine power cables

In comparison with a standard land AC cable, power cables designed for submarine application need to take into account a number of special factors and address different physical and installation constraints and opportunities. This is explained in Table 1 and Fig 2.

Fig 2: Illustration of a three-core AC submarine power cable and its components (sketch: Courtesy of Prysmian)

1 – conductor, 2 – conductor screen, 3 – insulation, 4 – insulation screen, 5, 6, 7 – water barrier and metal or semiconductor sheath, 8 – fillers, 9 – bedding tapes and inner sheath 10 – wire armour, 11 – outer serving and 12 – optical fibres

Special factors

Explanation and solutions

More intensive need of water tightness and blocking of water ingress and anticorrosion

Submarine power cables require a high degree of water tightness measures to prevent water ingression into the cable during normal operation or after a cable fault or from any minor damages.

To stop water migration, swelling agents in the shape of powder, tapes or yarns are often inserted in between the conductor layers, unless the water tightness is already sufficient thanks to other techniques in cable production. These agents swell considerably and block the passage for water efficiently upon contact with water. As most swelling agents work much better in contact with fresh water rather than salt water, submarine power cables requires extra attention to manage water tightness.

Need of armour and extra mechanical strength

Submarine cables need a structural element, such as a layer or layers of metallic tapes or wires (typically galvanised steel wires) to withstand the mechanical forces that are generated during the cable laying offshore. The metallic armour also provides anchoring of the power cable to the offshore platform in order to transfer all mechanical loads directly from the armour to the platform without endangering any of the internal elements of the cable (conductor, insulation and screen).

It should be noted that although armour wires reduce the risk from external mechanical damage, they do not provide protection against major external events such as a dragged ship's anchor, which requires external protection, such as burial with rock placement or mattresses only used where target burial depths cannot be achieved. Armour wires should also come with some corrosion protection.

Need of multi-purpose outer sheath

A multi-purpose outer sheath, normally referred to as outer serving, is required for submarine application. The outer serving layer is typically formed using bitumen bonded polypropylene yarn. Its fricative surface facilitates cable handling during loading, unloading and installation. The use of bitumen also improves the corrosion resistibility to sea water.

Opportunity for 3-core in AC

When cables are installed across seas, it is feasible and also preferable for AC submarine cables to have three cores built together to save cost thanks to the fact that seawater can mitigate the resultant heat dissipation problems. This also gives an opportunity to have a bundle of optical fibre data and communication cables built-in and offer effective temperature measurement.

A structure sketch of a three-core cable is shown in Fig 2.

Three single-core cable systems may still be used in AC submarine application if a three-core structure makes the overall diameter of the large ampacity cable too large to handle and becomes technically and economically undesirable.

Length of cable sections deliverable

Main considerations on length are the limitations on logistics. For land cables these refer to weight and size of the cable drum limited by transport. Typically single core land cables are in lengths of around 1 km but submarine three-core cables can be in sections of 80 km. However, submarine cables may sometimes be forced to cut short during the installation and unable to achieve the maximum length available if the weather window is too narrow to complete cable laying in a single installation campaign. When this happens an offshore joint with the repair splice has to be introduced.

Uncompensated capacitive charging current build-up in AC cables

Being submarine makes en-route mitigation difficult in relation to capacitive charging current build-up and reactive power in AC cables. This will (a) limit the AC transmission distance achievable in submarine conditions, (b) restrict the AC voltage level a cable can operate at regardless whether higher voltage cables are available and (c) necessitate special switching/earthing procedure to ensure full discharging/charging when de-energising and energising an AC submarine power cable.

Table 1: Special factors of using power cables in submarine environment

In summary, like basic land power cables, submarine power cables also have a metal conducting core(s) surrounded by insulation and screens, they have further strengthened water blocking barriers and one or more armouring layers protecting against mechanical damage. In addition, being offshore may under certain circumstances bring useful opportunities, including longer section length, three AC core structures and combination with optical fibre cables and so on.

Impact of AC and DC operation on conductors and insulation of power cables

By instinct, one may not expect much difference between a power cable for AC application and DC operation, other than that AC normally involves three phases. However, the reality is that AC and DC power cables do have fundamental differences right into the conductors and insulation materials and configuration.

Impact of AC and DC operation on conductors of power cables

Most conductors of large capacity power cables are stranded from round wires. Although the same compressed round wire conductors can be used for both AC and DC applications, it would be more economic if the final formation of the conductor cores takes into account the different characteristics of AC and DC.

As is known, when the AC flows in a conductor, an alternating magnetic field is generated which produces an electromagnetic force that concentrates the current flow lines into the peripheral parts of the conductor (the ‘skin’). As a result of this ‘skin effect’, the current density in the inner part of the conductor decreases causing reduction of the effective cross section area of the conductor which leads to an increase of the resistance. Therefore, the manufacturing of AC cables should avoid and mitigate the impact of the skin effect, whereas this does not need to be a concern when manufacturing a cable for DC application.

For this reason, the conductors for AC power cables are produced in a segmental manner following a so called Milliken method whereby a number of segment stranded sub-conductors are first formed from individual wires then rolled, twisted and laid up into a round conductor [2]. This is shown in Fig 3.

Fig 3: Conductor manufactured using the Milliken method [1]

1 – conductor in Milliken segments, 2 – semi-conductive or insulating tapes separating the Milliken segments

In this way, each individual wire changes its radial position from close to the centre to far from the centre as the wire proceeds along the conductor. As the electromagnetic force causing the skin effect at the centre and at the periphery have opposite signs and cancel each other to certain extent, the skin effect is reduced. The application of extra insulation, such as oil film, thin paper strips or lacquer between the wire layers or between individual wires can further help reduce the skin effect [2]. All of these measures mitigating the skin effect will inevitably increase the cost of conductors for AC cables.

The conductors for DC power cables on the other hand can be produced without sophisticated process although stranded wires will still be used.

Incidentally, following the analysis above, it can be expected that the conductor for a low frequency application, such as 16.7 Hz power transmission, can be manufactured at a cost lower than that for a 50 Hz application.

Impact of AC and DC operation on insulation of power cables

When cable insulation is paper based, an operation in AC will incur dielectric losses as a result of the AC, like with any other insulation. In order to reduce such dielectric losses, the pulp of the insulation paper is washed with de-ionised water and only low density paper is used.

For DC application, as AC related dielectric losses are no longer an issue, DC application may use higher density paper to achieve the maximum dielectric strength. This also leads to better mechanical flexibility in the cable.

When cable insulation is formed using polymeric materials, the AC against DC comparison will be the opposite of that for paper-based insulation. In other words, it is less problematic to use polymeric insulation for AC cables than for DC cables. This is because in DC application of polymeric insulation, space charge (a continuum of electric charge distributed over a region of space) within the polymeric materials can become trapped under DC electric field. A continuous build-up of such space charge across the insulation thickness will have a negative impact on the cable performance and its serviceable life because the uncontrolled increase of the electric field as a result of space charge build-up could lead to the breakdown of the insulation.

There may be two options to mitigate the problem: one is to develop special polymeric compound formulation through comprehensive composition redesign to reduce the accumulation of space charge; the other option is to tolerate the unavoidable space charge build-up, but carefully de-rate the designed life and running conditions through testing and simulation of the insulation performance against internal field distributions under service and fault conditions. In addition, some mitigations of marginal nature may also help, such as allowing a longer degassing process at the end of manufacturing and so on.

A further issue related to space charge of polymeric insulation in DC application is that the deployment of polymeric insulated cables is sensitive to the DC technologies used. There are two different HVDC (high voltage direct current) technologies depending on whether current source converter (CSC) or voltage source converter (VSC) is used. Polymeric insulated cables can only be deployed to projects using VSC converters, but not for those using CSC converters. This is because transmission using CSC converters will need to reverse its polarity when it reverses the power flow and therefore would require the cable to be able to withstand polarity reversal. The capability of heavy duty polymeric insulated cables to withstand polarity reversal is not proven yet as space charges could potentially cause excessive dielectric stress within the cable insulation and lead to insulation breakdown in case of sudden polarity reversal.

Power cable technologies

Power cables can be divided into many different types based on application and insulation materials and technologies. The dominate cable types may include oil filled paper insulated cable, gas filled paper insulated cable, mass-impregnated (MI) paper insulated cable and extruded polymeric insulated cable. The key features of each of these types of power cables are discussed in the following.

Oil filled paper insulated power cable

Oil filled or fluid-filled paper insulated cables, also known as SCFF (self-contained fluid filled) cables, are a veteran in power cables and have been in use since 1920s. An indicative structure of an oil filled paper insulated single core power cable is given in Fig 4.

Fig 4: Structure of an oil filled paper insulated single-core power cable [3]

The insulation paper tapes are made from conifer pulp (kraft paper) and are pre-impregnated then lapped or stacked to form an insulating wall around the conductor. Thinner paper with higher dielectric strength is used closer to the conductor as electric stress is higher in the inner part of the cable. Lesser layers of thicker paper tapes are used in the outer parts of the cable. The paper tapes in the insulation wall also provide great mechanical strength.

The oil used during the operation stage is a very low viscosity (lower viscosity than water) synthetic hydrocarbon oil, which is contained and flows in a duct in the conductor centre. The purpose of the oil is to fill any gaps between paper laps which may grow with thermal expansion and between the insulation and the conductor, suppressing the formation of any ionisation or voids. The continuous flow of oil is fed into the cable through the end terminations using pumping stations.

For voltages above 275 kV AC, paper polypropylene laminate (PPL), a sandwich structure inserting one sheet of polypropylene laminate between two sheets of paper, has been adopted on some occasions to improve electrical performance, reduce losses and increase the transmitted power, thanks to the special low permittivity and low loss of the PPL insulation [4].

Oil filled paper insulated power cables are usually in a single core construction, however, three-core constructions are also available in which each core has a compact stranded conductor insulated with paper tapes and the cores and oil ducts are laid up together under a common metallic sheath [4].

Gas filled paper insulated power cable

Gas filled paper insulated cables were first developed in 1937. A structure of gas filled cables is illustrated in Fig 5.

Fig 5: Structure of a gas filled paper insulated three-core power cable [3]

Their design elements are largely the same as those in a standard oil filled paper insulated cable and again the cable insulation is provided by pre-impregnated and lapped paper tapes. The difference here is that pressurised nitrogen gas is used to fill the gaps in between the paper tapes. The nitrogen gas is pressurised and injected from both ends once the cable is installed and vacuum treated.

It should be noted that gas filled paper insulated cables are different from gas insulated (transmission) line. The former uses gas to facilitate paper insulation whereas the latter uses gas (a mixture of mainly nitrogen and a smaller percentage of SF 6, typically in 80:20) as the insulation.

MI paper insulated power cable

An example of MI paper insulated power cables (or, in short, MI cable) is presented in Fig 6.

Fig 6: MI paper insulated single-core power cable (Courtesy of ABB)

The key feature of a MI cable is that the insulation is formed onto a conductor core using a large number of layers of lapped high-density paper tapes and the entire mass (i.e. the core and insulation) is loaded into an impregnation tank and filled with a high-viscosity compound. The purpose of the impregnation is to ensure that the gaps between adjacent turns of paper tapes are permanently filled. The compound also provides a degree of water blocking between the insulation and overlying metallic sheath in order to restrict flow of water in the event of a breach of the metallic sheath [5]. The manufacturing process should ensure that there is no draining of the impregnation fluid from the cable and the compound should remain highly viscous throughout the expected life of the cable. There is no need of oil feeding during operation. For this reason MI cables may be fully called ‘mass-impregnated paper lapped non-draining cable’. MI cables can be used for very long lengths thanks to no need of oil feeding.

MI cables have a higher requirement for factory space and longer manufacturing time because of the need to immerse the cables in large tanks for several months [5].

The maximum operating temperature of the insulation is around 55–60°C owing to the electrical and thermal performance of the insulation paper. Continuous improvements in MI cables are to develop new impregnation compounds, including those with non-linear viscosity and temperature characteristics, so that the operating temperature can be increased so does the cable ampacity.

Following the use of PPL in oil filled paper insulation cables, it is also feasible to use PPL insulating tapes instead of paper tapers in MI cables to increase operating temperature and power transfer capabilities.

MI cables have been used in the past for medium voltage AC applications, but are presently used only in high capacity DC application hence are only in single core structure, although two cores bundled in parallel or in a concentric configuration are possible.

Extruded polymeric insulated power cable

Polymeric materials used in power cables include polyethylene (PE), polypropylene and polyvinyl-chloride. The most widely used polymeric insulation material in higher voltage power cables is PE which is hydrocarbon material consisting of long chains of CH 3and CH 2 molecules. PE is non-polar, thermoplastic and semi-crystalline material and has lower dielectric losses than paper insulation does.

Three typical types of extruded polymeric insulated power cables are discussed below.

Single core extruded AC cable

A typical structure of single core extruded polymeric insulated AC cables, shown in Fig 7, is broadly similar to any other single core cables discussed so far.

Fig 7: An illustration of a single-core extruded AC cable (Courtesy of Prysmian)

The unique feature of extruded polymeric insulated cables is the formation of the polymeric insulation layer (or wall). This is carried out by following an extrusion process at the same time as the screens over the conductor and the insulation. The extrusion process generates a number of by products including water vapour and methane gas to be removed by heating and drying the insulated core.

There are two main variations from the basic extruded PE insulation materials: cross-linked PE (XLPE) and ethylene propylene rubber (EPR).

  • XLPE: It has been used for submarine cables since 1973 and for underground land cables even earlier. The cross linking refers to cross linking of the PE molecules after extrusion to form three-dimensional networks. This is to increase the tolerable temperature of the insulation to increase the power transfer capability of the cable. The cross linking is an irreversible process and gives the polymer a higher melting point higher than the normal limit of 80–110°C and in fact XLPE may not be pyrolysed until at 300°C. This allows the conductor to be loaded until it attains a temperature of 90°C.
  • EPR: Some manufacturers use EPR in place of XLPE for submarine cables. EPR has a higher loss factor and permittivity values than XLPE hence incurs higher dielectric losses. This makes EPR cables less attractive than XLPE cables in terms of cable losses. However XLPE is more vulnerable to water treeing in case of moisture ingress and requires an impervious metallic lead sheath to be applied over each core to avoid direct contact with water. EPR compounds on the other hand can be formulated to give a satisfactory performance in terms of both electrical reliability and ageing, thus removing the need for an impervious metallic sheath thus qualified to a so called ‘Wet Design’.

Hence the choice between XLPE and EPR seems to be a choice of lower operating losses against lower manufacturing cost. It seems common to use XLPE for higher voltage, such as 132 kV and above as the need of lower losses prevails over the extra cost of ‘Dry Design’. In other words, submarine transmission power cables are normally in ‘Dry Design’ but offshore wind farm array cables tend to be in ‘Wet Design’ [6].

Three core extruded AC cable

The discussion on the polymeric insulation for single core extruded AC cables above is fully applicable to three core extruded AC cables. The uniqueness here is that once the three cores and their insulation and screens have been manufactured, they are twisted together along with any optical fibre cables. The bundle is then bound together after fillers are put. A three-core power cable is illustrated with optical fibres, lead sheath and wire armour in Fig 8.

Fig 8: Three-core power cable with optic fibres, lead sheath and wire armour (Courtesy of ABB)

In some constructions a layer of bedding for the armour is applied. A continuously extruded anti-corrosion layer of semi-conducting PE may also be made to provide corrosion protection and prevent the lead sheaths from binding together. The extruded layers over each core are in contact to allow the cancelling of the three-phase charging current and the sharing of any fault current among the sheaths [5].

Single core extruded DC cable

Extruded DC cables are normally in a single core construction, although two cores bundled in parallel or in a concentric configuration are available. Extruded DC cables have components similar to those for extruded AC cables. The key feature of extruded DC cables remains to form the polymeric insulation using an extrusion process.

Fig 9 shows the principal composition of DC cables for both land and submarine applications.

Fig 9: Illustration of a single-core extruded DC cable [7]

A Land application

B Submarine application

1 – conductor, 2 – insulation, 3 – water barrier, 4 – wire armour and 5 – outer sheath/serving

As discussed in the Impact of AC and DC Operation on Conductors and Insulation of Power Cables, there are different implications of the manufacturing process of the conductors and insulation for DC application in comparison with that for AC application. In particular, there is a unique problem with the DC polymeric insulation, that is, space charge building up across the insulation thickness affects the life longevity of the insulation and hence the cable. With space charge issues being resolved and mitigated, extruded DC cables can be expected to reach an operating temperature of 90°C.

Choices of power cables for submarine application

Suitability of oil filled paper insulated cables

Oil filled paper insulated cables are presently suitable for voltages up to 600 kV DC and 1000 kV AC and hence can carry the highest levels of power in both AC and DC transmissions. They can be laid at depths down to ∼800 m without special precautions; with the use of special fluids and proper armour designs, depths of 2000 m can be reached.

This type of power cables has an inherent disadvantage of distance limitation because of the requirement of an oil feeding hydraulic system which may limit the transmission distance to 100 km or shorter depending on the oil feeding length. The longer the route, the larger the oil duct required. Furthermore, potential risk of oil leakage also gives rise to serious environmental concerns offshore as oil leaks are already a major issue for oil filled cables in land. For these two reasons, oil filled paper insulated cables are not a preferred choice for submarine application given that alternative types of power cables are available. Oil filled paper insulated cables continue to find a wide spread application onshore.

Suitability of gas filled paper insulated cables

Despite voids present in the pre-impregnated paper-lapped insulation, gas filled paper insulated cables can be used for both AC and DC submarine applications, but are limited to very short distance because of the need to maintain gas pressure and prevention from any gas leakage.

Suitability of MI paper insulated cables

MI cables are suitable for very long distances for DC links, up to several hundreds of kilometres, the typical distance of submarine transmission interconnectors, thanks to the avoidance of oil feeding.

The existing rating of MI cables is about 500 kV 800 MW corresponding to a maximum conductor temperature of 55–60°C limited by the electrical and thermal performance of the insulation paper. The mass impregnated paper polypropylene laminate (MI–PPL) cable, a new type of MI cable combining MI and PPL technologies, has a significantly improved electrical and thermal performance which makes higher temperature and high power capacity become feasible. The latest rating record of MI–PPL cables indicates a maximum conductor temperature of 70–80°C and a DC power rating of 600 kV 1200 MW per cable or ±600 kV 2.4 GW per bipolar circuit. The first project at this rating is being installed in the UK for completion in 2016–2017. In terms of water depth, up to 1000 m is feasible, although deeper waters can be reached by applying special design features.

For cost efficiency, the use of MI cables in submarine applications is largely confined to large capacity HVDC interconnectors, although they have been used for AC transmission below 100 kV. For information, the time required to joint sections of a MI cable is normally 5 days while only 1 day is required for XLPE cables.

Suitability of extruded polymeric insulated cables

Extruded polymeric insulated cables typically have a higher limit of electrical stresses and can operate at a higher operating temperature (70–90°C) thus deliver higher power density. They are also flexible, relatively light and easier to manufacture, handle and joint (hence cable laying and repairing is easier, especially in submarine conditions) and free of oil leakage. Therefore, it is preferred for a submarine application.

This technology has been widely used in large capacity AC applications over the last several decades. XLPE cables at 110–150 and 220–275 kV have been in use in land since before 1985 and 1990, respectively. The first 400 kV XLPE cable was commissioned in Copenhagen in 1996 and the first 500 kV XLPE cable in Japan in 2000 [8], albeit both were onshore. For submarine AC application, one of the world's first AC 380 kV submarine cable links using XLPE insulated cables is being installed in Turkey, linking its European side with its Asian side due for completion in 2016. The system is composed of a double AC power transmission circuit measured at ∼4 km, with a rating of 1000 MW for each circuit.

However, because of the issue with space charge accumulation in DC operation, DC polymeric cables have a voltage limitation. Currently submarine polymeric cables for DC application have only been qualified for voltages up to 320 kV (500 MW) per cable. It is reported that XLPE DC cables of 525 kV have recently been developed and completed type tests [7].

In addition, as mentioned earlier, extruded polymeric insulated cables in DC are limited to HVDC-VSC only. For this reason, HVDC transmission projects associated with offshore generation and those associated with interconnectors tend to use different types of cable technologies. This means that their pressures on supply chains are not necessarily overlapped, thus the impact of interconnectors on offshore generation driven HVDC transmission may not be as serious as the industry normally fears.

Summary of suitability for submarine application

On the basis of the discussion above, the suitability of different types of power cables for submarine application is summarised in Table 2 [9].

ower cable type

Application

AC (single core or three-cores)

HVDC-CSC (for interconnection)

HVDC-VSC (for export of offshore generation or for interconnection)

Oil filled paper insulated cable

Usable but not preferred owing to need of oil feeding and risks of potential oil leakage.

Usable but not preferred owing to need of oil feeding and risks of potential oil leakage.

Usable but not preferred owing to need of oil feeding and risks of potential oil leakage.

Gas filled paper insulated cable

Usable but not preferred owing to need of maintaining gas pressure and risks of potential gas leakage.

Usable but not preferred owing to need of maintaining gas pressure and risks of potential gas leakage.

Usable but not preferred owing to need of maintaining gas pressure and risks of potential gas leakage.

MI paper insulated cable

Not normally used above 100 kV

Usable

Usable

Extruded polymeric insulated cable

Usable

Not usable (as unable to cope with polarity reversal)

Usable

Table 2: Suitability of power cables for submarine application

Conclusion

As a concluding remark, some of specialities as a result of using power cables in submarine environment under either AC or DC conditions are summarised in Table 3.

Special features

Consequential impact

Need of water tightness to block water ingress

Need of swelling agents (powder, tapes or yarns) suitable for salt water.

Need of stronger armour and extra mechanical strength

Application of metallic tapes or wires to withstand the mechanical forces and reduce the risk from external mechanical damage (not sufficient to resist anchor damages although).

Opportunity for 3 cores per cable in AC

Thanks to seawater's heat dissipation, three cores can be built and installed together to save cost. Also optical fibre data and communication cables can be built in as a by-product.

Longer length of cable sections deliverable

Land cables are in lengths of around 1 km but submarine cables can be in sections of 80 km.

Implication of operation in DC

Operating in DC gives rise to a better operating condition for conductors than that for under AC, thanks to being largely free from typical AC related phenomena (such as the skin effect and proximity effect). This allows simpler manufacturing of conductors for DC.

DC operation also inherently avoids AC related dielectric losses in paper based insulation. This will bring advantage to manufacture and operation of paper insulation for DC.

However, when polymeric insulation is used, operating in DC will give rise to space charge accumulation not seen in AC operation. This will require additional enhancement in the formation of the insulation or result in reduction of operating voltage for DC or reduction of operating life.

Implication of operation in AC

Being submarine makes en-route mitigation difficult in relation to capacitive charging current build-up and reactive power in AC cables. This will (a) limit the AC transmission distance achievable in submarine conditions, (b) restrict the AC voltage level a cable can operate at regardless whether higher voltage cables are available and (c) necessitate special switching/earthing procedure to ensure full discharging/charging when de-energising and energising an AC submarine power cable.

Table 3: Specialities of using power cables in submarine environment under either AC or DC conditions

References

  1. Nexans: ‘60–500 kV high voltage underground power cables’ (Nexans, 2011), pp. 1–64.
  2. Worzyk T.: ‘Submarine power cables – design, installation, repair, environmental aspects’ (Springer, 2009).
  3. Harker K.: ‘Power system commissioning and maintenance practice’ (IEE, 1998, 1st edn.) .
  4. ‘Prysmian – Submarine Energy Systems’, http://www.prysmiangroup.com/en/business_markets/markets/hv-and-submarine/downloads/Brochures/LOW-A4-Leaflet-Submarine.pdf, accessed December 2014.
  5. Cable Consulting International Ltd: ‘Cable manufacturing capability study’ (The Crown Estate, 2012), pp. 1–62.
  6. CIGRE (International Council on Large Electric Systems): ‘Technical brochure 483: Guidelines for the design and construction of AC offshore substations for wind power plants’ (CIGRE, 2011), pp. 1–378.
  7. ABB: ‘525 kV extruded HVDC cable system – doubling power transmission over longer distances’ (ABB, 2014), pp. 1–8.
  8. ENTSO-E and Europacable: ‘Joint paper: Feasibility and technical aspects of partial undergrounding of extra high voltage power transmission lines’ (ENTSO-E and Europacable, 2010), pp. 1–25.
  9. Zhang C.: ‘Offshore transmission associated with connecting offshore generation’, IET Eng. Technol. Ref., 2014, pp. 1–17, doi: 10.1049/etr.2014.0003.
  10. BICC Cables: ‘Electric cables handbook’ (Blackwell Science, 1982, 3rd edn., 1997).

Appendix

Brief timeline of power cables

A brief development history of power cables of interest is presented below:

Power cables for land application [10]

  • 1980s first power cable using rubber and vulcanised bitumen insulation (<10 kV)
  • 1890 first power cable using paper insulation (10 kV)
  • 1926 first power cable using oil filled paper insulation at 66 kV inland
  • 1937 first power cable using gas filled paper insulation
  • 1930s first power cable using PVC insulation
  • 1949 first power cable using mass impregnated non draining paper insulation
  • 1954 first power cable using oil filled paper insulation at 275 kV onshore
  • 1969 first power cable using oil filled paper insulation at 400 kV onshore
  • 1970s first power cable using XLPE insulation at 15 kV onshore
  • 1980s first power cable using XLPE insulation up to 275 kV onshore
  • 1990s first power cable using oil filled paper insulation with PPL
  • 1996 first power cable using XLPE insulation at 400 kV onshore
  • 2000 first power cable using XLPE insulation at 500 kV onshore

Power cables for submarine application

  • 1961 first DC cable at ±100 kV (between England and France)
  • 2000 first AC cable that went longer than 100 km (105 km, 90 kV, XLPE, 3×300 mm2)
  • 2012 first AC three-core XLPE cable at 220 kV (in Denmark, for a 400 MW offshore wind)
  • 2013 first DC cable for connecting offshore wind export (in Germany at ±150 kV, 400 MW)
  • 2016 first AC cable using XLPE insulation at 380 kV (in Turkey)
  • 2016 first DC cable using MI-PPL insulation at ±600 kV and 2.4 GW (via the Irish Sea)

Incidentally, as of 2014, the following typical DC cable projects may be noted:

  • The longest DC submarine cable system is a pair of 580 km cables (NorNed).
  • The deepest DC submarine cable system is the link between Italy and Greece built in 2000 with a maximum depth of 1000 m in the middle of the Otranto channel. The link is 163 km long using paper insulated cable of 1×1250 mm2 at 400 kV DC manufactured by Prysmian.

Only three projects have been completed using DC cables for connecting offshore wind [Borwin 1 (±150 kV), Helwin 1 (±320 kV) and Borwin 2 (±320 kV, 800 MW), all in Germany].

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Chuan Zhang

General Manager and Technical Director in power/energy sector , GAELECTRIC HOLDINGS PLC

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