Investigation of Subsea Power Transmission Cable Technology Rev D

EG5085 Advanced Topics 2013-14

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Investigation of Subsea Power Transmission Cable Technology Stephen Gabriel McCann

Abstract: Electric power transmission technology has grown to meet the demands of industry and consumer. The advent of reliable electrical generation seeded the growth of power cable design and manufacture which grew from simple compositions for terrestrial installations to complex structures to meet the demands of the submarine environment. Literature spanning more than a century of development into power cable is examined with focus on subsea power transmission. Several huge subsea installations in development at the time of writing which seek to push the boundaries of conventional power delivery are detailed. AC and DC power subsea transmission cable is analysed in detail on construction and their materials, detailing their individual merits and inherent problematic characteristics. Installation, inspection and repair as well as supporting technologies used to support subsea cabling are investigated which includes some analysis of typical installation scenarios and a basis for simple stress analysis of cabling. A brief look forward into the possibilities of HVDC and superconductivity with integration into the renewable energy sector is examined.

Keywords: cable, power, subsea

1. Introduction

Transmission of electrical power using subsea cables has remained fundamentally unchanged throughout its 170 year tenure. Supply of electrical power using subsea cabling is a requirement for contemporary society, with the more common static cable installations run from generators to islands, across bodies of water between nations and to remote offshore installations. There is a less frequent but growing demand for dynamic installations run from seagoing vessels and installations to subsea equipment and machines, most commonly seen within the oil and gas industry. Subsea cables are capable of supporting AC and DC power as well as a myriad of communications technologies relying on metallic conductors as well as additional optical fibres for data. Component structure for contemporary subsea cable design remains in principle the same as the first manufactured conductive subsea cables; a conductive medium at the core, a dielectric insulator, protective sheaths and outer steel armour all encompassed in a non-permeable membrane.

Advancements in subsea cabling come from manufacturing capability, materials science and chemistry, and advancements in deep sea installation and inspection, repair and interconnectivity at depth. Manufacturers such as ABB, Nexans, Prysmian, NSW, General Cable, and Hanhe Cable Company have the capabilities to create continuous lengths of homologous cable, with assured integrity and consistent electrical and magnetic characteristics. Purpose built vessels are used to install cable on the seabed, with the first being CS Hooper, built in 1873 in Newcastle, United Kingdom [1]. Todays cable laying vessels consist of those owned by manufacturers and energy industry contractors such as Subsea 7, EMAS, and Tyco. These vessels operate with precision and have the capability to fully realize the cable lay in advance through use of advanced 3D survey imaging technologies, utilising remotely operated underwater vehicles (ROV), specialised trenching machines, and divers to complete installations.

Traditional subsea power cables have primarily been devoted to AC power delivery due to the limitations of DC in terms of the delivery medium. Advancements in power electronics are creating viable alternatives using HVDC with many projects currently in development such as an ABB contract awarded for a 158 km HVDC run to Finland [2], and the proposed 1000 km HVDC link between Iceland and the UK [3]. The future of HVDC transmission is dependent on inverter technology and development of networks which can operate beyond the point to point historical limitations of DC. Ultimately, a HVDC grid using superconducting cables will allow the world to be connected with much of this development currently being invested into offshore wind farms. A subsea super grid interconnecting all of Europe and further is proposed to bring power from renewables to


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the consumer, deriving a lot of power from the proposed offshore renewable energy schemes in development.

2. Historical Perspective

Development of power cable became a necessity with the invention of the incandescent bulb in the 1880s. Initially DC power was transmitted using multiple wound telegraph conductor with gutta-percha for the sheath until the development of vulcanized rubber. Growing power demand also saw the development of the conductor into different geometries and material. Overhead land power networks grew to meet demand but became unsafe for the communities and utility workers who worked around them, creating a requirement for subterranean power transmission. In 1890 Ferranti developed a viable dielectric for power cable which consisted of oil impregnated paper, which was wound or lapped over the conductor, which is still in use today in low power cable applications. This was a good solution, however, as voltages increased in the power networks the small gaps and voids which developed within the oil impregnated paper and conductors gave rise to a phenomenon known as corona discharge, and rising currents saw significant ohmic heating effects, both of which resulted in degradation of the dielectric and ultimately failed cables.

Durability was improved though the addition of lead sheaths to keep out water and the addition of outer steel armour to protect from mechanical abrasion and impacts. In 1917 Emanueli developed a system for power cables which used a centrally located hollow core within the conductor in which a pressurised dielectric oil compound was injected [4]. This served to prevent the damaging effects of air voids within the oil impregnated paper and conductor and is still in use today in some applications. This was particularly useful for subsea cables of the era as they were laid on top of the sea bed and were subject to movement from tides and currents. Power cable manufacturing methods, materials composition and installation techniques were developed which facilitated power delivery across vast expanses of ocean. Todays power cables remain essentially unchanged in principle but advances in power electronics, chemistry and manufacturing methods have greatly increased the voltage and ampacitity, and has allowed for installations over greater distances and increased depths.

3. Emerging Power Transmission Technologies

Typically AC systems are limited by such factors as reactivity in the line from induction or capacitive interactions with the conductor, shielding and armour, and influence of external electromagnetic fields and adjacent structures. Other factors affecting the quality of transmission are limitations on the geometry of the conductor due to skin effect which is frequency dependent, configuration and installation of cabling, and manufacturing capabilities. These limitations introduce boundaries on the carrier frequency, current and voltages. Improvements in power transmission networks have been made through the utilisation of power electronics. These technologies have created the potential for large efficiency gains within industry and continue to be researched heavily across the private and academic sectors. Traditional power transfer has been dominated by AC systems, however, advancements in power electronics is beginning to create a viable economic basis for switching some networks to HVDC transmission systems, particularly in the area of subsea transmission and offshore wind power generation. High voltage and high current capable insulated-gate-bipolar-transistors and thyristors (IGBT) have made it possible to introduce various improvements into both AC and DC systems.

For AC power, the most beneficial technology in development is FACTS [5]. The FACTS system is power electronics based consists of series or shunt switching circuits with reactive elements which will facilitate a better power transfer over the line through continual optimisation of the reactivity of the line. Advancement in power electronics and sensing systems allows for compensatory inductive or capacitive elements, or sometimes simple ground switches, to be intermittently added to a transmission circuit to reduce reactive power losses. AC transmission has traditionally held advantage over DC networks due to its ability to be transformed into varying current and voltage levels, tapped and branched off with relatively simple technology, and for networks capabilities to be simply


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compensated for in the event of faults. AC transmission is very susceptible to reactive losses; however, these can be militated through use of compensatory equipment throughout the network. Power losses are kept at a minimum through stepping up voltage levels which reduces I2R losses. The only major hindrance between differing AC networks is frequency, which can also be adjusted through use of cyclo-convertors and various intermediate DC inverter schemes, but more often than not competing utilities operating on differing frequencies do not share grid.

Figure 1: VSC Transmission Configurations For the DC system, the development of voltage source converters (VSC) and multi-terminal DC (MTDC) technologies is enabling engineers to create stable DC grids which are analogous to their AC counterparts in that they can be stepped up and down in voltage and be tapped off. VSC allows for frequency independent rectification from AC courses using IGBTs which follows that convertor operation is possible across all quadrants. These systems can provide both active and reactive power production and absorption. VSC can operate in various transmission modes, which include mono-polar, bi-polar, and back to back systems [6]. Their various configuration are detailed in figure 1.

Figure 2: MTDC Configurations Traditional DC networks have strictly been point to point, allowing for a single voltage level between source and load. MTDC technology allows for a HVDC circuit to be tapped into intermediately, akin to traditional AC power networks, with either a series or parallel configuration as shown in figure 2. The main drawback for large scale use of MTDC are the inability to draw off differing voltages directly from the network as can be done with transformers and AC. HVDC networks operate with tens or hundreds of kilovolts, requiring on expensive DC-DC inverters and choppers to step down and draw out useful voltage levels for localised machinery. Both AC and DC systems have seen a lot of advancement and are continually being improved upon within the utility industries as shown. However, the majority of cable installations across the world have been AC and will remain so due to reduced cost of and simplicity. There are limiting factors strictly within cable for both types of voltage, which will be expanded upon in section 8.

4. Installation Regardless of the cable configuration there is a primary requirement to lay electric cabling safely and efficiently on the ocean floor. An ever larger component of offshore engineering is specialising in the subsea environment where the lions share of investment capital originates from the oil and gas sector. A broad spectrum of contemporary engineering disciplines within industry and academia cooperate and develop state of the art technologies and solutions for subsea power systems and cabling. A fleet of technologically advanced cabling ships operate over all the worlds oceans and lakes carrying out submarine installations using state of the art equipment.


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These ships are purpose built utilising geo-positioning thruster technology to stay on station during operations, often to within a few metres even in considerable sea states. They are equipped with integral horizontal or vertical spooler machines, cable handling and conveyance machinery and a multitude of support systems including high tonnage hydraulic articulated pedestal cranes as well as other smaller support cranes, ROV deployment and support facilities, moon pool lifting devices and diving support. Cables in excess of 160 km can be stored on the vessel spooling machine, which can either be fully reloaded or individually spliced while at sea, either on board or with assistance of specialist support vessels.

Figure 3: Lewek Constructor Schematic [7] Cabling can be placed directly on the seabed but is more often installed below the surface of the seabed to a depth of several meters to mitigate damages caused by currents, fishing nets and anchors. Burying the cable is facilitated through use of specialised trenching machines which use mechanical excavators or pressurised jets of seawater to displace the underlying materials. The cable is laid into the trench through use of the trencher machines or auxiliary equipment and allowed to backfill by natural means of ocean currents and movement of seabed material. A control system will provide communication between the trenching machine and vessel turntable and feeder mechanism to ensure the cable is paid out at the correct rate. Normally the entire process is observed from a control room in the vessel through use of cameras mounted on the trenching machine and ROVs.

No cable is laid without a thorough assessment made of the seabed geography and composition, which may be completed using advanced 3D surveying technology, inspection through ROV or divers. Every part of the cable run will be assessed and solutions engineered to minimise or prevent exposure of cable to currents and other hazards and to ensure the cable is installed with its nominal spacing between other nearby power and telecommunication cables, oil and gas pipelines, galvanic corrosion equipment or any other sources of magnetic fields, induction, or temperature gradients. Further, it must be noted that these cables are sensitive to their laying methods due to their internal component substructures, particularly in the particular lay of the armour and conductors with respect to bending radius. When obstacles such as pipelines, machinery and trenches exist in the proposed laying scheme solutions using concrete mattresses and more advanced underwater constructions can be utilised to complete an installation.

Cable laying vessels are owned by states, cable manufacturers, and the larger naval merchants and services. These can leased on long or short term contract to exterior utilities and states, or chartered directly for single installs or support roles, where daily intervention capital budgets may range from several hundred thousand to several million pounds per day. Deployments are often across the globe from their registered flag state which may see a vessel at sea for several years before returning to dry-


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dock for major overhaul and re-commissioning work scopes. It is a normal consideration for newly charted vessels to be fitted out with specific support equipment and specialist crew to the type of cable installation, operating environment, and nationality of the waters. The crew will consist of the ships company who are responsible for the operating of the vessel and its maintenance. In addition there will be specialists on board who are responsible for the cable laying operation, ROVs and other equipment, crane operators and divers.

Figure 4: Typical Trenching Machine The particular laying scheme for cable factors in many considerations such as proximity to other emf field emitters, the ocean currents and potential for contact with fishing nets. Figure 5 shows a number of common configurations all of which are buried in these examples. It is not uncommon for extra redundant cables to be laid at the same time as the primary cables due to the cost implication of running a cable after finding a fault and the reduced downtime for a transmission network. It is common for DC cables to be laid with spacing in the region of hundreds of metres to reduce field interactions and to remove the field components from complementary cables which may induce corona losses [8]. Consideration must be made to the depth of the cable and what the constituents of the seabed are and whether the backfill will cause issues, especially when regions of increased currents are known to exist as in the case of a region of shallow water or approach to land. Other important considerations are the environmental impact of a cable installation and the particular legislation which will be region dependent and set by the host state.

Figure 5: Typical Cable Laying Configurations


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Further technology is involved in bringing the cables ashore, known as a landfall, which normally consists of conduit installed using horizontal drilling equipment in which the cable is pulled through, or it is installed in a trench. The cable is connected to the land based grid using specific interconnections which may be housed in a pit or structure.

5. Construction

Subsea power cable construction is a mature technology which has had almost two centuries of development drawing from advancements made in land based power cables and various types of telecommunications cables, as well as those specific to the submarine environment. The major difference between land and subsea cabling is in the overall dimension, but also in more subtle areas dealing with shielding and the obvious necessity for significant armouring and sheathing to prevent water ingress as land based cables do not face the extremes of subsea hydrodynamic stress and pressures. Construction, design, testing and safety standards are detailed by governing bodies such as British Standards Institution (BS-EN), International Organisation of Standardisation (ISO), International Electrotechnical Commission (IEC), Underwriters Laboratories (UL) and the Institute of Electrical and Electronic Engineers (IEEE).

Figure 6: Typical Cable Construction in Section

5.1 Conductor

At the core there is a conductor which will be selected to match the voltage, current, and frequency requirements of the system. The composition of the conductor is dependent on its rating and is generally referenced by its cross sectional area and number of strands or braids, usually in units of mm2 or also commonly referred to in other formats such as gauge; with the American, Standard, or Birmingham wire gauge systems (AWG / SWG / BWG). This is not strictly the case as other configurations are available, as shown in figure 7. The conductor may be copper or aluminium, plated in silver, tin, or nickel depending on service and the braiding is selected for its characteristics particular to its application. Ultimately the price of metal on the market at the time of fabrication and a clients budget will narrow selection criteria.

Figure 7: Conductor Configurations Conductors are generally arranged in several common configurations which are solid, stranded, profile wire, or Miliken [8]. Solid core and profile wire configurations are commonly used in HVDC installations, with the option to have a hollow core to allow the addition of a centrally located oil


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channel in both the solid and profile configurations. Solid conductors can take various shapes but are most commonly found in circular round or oval and elliptical shapes. The profile wire configuration uses segmented sections of solid wire which are pressed or rolled into shape, specific to the layer on which they are positioned, allowing for several concentric layers to make up the required volume. This segmented design allows for more structural flexibility on unit lengths of cable and also reduces the cost of a section as it is less expensive to create several layers.

Figure 8: Single Core Subsea Cables Alternating current has particular qualities within a conductor which can create detrimental effects due to skin effect and reactivity, excluding solid geometries from consideration. Normally stranded cabling is used, which consists of a twisted bundle of smaller round conductors rolled and pressed into the least volume. A final configuration exists which is the Miliken, which consists of circular pressed bundle of smaller conductors in the centre, with a further layer of segmented pressed bundles of similar gauge conductors. The Miliken reduces the potential air gaps and increases the bundle volumetric efficiency.

Figure 9: Alternate DC Cable Configurations

5.2 Screens

Conductive and semi-conductive screening is placed over the conductor and insulation layers within the cable, around 2 mm thick of continuous tape and wrapped up in layers to match the required properties of the cable [8]. These can be made of carbon-black, semi-conductive XLPE or even multi-layered tapes constituted with copper, oil impregnated paper and the previously mentioned semi-conductive layers materials, added to the cable using specialist lapping machinery. Screening is used for several purposes and is to be considered integral to the overall dielectric properties for a cable. For the conductor screen, it is normally used as a container to hold the conductor geometry in the production line prior to the addition of the dielectric insulation through and extrusion process, preventing ingress of the dielectric into the conductive cavity. Screen on the outer portion of the


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insulation will sometimes be grounded to pull off reactive imbalances in AC cabling and as a pathway for any short circuit currents which may be introduced within the outer layers of the cable.

5.3 Insulation

A dielectric material is polarised when subject to an electric field, creating intermolecular dipolar moment within the material which resists the free flowing of electrons making it a good insulator. Most power cable in use today relies on XLPE as the dielectric as well as PE and EPR, and less commonly PVC. XLPE was developed in the 1950s and has advantages over other insulators in that is mechanically stronger, has a higher dielectric strength coefficient, and is thermally stable over a wider range of temperatures. It also does not have a real melting point and therefore can be used in very high temperatures without breaking down and is capable of insulating carriers in excess of 500 kV [9]. This means it can be used on high current applications without detriment to the cable or its dielectric qualities over time. XLPE and the other common dielectrics are normally extruded over a conductor during the manufacturing process and will have a setting and curing time that has to be factored.

Figure 10: 3-Phase Subsea Cables

5.4 Sheath

The sheath provides a physical barrier to the inner components of the cable, preventing ingress of water and humidity into the dielectric and conductors, also giving protection against mechanical abrasion and impacts and adding weight to a cable. Various materials are used for the sheaths with the first developed and still most common being lead. Other materials are also used such as copper and polymers depending on how dynamic or flexible the service of the cable is to be designed for. Lead sheath is added as a continuous wrapped layer and soldered or friction welded on the seam while in production. For copper, it normally added as a layer akin to the lead type or can be installed as a continuous spiral wrap of corrugated sheeting which is welded or brazed to provide the seal. A further component of the sheath is a water resistant polymer outer layer which guarantees a water tight barrier for the inner cable, normally consisting of high or low density PEs, PVC, nylon, or polyamide. These can infused with carbon black for voltage equalisation purposes. As the sheath is a conductive material it is susceptible to currents from the core conductor and exterior sources, therefore plays into the electrical characteristics of cable which often requires intermediate grounding in an installation.


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5.5 Binders and Fillers

These are generally non-conductive elements which serve to fill out the voids within the cable geometry and provide a stable structure on which the armour and outer serving can be applied. They can be comprised of solid sections of polymer or woven threads and are applied in the process line. Materials normally consist of woven fibreglass fibres, which can sometimes have metallic fibres or polymers for the addition of specific mechanical and electrical properties [9].

5.6 Armour

Perhaps the most critical element of the subsea electric cable is the armour. This serves as the backbone of the cable which protects it from mechanical damages on the sea bed and provides the necessary mechanical tensile strength as it is installed, particularly when being paid out from a cable laying vessels into very deep seabed. The armour itself consists of galvanized carbon or stainless steel strands which lie over the outer binding layer of a cable, and can be laid in left or right hand lays and are normally bathed in bitumen just prior to installation on the process line. The gauge of the armouring wire and the number of layers of armour is determined by the cable manufacturer and industrial or state client, based on the service requirements and potential damage factors. When more than one layer of armour is required it is often the case that the second and further layers are lain on top of the previous layer with an alternate lay, ie right lay over a left lay. The particular composition of the armour plays heavily on the installation methodology as it can restrict how a cable is laid on the spooling machine on the vessel, or whether it is possible that the cable can be coiled, and what is the acceptable bending radius [8].

5.7 Serving

This is the outside, weather facing component of the subsea cable. Normally it consists of a mix of bitumen or similar compounds impregnated into a woven fibre covering or can be a polymeric extrusion over the armour. Normally the yarns are woven in the same lay as the outermost armouring to prevent bird caging and delinking from the cable exterior. The outer serving is chosen to be non-conductive and non-reactive to the seawater environment, and considerations such as its coefficient of friction need to be assessed as this will affect the installation methodology. The outer serving is also used to place cable markers and identification.

5.8 Additional Components of Subsea Cables

The requirement for data communications has grown significantly as has power delivery, so it is very often the case that power cables will have integral fibre optic carriers within their construction. The fibre carrier will be a smaller version of the main cable, with a hollow core for the fibre optic lines, a sheath layer and armour outer layer. These are placed into the voids in multi-core cables as is the case with three phase AC cables or dual conducting HVDC cables. Other components which may exist within a power cable are oil carrying and gas lines. These are used to hold inert gasses or dielectric oil compounds which prevent corona losses and add structural rigidity and filling voids with the pressurised fluids or gasses.

For shorter installation lengths sometimes the cable can be run in a conduit which may consist of a solid section of pipe filled with an oil, air or gas. Conduits can also be made of polymers or concrete. Other components that will be found on subsea cables are grounding points in AC cables and splices. Grounding points will be routed out from the inner cable layers or armour to allow for intermittent bleeding off of reactive elements developed over the cable, and the splices which are detailed in the following section are used both in factory and installation to create continuous lengths of cable. Finally there can also be the addition of specialised wet and dry mate connectors which can facilitate land based and subsea connections to equipment, transmission and delivery points.


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5.9 Splices

Flexible factory joints (FFJ) and flexible installation joints (FIJ) are used to splice together two sections of cable normally used when defects occur during the manufacturing process, when process lengths have been met during production, or a vessel spool capacity has been met [8]. Firstly the free ends of the cable are stripped back and the armouring is offset from the outer serving and inner shielding and components. The main dielectric and its shields are formed into conical shapes with a free length of the core conductor exposed. For smaller conductors it is sometimes possible to splice using solder or crimp terminations. Subsea cables are usually large, ranging from 95 mm2 upwards, which require alternate methods to join. For the larger cables TIG and MIG welders and sometime friction welding devices are used to join the conductors of the two cable sections. The next part of the repair involves lapping conductive and dielectric amalgamating tapes in the conical sections as required, building up in layers matching the diametrical constituent parts until outer diameter slightly in excess of that of shield bedding is matched. Normally these tapes are then set to with a thermal blanket and pressure device to heat and shape the tapes which normalises the properties throughout by removing air gaps and matching densities. The outside of the built up cable is shaped to match that of the existing armour bedding once cooled and properties are assured.

Figure 11: Typical Splicing Technique

A critical component of the repair follows where an amour section is shaped and set into the cable, where it will be soldered or welding into position. The final stage of the splice involves creating an outer barrier which is composed of a flexible, machine injected polymer. While splicing of the cable in such a manner enables a factory to construct essentially endless lengths of cable, each splice adds potential for increased service issues such as localised hotspots due to changes in resistance, inductance and capacitance and the potential for unwanted air gaps and subsequent corona losses. Most importantly are the localised issues of current density at the conductor joints specifically due to skin effect. Other notable problems occur from losses in tensile and strength and decreased flexibility which may have effect on how the cable can be handled, spooled, and installed, especially when considering shear as cable will tend to turn as it is taken on or off the spool.

6. Stress Analysis

An ideal subsea laying operation would see the entire cable buried, however this not very often the case. Both manmade and naturally occurring trenches as well as obstructions and non-trenchable areas of seabed will need to be spanned with cable creating potential for tension within the cable and leave sections of cable exposed to currents. A hanging section of cable in a trench can be analysed using formulae developed by Bernoulli for the catenary problem [10], defined as a suspended chain or rope hanging under its own weight. This follows a hyperbolic curvature which sees the maximum tension


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in the vertical direction at the end of the rope as determined by the half section. Tension at the ends of the cable as shown in equation (1.6) is deduced by using Pythagoras theorem for resolving the horizontal (To) and vertical (V) tension vectors. The static stresses can be simply solved using these formulae; however, evaluation of cable stresses due to the combined forces of gravity, currents and tidal movements requires a more in-depth treatment using Morisons equation, evaluation of drag forces, and numerical computation of the cable using fluid dynamics.

= cosh 2 1 (1.1) = 2 sinh 2 (1.2) = 8 4 (1.3) = 2 sinh 2 (1.4) = 2 (1.5) = + 2

(1.6)

Figure 12: Typical Subsea Obstacles

= !"#$ &'( (1.7) Analysis of the cable being suspended from the vessel to the sea bed and trenching robot can be taken from the simplest case where the cable is vertical with the tension on the cable at a maximum tension at the feeder mechanism interface on the vessel. An integration as shown in (1.7) is a good starting point estimate of the load on the cable and the feeder. Cables suspended in such a manner will of course be subject to currents, tides, and movement of the vessel and trenching machine. Calculation for more accurate results will involve more in-depth analysis using similar tools as previously discussed.


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7. Inspection and Maintenance

Inspection of power cables is carried out using various technologies such as deep sea divers, ROVs and autonomous underwater vehicles (AUV) and through use of specialist, ad-hoc machines which traverse the length of an installation looking for faults. Faults and condition monitoring within a power cable is commonly carried out using integral optical fibre lines and specialist optical equipment which is able to discern temperature gradients and vibration detection through measurement of scattered light in the fibre due to Raleigh scattering effects which can be analysed with advanced detection equipment. Temperature monitoring allows for the utility to detect the potential short circuits and the presence of hot spots in due to breakdowns in cables, the presence of nearby geographic anomalies such as volcanic activity or industrial faults such as pipeline ruptures. Vibration monitoring can detect the presence of similar anomalies, such as earthquakes or dropped objects like anchors, movement of cable due to tides and other natural occurrences using integral Bragg gratings in the fibre [11]. The optical systems are good for many purposes but they will not detect any electrical faults, such as emf or breaks in conductors. When a cable does become damaged and breaks occur, equipment such as time domain reflectometers in electrical and optical variants can be used to inject a signal in the cable and calculate where the break is to within metres. Normally a damaged cable will simply be removed from service by disconnecting from each end as it is common practice to run redundant cables during installation. However, in the case with 3-phase cable and other multi-core cable arrangements, repairing a damaged cable can be a requirement as laying redundant cables may have been too costly for the original installation budgetary constraints. When it comes time to repair a cable there will be a large engineering investment in terms of capital and resources as the potential savings of repair versus running extra lines will be significant. Repairs can be instituted both above and below the sea surface using divers and cranes to lift portions of a cable out of the seabed, to the surface if possible or into a contained subsea workshop, where a splice or series of splices can be put in place. The complexity and cost of repairing a subsea cable will of course grow exponentially as the depth increases.

8. Limitation Analysis for AC and DC Subsea Power Transmission

The main drawbacks to AC transmission come from the nature of alternating current and energy storage characteristics of the power cable itself. By design, a subsea power cable is fundamentally capacitive in that there is a dielectric medium between the conductor and shield, which extends to the water outside the cable. This allows for a charge to develop within the dielectric each half cycle, creating a positive reactive current vector in the line. Additionally, with the various shielding and armouring properties of the cable it follows that induction will take place between these elements and the conductor, with a negative current vector each half cycle. The vector quantities of the reactance are frequency dependent and can create huge losses in a transmission line if not compensated for at the source and load. It is possible to reduce some of the inductive reactive in the line by bleeding off acquired emf induced potential with intermediate grounding of the shield. DC cables will also be subjected to capacitive reactance, however, this will only be an issue when the line initially energised or transients are introduced. Both AC and DC lines can be affected by inductance within the armour and shields drawn from external fields, such as those from other power lines or other nearby emf emitters. Determination of the capacitive and inductive properties can be made using the known properties of the materials as a basis for installation analysis, with capacitance and inductance normally expressed in units of F/km and mH/km respectively.

Nominally a 3-phase load cable installation will see zero sequence currents within the core, ground and sheath due to load imbalances or self-induction and capacitance in a line, or by introduction of fields. These need to be designed for as they can introduce unwanted harmonics which can add to the heating effect on cable. Normally these are removed from transmission in a filtering system in a transformer but in a very long AC subsea transmission these can become a serious problem [9]. A further detrimental characteristic of AC transmission stems from the skin effect which is a property of


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conductors which states that current density will follow an exponential distribution from a minimum at the core to maximum on the outer skin of the conductor [12]. In short, with AC transmission the maximum electron flow will occur on the outside of the conductor. The skin depth, shown in equation (1.8) and measured in metres is a defined characteristic for AC lines and limits the size and geometry of conductors, which has a knock on effect in limiting overall ampacity. AC lines are therefore made of multiple braided small gauge wires to maximize the surface area within a conductor, which can sometimes be improved upon by putting materials of higher conductivity on the surface such as silver. The skin depth is frequency dependent much like the reactance as seen with the angular velocity term in the denominator.

) = * 2+,,-. (1.8) Both AC and DC transmission lines are subject to the characteristic resistance of the conductor itself which also extends to the screen and armour. Evaluation of resistance is relatively simple as shown in equation (1.9) which is normally expressed in units of /km. Rho is the electrical resistivity of the material and can readily be determined by referring to well established tables. While the choice of conductive material can make improvements online resistivity, there will always be a voltage drop across a transmission line with the conversion of electrical energy into heat.

/ = ! 0# (1.9) Additionally, AC and DC [13] power lines can be subject to corona losses, which is a phenomenon whereby the high potential electric fields in the line cause an ionization of the surrounding fluid, commonly air, which causes the fluid to become conductive creating a plasma discharge. This reaction creates a high temperature zone within the plasma and can cause significant damages to the surrounding cable structure. Any air gaps in a cable will be subject to potential fluid ionization which must be eliminated.

Table 1: Comparison of AC and DC Transmission Cables

AC Transmission Cable DC Transmission Cable Pros Pros Power transfer efficiency of 1/2 Reactive losses Simpler networks 3-Core Cable Simple armouring for 3 core [4] Single Core Cable More armouring for 3x single core [8] Single core has increased bending radii Single core subject to increased induction [9] Single core easier to repair [8] Cost effective to run redundant cables

Power transfer efficiency of 1 Complex networks Reduced losses 2-Core Cable Simple armouring for 2 core Single Core Cable Single core subject to increased induction Single core easier to repair [8] Cost effective to run redundant cables

Cons Cons More complex laying methods for 3 core Line impedances, zero sequence currents [9] 3 core cable very expensive [8] 3 core cable heavy, limits spool capacity [8] 3 core has increased bending radius [4] 3 core repairs are extremely difficult in

service [8]

More armouring for 2x single core [8] More complex laying methods for 2 core [8] Dual core has increased bending radii [4] Large capital requirements for inverter and

multi-point distribution technology [5]


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AC lines are much more susceptible to corona losses due to the exponential current density function from the skin effect and the much higher potential at the surface of the conductors. A set of empirical formulas were developed by Peek [14] which can predict the onset of a corona discharge, with studies into its damaging effects included in this work as well as compensation for weather effects, which could be adapted for influence of subsea currents and water temperatures.

Other considerations for choice of cable and its limitations come from the required voltage and current capacity. While conductors can be chosen to carry large levels of voltage and current there are factors such as the temperature ranges and breakdown limitations in the dielectric or other cable components which limit selection. All these factors will weigh into the economy of a project and ultimately make a project unfeasible if costs grow beyond budget restraints. 9. Conclusions

Subsea power cable is an essential component of contemporary society, bringing ever increasing power demand. Development of power electronics and high temperature superconductivity (HTS) will see a growth in DC transmission understanding that there are certain obstacles which must be considered prior to any large scale implementation. Transients in AC or DC superconducting networks introduce oscillations of much longer settling time due to the lack of damping which will require compensation at the source whereby adding to system complexity. AC networks can absorb transients through the nature of their design with little upgrading while DC networks will require extensive filtering and compensation, all requiring significant technology and investment capital. AC superconductivity still deals with the issue of reactance with the other phases and shielding, where one solution is to provide a superconducting shield which is shorted at each end [18]. In either case, distribution of power using HTS allows for cables to carry near limitless currents which will be subject to only external heating effects. This will allow for all regions of the world to receive electrical power and minimise the requirements for operators to produce energy as losses can be hypothetically reduced to near zero.

Carbon reduction and impact on wildlife and the environment are a very serious concern with politicians, lobbyists, and industry all pressing forward with legislation and policy to reduce reliance on fossil fuels. The proposed European Supergrid has seen huge investment and development which seeks to integrate the current and future energy supply with an all-encompassing power grid, uniting the European member states and further. Targets for 20% renewable energy have been set by the European community with much of this coming from offshore wind. Supergrid seeks to enable this network within Europe which will require a large scale implementation of subsea DC power transmission cables, MTCD and VSC sub stations, and integration into the existing AC grids [19]. It has been discussed in this paper that the underlying principles and technology for subsea power cable is a mature one. This does not equate to a lack development in manufacturing and the component materials which is in fact a going concern. The market leading cable manufacturers are continually pushing boundaries in respect to component lengths and consistency in their cables as well as increasing their carrying capacities, flexibility, and strength in the materials. Such is to be expected as the value of current and future installations often carry budgets of billions. Industry is also developing wet-mate interconnections for subsea equipment and transmission points with the oil and gas industry at the forefront. Advances in robotics through materials innovation and near autonomous software algorithms will make installation of a future subsea grid a much simpler task. Trenching machines, inspection and maintenance robots can ensure accurate placement of cables at ever increasing depths, making the impossible today possible tomorrow. In parallel there is development in heave compensatory vessel cranes, stable thruster technology, increased education standards and specialisation amongst crew members on the cable laying and maintenance fleet.

10. Acknowledgement

The author would like to thank the supervision of Dr. Quan Li for this advanced topic paper.


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Investigation of Subsea Power Transmission Cable Technology Rev D