Deployment of submarine power cables

With the increased interconnection between lands and integration with power generation from offshore renewable energy, more and more submarine power cables are deployed and in operation.

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Jul 20, 2017
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Author: Dr Chuan Zhang

Abstract

With the increased interconnection between lands and integration with power generation from offshore renewable energy, more and more submarine power cables are deployed and in operation. This article discusses the deployment aspects of submarine power cables and investigates the physical considerations, installation and operational issues of high-voltage submarine power cables. Typical problems and lessons learned from actual projects are highlighted. This article builds the discussions on a sister article by the author on ‘Power Cables and Submarine Power Cables’ focusing on the design and technical aspects surrounding submarine power cables. 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 (Great Britain) and above, for both AC and DC applications. However, the content of this article should largely be valid for lower voltage power cables as well.

Specifications of submarine power cables

Submarine power cable layout

AC (alternating current) submarine power cables are normally in one cable with three cores for cables up to 220 kV. Optical fibre cables are integrated into the cable bundle to transmit measurement and communication data. For comparison, the three phases of a land AC power cable are normally in three separately single-core cables. This is primarily for coping with increased heat dissipation requirement and road transport logistics. For in-land transmission, optical fibre cables are normally outside of the three single-core power cables but located close-by.

In the case of DC (direct current) application, both the submarine and in-land portions of the transmission cables use single core cables. For both in-land and offshore, a DC circuit will usually need two cables to form a loop or ‘circuit’, one with a high positive voltage, and the other with a high negative voltage relative to the earth, however, many other DC transmission configurations [1] are possible, such as using a separately insulated metallic return conductor or an earth return. The two DC cables and associated optical fibres may be bundled together and buried in the same trench until they approach the landfall location. DC cables can also be manufactured in two-core cables or even coaxial cables containing two integrated and concentric conductors.

For both AC and DC, the land portion of the submarine transmission cables will likely be light-armoured (or unarmoured if installed in ducts), aluminium or copper conductor cables, suitable for underground installation but the submarine portions will need to be armoured with extra protection [2].

It should be noted that when the transmission capacity is high (say over 300 MW) the overall diameter of a three core AC cable may become physically too large. Under those circumstances, three single-core cables will have to be used for AC submarine application.

Typical parameters and dimensions of submarine power cables

An illustrative set of cable conductor sizes and overall dimensions of typical XLPE AC submarine power cables are shown in Table along with their land counterparts [3].

CABLE VOLTAGE, KV132132132400400
submarine or land applicationsubmarinesubmarinelandsubmarineland
number of cores per cable31111
cross sectional area per core mm 2 (copper)100010001200300400
overall external diameter (mm) per cable20612089131109
ratio of overall diameter to core diameter5.83.42.36.74.8
cable weight (kg/m) per circuit8536 × 316 × 333 × 314 × 3
cable power capacity (MW) per circuit200300300500500

Table 1: llustrative parameters and dimensions of AC submarine power cables

It is interesting to note the following points:

  • Submarine power cables (single core) are almost three times as heavy as land power cables. Most of the extra weight of the submarine power cables is for the armour and water blocking.
  • For the same current capacity (ampacity), the conductors of submarine power cables can be smaller than land power cables as submarine cables have better heat dissipation conditions owing to the installed environment.
  • The overall diameter of an AC 3-core XLPE submarine power cable is about 5–10 times of the core conductor diameter.

For comparison, a typical ±320 kV 500 MW DC circuit has an overall external cable diameter of 99 mm (for a core section area of 500 mm 2) per cable at a weight 2 × 22 kg/m per circuit [3].

Cable ampacity and rating are normally provided by the manufacturers. When used in actual projects, some de-rating and capacity reduction by as much as 15% of the nominal rating would be required to take into account sharp bends and thermal dissipation conditions, such as when arriving at the shoreline (sea defence), or at the J-tube bends leading to offshore platforms [4]. This is one of the reasons that these two sections often use cables with a larger cross-section area.

Tailor-made cables against standardised cables

When choosing cable parameters, project developers may prefer to specify a bespoke cross-section area and even voltage level based on their project need, such as specific resistance or losses, rather than use standardised submarine power cables already available on the market. Cable manufacturers would accept orders of both standardised and bespoke specifications, but there will be different implications on capital and operating expenditures. In other words, whether to go for a bespoke or standardised specification is a balancing call between tangible benefits and downsides.

In terms of cable standardisation itself, potential aspects to be considered include some of the following:

  • Cable voltage, ampacity or power ratings
  • Cable installation process
  • Joints and accessories
  • Conductor cross-section area or resistance or dielectric materials and purity or conductor production method. During manufacturing, conductors are subject to different degrees of cold working. Cold worked and compressed copper and aluminium have a higher specific resistivity than annealed metal. The presence of water-blocking compounds in the conductor also increases the resistance as contact points between individual wires vanish. Furthermore, lay length of the strands, the number of strands and the compression during lay-up all have an impact on the cross-section area and resistance [5].

Electromagnetic and Heat Impact of Submarine Power Cables

The next step of the project development is to conduct a comprehensive environmental impact assessment (EIA) of cable deployment. This section discusses two technical aspects of the EIA.

Electromagnetic field (EMF) generation from power cables

As known, a flow of electric current generates a magnetic field and AC generates a time varying magnetic field. On the other hand, a constant magnet field cannot induce any electric current but a time varying magnetic field will induce an electric field.

In the case of power cables, the electric field can be effectively confined (or shielded) within the cable by earthing of the insulation screen and metallic sheath through solid bonding at the termination points and selected joints. Therefore, power cables will not directly generate electric fields outside of the cable, except for the case of DC cable deployment involving sea electrodes (which is not considered in this article). However, the magnetic field cannot be effectively shielded using earthing or other means. As a result, there exists a magnetic field outside the cable and radiating into the surrounding medium, such as ground, seabed or sea water. Furthermore, while the electrical field directly generated by cable current can be shielded through earthing, there is a secondary and induced electric field outside the cable induced by different sources. The magnetic fields and induced electric fields are further discussed below.

In the case of AC operation, the magnetic fields of the three conductors tend to be offset to a large extent, because the conductor cores of the three phases are built in a three core structure or laid together in trefoil or at least in close proximity and the currents in the three phases are normally balanced, in other words, the magnetic field is largely cancelled out. Therefore, the magnetic field intensity in the proximity of an AC cable is expected to be very low. The nature of being alternating also means that the magnetic field is time-varying and as a result an electric field will be induced outside of the cable at reducing intensity as the distance from the cable increases.

In the case of DC operation, the two single-core DC cables should, where practicable and in particular in shallow water, be laid in close proximity (ideally bundled) to minimise the net effects of magnetic fields during operation. In one extreme, when an integrated cable comprising of two coaxial or concentric conductors is used, the magnetic field intensity outside the cable will be zero. Magnetic fields from DC cables can also be affected by the orientation of the cable system with respect to the north–south geomagnetic fields of the Earth.

DC cables will not directly induce any electric field outside of the cables because its magnetic field is constant. However, the presence of a DC cable causes disturbances to the net strength of the Earth's geomagnetic field. As a result, a background electric field that is naturally induced through the movement of charges in seawater with respect to the Earth's static magnetic field is disturbed. For this reason, DC cables have an impact on induced electric fields [6].

Table summarises the discussions above on EMF generation from power cables and gives a high level quantitative indication.

EMF generation from power cables


FIELDS OUTSIDE OF POWER CABLESAC POWER CABLESDC POWER CABLES
magnetic field directly generated by power cablesin existencein existence
Likely 20 μT in maximum and decreasing with the distance away from the cable.Likely 10–300 μT in maximum, chiefly depending on the proximity of the return cable.
electric field directly generated by power cablesnilnil, except for the case using sea electrodes
induced electric fieldin existence, induced by time varying magnetic field.causing changes to the background naturally induced electric field.
Likely 1 mV/m in maximum and decreasing with the distance away from the cable.The variation could be 0.2 mV/m and decreasing with the distance away from the cable.

Table 2: EMF generation from power cables

It may be noted that the ‘reference levels’ for human exposure to the EMFs given in the internationally accepted guidelines as published by the International Commission on Non-Ionizing Radiation Protection are:

  • 100 μT for magnetic fields,
  • 5 kV/m (kV per metre) for electric fields.

For comparison, the strength of the static geomagnetic field generated by the Earth is ∼50 μT. It should be noted that the magnetic fields generated by AC power cables are 50 Hz varying fields that marine organisms will perceive differently to the geomagnetic field which should be similar to the magnetic field generated by DC cables.

In terms of any likely consequence of EMFs, the orientation and direction finding ability of marine organisms may be affected by unnatural and persistent confounding magnetic fields if these marine organisms are magnetically sensitive. Separately, the magnetic field from a DC cable should be mitigated to avoid potentially causing the navigational compass to give incorrect direction. Furthermore, the installation and burial of power cables offshore may have potential to impact upon the natural fish resource and marine life in a number of ways, including habitat disturbance, noise and vibration, smothering and contamination; and EMF generation.

It is important to note that to date, in the light of current information, there has been no evidence to indicate any tangible impacts on fisheries resources or marine organisms due to EMF from submarine power cables. However, there is still a need to further ascertain and mitigate EMF generation associated with power cables and power transmission in general to avoid any hazard of shock and other health concerns and reduce unnecessary safety margin intentionally created to cover unknown effects. An actual offshore wind export cable project in the UK was asked to be buried to a depth of 3 m mainly to mitigate any impact of EMFs on migratory fish (as well as to deal with concerns with mobile sands).

Heat generation from power cables

The electric current flowing in power cables will generate heat and this heat normally dissipates into the surrounding medium immediately outside the cables, including ground, seabed, air or sea water.

During normal operation, the cable temperature will rise dependent on the current (load) carried and the durations of each loading level. The conductor of the XLPE cable may reach up to 90°C, but at the cable outer sheath and outer serving, the temperature may not be above 25°C depending on heat dissipation efficiency.

There are two separate issues to be considered when ‘temperature and cable’ is discussed. One is about the impact of cable operating temperature on cable ampacity (current) rating and cable life; and the other is about the thermal impact of power cables on the surrounding medium. The latter is briefly discussed below and the former will be discussed as part of cable operation in intensive monitoring of operating temperatures.

It is possible to work out, using a finite element analysis, for example, the thermal effects of cables on the surface temperature of the ground or seabed or sea water.

An actual study has been reported which was for the Cross Sound Cable Interconnector project, a high-voltage DC buried cable system between New England and Long Island New York of the US. This study found that a rise in temperature at the seabed immediately above the buried cable is to be 0.19°C and an associated increase in seawater temperature of 0.000006°C [7]. The potential rise in temperature is therefore considered to be negligible.

Separate reports indicated that under long-term full load conditions soil directly over a land cable trench could be heated up by as much as 2°C [8], but in partial load operations this value is lower. When the level of soil temperature rise becomes unacceptable, the thermal impact can be mitigated with the use of a cable with a larger conductor size.

It should also be noted, the ambient soil temperature – a critical variable for the capacity to dissipate heat from the cable – changes from winter to summer, which may mean cable ampacity limit is also seasonal. For information, in central Europe, the ambient soil temperature varies between 5 and 23°C approximately.

Routeing, corridors and installation of submarine power cables

Routeing and corridors of submarine power cables

A careful and rigorous geophysical and geotechnical survey is required to define a likely submarine cable route. When necessary, cable routes will have to be altered after micro-siting. Once the routeing has been defined successfully, a corresponding cable corridor which may typically be 500 m wide of the centreline of the planned cable route will be specified within which the submarine power cables are to be installed.

In the UK, the owner of the territorial seabed (The Crown Estate) normally specifies a cable corridor to have a width of N × 30 m + 2 × 235 m, where N is the number of power cables of the same project. The formula basically allows a 250 m space from each side of the cable bundle.

The reason to allow space at each side of the cable or cable bundle is to provide a degree of installation flexibility to: (a) avoid any localised obstructions, such as wrecks or unexploded ordnance which are only discovered in the final pre-installation surveys; (b) avoid localised areas of difficult ground conditions, such as small rock outcrops, boulder formations or glacial till outcrops; and follow the troughs of sandwaves in areas where mobile seabeds are known to exist and (c) avoid sensitive locations, such as areas with dense aggregations of a particular marine life.

Another important reason to reserve space alongside a submarine power cable is to facilitate repair during the operating phase when necessary. This is because in nearly all the cases cable repairs will need to involve cutting away the faulty section and replacing it with a section of new cable long enough to allow jointing at a vessel deck or at least at a level out of water. As a result an omega (Ω)-shaped loop or repair bight will be formed as a result of the repair. This is illustrated in Fig.  .

Fig 1: Illustration of the omega bight for cable repair [9] (note: the additional space on the right hand side of the bight is for future repair needs)

he size of the repair bight depends on water depth, repair vessel deck height, deck length required and crown radius of the bight. For a 35 m water depth, the repair bight height may be above 90 m. In this case, the cable length to form the omega-shape can reach around 250 m. In fact an actual project had a length of 400 m in the omega loop. This also means that a spare cable up to the omega length or longer is required to be kept on stock.

Installation of submarine power cables

Once the routing and corridor have been finalised and the submarine power cables have been delivered to site, the installation of the cable can be carried out step-by-step starting from the final pre-installation surveys, to route clearance, cable lay, then cable burial and cable protection, and finally post-installation survey.

Cable laying operation can be started from either ends of the cable route. For a project containing two parallel cables, it may well have one cable installed from one end and the other cable installed from the other end at the same time. It should be highlighted that during cable installation, careful attention must be drawn to avoid exceeding minimum cable bending radii and avoid mechanically overstressing the cable. It is also worth noting that typical cable length that can be installed offshore can be up to around 80 km subject to weather window and other restrictions [2].

A diverse range of cable burial techniques is available, including cable burial ploughs, tracked cable burial machines, free swimming cable burial machines, cable jetting machines, mass flow excavation and burial sleds and so on [7]. No matter which burial techniques are used, it is important to specify an adequate burial depth. This can be achieved by carrying out a quantitative risk assessment, taking into account cable routing, seabed conditions and probability of anthropogenic risks, threats and hazards (such as anchor striking and fishing).

Submarine power cables are protected inherently through burial. When cable burial is not achievable, for example, owing to hard clay, or for operational reasons cable burial is not the preferred method, other cable protection measures should be considered, especially in the areas of shipping lanes and where trawling takes place. Common cable protection measures beyond cable burial include concrete mattresses, rock placement, grout bags or sand bags, frond mattresses, articulated metal shell connectors and so on [7]. Some modelling and testing may be carried out to assist in the specifications of potential protection choices [10].

Each of these cable protection methods has its relative merits and detractions. For example, fishing industry may consider concrete mattresses to be potentially less damaging to their fishing gear than rock placement.

Submarine power cable crossing with another installation

Where a submarine power cable needs to cross an existing cable or pipeline, a crossing arrangement should be implemented to ensure adequate separation between the new cable and the existing installations and put in place a robust protection for both of them including legal limits of liabilities and procedures where work is required on either cable or pipeline.

A common approach for crossing a naked unburied existing cable or pipeline is to place concrete mattresses over the existing unburied cable or pipeline, cumulatively 10–20 m long, and then lay the new cable perpendicularly on top of these mattresses. Finally, adding a further layer of mattresses or rock berm over the new cable forms an overall protection for all of the installations at the crossing.

To cross a buried existing power cable, a similar method can still be used, that is, firstly concrete mattresses or slabs or a filter layer are laid on the seabed under which the existing cable is buried, then the new cable is laid perpendicularly on top of these materials with a further layer of mattresses or rock berm added over the new cable.

Landfall, transition joint bays and cable joints

Landfall of submarine power cables

Submarine power cables will come ashore at landfalls where cable landing sites and transition joint bays are normally constructed to enable submarine power cables to be connected to onshore underground cables or overhead lines.

Selection of landing sites needs to take a number of factors into consideration and avoid areas unsuitable for power cable landing due to physical constraints, such as the geology of the landfall point, the width of the beach, the height of the cliffs under which the landfall cable has to pass and other local environmental considerations. Each landing site may face different challenges and require bespoke solutions.

When the topography of a landfall site makes open trenching difficult, or where environmental concerns prevent any open trenching, a horizontal directional drilling (HDD) will often be used to enable the cable installation to bypass the critical areas and only cause localised disruption at the drill site which can be reinstated when the drilling equipment leaves the site. The HDD is a steerable trenchless method of installing underground pipes, ducts and cables in a shallow arc along a prescribed bore path by using a surface launched drilling rig. For power cable installations using HDD, it is normal to install multiple pipes or ducts of the cable diameter and then pull power cables through the pipes or ducts.

Heat dissipation from the power cables may be an issue on dry soils at landfalls and in ducts. For this reason, a larger cable size is often used for the landfall section.

Transition joint bays and cable joints

Once submarine power cables have passed the landfall, they will normally be connected to onshore cables in specifically designed underground joint bays or pits, referred to as transition joint bays. The transition joint bays are located on dry land and buried below ground.

One of the purposes of the transition joint bays for AC power transmission is to facilitate the transfer from three core submarine cables (if they are in three cores) to single-core land cables, and allow a potential change in conductor type from copper offshore (used to minimise cable size) to aluminium onshore to reduce cable costs. In the case of DC cables, single core cables are normally used both onshore and offshore so the transition joint bay for DC is not too different from a normal onshore joint pit for DC cable joints although there is normally a change of cables from a submarine type to a land type.

In terms of layout, transition joint bays consist of a shallow chamber in which the submarine cables can be connected to land cables. Typical dimension of transition joint bays are 1.5 m deep, 3–4 m wide and 10–14 m long. The construction of transition joint bays is similar for both AC and DC application with a reinforced concrete slab base of about 300 mm thick. After installation and jointing of the cables at the transition joint bay, the transition joint bay is typically backfilled with sand and protected by a suitable cover which may be a concrete tile or a reinforced concrete slab of 250 mm thick. The whole structure may be up to 1 m below the ground level with the base being about 2–2.5 m below the ground level.

Future access to the transition joint bays may be needed for repairs, but it is not envisaged that regular maintenance will be required and hence the transition joint bays will not require any inspection chambers or access covers. Therefore, on completion of the jointing, the ground surface of the joint bays should be fully reinstated using the excavated soil. There should not be any above ground structures, but the ground may be used for other purposes, for example, as part of a car park and so on.

It should be noted that along with each transition joint bay, two auxiliary pits (each about 1.5 m long, 1.5 m wide and 2 m deep) are also required to be constructed. One is to house a metal link box for terminating the power cable screens and earthing metallic sheaths, including sheath voltage limiters, in order to minimise sheath currents and protect the cable sheaths against overvoltage. When jointing AC power cables further along the onshore route, in addition to earthing the metallic sheaths, cross-bonding or phase sheath transposition are also required to use similar link boxes en-route to minimise any circulating currents in the cable sheaths and also reduce the intensity of any residual magnetic fields. Cross bonding is not required when jointing DC cables. Incidentally, there is likely a cable joint en-route in land around every 1 km or shorter (as in-land cable section length is limited by the amount of cable that a cable drum and transport on the roads can manage), but it is not necessary to construct this auxiliary pit for every single cable joint.

The other auxiliary pit is to accommodate optical fibre cable joints and onsite data access. Access to these auxiliary pits must be maintained. Normally, these pits should be positioned no more than 10–15 m from their corresponding transition jointing bays. For protection purposes, there should be a concrete or metal manhole cover at the ground level. It is possible to combine these two auxiliary pits into one if necessary.

In the case involving multiple circuits, each circuit is likely to require a separate transition joint bay and each transition joint bay will have its own auxiliary pits. All of the transition joint bays are normally located in parallel with a spacing of about 5 m.

It should be noted that, should there be any need, it is feasible for the submarine power cables to be connected directly to an overhead power line, rather than to buried land power cables. In this case, the submarine power cables would be led up out of the ground onto an overhead line tower (pylon), terminated using cable sealing ends, and connected to the overhead line conductors.

Testing and operational monitoring of submarine power cables

Submarine power cable testing

Following the completion of manufacturing a cable, but before cable leaving the manufacturing plant, all cable system components should undergo a thorough verification procedure and tests. These final tests, usually referred to as Factory Acceptance Test (FAT), are to examine compliance with homogenous quality according to international standards and confirm the quality of the cable and the accessories. FAT should include dimension checks, verification of voltage and capacitance of the cable, outer sheath test and partial discharge (PD) test. If the cable involves any design changes or new materials, type tests should also be carried out to establish performance characteristics by testing the cable to destruction in specialist test stations to ensure compliance with all electrical specifications, such as short circuit and lightning impulse.

After transportation and installation, but before energisation, the power cable system must go through a series of site acceptance tests (SATs) which normally include thermal and electrical stress level test, AC insulation withstand test (at both power frequency and lower frequency), DC test on the outer sheath and PD test to detect any defects that might occur during storage, transportation or installation. These SATs are also a test of the completed cable system in order to ascertain the integrity and proper installation of the cable system after the cable pulling, the joints and terminations mounting have been completed. The SAT tests are typically carried out using a mobile test set and take one week. Cable earthing should be checked before energisation.

Once energised and in particular at a high load condition, the power cables should be checked for any signs of overloading, overvoltage and over temperature. Furthermore, for the submarine power cable systems associated with offshore wind power generation, cables should also undergo a series of cyclic load tests to ensure the integrity of the installed cable systems after energisation, given the cyclic nature of wind generation.

Monitoring and inspection campaign during operation

During operation, it is imperative to have a continuous campaign of monitoring and inspection. Typical monitoring should include online tracking of cable temperature using distributed temperature sensing (DTS) to ensure no overheating of the cable system. Typical inspection activities normally include periodic bathymetric and depth of burial surveys (seabed surveys) along the subsea cable route and for the land section periodic ‘walking’ of the route to inspect for any third party encroachment. Corrosion protection test at 5 kV DC voltage for the outer sheath may be carried out as necessary.

To further enhance the detection of any damage or weakening of the cable insulation, there have been new developments in cable monitoring over the last decade. These include ‘fingerprinting’ techniques (using time domain reflectometry or line resonance analysis), online PD surveillance or more recent acoustic fault location techniques.

PD related tests and monitoring may play an increasingly important role in future monitoring and inspection campaign. In electrical engineering, PD is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high-voltage stress which does not bridge the space between two conductors. Protracted PD can erode the solid insulation and eventually lead to breakdown of insulation. PD monitoring of short duration impulses in micro or even nanosecond that propagate in both directions away from PD location can help to detect early symptom of any insulation deterioration and any incipient defects in the insulation. Some installation errors and damages can also be detected by analysing PD results. PD can be detected and monitored without the need to de-energise the cable system and can be located if the signals are monitored at both cable ends. However, it should be noted that in practice the use of PD for submarine cables remains relatively rare at this stage and low levels of PD in a subsea would not be acted upon due to the cost implications of inspecting and insurance not paying out if no genuine fault is found. PD is great at risk mitigation and for onshore equipment where the costs of inspection to find source of PD is relatively inexpensive.

Intensive monitoring of operating temperatures

Cable operating temperatures are the most important operating parameter deserving close and constant monitoring. This is for two main reasons: one is to enable full exploitation of the cable power transmission potential which mainly depends on the cable operating temperature and the other is to ensure this is done without the sacrifice of cable usable life. This is because cables ‘age’ predominantly through deterioration of their insulation material under the influence of electrical stress, mechanical stress, temperature and chemical aggression (including humidity and ultra-violet irradiation). Well protected submarine power cables can achieve a service life of 30–40 years. A rule of thumb is that the useable life of most insulation materials (either paper or polymeric materials) could be reduced by half by either increasing the operating temperature above its designed operating limit by up to 10°C or increasing the operating voltage above its designed operating limit by up to 10%. It is important to observe these limits.

To effectively monitor and make use of operating temperature information, it is critical to establish a starting point by verifying that a cable system can safely transmit the required power for a sustained period (say, 6–8 h) without exceeding the maximum permitted conductor temperature (e.g. 90°C for XLPE insulated cables). The measured maximum operating temperature of a power cable should be checked against the theoretical maximum permitted temperature, which can be derived from the maximum design temperature and then corrected using correction factors related to seabed/ground temperature, sediment/soil property, proximity to other cables and burial depth [11].

It should be made clear that there are many different temperatures within a power cable, such as temperature of the optical fibre, or temperature of the core, or temperature of the insulation. Caution should be exercised to avoid confusion among different temperatures. If an operator indicates that their XLPE transmission cables were running at 20°C at full power, it is important to understand if the reading of 20°C is corrected from the temperature of the optical fibre for the conductor of the cable or the temperature of the optical fibre. Attention should also be drawn to avoid confusion whether a temperature reading is obtained through direct measurement or indirect calculation.

Correlation between the measured temperature of the optical fibres and the actual temperature of the conductors is a delicate and complex matter. A sophisticated calculation involving thermal modelling and multiple variables/factors (such as power loading levels and ramping rates, time delay factors and insulation materials/conditions) would be required to derive the temperature of the conductors from readings from the optical fibres. The higher the load gets, the higher the difference between the two temperatures will be. The time delay aspect is because heat needs to propagate through the insulation and as a result the fibre sees a delay.

Fig.  illustrates an example of various temperatures during load cycles at different positions of the sensor for a single-core land cable [8]. The optical fibre marked as position 2 in green in the figure is located in the insulation screen area of the cable.

Fig 2: Illustration of temperature readings for a power cable [8]

In terms of physical implementation of temperature measurement, optical fibres normally obtain temperature data by using the DTS technique based on the Raman effect [12]. This is because an optical laser pulse sent through the fibres results in some scattered light reflecting back (referred to as ‘Raman scattering’) to the transmitting end and the intensity of the Raman scattering is a measure of the temperature along the fibres. The location of the temperature reading is determined by measuring the arrival timing of the returning light pulse similar to a radar echo. DTS can function continuously regardless whether the power cables are energised or out of service.

The sensing fibres are either embedded in three-core power cables or deployed along the outside of a single-core cable. Naturally the data from embedded fibres is more useful for calculating the conductor temperatures (with an accuracy of around 5°C or 5 K) than measurements taken using the fibres outside power cables. In fact the data from the fibres in a land duct alongside land power cables may be of very limited use owing to poorer correlation accuracy.

The other limitation of DTS based temperature monitoring is that it currently may only be able to cover a length of 30 km from the signal transmitting end, or a power cable of 60 km if the sensing signals are transmitted from both ends [12].

In other words, although DTS has the potential to offer effective temperature measurement and detect hot spots and temperature anomalies, further developments and innovation are required to resolve the two limitations discussed above in order to obtain a full and accurate temperature of conductors. Accurate information on conductor temperatures becomes more essential when a power cable is operated to its maximum temperature rather than its nominal ampacity limit to enable better utilisation of the cable potential without causing overheating. This would be the prerequisite for any attempt to operate power cables to their power ratings in a real-time manner (so called dynamic power ratings).

It should also be noted that DTS information has the potential to be used for wider purposes, for example, using DTS information from strain sensing optical fibres to detect cable installation issues or fault locations or using DTS information about changes in temperature to detect cables becoming unburied or further burial.

Power Cable Failures and Prevention

Within the offshore transmission and offshore power generation sector, the most serious failures in terms of impact on power transmission and generation are related to power cable faults. An insurance company indicated that in the European offshore wind sector alone, during the 7 year period from 2007 to 2014, there have been over 60 submarine power cable incidents ranging from £0.5 million to £13.5 million in claims and in total costed insurers £75 million and project owners £20 million [13], although it must be stated that the vast majority of insurance claims occur during installation or are caused by latent defects from installation. One recent case is a submarine power cable in the UK that failed five times over a period of 3 years, probably owing to the cable being over stressed during installation.

Faults and problems with submarine power cables include: defects in cable design and manufacturing, poor transportation and storage, difficulties with burying the cable to specified depth, installation errors, damages from jack-up vessels and anchors, poor landfall design, and poor termination workmanship at the offshore substation and so on. In broad terms, cable related problems can be divided into two categories: one is of a technical nature and the other is due to poor project management. Table  attempts to list some of the cable faults/failures, their causes and prevention of a technical nature, although industry practice seems to indicate that project management and quality control aspects have a greater share of responsibility for cable faults and failures.

Table 3: Selected cable faults/failures, their causes and prevention

It should be noted that once a cable fault has occurred offshore, the time taken to get the cable repaired may be much longer than the actual duration taken to do the repair. Three months or longer of downtime is not unusual for a submarine power cable incident. To replace an in-line submarine joint alone may take a month. For comparison, a cable fault onshore can be attended and repaired much quicker. It is reported [8,14] that more than one-third of the cable faults in land were repaired and re-energised again within one week and more than 75% within one month. This includes fault location, repair and testing.

The reason why offshore repairs take much longer is because a number of other factors will substantially increase the overall downtime. These factors include the following, some of which can be progressed simultaneously:

  • Obtaining marine and/or port authority licence(s).
  • Safe access to site and availability of weather windows.
  • Fault investigation and fault location.
  • Clarification time required for transmission system operators and, if required, independent experts from the insurers or other stakeholders to undertake a thorough investigation to assess the cause of a failure. In exceptional circumstances this alone can last several months.
  • Decisions on counter measures to prevent future failures.
  • Availability of vessels to do the repair.
  • Availability of parts and spare cables.
  • Availability of specialist cable jointers.
  • Engineering, preparation, site mobilisation, fault location, de-burial, laying spare cable or replacing joints and cable protection.
  • Delay between repair completion and re-commissioning awaiting operational clearance.

For fault location, a two-step approach is usually used with the first step being to find the area of a cable fault within several hundred metres by either, a bridge circuit, time domain reflectometry, acoustic analysis or line resonance analysis. The precise location of the fault is then found using equipment measuring the magnetic flux close to the cable in the vicinity of the pre-located fault when current pulses are injected at the end of the cable. In some cases a high frequency current may be injected from one end of the cable.

Conclusion

This article has discussed submarine power cable deployment and covered the following aspects in particular:

  • high level specifications and configuration of submarine power cables;
  • electromagnetic and heat impact of submarine power cables as part of EIA;
  • routeing, corridors and installation of submarine power cables;
  • landfall, transition joint bays and cable joints;
  • testing and operational monitoring of submarine power cables; and
  • power cable failures and prevention.

Typical problems and lessons learned seen in actual projects have been highlighted.

Acknowledgement

The author wishes to thank Gary Thornton, an IET Fellow active in the offshore transmission sector, and other reviewers for their valuable comments and suggestions during the IET Peer Review Process which have greatly enhanced the quality of the article.

References

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Go to the profile of Chuan Zhang

Chuan Zhang

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

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