Modeling Subsea Cables for Offshore Wind Power at Hellenic Cables

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“Legal guidelines, Whitehouse acquired 5 minutes sign. Coil indicators too weak to relay. Strive drive sluggish and common. I’ve put intermediate pulley. Reply by coils.”

Sound acquainted? The message above was despatched by way of the primary transatlantic telegraph cable between Newfoundland and Eire, manner again in 1858. (“Whitehouse” refers back to the chief electrician of the Atlantic Telegraph Firm on the time, Wildman Whitehouse.) Quick ahead to 2014: The underside of the ocean is residence to almost 300 communications cables, connecting international locations and offering web communications world wide. Quick ahead once more: As of 2021, there are an estimated 1.3 million km of submarine cables (Determine 1) in service, starting from a brief 131 km cable between Eire and the U.Ok. to the 20,000 km cable that connects Asia with North America and South America. We all know what the world of submarine cables appears to be like like at this time, however what concerning the future?
Determine 1. Submarine cables preserve the world related.Transferring Wind Energy OffshoreThe offshore wind (OFW) trade is likely one of the most quickly advancing sources of energy world wide. It is smart: Wind is stronger and extra constant over the open ocean than it’s on land. Some wind farms are able to powering 500,000 houses or extra. At present, Europe leads the market, making up nearly 80 p.c of OFW capability. Nevertheless, the worldwide demand for vitality is predicted to extend by 20 p.c in 10 years, with a big majority of that demand equipped by sustainable vitality sources like wind energy.Offshore wind farms (Determine 2) are made up of networks of generators. These networks embrace cables that join wind farms to the shore and provide electrical energy to our energy grid infrastructure (Determine 3). Many OFW farms are made up of grounded buildings, like monopiles and different sorts of bottom-fixed wind generators. The foundations for these buildings are costly to assemble and troublesome to put in in deep sea environments, because the cables must be buried within the seafloor. Set up and upkeep is simpler to perform in shallow waters.Wind generators for offshore wind farms are beginning to be constructed additional out into the ocean. This creates a brand new want for well-designed subsea cables that may attain longer distances, survive in deeper waters, and higher join our world with sustainable energy.The way forward for offshore wind lies in wind farms that float on ballasts and moorings, with the cables laid straight on the seafloor. Floating wind farms are an important resolution when wind farms located simply off the coast develop crowded. They will additionally reap the benefits of the larger and extra highly effective winds that happen additional out to sea. Floating wind farms are anticipated to develop extra widespread over the subsequent decade. That is an particularly enticing possibility for areas just like the Pacific Coast of the USA and the Mediterranean, the place the shores are deeper, versus the shallow waters of the Atlantic Coast of the U.S., U.Ok., and Norway. One essential requirement of floating OFW farms is the set up of dynamic, high-capacity submarine cables which are capable of successfully harness and ship the generated electrical energy to our shores.Determine 2. Offshore wind farms are anticipated to assist meet growing calls for for sustainable vitality.Picture by Ein Dahmer — Personal work. Licensed beneath CC BY-SA 4.0, through Wikimedia CommonsDesign Elements for Resilient Subsea CablesEver skilled slower than common web? Failure of a subsea cable could also be responsible. Cable failures of this type are a typical — and costly — incidence, whether or not from the injury of mechanical stress and pressure brought on by bedrock, fishing trawlers, anchors, and issues with the cable design itself. Because the offshore wind trade continues to develop, our must develop energy cables that may safely and effectively join these farms to our energy grid grows as nicely.Earlier than fixing or putting in a submarine cable, which might value billions of {dollars}, cable designers have to make sure that designs will carry out as supposed in undersea situations. At present, that is sometimes completed with the assistance of computational electromagnetics modeling. To validate cable simulation outcomes, worldwide requirements are used, however these requirements haven’t been capable of sustain with latest developments in computational energy and the simulation software program’s rising capabilities. Hellenic Cables, together with its subsidiary FULGOR, use the finite ingredient technique (FEM) to investigate their cable designs and evaluate them to experimental measurements, typically getting higher outcomes than what the worldwide requirements can provide.Determine 3. Examples of three-core (3C) submarine cables obtainable from Hellenic Cables.Up to date Methodology for Calculating Cable Losses
The Worldwide Electrotechnical Fee (IEC) supplies requirements for electrical cables, together with Commonplace 60287 1-1 for calculating cable losses and present rankings. One drawback with the formulation utilized in Commonplace 60287 is that it overestimates cable losses — particularly the losses within the armor of three-core (3C) submarine cables. Cable designers are compelled to undertake a brand new methodology for performing these analyses, and the workforce at Hellenic Cables acknowledges this. “With a extra correct and life like mannequin, important optimization margins are anticipated,” says Dimitrios Chatzipetros, workforce chief of the Numerical Evaluation group at Hellenic Cables. The brand new methodology will allow engineers to cut back cable cross sections, thereby lowering their prices, which is the paramount aim for cable manufacturing.

An electrical cable is a posh system to mannequin. The geometric construction consists of three fundamental energy cores which are helically twisted with a selected lay size, and a whole lot of further wires — display screen or armor wires — which are twisted with a second or third lay size. This makes it troublesome to generate the mesh and clear up for the electromagnetic fields. “It is a tedious 3D drawback with difficult materials properties, as a result of a number of the components are ferromagnetic,” says Andreas Chrysochos, affiliate principal engineer within the R&D division of Hellenic Cables.

Lately, FEM has made a large leap in terms of cable evaluation. The Hellenic Cables workforce first used FEM to mannequin a full cable part of round 30 to 40 meters in size. This turned out to be an enormous numerical problem that may solely realistically be solved on a supercomputer. By switching to periodic fashions with a periodic size equal to the cable’s cross pitch, the workforce diminished the issue from 40 meters all the way down to 2–4 meters. Then they launched short-twisted periodicity, which reduces the periodic size of the mannequin from meters to centimeters, making it a lot lighter to resolve. “The progress was large,” says Chrysochos. (Determine 4)
Though the enhancements that FEM brings to cable evaluation are nice, Hellenic Cables nonetheless must persuade its shoppers that their validated outcomes are extra life like than these offered by the present IEC commonplace. Purchasers are sometimes already conscious of the truth that IEC 60287 overestimates cable losses, however outcomes visualization and comparability to precise measurements can construct confidence in challenge stakeholders. (Determine 5)Finite Component Modeling of Cable SystemsElectromagnetic interference (EMI) presents a number of challenges in terms of designing cable programs — particularly the capacitive and inductive couplings between cable conductors and sheaths. For one, when calculating present rankings, engineers must account for energy losses within the cable sheaths throughout regular operation. As well as, the overvoltages on cable sheaths must be inside acceptable limits to fulfill typical well being and security requirements.As Chrysochos et al. focus on in “Capacitive and Inductive Coupling in Cable Programs – Comparative Examine between Calculation Strategies” (Ref. 3), there are three fundamental approaches in terms of calculating these capacitive and inductive couplings. The primary is the complicated impedance technique (CIM), which calculates the cable system’s currents and voltages whereas neglecting its capacitive currents. This technique additionally assumes that the earth return path is represented by an equal conductor. One other frequent technique is electromagnetic transients program (EMT) software program, which can be utilized to investigate electromagnetic transients in energy programs utilizing each time- and frequency-domain fashions.The third technique, FEM, is the inspiration of the COMSOL Multiphysics software program. The Hellenic Cables workforce used COMSOL Multiphysics and the add-on AC/DC Module to compute the electrical fields, currents, and potential distribution in conducting media. “The AC/DC Module and solvers behind it are very sturdy and environment friendly for some of these issues,” says Chrysochos.The Hellenic Cables workforce in contrast the three strategies — CIM, EMT software program, and FEM (with COMSOL Multiphysics) — when analyzing an underground cable system with an 87/150 kV nominal voltage and 1000 mm2 cross part (Determine 6). They modeled the magnetic subject and induced present density distributions in and across the cable system’s conductors, accounting for the bonding kind with an exterior electrical circuit. The outcomes between all three strategies present good settlement for the cable system for 3 totally different configurations: strong bonding, single-point bonding, and cross bonding (Determine 7). This demonstrates that FEM could be utilized to all sorts of cable configurations and installations when taking into consideration each capacitive and inductive coupling.The Hellenic Cables workforce additionally used FEM to review thermal results in subsea cables, akin to HVAC submarine cables for offshore wind farms, as described in “Evaluate of the Accuracy of Single Core Equal Thermal Mannequin for Offshore Wind Farm Cables” (Ref. 4). The present IEC Commonplace 60287 1-1 features a thermal mannequin, and the workforce used FEM to determine its weak spots and enhance its accuracy. First, they validated the present IEC mannequin with finite ingredient evaluation. They discovered that the present requirements don’t account for the thermal influence of the cable system’s metallic display screen supplies, which implies that the temperature could be underestimated by as much as 8°C. Deriving analytical, correcting formulation primarily based on a number of FEM fashions, the workforce diminished this discrepancy to 1°C! Their evaluation additionally highlights important discrepancies between the usual and the FEM mannequin, particularly when the corresponding sheath thickness is small, the sheath thermal conductivity is excessive, and the facility core is giant. This subject is especially essential for OFW initiatives, because the cables concerned are anticipated to develop bigger and bigger.Additional Analysis into Cable DesignsIn addition to finding out inductive and capacitive coupling and thermal results, the Hellenic Cables workforce evaluated different points of cable system designs, together with losses, thermal resistance of surrounding soil, and grounding resistance, utilizing FEM and COMSOL Multiphysics. “On the whole, COMSOL Multiphysics is rather more person pleasant and environment friendly, akin to when introducing temperature-dependent losses within the cable, or when presenting semi-infinite soil and infinite ingredient domains. We discovered a number of methods to confirm what we already learn about cables, their thermal efficiency, and loss calculation,” says Chatzipetros.LossesThe conductor measurement of a subsea or terrestrial cable impacts the price of the cable system. That is typically an important facet of an offshore wind farm challenge. To optimize the conductor measurement, designers want to have the ability to precisely decide the cable’s losses. To take action, they first turned to temperature. Currents induced in a cable’s magnetic sheaths yield additional losses, which contribute to the temperature rise of the conductor.When calculating cable losses, the present IEC commonplace doesn’t contemplate proximity results in sheath losses. If cable cores are in shut proximity (say, for a wind farm 3C cable), the accuracy of the loss calculation is diminished. Utilizing FEM, the Hellenic Cables workforce was capable of examine how conductor proximity results affect losses generated in sheaths in submarine cables with lead-sheathed cores and a nonmagnetic armor. They then in contrast the IEC commonplace with the outcomes from the finite ingredient evaluation, which confirmed higher settlement with measured values from an experimental setup (Determine 8). This analysis was mentioned within the paper “Induced Losses in Non-Magnetically Armoured HVAC Windfarm Export Cables” (Ref. 5).Thermal Resistance of SoilDifferent soil varieties have totally different thermal insulating traits, which might severely restrict the quantity of warmth dissipated from the cable, thereby lowering its current-carrying capability. Which means bigger conductor sizes are wanted to transmit the identical quantity of energy in areas with extra thermally antagonistic soil, inflicting the cable’s value to extend.Within the paper “Rigorous calculation of exterior thermal resistance in non-uniform soils” (Ref. 6), the Hellenic Cables workforce used FEM to calculate the efficient soil thermal resistance for various cable varieties and cable set up eventualities (Determine 9). First, they solved for the warmth switch drawback beneath steady-state situations with arbitrary temperatures on the cable and soil surfaces. They then evaluated the efficient thermal resistance primarily based on the warmth dissipated by the cable floor into the encompassing soil.Simulations had been carried out for 2 sorts of cables: a typical SL-type submarine cable with 87/150 kV, a 1000 mm2 cross part, and copper conductors, in addition to a typical terrestrial cable with 87/150 kV, a 1200 mm2 cross part, and aluminum conductors. The workforce analyzed three totally different cable set up eventualities (Determine 10).The primary situation is when a cable is put in beneath a horizontal layer, akin to when sand waves are anticipated to regularly add to the seafloor’s preliminary degree after set up. The second is when a cable is put in inside a horizontal layer, which happens when the set up takes place in a area with horizontal directional drilling (HDD). The third situation is when a cable is put in inside a backfilled trench, typical for areas with unfavorable thermal habits, with a view to cut back the influence of the soil on the temperature rise of the cable. The numerical modeling outcomes show that FEM could be utilized to any materials or form of multilayer or backfilled soil, and that the tactic is appropriate with the present score methodology in IEC Commonplace 60287.Grounding ResistanceThe analysis of grounding resistance is essential to make sure the integrity and safe operation of cable sheath voltage limiters (SVLs) when topic to earth potential rise (EPR). With a purpose to calculate grounding resistance, engineers must know the soil resistivity for the issue at hand and have a strong calculation technique, like FEM.The Hellenic Cables workforce used FEM to investigate soil resistivity for 2 websites: one in northern Germany and one in southern Greece. As described within the paper “Analysis of Grounding Resistance and Its Impact on Underground Cable Programs” (Ref. 7), they discovered that the obvious resistivity of the soil is a monotonic operate of distance, and {that a} two-layer soil mannequin is adequate for his or her modeling drawback (Determine 11). After discovering the resistivity, the workforce calculated the grounding resistance for a single-rod situation (as a method of validation). After that, they proceeded with a posh grid, which is typical of cable joint pits present in OWFs. For each eventualities, they discovered the EPR on the substations and transition joint pit, in addition to the utmost voltage between the cable sheath and native earth (Determine 12). The outcomes reveal that FEM is a extremely correct calculation technique for grounding resistance, as they present good settlement with each numerical information from measurements and electromagnetic transient software program calculations (Determine 13).A Vivid and Windy Future
The Hellenic Cables workforce plans to proceed the essential work of additional bettering all the cable fashions they’ve developed. The workforce has additionally carried out analysis into HVDC cables, which contain XLPE insulation and voltage supply converter (VSC) expertise. HVDC cables could be extra value environment friendly for programs put in over lengthy distances.

Just like the wind used to energy offshore wind farms, electrical cable programs are throughout us. Though we can’t all the time see them, they’re working arduous to make sure we have now entry to a high-powered and well-connected world. Optimizing the designs of subsea and terrestrial cables is a vital a part of constructing a sustainable future.
ReferencesM. Hatlo, E. Olsen, R. Stølan, J. Karlstrand, “Correct analytic components for calculation of losses in three-core submarine cables,” Jicable, 2015.S. Sturm, A. Küchler, J. Paulus, R. Stølan, F. Berger, “3D-FEM modelling of losses in armoured submarine energy cables and comparability with measurements,” CIGRE Session 48, 2020.A.I. Chrysochos et al., “Capacitive and Inductive Coupling in Cable Programs – Comparative Examine between Calculation Strategies”, tenth Worldwide Convention on Insulated Energy Cables, Jicable, 2019.D. Chatzipetros and J.A. Pilgrim, “Evaluate of the Accuracy of Single Core Equal Thermal Mannequin for Offshore Wind Farm Cables”, IEEE Transactions on Energy Supply, Vol. 33, No. 4, pp. 1913–1921, 2018.D. Chatzipetros and J.A. Pilgrim, “Induced Losses in Non-Magnetically Armoured HVAC Windfarm Export Cables”, IEEE Worldwide Convention on Excessive Voltage Engineering and Software (ICHVE), 2018.A.I. Chrysochos et al., “Rigorous calculation of exterior thermal resistance in non-uniform soils”, Cigré Session 48, 2020.A.I. Chrysochos et al., “Analysis of Grounding Resistance and Its Impact on Underground Cable Programs”, Mediterranean Convention on Energy Technology, Transmission , Distribution and Vitality Conversion, 2020.