Understanding Submarine Power Cables for Offshore Wind Farm Power Transmission: Design, Conductors, and Grid Impact

Offshore wind farms are playing a critical role in the global transition to renewable energy, with the power generated needing to be transmitted efficiently to onshore grids.

Submarine power cables are the arteries that transport this energy across often vast distances under challenging marine conditions.

Today I shall discuss the design considerations of submarine power cables, particularly focusing on conductor types, and explores how these affect HVDC (High Voltage Direct Current) power transmission. Additionally, the impact on power quality, grid voltage, frequency, and impedance will be discussed, along with the importance of effective planning under the NEC4 Contract. The latest innovations in submarine cable technology will also be highlighted.

A. Design of Submarine Power Cables

Submarine power cables are essential components in transmitting electricity generated from offshore wind farms to onshore grids. These cables are designed to withstand the harsh marine environment, including high pressure, temperature variations, and potential mechanical damage from fishing activities or anchors.

The design of these cables is complex and involves several layers, including conductors, insulation, armoring, and outer sheaths.

The conductor is the core component responsible for carrying the electrical current, and its design significantly impacts the overall performance of the cable, especially in HVDC applications.

B. Conductor Types in Submarine Power Cables

The conductor’s design and material are crucial in determining the efficiency, reliability, and longevity of submarine power cables.

Different types of conductors are used based on the specific requirements of the transmission system.

B.1 Solid Conductors

Solid conductors are made from a single piece of metal, typically copper or aluminum. These conductors have a simple construction and are easier to manufacture, but they are generally inflexible, making them less suitable for applications where flexibility is required. In submarine cables, solid conductors are often used in situations where mechanical robustness is more critical than flexibility.

Impact on HVDC Transmission: Solid conductors provide low electrical resistance, which is beneficial for HVDC transmission as it reduces energy losses. However, their lack of flexibility can pose challenges during installation in uneven seabed terrains.

B.2 Conductors Stranded from Round Wires

Stranded conductors are made by twisting together several small round wires. This construction increases the flexibility of the conductor, making it easier to install, especially in situations where the cable needs to bend or follow irregular paths. Stranded conductors are commonly used in submarine cables because they offer a good balance between flexibility and electrical performance.

Impact on HVDC Transmission: The increased flexibility of stranded conductors can ease the installation process of HVDC submarine cables. However, the increased surface area between the strands can lead to slightly higher resistance and, consequently, higher energy losses compared to solid conductors.

B.3 Profiled Wire Conductors

Profiled wire conductors are an advanced form of stranded conductors, where the wires are not round but have a specific profile that allows them to fit together more closely. This design minimizes the gaps between the wires, reducing the overall resistance of the conductor and improving its electrical performance.

Impact on HVDC Transmission: The reduced resistance in profiled wire conductors leads to lower energy losses, making them ideal for long-distance HVDC transmission where efficiency is paramount.

B.4 Hollow Conductors for Oil-Filled Cables

Hollow conductors are used in oil-filled cables, where the conductor is designed with a central hollow space that can be filled with insulating oil. This oil provides additional cooling for the conductor, allowing it to carry higher currents without overheating.

Impact on HVDC Transmission: In HVDC applications, hollow conductors help in managing thermal loads, which is crucial for maintaining cable efficiency and longevity under high current conditions. However, the complexity of oil management can add to the maintenance challenges.

B.5 Milliken Conductor

The Milliken conductor is a type of segmented conductor where the total cross-section is divided into several smaller, insulated segments. This design reduces the skin effect (where AC currents tend to flow near the surface of the conductor) and proximity effect, which can cause increased resistance at high frequencies.

Impact on HVDC Transmission: While the Milliken conductor is typically more relevant for HVAC (High Voltage Alternating Current) systems, its design principles can benefit HVDC systems by reducing losses and improving current distribution across the conductor.

B.6 Conductor Resistance

The resistance of a conductor is a critical factor in the efficiency of power transmission. Lower resistance means less energy loss and higher transmission efficiency. Various factors, including material, cross-sectional area, temperature, and conductor construction, influence resistance.

Impact on HVDC Transmission: In HVDC systems, minimizing conductor resistance is crucial as it directly impacts the efficiency and cost-effectiveness of long-distance power transmission. Advanced conductor designs, such as profiled or Milliken conductors, can help reduce resistance and improve overall system performance.

B.7 Watertightness of Conductor

Watertightness is a key requirement for submarine cables as water ingress can lead to insulation failure and reduced cable lifespan. Conductors are designed with specific water-blocking features to prevent moisture from entering the cable.

Impact on HVDC Transmission: Ensuring the watertightness of conductors is essential in maintaining the reliability and longevity of HVDC submarine cables. Water ingress can lead to increased resistance, insulation breakdown, and ultimately, system failure.

B.8 Superconducting Conductors

Superconducting conductors offer zero electrical resistance when cooled below a certain critical temperature. They represent the future of power transmission, with the potential to significantly reduce energy losses and increase transmission capacity.

Impact on HVDC Transmission: Superconducting conductors could revolutionize HVDC transmission by eliminating resistive losses entirely. However, the technology is still in its early stages, with challenges related to cooling and material costs that need to be addressed.

C. Impact on Power Quality, Grid Voltage, Frequency, and Waveform

The choice of conductor in submarine cables has a direct impact on power quality, grid voltage, frequency stability, and waveform integrity in HVDC transmission systems.

C.1 Power Quality

Power quality refers to the voltage stability, frequency stability, and the presence of harmonics or other disturbances in the power supply. Conductor design can influence power quality by affecting resistance, inductance, and capacitance in the transmission line.

Impact: High-resistance conductors can cause voltage drops and energy losses, leading to poor power quality. Conversely, low-resistance, high-efficiency conductors help maintain voltage levels and reduce harmonic distortion, resulting in better power quality.

C.2 Grid Voltage

Grid voltage is critical for ensuring the efficient operation of electrical equipment. Conductor resistance and inductance can cause voltage drops along the transmission line, which must be compensated for to maintain grid stability.

Impact: Conductors with low resistance and optimized inductance reduce voltage drops, helping maintain stable grid voltage levels. This is particularly important in HVDC systems where long transmission distances can exacerbate voltage stability issues.

C.3 Frequency Stability

While HVDC systems inherently operate at a constant frequency (0 Hz in DC systems), the interaction with HVAC systems at grid interfaces can affect frequency stability. Conductor impedance plays a role in this interaction.

Impact: The choice of conductor can influence the impedance of the HVDC system, which in turn affects how the system interacts with the AC grid. Proper conductor design can minimize adverse effects on frequency stability at the grid interface.

C.4 Waveform Integrity

In HVDC systems, the waveform is typically a smooth DC signal, but at the points where HVDC is converted to HVAC, maintaining waveform integrity is crucial. Conductor properties can impact the quality of the converted waveform.

Impact: Conductor designs that minimize resistance and inductance help maintain the integrity of the waveform during DC-to-AC conversion, reducing the likelihood of harmonic generation and ensuring a clean power supply.

D. Grid Impedance and Its Management

Grid impedance is the opposition that a grid presents to the flow of electric current. It is influenced by the inductance, capacitance, and resistance of the transmission lines, including submarine cables.

D.1 Impact of Conductor Design on Grid Impedance

The choice of conductor affects the overall impedance of the transmission line. Higher resistance or inductance can increase grid impedance, leading to issues such as voltage drops and reduced transmission efficiency.

Management Strategies: To manage grid impedance effectively, conductors with low resistance and optimized inductance should be selected. Additionally, reactive power compensation devices, such as capacitors or reactors, can be used to counteract undesirable impedance effects.

D.2 Impedance Matching

Impedance matching is crucial in ensuring efficient power transfer between the HVDC transmission system and the AC grid. Mismatched impedance can lead to reflections, energy losses, and reduced power quality.

Management Strategies: Proper conductor design, along with the use of impedance-matching transformers or other devices, can help ensure that the HVDC system is well-matched to the AC grid, optimizing power transfer and minimizing losses.

E. Planning and the NEC4 Contract

Effective planning is essential for the successful deployment of submarine power cables, especially in complex projects like offshore wind farms. The NEC4 Contract, a standard form of construction contract widely used in the UK and internationally, provides a framework for managing such projects.

E.1 Ensuring Effective Planning

Under the NEC4 Contract, careful planning is required to manage risks, coordinate between multiple stakeholders, and ensure that the project is delivered on time and within budget. Key considerations include:

• Risk Management: Identifying and mitigating risks associated with the harsh marine environment, such as cable damage or installation challenges.
• Stakeholder Coordination: Ensuring that all parties, including designers, contractors, and regulators, are aligned and working towards common goals.
• Budget and Schedule Management: Developing a realistic budget and timeline, considering the complexities of submarine cable installation.

The field of submarine power cables is continually evolving, with new technologies and materials being developed to improve performance, reduce costs, and increase reliability. New insulation materials, such as cross-linked polyethylene (XLPE), offer better performance at higher temperatures and voltages, extending the lifespan of submarine cables and improving their efficiency. High-Temperature Superconducting (HTS) Cables, which use superconducting materials cooled to cryogenic temperatures, are being developed for future HVDC systems. These cables offer the promise of zero electrical resistance, which could revolutionize long-distance power transmission. In addition, Integrated monitoring systems are being developed to provide real-time data on the health of submarine cables. These systems use sensors to detect temperature, pressure, and mechanical stress, helping to predict and prevent failures before they occur.        

Submarine power cables are a critical component in the transition to renewable energy, but they come with their own set of challenges and considerations. Understanding the nuances of conductor design and its impact on HVDC transmission is essential for anyone involved in the planning, design, or operation of offshore wind farms.

How do you think advancements in superconducting technology will change the future of HVDC submarine cable systems, particularly in terms of efficiency and cost-effectiveness???

This post reflects my personal knowledge and is for educational purposes only.

#RenewableEnergy #PowerCables #OffshoreWindEnergy #HVDCPowerCables #PowerQuality #BESS #GridCodeComplianceStudies

Reference:

Recommendations for the electrical and electronic equipment of mobile and fixed offshore installations, Institution of Electrical Engineers, 2nd. 1992

Electric Cables Handbook Third Edition; G.F. Moore, 1997

“BorWin1 | Hitachi Energy.”?Hitachienergy.com , 2024, www.hitachienergy.com/uk-ie/en/news-and-events/customer-success-stories/borwin1#:~:text=BorWin1%20wind%20farm%20consists%20of . Accessed 17 Aug. 2024.

Anaya-Lara, O., Campos-Gaona, D., Moreno-Goytia, E. L., & Adam, G.P. (2014). Offshore Wind Energy Generation: Control, Protection, And Integration to Electrical Systems.