Offshore wind turbines

Integrating offshore wind power

Executive summary

Offshore wind power represents one of the fastest-growing renewable energy markets globally. Capacity installed worldwide is now 19 GW, of which 4.3 GW was installed in 2017 mainly in Europe and also China. In the US, the offshore wind market is now picking up pace with the first 30 MW operational, 1.2 GW awarded and >10 GW initiated. Offshore wind power with bottom-fixed foundations is now increasingly regarded as a mature technology with high scalability. The financial feasibility has improved significantly in recent years. European tenders applying WTGs in the 8-10 MW range now yield power price ranges competitive with natural gas and coal-generated electricity.

Offshore wind power brings a relatively new set of challenges regarding the integration in electrical networks. Upgrading existing transmission lines and installing new transmission capacity onshore is a critical factor for the timely connection of new offshore wind projects. For offshore wind, there are also specific technical challenges related to the design of a robust and reliable electrical system. The grid connection of wind power is always unique and therefore the financial and technical viability should be considered at an early stage when developing offshore wind opportunities.

Worley has over 100 years of international experience and provides services to support all sizes and phases of offshore energy projects. The Worley Group has recently partnered with Ventolines, a leading supplier of consultancy services to the Dutch wind energy market. Ventolines covers all project phases, from development to contracting to asset management and supported Deep Water Wind during the construction of Block Island, the US’ first offshore wind project. Together, Worley and Ventolines present a unique offering to the US market with an integral perspective on offshore wind projects.

Grid integration challenges and offshore wind

Generally, renewables such as wind and solar challenges the grid with a power supply that is variable and partly unpredictable. On the other hand, as experience with onshore wind in the US clearly demonstrates, power systems are technically capable of balancing out such variations. In addition, the electrical equipment available in modern wind turbines (onshore and offshore) is technically capable of supporting the grid by providing services such as frequency- and voltage control. However, to design and connect a batch of wind turbines that reliably operates as one electrical system (a grid-connected offshore wind farm) it involves more than summing its individual parts. The specific technical challenges are related to the fundamentals of offshore wind: high power, remote locations, and costly equipment.

Power transmission concept choice is the first and main electrical design consideration for any wind project at an early stage. Offshore wind requires a particularly efficient, robust and reliable technical solution to get the wind power from sea to shore. The design concept must be right from the start, capturing all project-specific technical, environmental, network and economic factors.

Alternating current (AC) solutions have the benefits of reliable technology and relatively low investment cost but at the cost of electrical losses, which increase with distance. Medium voltage ranges (33-66 kV) allow for the connection of tens of MW for distances up to several tens of miles. High voltage ranges (110-220 kV) are available for the connection of hundreds of MW for distances up to around 60 miles offshore / from the grid connection. For longer distances, AC may no longer be technically feasible due to reactive power compensation challenges.

For long-distance power transmission, only direct current (DC) solutions may be feasible. HVDC does not require reactive power compensation and the electrical loss can be lower, although AC-DC-AC conversion losses apply. However, the capital and operational costs are significant. The choice for AC vs. DC and the voltage level ties in to other design considerations, such as the specification and layout of infield and export cabling, but also the number and electrical design of transformer stations (possibly offshore).

Long-distance subsea transmission is based on medium- and high-voltage electrical cables running over or under the seabed. Apart from the challenges of physically installing cabling offshore, the electrical characteristics of long cables are different compared to overhead lines. This may require the use of additional electrical equipment, such as shunt reactors for reactive power compensation or harmonic filters to mitigate harmonic resonances between the wind farm and the grid. The challenges are relevant depending on the electrical interaction between the offshore wind project and the grid, which is a unique characteristic sensitive to project-specific conditions. Focused engineering efforts made during the development stage allow for an electrical design that is validated, which in return reduces overall project cost and risk.

Low short-circuit levels and network constraints are a frequent given for offshore wind farms due to site selection favouring remote locations where the public grid is not particularly strong. While modern wind turbines offer superior electrical controllability – including good fault ride-through capability – they have low short-circuit current contributions. That may lead to an increased power system vulnerability to voltage instability in the case of sudden changes, such as a load disconnection or system faults. Also, network constraints can pose (temporary?) limits to power production by the offshore turbines. These aspects shall be explored timely and in cooperation with the local transmission system operator, as to prevent unexpected costly delays and/or loss of project revenue.

Demonstrated experience

Worley and Ventolines offer offshore wind project developers solution strategies for the integration of offshore wind into electrical grid networks. Our range of services includes electrical design studies, reliability and cost-benefit analyses, short-circuit current and fault analysis and dynamic stability analysis. We are able to model the full range of electrical operation aspects of wind projects, ranging from voltage- and frequency control to the integration of energy storage in the form of pump storage, compressed air or chemical storage (battery systems).

The partnership between Worley and Ventolines presents world-leading renewable energy consultancy and engineering delivery services. Our unique service offer draws from a wealth of experience in grid connection studies, tailored to meet the specific challenges posed by offshore wind power projects. Based on solid electrical engineering, we maintain a long-term perspective optimizing project financial performance by smart design choices and minimizing risk across the entire project cycle. The track record of Worley and Ventolines builds on wind projects, including:

  • Block Island, 30 MW offshore (US): WTG installation management and electrical engineering advice
  • South Fork and Skipjack, ~200 MW (US): Client technical contracting support for wind turbines
  • Westermeerwind, 144 MW nearshore (NL): Full-service owner’s engineer during all project phases
  • Windpark Fryslân, 341-383 MW nearshore (NL): Full-service owner’s engineering for all phases
  • Beaufort, 340 MW offshore (NL): Cable crossing design, tender technical documentation and support
  • > 1 GW of onshore and offshore projects in Europe and the United States

Contact us

Eoghan Quinn | Worley | +44 78 7613 7131

Boy Koppenol MSc | Ventolines | +31 6 55 481 555


Subscribe to receive more perspectives