Index Terms—wind energy, wind plants, offshore installations, transmission, HVDC, HVAC, wind energy integration, costs, substations, submarine cables
To accommodate this new electricity generation will require significant new transmission from the offshore wind plant to the onshore load centers. The costs of this transmission and integration plays an important role in ensuring the wind project is economically feasible, i.e. can compete with other generation source by having a comparable levelized cost of energy (LCOE) and ensuring competitive electricity process for consumers. Luckily, a majority of the U.S. population lives near the coastline and 26 states have high offshore wind resources. This allows offshore wind plants to be sited near urban areas where demand is located, electricity prices are often higher, and space for land-based generation and transmission is limited . Both offshore and onshore transmission with play a major role in the cost of offshore wind energy. Studies have focused on the cost of transmission infrastructure, which technologies are most cost-effective as well as how strategic offshore transmission planning can ensure the lowest possible offshore transmission and integration costs. In addition, studies have been conducted to better understand how this enormous offshore wind energy potential can cost effectively be added to the electrical grid. Both are key aspects to ensuring offshore wind continues to decline in price to help accelerate the deployment of offshore wind energy.
An NREL report assessing the U.S. wind resource, determined there is large resource to accommodate the 86 GW DOE goal. The technical resource capacity for offshore wind is 2,658 GW. This takes into account offshore areas that are commercially viable today. In addition, the best offshore wind resource in terms of quality and quantity was determined to be located in East Coast in the Atlantic Basin, an area close to high population centers that could use this additional generation, including states such as Maine, Massachusetts, Rhode Island, New York, and New Jersey . Because of the Atlantic has the greatest resource potential and near term offshore development, activity is concentrated off the east coast, and many states in the area are offering incentives and procurement targets for offshore wind, a review of the transmission and integration costs will focus on this region.
Another consideration for this review of transmission and integration costs for offshore wind, is the trend of developing wind projects at a greater distance from the shore. Many global wind farms are developing outside the 50nm distance and expanding to 200nm. The latest U.S. wind resource assessment, discussed above, expanding the resource boundary potential from 50nm to 200nm. Greater distance from the shore means increased costs for infrastructure as cables must be longer and designed for deeper water. A central theme of this paper will focus on what the optimal transmission system is to transmit the power onshore. The resource assessment notes that the economics of using a high-voltage direct current (HVDC) transmission system increases as offshore wind plants move further from land .
The National Offshore Wind Strategy Report, concluded that “impacts of significant offshore wind deployment on local grids need to be better understood, and the costs and benefits associated with offshore transmission infrastructure need to be characterized” to support near-term deployment through cost reductions . This paper will follow the “cable line” from the offshore wind turbine platform to the onshore transmission electrical grid, analyzing the current technology and planning strategies for onshore wind energy transmission and integration. Lowering costs for this transmission infrastructure will be a primary consideration to achieve an industry that in the future can provide competitive rates for electricity generation.
II. Collection System
As an array of offshore wind turbines convert wind energy into electricity, the collection system connects the turbines in the wind plant together through an underwater array cable system. At the base of each wind turbine is a transformer steps up the generation voltage from 690 volts (V) to 25-40 kilo-volts (kV). This higher voltage transmits through an array cable system to an offshore substation.
[Talk about WindSTAR transformer]
HVAC Transformer (in Lit)
A 500 MW offshore wind farm was used a case study to determine how to lower the transformer, collection cables and collection cable installations costs. Costs include cable cost, cable shipping and installation, tower transformers, offshore transformer substation, and onshore transmission and substation. Typical cost considerations for the turbine transformer costs is 3.5% of the total electrical costs for delivering energy onshore. The collection cable costs shipping and install cost is 15% and the actual collection cable cost is 35% of the total electrical costs.
To reduce these the electrical costs, layout of the wind plant is an important consideration. For the 500 MW wind plant case study, a 21-row layout with eight turbines per row was found to be the most cost-efficient. Siting each turbine in a configuration that reduces the length of submarine cables while still maximizing the wind plant output will be an important consideration for plant design.
[OWGIS p. 19 for cable array information]
However, increasing the distance between turbines increases the length and cost of the required inter‐array cabling as well as the required space and therefore costs for that space. (For between turbines) MAOIT
In addition, to reduce costs from the collection system cabling, laying cable on the sea floor instead of burying the cables is recommended. Since the win farm creates a barrier that prohibits boat access, the cables laid on the sea floor are at less risk of being damaged from boat anchors and fishing nets. Another consideration to reduce the costs of collection system cables include: aluminum conductors could be used in place of copper along with a lead shield to prevent saltwater corrosion, could reduce cable costs by 10-20% . The collection system is the first step in the transmission system to the onshore electrical grid. To ensure cost reductions, reducing the cost of transformers and the cable system connecting each turbine is essential, especially as developers deploy more turbines per plant.
[Talk about need to increase cable array to 66 kV as larger turbines are used (2016 Offshore Wind Technologies Report)]
[Highlight costs of power loss from different submarine cables]
Cable developments to lower cost
These medium-voltage submarine cables then connect to an offshore substation. This offshore substation will step up the voltage to an appropriate level to transmit the wind energy to the onshore substation.
development of compact high-voltage direct-current (HVDC) converters can help cost reductions
III. Delivery System: Offshore to Onshore
Appropriate technologies to transmit energy collected at offshore substations to the onshore electoral grid already exist to facilitate the large offshore wind potential. The two primary technologies to deliver offshore wind to the onshore grid, include transmission technologies based on high-voltage alternating current (HVAC) and high-voltage direct current (HVDC). The cost-effectiveness of each technology is based on the system design and plant siting characteristics .
Offshore delivery systems can be radial, split, backbone, or grid connections for both HVAC and AVDC. Figure # shows the basic design of each system. Radial connections multiple wind plants or turbines into one substation and delivers the energy to an area with an onshore substation. Split connections use two power cables to connect two different points onshore at different substations. Backbone connections connect multiple wind plants and/ or substations on one larger power cable and connect to multiple substations onshore. Finally grid connections where multiple wind plants or substations can connect with each other and move power more freely and connect to multiple onshore substations. Each delivery system can be used for HVAC or HVDC but require unique considerations in choosing the delivery system to best fit the transmission technology .
A. High-voltage alternating current (HVAC)
HVAC transmission technology is currently the most widely used technology for offshore wind farms to deliver energy to the grid. Since wind turbines produce energy in alternating current (AC), a converter is not required to change the current from alternating to direct.
A wind plant’s distance from shore is a primary consideration in which transmission technology is most practical and cost-effective. As described earlier, siting for offshore wind plants has been trending upward from 50nm to 200nm. HVAC transmission systems are best most economical for wind plants that are less than 50km from the offshore and are similar in costs to HVDC for plants 50-80km. These distances are important because energy losses increase and capacity decreases as the HVAC based-transmission cable increases in length, primarily due to the capacitive and inductive characteristics of the cable . This energy loss and capacity constraints can be overcome by adding additional AC-based transmission cables or adding in reactive power compensation equipment on a platform, which increases the amount of active power that can transmitted. However, this additional infrastructure will increase the costs of the offshore delivery transmission. This additional infrastructure is one reason at longer distances HVDC is more economically advantageous .
Comparing the different system designs for HVAC, such radial, split, backbone, or grid connections has important costs, reliability, and efficiency considerations. Currently, the connect a single wind plant to the onshore grid, deploying a radial HVAC connection is the lowest cost. A basic HVAC connection also has higher reliability than a basic HVDC connection, up to 30% more reliable. However, as the distance from the shore increases, and reactive power compensation is required, the reliability of HVAC and HVDC connection becomes comparable. A HVAC backbone or grid system design also require reactive power compensation to transmit the wind generation over longer distances. This would raise the cost of the project because of the need for additional equipment, platforms, and ancillary equipment to have an HVAC backbone or grid system design . A backbone or grid system design is desired because studies have shown that in terms of efficiency, HVAC radial connections cause 20% more “spilling of offshore wind power” than deploying a HVDC backbone system design and could lead to additional wind plant curtailment .
Using HVAC for single offshore wind plants, especially with demonstration projects such as Block Island, lead to lower costs. As U.S. wind industry grows and takes advantage of the enormous offshore wind potential further offshore, the benefits and potential cost reductions from sharing a HVDC backbone or grid system for multiple wind plants, may move the wind industry away from HVAC and toward HVDC.
B. High-voltage direct current (HVDC)
Offshore wind plants are moving further offshore. As this trend continues, HVAC transmission systems become more expensive and have greater costs than benefits, leading HVDC to be a superior and cost-effective transmission technology.
HVDC operates in the 320-800kV range, allowing the transmission line to carry more power over an infinite distance. An HVDC system can also be more stable because of fast power control and the power flow direction can be changed efficiently . In the 50-80nm offshore transmission range, the costs of HVAC and HVDC are comparable but at distances greater than 80nm, the HVDC becomes the lower cost option because it does not require the use of additional HVAC transmission cables, reactive power equipment, and does not suffer from power losses. Installing a HVDC system would minimize the losses from the distance constraints of HVAC .
Backbone and grid systems are the proffered designs for HVDC transmission. HVDC allows these systems to be used for the greatest benefit to moving generation onshore. These system designs can control the power flow amount and direction making this system design more flexible than HVAC, which enables HVDC to manage onshore transmission constraints. HVAC cannot control power flow without installing additional equipment. Furthermore, since HVAC backbone and grid designs requires reactive compensation equipment the costs of a HVDC backbone system is less costly. Backbone systems are more reliable then radial systems and HVDC allows the benefits of the backbone system to be realized. In the backbone system design, if one onshore connection went could not accept the offshore wind generation or the substation failed, the power could be transferred to another connection since backbone and grid have multiple onshore connections, which is unlike HVAC radial designs. HVDC allows power flow control over greater distances so the wind plant would not need to be curtailed. Instead the generation would be transferred to another onshore connection point, decreasing the risk of production loss from the multiple offshore wind plants. While backbone and grid design both have similar benefits, HVDC grid design are more flexible and reliable than backbone. However, grid systems require more infrastructure than a backbone system so the economics of installing a grid system must be analyzed in terms of the capital costs and generation potential of each wind plant connected to the system .
Since generators in wind turbines produce AC electricity and the U.S. grid system is also designed to carry AC electricity, converters are required to change AC to DC to carry energy through an HVDC transmission line. This is additional infrastructure costs required for HVDC transmission systems. Historically, onshore HVDC transmission systems have used line commutated converter (LCC) but these require strong interconnecting systems, which is often impractical for offshore wind. LCC require large offshore platforms to accommodate the LCC converter and components. The costs and size of a LCC converter is much larger compared to a conventional transformer for HVAC . However, recent innovations in voltage source converters (VSC) for HVDC systems have allowed for cost reductions to convert AC to DC and vice-versa. Recent innovations for VSC include transmitting higher voltages and no longer needing a large filter to remove harmonics from the AC voltage from the wind turbine . HVDC-VSC systems are also more appropriate for offshore wind because VSC proved more reactive power control than LCC, allowing the DC system can more easily connect to a weaker onshore AC transmission system. Other benefits from VSC improvements, which help reduce the costs of converters for HVDC transmission and allow for improvements in backbone and grid system design, include allowing independent control of active and reactive power at each converter station in the backbone or grid system, there is no risk of commutation failure as VSC can change the direction of the AC to DC power flow, and VSC systems require fewer components, capacitors, and fewer AC voltage filters of the offshore platforms for HVDC-VSC converters is much smaller than HVDC-LCC converters. HVDC-VSC allows for multiple connection points in the backbone and grid transmission system designs .
As wind plants move further offshore, HVDC systems become more economical. Furthermore, HVDC opens up more possibilities to connect multiple wind plants to multiple onshore connections. With strategic planning, this may help facilitate cost sharing and per project cost reductions for offshore transmission. This will be explored further below. Finally, innovation in HVDC technologies, such as VSC are continuing to add benefits at a lower cost to encourage HVDC adoption over HVAC as the U.S. builds the offshore wind industry.
IV. Integrating to the Electrical Grid
As electricity is generated at the turbine, the energy is carried through cables in the collection systems, and is transmitted across the ocean floor by HVAC or HVDC in the delivery system. This generation will connect to the onshore electrical grid where transmission system will need a demand to accept this additional generation. Two primary studies have been competed in the Atlantic region to understand the effects on the onshore grid if a large amount offshore generation were added to the grid. These two studies are the Mid‐Atlantic Offshore Wind Interconnection and Transmission (MAOWIT) and Eastern Wind Integration and Transmission Study (EWITS).
The MAOWIT study focuses on the PJM Regional transmission organization. The PJM territory includes Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and the District of Columbia . PJM is poised to integrate the large resources of offshore wind of the coats of New Jersey, Delaware, Maryland and Virginia to support the large population centers in its territory. The MAOWIT study looked at the “near‐term, already‐planned build‐out level” of offshore wind and build out of offshore wind at 15%, 25%, and 35% of the average PJM load, and a “maximum build-out assuming bottom‐mounted turbines filling un‐conflicted space up to 60 m depth.” This study compared a HVDC delivery system to a piecemeal radial ties system . This study found that the “near-term, already planned build-out level,” if there were 1-2GW of additional synchronized ramp-up and down reserves during peak summer periods, 7.8 GW of offshore wind could be integrated into the PJM territory. The study also found that to increase installed offshore wind capacity to around 35.8 GW, a high penetration scenario, PJM would need to conduct onshore transmission upgrades, increasing the capacity of current lines in the PJM system in addition to around 8 GW of reserve in peak summer periods. Furthermore, comparing an HVDC offshore backbone system to a HVAC radial connection, PJM determined that by using a HVDC backbone system, wind curtailment would be reduced on an order of 20% .
EWTIS modeled four wind scenarios to determine the effects of integrating both onshore and offshore wind in the Eastern Interconnection. Since the Eastern Interconnection includes the entire Atlantic coastline, one scenario included a 20% wind penetration scenario by 2024, with offshore wind supplying the majority of required generation to meet demand closer to load centers in the Eastern Interconnection. Key findings from this study for the 20% offshore scenario include: new transmission is required in the Eastern Connection to integrate increased wind energy or curtailment of generation will be required; since the Eastern Connection has large regional power pools, costs can be shared, leading to manageable new transmission costs; additional transmission will lead to lower integration costs and a more reliable grid because more transmission will spread out the variability of wind generation. The EWTIS also noted that planning the build out of this required transmission was essential because it takes longer to build transmission lines than wind plants. The EWTIS modeled that the lowest onshore transmission and interconnection costs were attained in the 20% majority offshore wind scenario. However, the majority offshore also was the highest overall project cost because offshore has a higher wind plant capital cost. The study found that five 800-kV HVDC and one 400-kV onshore transmission would allow the Eastern Connection to accommodate the offshore wind energy growth .
The key takeaways from the MAOWIT and EWTIS is that integrating a high penetration of offshore wind energy is possible today with an additional onshore transmission build-out. Both studies considered HVDC the preferred technology both onshore and offshore to carry increased offshore wind generation.
V. Strategic Offshore Grid Planning
Strategic planning of an offshore transmission system for future wind plants will help lower overall project costs and ensure the load centers can use the large offshore wind resource. Coordinating the offshore transmission network in the North Sea in Europe is expected to allow planners to fully utilize each transmission line in the network, plan each path to a connection onshore where the market can fully utilize the wind generation without having to curtail, and reduce the total cost of connecting each wind plant to the shore. If this offshore transmission network was panned as a multi-terminal, interconnected HVDC grid network instead of radial connections, more wind power could be brought to the shore, power could be exchanged from different regions in Europe, higher cost generators would be used less and wind would not have to be curtailed as often . The offshore wind energy in the United States is similar to the North Sea in Europe in that a large wind resource is located near high population centers. The U.S. offshore wind energy should take note of the importance of coordinating the offshore and onshore transmission systems to ensure the most efficient utilization of future offshore wind generation.
By strategically planning the offshore transmission system for wind energy, planners can use the best available technologies reviewed in this paper. Planners can ensure transmission projects use a 66-kV cable array to connect each wind turbine. These wind plants could then connect together using utilizing a backbone transmission grid system that carries electricity onshore through a HVDC-VSC connection. Once onshore, the additional onshore HVDC transmission lines, which were planned and built before the offshore wind plants, will carry the offshore wind energy to its consumer.
The Atlantic Wind Connection (AWC) project is a proposed transmission plan to support growth in the U.S. offshore wind industry. When completed the offshore transmission line would span the mid-Atlantic Coast from New Jersey to Virginia, supporting up to 7,000 MW of offshore wind growth. The project will use a multi-terminal 340 kV transmission HVDC system with a backbone design. This HVDC system will also use VSC, allowing multiple wind farms to connect into the system and transfer their generation to up to seven onshore transmission locations . AWC is expected to have numerous benefits for offshore wind in the Atlantic, including the ability for multiple offshore wind plants to connect and increasing control over their generation, since its based in HVDC this system will avoid the electrical losses that spur from HVAC and distance, the offshore transmission will be able to connect into multiple of the strongest and highest capacity parts of the onshore transmission system, and the system will have the ability to easily expand to the North, South, or East in the future by extending more HVDC lines into the sea. With these benefits, the AWC is expected to significantly reduce the cost of each offshore wind plant because it will not need to provide its own individual offshore line to an onshore connection, despite the initial higher capital costs of HVDC lines .
The AWC is an example of the necessary strategic planning required to reduce the cost of offshore wind, plan for the future, and use the best available technology to optimize the offshore transmission system. The Federal Energy Regulatory Commission (FERC) approved several incentives for the AWC because this transmission project was included in the PJM regional transmission plan . However, despite the incentives and a $5 billion investment from Google, the future of the AWC is uncertain. Without this project the future of offshore wind energy in the U.S. might be more sluggish and developers may stick with AC lines on an individual project basis, which may argue will hurt the long-term efficiency and cost-effectiveness of the U.S. offshore wind industry .
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