Offshore Wind Transmission Study

The Massachusetts Clean Energy Center (MassCEC), working with the Executive Office of Energy and Environmental Affairs (EEA) and its Office of Coastal Zone Management (CZM) and other state agencies commissioned this Offshore Wind Transmission Study report to analyze and understand the transmission infrastructure necessary to interconnect future Massachusetts offshore wind projects to the regional electric grid. This report examines the technical aspects of offshore wind transmission interconnection and analyzes
scenarios that minimize cost and environmental impact.
Since 2009, the Commonwealth of Massachusetts has been leading a planning and stakeholder processwith the U.S. Department of the Interior’s Bureau of Offshore Energy Management (BOEM) for the Massachusetts Wind Energy Area (MA WEA), the largest offshore wind planning area along the East Coast.
The National Renewable Energy Lab estimates the area can host 4,000 to 5,000 MW of installed offshore wind capacity that could produce enough electricity to power the majority of homes in Massachusetts. The federal government is expected to conduct an auction of the Massachusetts Wind Energy Area later this year. Additionally, Massachusetts has been working with BOEM and the State of Rhode Island on the Rhode Island–Massachusetts WEA (RIMA WEA).
Development and growth of the Commonwealth’s offshore wind sector will be driven largely by policy and market factors. This study was undertaken to explore the technical characteristics of offshore wind transmission infrastructure independent of these factors. The results of the study will support EEA and CZM in the current update of the 2009 Massachusetts Ocean Management Plan, which will examine potential transmission cable routes within the context of critical marine habitat areas, other natural resources, and
marine water-dependent uses.
Four build out scenarios were developed to represent potential stages in the development of the federaloffshore wind planning areas (RIMA WEA and MA WEA). The center of the RIMA WEA is approximately 30 miles from the mainland coast of Massachusetts and the center of the MA WEA is approximately 50 miles off the coast. Together, the build out scenarios provide a framework to describe and evaluate thetransmission infrastructure necessary to connect future Massachusetts offshore wind projects to the New England electric grid.
The four scenarios for build out of the RIMA WEA and MAWEA areas are as follows:
  • Scenario 1: 500 MW
  • Scenario 2: 1,000 MW
  • Scenario 3: 2,000 MW
  • Scenario 4: 3,000 MW
Specifically, the study addresses (i) technical approaches for building offshore transmission lines, including transmission system components and design factors, (ii) identification of potential interconnection points to the existing electric grid, including generally the improvements and upgrades required to accept this energy, and (iii) the potential for expansion of offshore transmission as development advances in the federal wind energy areas WEAs.
While formal electric system impact studies (load flow and interconnection engineering) were beyond the scope of this effort, the study did result in a number of important high-level findings. Prior to finalizing this report, the Project Team reviewed these findings with key industry stakeholders that included ISO New England, electric utilities and private offshore wind and transmission developers. Key findings of the study include:
  1. Transmission cable distance will range from 40 to 130 miles or more, which favor the use of high-voltage direct current (HVDC) technology. HVDC technology offers advantages to high-voltage alternating current (HVAC) including reduced line losses, highly controllable power flow and lower cable costs due to fewer conductors and typically smaller cables.
  2. Any transmission system will require one or more offshore collector stations to aggregate power from the wind turbines for transmission to land,
  3. If an HVAC system was designed for a project location in the MAWEA close enough to shore and with operating characteristics to make it feasible for a project in the 250 MW range, voltage compensation and system protection equipment would be required and could be located on the offshore platforms, as well as the land based interconnection station.
  4. HVDC transmission systems include offshore collector station(s) which aggregate alternating current (AC) electricity from the turbines, an offshore converter station to convert the electricity from AC to direct current (DC) for transmission over longer distances, the undersea transmission cable bundle, and an onshore converter station located adjacent to the interconnection point to convert DC to AC.
  5. The offshore converter station platform is a limiting factor for offshore wind energy facilities that rely on HVDC technology to deliver the electricity to the grid. Current technology limits the size of the converter stations to 1,000 MW.
  6. There are a number of potential interconnection points in Massachusetts and coastal southern New England where offshore wind projects can interconnect to the grid.
  7. The interconnection would be at the 345 kV level to integrate the large electric generating capacity anticipated from offshore wind projects with the existing electric grid. If a smaller (250 MW) project could be designed to operate in the MAWEA, it is possible that, in addition to the 345 kV substations, an interconnection could be made at a 115 kV sub. This report did not evaluate interconnection at this level.
  8. It is technically feasible to interconnect 500 to 1,000 MW, and in certain cases up to 2,000 MW, ofoffshore wind capacity at each potential 345 kV interconnection point.2
  9. The offshore transmission system can be developed to accommodate the sequential or phased build-out of the wind energy areas.
These findings pertain to the technical characteristics of offshore wind transmission infrastructure independent of the market and policy factors widely recognized as principal drivers affecting the scale and pace of offshore wind development in the region. Accordingly, the increasing installed capacity captured in each build out scenario equates with the incremental addition of 500 to 1000 MW. While this development path is hypothetical, it does represent the optimal approach to achieve transmission-related economies of scale. However, market and policy factors may exhibit a greater influence on the size of offshore wind projects developed in the region. For example, projects in the 250 MW range could also be developed and are considered potentially more viable in the near term by some industry stakeholders due to the current status of policy, the market, and financing mechanisms.
The results of this study provide valuable insight for future planning efforts by the Commonwealth to help foster the development of offshore renewable energy. Further evaluation can be undertaken to help provide additional understanding. The next steps that could be undertaken to build upon this study could include:
  • Further refine the understanding of interconnection requirements for an offshore wind project to the grid. One identified location could be selected for an ISO New England “Feasibility” level study. Brayton Point substation may be a good candidate given the existing capacity interconnected at this location, the announced Brayton Point Generating station retirement and potential future re-uses.
  • Further investigation of the potential ownership scenarios described in Section 7 would help provide insight on the implications of the regulatory (i.e., rate impact) and cost advantages or disadvantages of the described options.
  • Constraints that need to be considered when siting cable routes, landfall locations, and converter station sites are generally described at a high-level in the report. A next step would be to expand the understanding of these factors and outline their potential importance to a developer.
  • The injection of large amount of offshore wind energy to the grid will have an effect on the environmental characteristics of the overall electric system and on the cost of power. A next step to evaluate the offset in greenhouse gas emissions due to the displacement of existing fossil fuel generation by offshore wind generation and the potential effect on cost of energy such as price suppression would help further define the economic and environmental benefits of energy provided to the grid by offshore wind-generating facilities.
  • The development of offshore wind energy and the transmission systems to bring the power to marketwill present employment opportunities and economic impacts in Massachusetts and the region in general. An analysis of these potential effects from the build out of the wind farms, could evaluate the potential benefits to manufacturing, construction and long term operations in terms of goods and labors services and the ability of this developing sector to help drive economies of scale in offshore wind energy while benefitting state and local economy.
  • Low frequency alternating current (LFAC) is a developing technology that may provide another option between HVAC and HVDC. However, because of the need to design and commercially-develop several new pieces of equipment, this technology is many years in the future.