Notably, in January 2023, the four New England states of Connecticut, Maine, Massachusetts, and Rhode Island requested funding from the US Department of Energy’s (DOE) Grid Resilience and Innovation Partnership (GRIP) programme (administered under the BIL) for the construction of multi-terminal HVDC (MTDC) transmission lines. For this, the states represented by the Connecticut Department of Energy and Environmental Protection (DEEP), Maine Governor’s Energy Office (GEO), Massachusetts Department of Energy Resources (DOER), and the Rhode Island Office of Energy Resources (OER) along with the support of two other New England states of New Hampshire and Vermont submitted their joint concept paper ‘Joint State Innovation Partnership for Offshore Wind’  to DOE’s Grid Deployment Office (GDO).

Although the concept of MTDC systems is new in the US, it has already been developed in Europe and are in advanced stages of development in China proving its viability. The proposed concept by New England states is intended to sync with and build on this work by advancing an innovative, multistate effort that would coordinate joint participants’ efforts to unlock OSW resources in New England with the DOE and others’ efforts to introduce HVDC standardisation in the US.

Specifically, the collaboration, which is an effort between the New England states, transmission providers, and wind developers, working closely with the New England grid operator (ISO New England or ISO-NE), has been formed to perceptively plan, recognise, and determine an initial portfolio of one or more HVDC transmission lines, and associated onshore system upgrades, to unlock the region’s significant OSW potential, improve grid reliability and resiliency, facilitate innovative regional cost allocation models and reduce innovative technology risk. The participants propose to explore a solicitation process that seeks a modular development structure to facilitate the initial deployment of offshore HVDC systems in the near term, while enabling upscaling of the system to accommodate a first-in-the-nation networked or ‘meshed’ MTDC system as that technology becomes available.

A fully networked MTDC system is expected to enhance regional (and eventually interregional) capacity transfers, increase reliability and resilience, and pave the way for the potential development of an innovative offshore backbone system along the Atlantic coast. Furthermore, by coordinating and planning OWT, it will be possible to connect projects together more affordably, lowering the costs associated with the states’ OSW targets for ratepayers. 

The OSW leasehold in New England has the capacity to support up to 14 GW of OSW, which could have significant regional and interregional benefits. Both geographically and technologically, these wind resources are limited. Particularly, the resources chosen so far have been those that are close to shore and primarily use AC tie lines. The closest points of landfall for these resources are on or near Cape Cod in Massachusetts, a slender peninsula with lower voltage grid infrastructure, high solar development, and periods of low load during shoulder months (which refer to the period between winter and summer peaks). As a result, about 2,400 MW of OSW is planned for Cape Cod and no significant additional wind will be able to interconnect without significant land-based upgrades that would add substantial costs to the OSW projects and would be extremely difficult to site or require costly offshore cabling around the Cape into Boston, Massachusetts. 

It has become clear that an OSW build-as-usual strategy will result in point-to-point developer generator lead lines using HVDC, which might landfall at less-than-ideal points of interconnection (POIs). The current regulations encourage developers to landfall their lead lines at POIs that are close to their generation projects and might need fewer upgrades. Developers are therefore less motivated to take into account ideal POIs or pay for high upgrade costs. Continued interconnection at sub-optimal POIs could result in curtailment or expensive upgrades to onshore reliability that would be paid for by ratepayers. This will ultimately prevent the region’s OSW capacity from being fully optimised to meet resource adequacy needs. 

To address these issues a proactively planned and integrated HVDC approach that leverages multi-state collaboration is required and offer significant benefits for resilience and dependability while also offering crucial grid support services that can facilitate the integration of other clean energy resources.

The participating states will competitively identify a broad set of transmission solutions, either through a request for proposals (RFP), or another competitive mechanism. The initial RFP will allow for the selection of one to three HVDC transmission lines with potential injections of up to 2 GW or other technologies, contingent upon receipt of DOE funding, to unlock constrained clean energy resources and inject power in the most efficient manner into the regional grid. Selection of any HVDC converters will be coordinated with developing standards to ensure the ability to interconnect and integrate the grid.

HVDC technology is generally considered to be the most reliable and efficient way to transport large amounts of bulk power over long distances. Huge investments are being made by industry, governmental organisations, and equipment manufacturers in advancing such HVDC technology, including the development of technology to network, or “mesh,” HVDC converters to permit converter-to-converter power transfers, either using an AC meshed model or a full MTDC model. 

The states have mentioned that MTDC technology is expected to bring significant advantages such as a reduced footprint offshore and onshore, flexibility in power flow control without the need for phase angle regulators, and ability to transport power across long distances with reduced losses, and lower offshore connection costs over long distances. HVDC converter stations can support the current onshore grid, reactive power, and voltage control (static synchronous compensator or STATCOM operation mode), black start capability, enhanced grid recovery and restoration, fault ride-through capability. HVDC systems with voltage source converter (VSC) technology provide important grid forming and resiliency functions. With the ability to couple two asynchronous grids, HVDC systems act as a firewall between offshore and onshore grid failures. Another possibility is to construct MTDC grids for different areas, states, or regions. 

By enabling the interconnection of sizable amounts of OSW, a backbone or integrated MTDC system for offshore wind in New England is likely to be a crucial component in facilitating the region’s decarbonisation.

While acknowledging that no current MTDC system exists in the US and that important technical standards for MTDC systems have not yet been established; the joint participants point to the federal and industry efforts to develop and institute HVDC standardisation protocols. Such protocols may be implemented as early as 2025. Similar timelines have been indicated for the commercial availability of HVDC circuit breakers and other critical equipment necessary to build an MTDC grid in the US. As it may take 8-10 years to plan, develop, and build major transmission projects, it would enable joint participants to integrate and incorporate the developing MTDC standards and technologies as the concept proposal moves forward. The plan is to interconnect the HVDC converters, if not initially, at the first technically permissible time. The concept paper also identifies an AC meshed grid approach as a feasible alternative should the necessary HVDC technologies and standards not develop in time. That said, the benefits derived from such a system may not be equivalent to that of MTDC.

HVDC projects, which will be imperative to developing OSW areas beyond the near shore environment, have not been successfully developed in the US. Past solicitations such as that conducted by Connecticut DEEP in 2019 as well as the recent innovative State Agreement Approach (SAA) solicitation by New Jersey in 2022 declined to select HVDC OWT solutions, citing high cost and risk. In this background, DOE funding will be central to accomplishing the proposed MTDC solution as it would aid in reducing the overall project cost, achieve technological standardisation and facilitate multi-state and regional coordination. 

Figure 1: Potential MTDC system for connecting OSW lease areas off New England coast

Source: New England states’ Joint State Innovation Partnership for Offshore Wind concept paper