As the US gears up to meet its clean energy goal of reaching 100 per cent carbon-free electricity by 2035, renewable energy is expected to play a significant role in the country’s energy mix. A significant part of this will be contributed by new solar and wind (both onshore and offshore) capacity coming up in the eastern and central regions of the country. Transmission systems, which traditionally take several years to build, have to be developed in tandem to keep pace with generation plans.

In this context, the Eastern Interconnection Planning Collaborative (EIPC), comprising 19 key transmission planning coordinators from Eastern and Central US, recently released a white paper calling for more transmission and cooperation to help achieve the country’s renewable energy targets. Titled ‘Planning the Grid for a Renewable Future’, the white paper is an outcome of extensive study and discussion, which highlights that large amounts of renewables make the grid more complex to manage than traditional power plants and disrupt the transmission planning process. It aims to help local, state and federal policymakers understand the challenges grid operators face as more renewable facilities come online and identifies the challenges and recommendations that policymakers should consider when making decisions that affect the transmission grid.

The white paper, which includes contributions from several grid operators including the Midcontinent Independent System Operator (MISO), PJM Interconnection, Southern Company, Duke Energy and Tennessee Valley Authority, focuses on the lessons learned through experience and studies of future conditions as they relate to the planning and operations of systems with a high penetration of renewables. In addition, it states that reforms to transmission planning, cost allocation and facility siting can overcome those challenges. These recommendations come at a time when the Federal Energy Regulatory Commission (FERC) is in the midst of consultations to bring about broader reforms in transmission planning, cost allocation and generation interconnection. Further, some of the issues related to siting have also been addressed by the recently passed Infrastructure Investment and Jobs Act (IIJA).

Renewable resource proliferation in the region

The Eastern Interconnection’s renewable wind and solar resources are rapidly expanding, and while this growth is happening in some form or another throughout the Eastern Interconnection area, it is not uniform in terms of adoption rate or technology type. This non-uniformity produces unique insights from different companies on their changing needs with more wind and/or solar resources entering their systems and how they foresee future challenges as this growth continues.

Each area of the Eastern Interconnection is enhancing its use of renewable generation as indicated in Figure 1:

 • The Northeast region, with limited available land, sees proposals for offshore wind resources built near population centres, high-voltage direct current (HVDC) transmission potentially bringing additional hydropower from Canada, and more solar from distributed resources from utility-scale facilities.

• The South and Southeast regions, with more limited wind resources, are realising their solar potential with an eye on possible low-speed and offshore wind.

• The Central Plains region, with high wind speeds, has driven large amounts of wind development, with expansion of additional wind resources moving forward aggressively.

• The Great Lakes region currently plans for a mix of solar and wind development.

Figure 1: Representative renewable growth in the Eastern Interconnection states 

Note: Representation is based on data provided by an EIPC member.

Source: EIPC’s report on Planning the Grid for a Renewable Future

Challenges experienced by system planners and operators

Due to the continually shifting resource mix, system planners face several challenges pertaining to renewable integration, resource adequacy risks and growing variability of generation. Further, additional transmission infrastructure is required to meet increased demands for electrification of the transportation and industrial sectors. 

Planning transmission necessary to achieve high renewable penetration  Grid planners have traditionally depended on legacy synchronous generators, which were backed up by transmission networks designed to transmit power from these resources to load centres. Renewable energy sources (RES), on the other hand, are clustered in areas far from load centres in some parts of the Eastern Interconnection, and they interconnect to the grid at the grid’s extremities at present. Transmission upgrades and/or non-traditional assessments, such as electromagnetic transient (EMT) studies, may be required to adequately integrate RES into the transmission system. 

Studies indicate a decline in grid performance for steady-state thermal loadings, voltage stability and transient stability when synchronous machines are replaced with inverter-based renewable resources. As technology advances, inverter-based solar and wind resource design will need to maintain system voltage and assist with primary frequency response due to system events. The load patterns are also fast changing due to new loads and behind-the-meter resources such as home backup generators, home chargers for electric vehicles, the conversion of gas and oil heating to ground-/air-source heat pumps, increased penetration of rooftop solar arrays, and whole-building battery backups.

Another issue that has developed over the last few years deals with distributed energy resources (DERs) and their impact on grid operations during system events. Due to existing interconnection standards at the time of installation, some DERs perform differently than other grid resources and trip offline before transmission-level resources, which can exacerbate outages or other grid disturbances.

The renewable generation interconnection queues have increased exponentially over the last decade. However, only part of these proposed projects finally come online, which increases the complexity of grid planning. Currently, the lack of standardised performance requirements for renewable resources with inverter-based control systems has caused significant delays in the interconnection study queues of system planners across the Eastern Interconnection. These factors result in significant delays in integration of RES into the grid. Balanced reforms to the interconnection processes are needed to address these challenges.

Operating the system reliably and efficiently – Operating the power system is becoming more challenging due to limited availability of transmission, the need for system operators to maintain the proper mix of generation resources to accommodate the variability of renewable resources, and load and resource uncertainty based on weather conditions. System operators must carefully manage the impacts of renewable resources ramping up and down due to wind or solar variability, with a limited amount of flexible generation resources quickly available to follow these variations to maintain system reliability. 

To ensure system reliability and realise the potential value of these resources for consumers and the grid as a whole, close coordination between transmission and distribution system operators will be required. The energy markets will have to find out how to provide ancillary reliability services like generator ramping, voltage support, reactive power, frequency response, and system inertia more effectively in the future. As resources vary, the reliable and efficient delivery of electricity will require the development of additional market products to effectively incentivise the grid’s ancillary services. The capacity of and energy produced by renewable resources varies a great deal across North America and is highly dependent on the geographic location and type of resource. This makes it difficult to align the availability of renewable resources to the times when they are needed.

Learning to plan and operate differently – The retirement of key personnel with deep industry experience impacts engineering, operations and regulatory activities. While today’s planners are familiar with control systems for legacy synchronous generation, inverter-based resources are a new and quickly evolving technology requiring operators to develop a better understanding of the wide variety of control schemes these resources employ to ensure continued reliability in the future. 

EIPC recommendations for policymakers

The Eastern Interconnection is a single, interconnected grid with constant power flows between neighbours and within individual systems, as well as across state and international borders. The electricity market, unlike other forms of interstate markets, must always maintain a perfect supply and demand balance. It is now harder to separate policies in one jurisdiction from those in another. This rising interdependence necessitates sufficient time for system planners and operators to complete the necessary analyses in order to identify and implement relevant fleet adaptations. In this context, EIPC makes the following suggestions:

Enhance policy coordination across the three-legged stool of planning, cost allocation and siting – Enhancing planning alone will not result in the significant transmission required to achieve a high-renewable future unless policymakers address issues such as who pays for new transmission and challenges in siting new transmission, including property rights, land use, and environmental and social justice.

Current processes to allocate costs and site transmission are increasingly being challenged in some regions as the amount, size and complexity of new transmission infrastructure grows. The cost allocation issue necessitates close coordination between FERC and state public service commissions or public utility commissions (PSC/PUCs), as FERC regulates wholesale transmission rates while the PSC/PUCs regulate bundled retail rates.

The siting issue is one that is primarily within the authority of states and, in some cases, local authorities, except on federal and tribal lands. As policymakers consider future initiatives to develop renewables or advance other policies, they must recognise the need for additional transmission investment to ensure that new generation can actually deliver electricity to customers.

Establish a system of monitoring and course correction as events unfold – According to EIPC, any policy initiative should clearly provide regulators, industry and stakeholders the ability to monitor and correct course in a timely manner if a specific path is leading to excessively higher costs, reduced customer choice, or poor reliability implications. Regulators could consider a ‘reliability safety valve’ mechanism in any future legislation. The purpose of this mechanism is to recognise that any timeline embodied in statutes or regulations can potentially create unintended consequences that could impact the reliability of the bulk power system. 

Enhance collaboration – EIPC recommends that policymakers considering renewable portfolio standards (RPS), carbon dioxide standards, or other similar energy-related goals take the affirmative step of inviting system planners and operators to provide clear elaborate inputs regarding the full range of planning and operational challenges, costs and trade-offs associated with the proposed set of standards. System planners must assist policymakers to ensure their decisions are informed by reliable, unbiased information provided by the system operators who will be responsible for carrying out the policy directives. Policymakers should support assurance of grid reliability as reliability challenges arise. To safeguard, monitor and correct for reliability issues, reliability standards, detailed planning and verification are required.

Related NREL study recommends uniting Eastern and Western US electric grids

In a separate development, in October 2021, National Renewable Energy Laboratory’s (NREL) Interconnections Seam Study concluded that uniting the Eastern and Western US electric grids could offer significant value in strengthening the power system’s ability to share generation resources and flexibility across regions. The study, conducted by NREL in collaboration with the US Department of Energy (DOE), Iowa State University and the electricity industry, quantified the overall value to support utilities, developers, regulators and policymakers in their efforts to prepare the grid for the future.

Researchers developed and examined four transmission grid designs to explore their possible costs and impacts using a novel multi-model method that cost-optimises both generation and transmission resources across the US. According to the results, increasing transmission capacity between the Eastern and Western Interconnections could enable more economically efficient and flexible power exchange across the country, with scenarios indicating up to USD2.50 in benefits for every USD1 spent.

The study’s findings provide new tools that assist decision-makers in evaluating strategies for strengthening the evolving grid. Future research might look into the technological and economic benefits of alternative grid designs, as well as assess system reliability, security and resilience, among other topics.

The way forward

Substantial policy reforms are required across all issues to cost-effectively integrate large amounts of renewable capacity. In the past, projects such as the USD7 billion Texas Competitive Renewable Energy Zones initiative (CREZ), which has already unlocked massive amounts of wind and solar energy, and MISO’s Multi-Value Projects show that with coordinated transmission planning and broad cost allocation, transmission lines can be built.

It is notable that policymakers and regulators acknowledge the challenges facing the US grid and are in the process of taking action to identify needed transmission builds along with other changes required to meet the country’s energy transition objectives.

This article has been sourced from Global Transmission Research