Power Engineering

Enabling Large-Scale Renewables in the Western U.S.

Power from the Prairie is a proposed 4,000 Megawatt HVDC transmission project in the  wind-rich energy resource of the Upper Midwest.

Power from the Prairie is a proposed 4,000 Megawatt HVDC transmission project in the wind-rich energy resource of the Upper Midwest.

The Northern Prairie region of the United States (i.e., Wyoming, Nebraska, the Dakotas, etc.) has an abundance of wind and wide-open spaces that could be used to develop large, wind-powered electricity generation resources. The opportunity is daily, bi-directional renewable energy swaps between California, the Northern Prairie, Chicago and points East.

Past studies by the Midcontinent Independent System Operator (MISO) have shown that a national high-voltage direct current (HVDC) power transmission grid could significantly improve the capability of wind to reliably and economically supply power to load. But the practical hurdles to developing a suitable nationwide power grid all-at-once are a barrier to such a solution. However, it may be possible to initially achieve such economies and reliable production at a more focused, regional scale, as building blocks toward an eventual nationwide system. Such regional scale must have an abundance of wind (and potentially solar energy too), access to significant markets for such electricity, and the capability to add large-scale, grid-level energy storage where available and cost-effective.

Such development could also be used to enable the State of California to achieve its planned high levels of Renewable Portfolio Standard (50 percent or more) by building sufficient renewables in-state. When California inevitably over-generates from its solar photovoltaic installations during the solar day, it could export the over-production to other states over the network, and then import a previously-contracted “swap” of renewable energy back from other states when the sun sets.

The Power from the Prairie Project

Power from the Prairie is a proposed 4,000 Megawatt HVDC transmission project in the wind-rich energy resource of the Upper Midwest (Figure 1). The new line would extend from Southeast Wyoming across Nebraska or South Dakota to Northwest Iowa or Sioux Falls or Omaha. It would feature a DC/AC/DC terminal in the middle to enable interconnection of large quantities (thousands of MW) of additional renewable energy in Nebraska and South Dakota currently blockaded and unavailable due to lack of transmission for access to markets. The effort may also include grid-level energy storage, if found to provide material benefits in addition to the HVDC line itself.

The goals of Power from the Prairie are:

  • Develop low cost electricity with wind resources in the Northern Prairie that will take advantage of the natural geographic and temporal diversity of customer loads as well as wind and solar generation resources across large, inter-regional areas;


  • Reduce utility generation capacity reserve requirements and associated costs without sacrificing reliability;


  • Enable the economic integration of massive quantities of economical renewable energy resources into the grid—supporting very high Renewable Portfolio Standards (RPS);


  • When combined with similar HVDC developments to the West and East, enable large-scale hourly and daily renewable energy swaps between California, Midwest and Eastern states to achieve high levels of renewable energy; and:


  • Provide a primarily-clean energy replacement alternative for legacy generation facilities as they are retired.


The result would be an inter-regional, renewable energy superhighway with multiple new ‘on-ramps” for renewable resources currently blockaded due to lack of transmission, enabling renewable energy swaps between regions to make intermittent renewables more reliable. Simply, when viewed along the span of a long, multi-state and efficient HVDC transmission system, renewable energy is always happening somewhere.

Relationship to Other Efforts

DOE/NREL Interconnections Seam Study

The U.S. Department of Energy (DOE) Grid Modernization Program includes aggressive initial efforts toward a national high voltage direct current (HVDC) transmission grid overlay over the existing alternating current (AC) system. As part of the Program, the National Renewable Energy Laboratory (NREL) is performing a $1.3 million “Interconnections Seam Study” examining the conceptual and economic feasibility of such a national transmission overlay, combined with renewable energy.

“Past studies by the Midcontinent Independent System Operator have shown that a national high-voltage direct current power transmission grid could significantly improve the capability of wind to reliably and economically supply power to load.”

The term “Seams” refers to the current boundaries between the Western Interconnection, Eastern Interconnection, and Electric Reliability Council of Texas (ERCOT) systems. See Figure 2. These three alternating current (AC) systems operate independently, and asynchronously. That is, their 60-cycle AC waveforms are not aligned. They are connected together only at seven relatively small (200 MW or less) AC/DC/AC terminals located along their seams. The Seams Study is examining how transfers across the Seams (and thus between the Interconnections) can be significantly increased in the future.

The use of HVDC transmission technology is important because: a) it is more efficient over long distances; b) its power flow is electronically controllable /throttleable (compared to AC systems where the power flows by the path of least resistance); and: c) it can be used to interconnect asynchronous AC systems. The latter benefit is critical to enabling large-scale renewable energy transfers across the Interconnection “Seams”.

The intended result is that the Seams Study will provide a global look at the generic, conceptual feasibility of a nationwide HVDC transmission overlay. Preliminary results of the Study, to be publicly released in early 2018, indicate such a national HVDC transmission grid combined with renewables would be a cost-effective resource for electric customers. NREL staff is sharing modeling data and study results with the Power from the Prairie team.

A follow-on Concept Development Study (CDS) described later in this paper will define and examine the economics and other key factors of a Power from the Prairie development—entailing a specific HVDC line route with specific project participants, and identifying who would benefit from such a project. The PftP CDS will provide a focused, project- and utility sponsor-specific look at one initial element of such a nationwide infrastructure, plus concepts of how such transmission elements could be accomplished.

Other HVDC Transmission Developments

The Power from the Prairie line would form a go-between for several currently under-development HVDC transmission line development projects. To the West in Wyoming, it would link with either the Anschutz “TransWest Express” project from South Central Wyoming to Delta, Utah to Las Vegas. This project is already on the Trump Administration’s’ priority list for infrastructure projects. Alternatively, PftP could link with the Duke-ATC Transmission “Zephyr” line from Southeast Wyoming to Delta, Utah.

Once at Delta, Utah, there is an existing HVDC transmission line from there to Los Angeles called the Southern Transmission System (STS). This 500 kVDC, 2400 MW line currently delivers baseload energy from the 1,900-MW coal-fired Intermountain Power Project at Delta to Southern California. The plant is scheduled to be retired in 2025.

To the East, Power from the Prairie would link to similar HVDC projects like the Rock Island Clean Line from Northwest Iowa to Chicago and PJM Interconnection. Although Rock Island is experiencing court challenges in Illinois, unfortunately such challenges are part of the business of such developments. In the spirit of DOE Grid Modernization and Seams Study efforts, the authors believe such projects will eventually proceed.

Large Scale Energy Storage

A Power from the Prairie effort could include two types of large, grid-scale energy storage: virtual and physical.

Virtual Storage

Virtual storage refers to a system that exhibits the characteristics of storage, but does not entail physical storage. Energy swaps between California and Upper Midwest States would be virtual storage. From California’s perspective, they would export energy to other states via the HVDC system during renewable overproduction time periods in California. Later, California would receive renewable energy back from those states to complete the swap. To California, it looks like storage is happening somewhere else. When in reality, the other States may in fact be consuming California’s renewable overproduction, and then returning their own surplus renewable energy to California.

This virtual storage approach is not unique to Power from the Prairie. Minnesota Power recently enacted a similar scheme to send surplus North Dakota wind energy northward to Manitoba. Manitoba later returns hydro-based renewable energy to Minnesota Power when Minnesota Power needs it. And the exchange does not necessarily involve physical storage. Instead, in this innovative way the parties take advantage of the time diversity between the output of their renewable resources (both dispatchable and non-dispatchable), and the timing of their respective customers’ needs for electricity.

Physical Storage

With regard to physical storage, we are all now accustomed to the ongoing discussion and potential of battery storage. The Tesla Powerwall 2 for example, offering residential battery storage applications of 14 kilowatt-hours (kWh) of storage can be installed for a capital installed cost of about $540 per kWh of storage. And battery costs are projected to continue to decline.

For Power from the Prairie purposes, larger scale and storage duration are required. Grid-level storage offers particular advantages because, if sited correctly, it can be used as a transmission asset, enabling twice the amount of renewable energy to be sent down limited transmission capacity compared to renewables integration with just conventional, gas-fired generation alone. Simply, you intentionally “super-size” the capacity of renewable energy located electrically behind the storage compared to available outlet transmission capacity. Then, when the renewable output is more than the outlet transmission capacity can handle, you store the excess and release it later when the renewables are producing less, or when the transmission constraint is otherwise relieved.

Schulte Associates LLC has recently performed feasibility studies for large-scale (1200 MW or more, 48 hours of storage) compressed air energy storage (CAES) installations. Such a CAES installation at Delta Utah, a world-class CAES site candidate, could be combined with Wyoming wind and HVDC transmission to replace the existing 1900 MW coal-fired Intermountain Power Project at Delta. Such CAES would have a capital cost of only about $40 per kWh stored. And if you are storing wind energy that is subject to extended lulls, 48 hours of storage at full output is a lot better than the eight to ten hours typically assumed for stationery batteries.

Schulte Associates LLC has also been involved in feasibility studies for the Gregory County pumped hydro storage project combined with renewables in Central South Dakota. At 1200 MW or more and 26 hours of storage, this site is located directly on the route of the Power from the Prairie line.

Such physical storage could offer additional cost and reliability benefits than just the virtual storage benefits of the HVDC line itself.

“A 50% RPS based on annual energy means California will need to install renewable production capacity meeting or exceeding its annual peak electric demand by 2030.”

The California Conundrum

The State of California along with Hawaii lead the nation in plans for renewable energy development. California law (Senate Bill 350) specifies a statewide Renewable Portfolio Standard (RPS) of 50% of total retail electric energy sales by the year 2030. That means, under penalty of law, California electric utilities must provide 50% of their total retail electric sales from renewable sources by 2030.

Recently, an even more aggressive bill, SB 100, was introduced that would have increased the RPS to 60% by 2030, with an intent to achieve 100% clean energy by 2045. This measure did not pass this year, but is sure to arise again in the next legislative session.

What does such a high RPS mean in terms of what practically needs to be done? Let’s do the math. California electric customers have an average annual load factor of about 55%. Renewable energy resources located in California (wind and solar) typically have an average annual capacity factor of about 25%. And California so far indicates a preference for having its renewable resources located in-state.

A 50% RPS based on annual energy means California will need to install renewable production capacity meeting or exceeding its annual peak electric demand by 2030. That’s the highest electric demand that happens on the hottest day of the year. That’s fine. But having that much installed production capacity that is non-dispatchable means California will be over-generating on all other sunny days during the year. Particularly in the spring, fall and winter, when electric loads are lower. The state is already experiencing periods when market prices go negative (that is, there is more generation than the market needs, and producers need to pay to generate; not get paid for it. And the 50% RPS level (much less the higher RPS levels being contemplated) is still many years in the future.

This California renewables over-production during the day needs to find a market. Conversely, California needs renewable resources to serve its loads when the sun is not shining there.

Making Renewables Distribution Bi-Directional

To-date, all of the proposed HVDC transmission developments designed to move wind energy from the Midwest to load centers have been designed to be one-way, unidirectional. Either from the Midwest to the East, or from the Midwest to the West. In contrast, in addition to enabling renewable energy from the Upper Midwest, Power from the Prairie exhibits an interesting feature for possible use elsewhere: bi-directional renewables distribution (See Figure 4). Renewable energy can travel on Power from the Prairie and its neighboring HVDC lines from the Midwest to the East or West. Or conversely, from California and the Southwest in general to the East.

The challenge of future renewables over-generation from California is probably larger than Power from the Prairie alone can resolve. Perhaps the other proposed HVDC transmission developments should be made bi-directional as well, by connecting them in the middle of the country like Power from the Prairie envisions for the Northern Prairie?

The Concept Development Study

As an initial step in such a regional approach, the Power from the Prairie team plans a Concept Development Study (CDS). The goal of the CDS is to develop initial definition, data and quantified benefits of a Power from the Prairie development to enable the study participants to determine if pursing such a development is in their customers’ and stakeholders’ best interest.

The CDS sponsors include multiple regional utilities and others interested in whether such a development would benefit their customers and stakeholders. The CDS will be performed from their perspective. It will also quantify the value of the proposed HVDC projects to West and East by themselves, and the market opportunities for Power from the Prairie to the West and East. The study, to be performed in mid-2018 as a follow-on to the NRES Seams Study, will entail four Tasks:

Task 1: Resource Modeling and Benefits. This Task will include modeling of the U.S. electric system from the Western Interconnection to the Eastern Interconnection, including the Western Electricity Coordinating Council (WECC), Southwest Power Pool (SPP), Midcontinent Independent System Operator (MISO), and PJM Interconnection (PJM); both with and without the Power from the Prairie development.

Task 2: Technology and Operations. This Task will define applicable HVDC technology options and market structures for operating such an inter-regional HVDC line. It will consider how such a line would operate in relation to regional and RTO/ISO markets. It will consider how automated the HVDC line operation can be, and how renewable and other facilities would request interconnection to it. It will also consider the types of cyber security provisions that need to be included in project design.

Task 3: Organization and Policy. This Task will define organizational options useful for creating a diverse “DC Federation” of participants for implementing such a project. It will also identify federal/state/local regulatory, policy and tariff issues and develop a repeatable roadmap for addressing them when planning and developing similar projects elsewhere.

Task 4: CDS Management, Coordination and Report Writing. This Task will ensure the CDS delivers quality and actionable products on-schedule.

The PftP team plans to complete the CDS by late 2018.

Grid-Level Storage to also be Considered

The CDS will also determine the economic and operational benefits of grid-level storage that could be provided with pumped hydro or compressed air energy storage (CAES) opportunities, and whether they would be incrementally beneficial in addition to the benefits of the HVDC transmission alone. It will also determine the reliability and economic improvements that may be possible through diversity between utility customer loads and available renewable resources across the region, and in conjunction with the large markets of the Midwest and West.

PftP would represent new inter-regional infrastructure in the form of an electric transmission “superhighway” for cost-effective renewable energy, with on-ramps for some of the richest renewable energy resources in the nation. Such resources do not appear in traditional power supply system studies that tend to only focus either on the needs of individual utility systems, or individual independent transmission owners, depending on who the project sponsors are.

In contrast, the PftP CDS will be done as an independent and objective due-diligence determination of who would benefit from such a project, and how much. It would not require development of a cogent, coordinated federal energy policy in advance.

Instead and more simply, let the project participants (utilities, individual states, and others) self-identify themselves in their own respective best interests as a federation of the willing.

Bob Schulte is a Principal in Schulte Associates LLC, an executive management consulting firm with offices in Raleigh, North Carolina. Fredric Fletcher is Chairman of Power from the Prairie LLC.

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