Studies Link Wind Transmission Methods

by Robert J. Henke, Ken Collison and Venkat Banunarayanan, ICF International

Two groundbreaking ICF International studies provide direction for the effective siting of high-power transmission lines and the design of collector systems. Conducted in Wyoming, the independent studies primarily address costs and reliability. Their findings improve the prospects of meeting energy needs through renewable sources in Wyoming and other states. Sponsored by the Wyoming Infrastructure Authority, both studies illustrate the link between renewable energy and transmission.

The following summarizes ICF’s innovative technical approaches for the studies. ICF recommends that application of the approaches be customized for the characteristics of the specific setting.

Study for Siting Power Transmission Lines

ICF’s first study encompasses the development and application of an analytic framework for determining the minimum separation distance for maintaining power system reliability in Wyoming. Applicable for other states, this analytical “Framework for Analyzing Separation Distance between Transmission Lines in Wyoming” and the results of its application to Wyoming are available online as a PDF (see link at the end of this article).

Study for Designing Collector System

The “Wyoming Collector and Transmission System Conceptual Design Study” (see link at the end of this article) was prompted by the state’s large, high-class wind energy potential and status as a major exporter of energy generated from coal and natural gas. Expectations for the state’s wind development and constraints on existing transmission lines have increased the need for a collector system to transport energy. Currently, no reliable, secure collector system exists in Wyoming.

Transmission Line Separation Study Challenges

Wyoming encounters multiple challenges in siting transmission lines that cross public lands administered by the Bureau of Land Management or other federal agencies. One challenge involves the tradeoff between cost and risk. Maintaining an adequate distance between lines reduces risk of simultaneous outages caused by storms and other natural disasters. Yet, as the separation distance increases, so do right-of-way acquisition costs and potential impacts on the environment and land use. ICF’s solution offers a means for determining the minimum separation distance for siting new transmission lines while meeting the applicable power system reliability criteria. The framework developed in ICF’s study is not intended to be either in whole, or in part, a substitute for processes that should be followed to obtain the regional entity’s approval.

Figure 1 shows pictorially various factors that impact the determination of separation distance between two parallel lines.

The terms “minimum” and “complex” are especially relevant in this study. No single separation distance works as the universal answer because of complexities associated with power system reliability, land acquisition and transmission costs, environmental and land-use considerations and other issues that differ among regions. ICF’s study developed a universally applicable analytic framework for determining the fundamental distance between high-voltage transmission lines. When systematically applied, the framework presents a robust methodology with which the minimum separation distance between two parallel transmission lines can be derived.

Study Design and Results

ICF’s framework determines the minimum distance based on the absolute minimum separation distance that can be adjusted by case-specific and regional factors. This minimum is expressed as:

SD = AB”MIN + CASE”MIN + REG”MIN
where SD = minimum line separation distance needed
AB”MIN = absolute minimum needed separation distance
CASE”MIN = change to AB”MIN needed case by case (incremental or decremental)
REG”MIN = change to AB”MIN because of regional factors such as weather (incremental or decremental)

The range of minimum line separation distance needed could vary from AB”MIN to the sum of AB”MIN, CASE”MIN and REG”MIN based on specific mitigation measures such as line de-rating or robust transmission line construction. This calculation is shown pictorially in Figure 2.

The AB”MIN component of the line separation distance depends only on industry codes and the types and characteristics of the transmission lines, which are relatively standard. Therefore, this value could be estimated without performing any region”specific analysis. The CASE”MIN component depends on the circumstances surrounding individual transmission line projects such as land topology and voltage level. The REG”MIN depends on regional factors such as frequency of weather”related line outages, current reliability of the power system, availability of operating guides, likelihood of airplane strikes and risk of damage from fires.

Application to Wyoming

As described, ICF developed a framework and specific methods to determine the minimum separation distance to mitigate the risk of a simultaneous outage of two parallel transmission lines. The analysis of available data for Wyoming indicated that the minimum line separation for new lines in eastern and southeastern Wyoming ranges from about 260 feet up to the longest span length (1,500 feet in this study) for parallel 500-kV transmission lines. The lower value of the range is dependent on the height of the transmission tower and line sag length. The upper value encompasses the lower value and equals the longest span length of two illustrative 500-kV transmission lines.

This analytical approach works for calculating lines of various voltages, tower heights and span lengths. Changes in one or more of the assumptions or constraints identified, however, could substantially increase the minimum separation range.

In addition, the recommended minimum range in ICF’s transmission report is only one of several factors that should be used in determining the actual separation of lines in Wyoming. Other factors that must be considered include the Western Electricity Coordinating Council path-rating process; costs; environmental permitting; land-use constraints; public and other stakeholder interests; and state, regional and national concerns.

Collector System Objectives

Multiple wind farms (WFs in Figures 3 and 4) are in various stages of implementation and planning to take advantage of Wyoming’s substantial wind resource. The transmission lines already mentioned are proposed to export that wind-based power to distant load centers. The second study investigates the missing piece in the transmission network: a collector system to serve as an intermediate transmission grid that connects the transmission export lines with the wind farms.

Study Design and Results

ICF developed conceptual designs and high-level cost estimates for a reliable system for collecting and transporting wind energy from Wyoming to distant load centers. The system could collect and transport up to 12 GW via the six high-voltage transmission lines identified in the first study. Comprising two scenarios with various collector system configurations and a staging process, the study shows the feasibility for designing reliable systems to transfer 12 GW of wind out of Wyoming.

To frame the analysis, ICF created two illustrative resource scenarios that are differentiated by the conceptual location and amount of wind power development in Wyoming. Resource Scenario No. 1 (see Figure 3) assumes development of wind generation in a relatively widely dispersed manner in qualified resource areas (QRAs) primarily west of the Laramie Mountain Range. Resource Scenario No. 2 (see Figure 4) covers wind generation within QRAs only to the east of the range in a narrow north-south band. While the study does not predict where wind energy will be developed in Wyoming, the two resource scenarios bracket the QRAs identified through the U.S. Department of Energy and Western Governors’ Association Western Renewable Energy Zones. For each scenario, ICF developed collector system designs for four configurations: radial-single circuit, radial-double circuit, networked wind hubs, and networked wind hubs and transmission export hubs.

A conceptual networked design and the various components of a wind farm, wind hub (WH), and a transmission export hub (TEH) are shown in the Figures 3 and 4.

ICF’s study demonstrated that reliable collector systems capable of transferring up to 12 GW of wind out of Wyoming can be designed. The designed systems will withstand contingencies of the collector network elements without significant overloads on the rest of the existing network in Wyoming.

The collector system design is conceptual at this point. More studies such as detailed contingency analyses and dynamic analyses and approval from the regional reliability authority are needed before finalizing a specific collector system design.

Conclusion

The two studies provide a framework and specific methods for addressing critical measures and systems related to renewable energy.

They are an important foundation for siting transmission lines and designing reliable and secure collector systems. As with all frameworks and methods, they must be systematically used with adequate knowledge and definition of the specific application.

In the case of the transmission study, multiple opportunities for further analyses exist.

For example, one study might weigh the effects of different future energy-supply scenarios on line separation distances and societal considerations.

Another could investigate the impact of complying with the necessary reliability rules, especially when they conflict with the development of renewable energy while meeting a federally mandated renewable energy standard.

A third study could examine the maximum range of line separation distances in environmental and land-use factors.

“Framework for Analyzing Separation Distances between Transmission Lines in Wyoming” PDF: http://icfi.com/docs/wyoming-transmission.pdf

“Wyoming Collector and Transmission System Conceptual Design” PDF: http://icfi.com/docs/wyoming-collector-transmission-final.pdf

Authors

Robert J. Henke is senior vice president of ICF International, a provider of consulting services and technology solutions to government and commercial clients. Reach him at rhenke@icfi.com or 303-792-7810.

Ken Collison is vice president of ICF International. He is an expert in electric power transmission and power system reliability.

Venkat Banunarayanan is senior manager of ICF International. He has expertise in power system technical and economic analyses, cost-benefit studies, estimating renewable generation performance and impacts, and energy market assessments.

 

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