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Abstract—Regional transmission planning has increased the

need for an expeditious way to analyze multiple transmission expansion plans under different potential futures. The PROMOD Analysis Tool (PAT) was utilized to screen numerous transmission portfolios to determine the most robust transmission portfolio across all hypothetical futures.

Index Terms—transmission planning, optimal power flow, transmission lines, Midwest ISO, transmission portfolio screening, PROMOD Analysis Tool

I. INTRODUCTION ncreased energy consumption and aging transmission infrastructure has increased the need for new transmission

infrastructure to be built in the United States. Each year, the Midwest ISO completes the Midwest ISO Transmission Expansion Plan (MTEP), which analyzes a portfolio of transmission projects to determine their effect on the reliability and economic operation of the Midwest ISO’s transmission footprint. Additionally, Midwest ISO has performed studies for regional transmission plans that analyze transmission expansion plans encompassing the entire Eastern Interconnection.

Midwest ISO performs both a reliability and economic analysis on the transmission projects proposed each year. The economic transmission analysis is performed with PROMOD IV, a security-constrained economic dispatch (SCED) software packag used in the power industry. The studies performed in PROMOD IV may analyze an incremental line addition or a complete transmission overlay. For example, Midwest ISO may study a utility’s proposed transmission line upgrade as well as a high-voltage transmission overlay, such as the Joint Coordinated System Plan which analyzes a number of high-voltage transmission additions to facilitate wind energy transfer. Studies performed in PROMOD IV typically analyze an entire year, hour-by-hour, for the entire Eastern Interconnection. Because of this, studies can take from 48-72 hours to complete. With the possibility of analyzing hundreds of projects, the analysis can quickly

Patrick Quinn is a Resource Forecasting Engineer for the Midwest ISO, St.

Paul, MN 55108 USA ( e-mail: pquinn@ midwestiso.org). Raja Thappetaobula is Technical Manager Real Time Operations for the

Midwest ISO, St. Paul, MN 55108 USA (e-mail: rthappetaobula@midwestiso.org).

become time and labor-intensive. To alleviate this problem, Midwest ISO determined that an

screening tool was necessary to analyze transmission projects and determine the most economical projects that warrant further evaluation in PROMOD IV. In order to reduce the time required for each study, Midwest ISO developed a methodology that selected a subset of representative hours from the full-year analysis to use with the screening tool. The methodology and the tool are the core components of the transmission portfolio screening process and are highlighted below.

II. INCORPORATION OF THE PROMOD IV ANALYSIS TOOL The ultimate goal in developing the transmission portfolio

screening tool was to develop a tool that would be able to mimic the results of a PROMOD IV analysis, albeit with a reduced set of hours. Additionally, Midwest ISO sought a tool that used similar input data as PROMOD IV to save time in preparing studies.

PROMOD Analysis Tool (PAT), developed by Robert Burchett, is a DC SCED tool that uses PROMOD IV files to analyze specific hours within a PROMOD IV study. Within Midwest ISO, PAT is normally used for detailed analysis of constraints and the effects of user-defined contingencies on the study. PAT can perform studies for a specific hour or 8760 hours, which gave Midwest ISO more flexibility in the number of hours chosen for the screening tool.

After PAT was identified as a potential screening tool, Robert Burchett was consulted to determine whether he could make the necessary modifications to PAT that Midwest ISO required for the transmission portfolio screening tool. The modifications that Midwest ISO requested were:

1. Modification of PAT to allow transmission projects

to be analyzed in different combinations using a Microsoft Excel spreadsheet as the input

2. Output of specific market data, such as adjusted production cost and load cost, in order to calculate benefit/cost ratios for different projects.

3. Allow PAT to accept a Microsoft Excel spreadsheet that lists the hours to be analyzed and their weighting, if any. This allows Midwest ISO to select a subset of hours and weight them, for example, by season without having to select individual hours in the PAT Graphical User Interface (GUI).

Transmission Portfolio Screening Using PROMOD Analysis Tool (August 2009)

Patrick Quinn, Member, IEEE, Raja Thappetaobula

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978-1-4244-6547-7/10/$26.00 © 2010 IEEE

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Midwest ISO performed numerous benchmarks and tests to ensure that the algorithms used in PAT were consistent with PROMOD IV and that the raw data output from PAT, such as adjusted production cost and load cost, was consistent with PROMOD IV.

III. TRANSMISSION PORTFOLIO SCREENING Midwest ISO performs numerous studies throughout the

year. These studies range from economic analysis within the Midwest ISO footprint, such as the Regional Generation Outlet Study (RGOS), to studies that span the entire Eastern Interconnection such as the Midwest Transmission Expansion Plan (MTEP) and the Joint Coordinated System Plan (JCSP). The primary purpose of the transmission screening tool was to screen hundreds of different project combinations under different scenarios and identify the top ranked projects that were the most economically beneficial. The top ranked economically beneficial projects would be further analyzed in PROMOD IV for detailed economic impacts.

In order to successfully perform transmission portfolio screening, Midwest ISO identified 3 principles that must be adhered to for any screening process to add value to the MTEP value-based planning process: 1. The screening tool must use algorithms similar to those

used in PROMOD IV. 2. The raw data output from the screening tool should be

consistent with the raw data output from PROMOD IV. 3. The screening tool should be able to analyze multiple

projects faster than PROMOD IV. These principles are fundamental components of the robustness testing that Midwest ISO uses for transmission line screening. Robustness testing is defined as the analysis of a transmission project under different scenarios or “futures” to determine which transmission project provides the most economic benefit under any hypothetical future. Midwest ISO typically analyzes projects under 3-4 different scenarios, which can range from a low-demand future to a high renewable energy penetration scenario. To rank a portfolio of transmission projects, Midwest ISO uses Regional Economic Criteria Benefits (RECB) II criteria to determine a benefit/cost ratio for each project. This methodology identified benefits as a combination of 70% adjusted production cost savings and 30% load cost savings. The calculation methodologies for adjusted production cost and load cost are described in section V. The savings are calculated by taking the difference in cost between the transmission overlay case and the base case.

IV. HOUR-PICKING METHODOLOGY To reduce the run-time of the transmission portfolio screening, Midwest ISO investigated the possibility of analyzing a small subset of hours in PAT that would be representative of an 8760-hour analysis in PROMOD IV.

During the investigation, Midwest ISO wanted to answer two questions:

1. What is the smallest number of hours that can be modeled while still producing the same project ranking results as an 8760 hour run?

2. Does a weighting factor need to be applied based on a logic (for example a seasonal or on-peak/off-peak weighting)?

Midwest ISO began their investigation with a 50 hour run and continued to increase the number of hours until an adequate benchmarking with PROMOD IV was reached. A load duration curve was created for each study and a set of hours was picked along the load duration curve, either equidistant or using a weighted methodology as indicated below. The number of hours selected ranged from 50 hours to 1000 hours, the only selection criteria being to pick the smallest number of hours that would produce results similar to a full-year PROMOD IV analysis. The primary weighting method used was an on-peak/off-peak weighting methodology, in combination with a summer/winter seasonal weighting. The first step in this methodology was to create a load duration curve for the study. This load duration curve was broken down into 4 separate load duration curves, one for each season/load-level (Summer On-Peak, Winter Off-Peak, etc). To compile the set of hours for analysis, Midwest ISO selected equidistant points along these individual load duration curves. The weighting was produced by selecting a weighted number of equidistant points from each seasonal load curve. For a 200 hour analysis with on-peak weighting, approximately 67 hours would be chosen along the seasonal on-peak load duration curves, and 33 hours would be selected along the seasonal off-peak load duration curves due to the fact that there are more on-peak hours than off-peak hours. The steps involved in weighting the hours can be seen in Figure 1. The study’s load duration curve was broken down into 4 individual load curves (the 4 red curves) and from those curves, a representative number of equidistant points were picked based on the weighting of that season/load level and the total number of hours for the study (200, 300, 1000, etc.)

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Fig. 1. Hour-Picking Methodology

V. PROMOD IV ANALYSIS TOOL The PROMOD IV Analysis Tool (PAT) is a tool that was

developed to provide valuable insight into the results of a yearly PROMOD IV analysis. It uses the same mathematical model as PROMOD IV to solve an hour-by-hour transmission optimization problem. PAT uses the same modeling files as PROMOD IV in addition to PROMOD IV-generated hourly files containing load and cost curve information.

PAT produces the requisite data for adjusted production cost and load cost calculations, some of which includes:

• Hourly Locational Marginal Price (LMP) for

buses and hubs, including energy, loss and congestion components

• Hourly unit generation and production cost • Hourly binding constraints and shadow prices • Hourly line flows • Hourly company purchase/sale information

To calculate the adjusted production cost and load cost,

PAT uses the following equations: Load Cost = Load LMP * Load Adjusted Production Cost = Production Cost + Import * Load Weighted LMP (or) - Export *Gen Weighted LMP

VI. TRANSMISSION EXPANSION TOOL Using the guidelines defined in development of the

transmission portfolio screening process, a module was developed within PAT, called the Transmission Expansion Tool. The Transmission Expansion Tool has two user-defined inputs:

1. Transmission Input Spreadsheet

2. Weighting Input Spreadsheet

The two input spreadsheets allow the user to identify the transmission projects they would like to analyze and the hours, weighted or un-weighted, that the study should comprise. This Weighting Input Spreadsheet can be considered an alternative to the Hour-Picking Methodology described in Section IV. Instead of creating seasonal load curves and picking a weighted set of hours, the user can take the original load duration curve for the study and individually weight specific hours, thus requiring less manual manipulation and data compilation. Once the projects and hours have been read in by the Transmission Expansion Tool, it creates project combinations based on user input. The Transmission Expansion Tool then runs the base case and transmission overlay case for the hours specified by the user to determine the adjusted production cost and load cost savings. These results can then be used to calculate a benefit/cost ratio and ultimately rank the projects across all futures. The two input spreadsheets are discussed below.

A. Transmission Input Spreadsheet The Transmission Input Spreadsheet is a Microsoft Excel

comma-separated value (.csv) file that defines transmission projects for individual or combinatorial analysis by the Transmission Expansion Tool. The fields in the spreadsheet that require user input include:

1. Project Category- Each project falls into a category (for example, PJM, MISO, SERC)

2. Project Number- Project number to differentiate projects

3. Add (a) or Delete (d)- Identifies whether the line should be added or deleted

4. From/To Bus- Bus number for the From and To Bus, identified in separate columns

5. Name- Name of the From or To Bus, identified in separate columns

6. Control Area- Control Area for the From or To bus, identified in separate columns

7. From/To Bus PM Area- The PROMOD-defined area the bus is located, identified in separate columns

8. kV- voltage level for the From and To bus, identified in separate columns

9. Ckt ID- circuit id for the line 10. Line Length (miles)- length of line in miles 11. Resistance (p.u.)- resistance of line 12. Reactance (p.u.)- reactance of line 13. Summer/Winter Normal/Emergency Rating-

Seasonal normal and emergency ratings for the transmission line

14. Cost- Total cost for the transmission line In its current form, the Transmission Expansion Tool can

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List of Hours for PAT Analysis

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perform a combinatorial analysis on three different project categories. The tool will combine every project within a category with every other project in another category. For example, if the project categories were represented by the Midwest ISO regions (West, Central, and East) and there was one project in each region, then a list of all combinations would look like:

West Project 1 Central Project 1 East Project 1 West Project 1-Central Project 1 West Project 1- East Project 1 Central Project 1-East Project 1 West Project 1-Central Project 1-East Project 1 The user can tailor their query so that project combinations

within the same region are included or excluded and also select whether they want to look at one-, two-, or three-project combinations. Once the project combinations have been created, the Transmission Expansion Tool performs an economic analysis for the base case and then adds the transmission combinations, one combination at a time, to determine the adjusted production cost and load cost savings. B. Weighting Input Spreadsheet

As part of their hour-picking methodology, Midwest ISO developed hourly samples based on seasons and weighted the economic benefits of these hourly samples with a different weighting factor for each season. In order to calculate the weighted economic benefits for the transmission projects, an hourly weighting input spreadsheet was developed as an input to the Transmission Expansion Tool.

Midwest ISO assigns a weighting factor to each hour as part of the input and PAT calculates the economic benefits of each transmission plan based on this user input, taking into consideration the weighting factor of each hour. A screenshot of the input spreadsheet, identifying date, hour, and weighting, can be found in Figure 2.

PAT Hour Selections Hour Weight 12/29/2003 3 0.7512/29/2003 4 1

Fig. 2 Weighting Input Spreadsheet

VII. BENCHMARKING Before implementing the PAT Transmission Expansion

Tool, a benchmark was conducted to ensure that PAT produced the same economic results as a PROMOD IV analysis. The two criteria that Midwest ISO identified as critical to the successful implementation of the Transmission Expansion Tool were:

1. PAT produces the same economic results as PROMOD IV when comparing the same hours.

2. The ranking of projects in PAT and PROMOD IV should be similar when a reduced-hour PAT run is compared to a full-year PROMOD IV run.

For the benchmark to work, Midwest ISO had to be certain that the unit commitment, dispatch algorithms and other internal calculations were comparable between PROMOD IV and PAT. During benchmarking, Midwest ISO determined that there was a discrepancy between the dispatch algorithms in PAT and PROMOD IV. The block-loading, used for unit commitment, was different between PAT and PROMOD IV, which created discrepancies in the production cost of the dispatched units between the programs. Additionally, a loss-penalty factor, used in PROMOD IV, prevented PAT from calculating the true production cost of a unit. Once these issues were resolved, Midwest ISO proceeded with the benchmarking. For the first benchmark, Midwest ISO performed a one-month analysis of five transmission projects in PAT and PROMOD IV. The transmission projects were projects proposed as part of the MTEP process and the month chosen was March 2021, which aligned well with the proposed in-service date of the transmission projects.

The same calculation methodology was used for the PAT and PROMOD IV economic analysis. PAT was run for all the hours in the month for the base system and again for each new transmission system. PAT and PROMOD IV calculated the load costs and adjusted production costs for all the cases which Midwest ISO used to calculate the RECB II benefit/cost ratio. Using the benefit/cost ratios to create a ranking of projects in PAT and PROMOD IV, an identical ranking was produced for both tools.

The next step in the benchmarking was to verify whether PAT would produce comparable ranking results with PROMOD IV for the hours that were selected using the hour-picking methodology. Benefit/cost ratios for the new transmission options would be calculated with PAT for all hours selected with the hour-picking methodology and compared to benefit/cost ratios calculated using PROMOD IV for a full study year using the same set of transmission projects. This benchmark was used to ensure that the hour-picking methodology was valid and the appropriate amount of hours was picked in order to save processing time. If the hour-picking methodology was valid, the top projects identified by PAT could be analyzed in-depth using PROMOD IV.

Midwest ISO selected eleven test transmission projects from the MTEP 08 2021 overlay and using the PAT Transmission Expansion Tool, developed eighteen different transmission project combinations for benchmarking purposes. The test projects were selected from the Midwest ISO West, Central and East regions. Because the PAT Transmission Expansion Tool was initially created to only handle two-category combinations, Central projects were grouped with West projects for the benchmark. Additionally, a number of PJM transmission expansion projects were added to the cases as existing transmission infrastructure. The

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project combinations spanned from West region to the East Region within the Midwest ISO footprint. Two different hour-picking methodologies were identified for the bench marking.

• Method M1: Set of 200 hrs (67 summer peak hours,

33 summer off-peak hours, 67 winter peak hours and 33 winter off-peak hours). This combination gives more weight to peak hours.

• Method M2: Set of 200 hrs (66 summer off-peak

hours, 33 summer peak hours, 67 winter off-peak hours and 33 winter peak hours). This combination gives more weight to off-peak hours.

To verify that the hour-picking methodologies worked for

scenarios used in MTEP analyses, Midwest ISO selected three futures from the MTEP 08 analysis: Reference Future, 20% Wind Future and Gas Future. As their names indicate, the Reference Future represents a base case, with no significant modifications to the proposed generation, transmission, and load in 2021. The Wind Future uses the Reference Future as a base, but increases the wind penetration in the Midwest ISO footprint by 20%. The Gas Future represented a model where the only generation expansion allowed

Analysis of the results indicates that the M2 methodology- which was weighted more towards off-peak hours- benchmarked well with the full-year Reference Future run performed with PROMOD IV. Neither Method 1 nor Method 2 benchmarked well with the 20% Wind Future. Because of the large amount of wind in the model, a variety of hour-picking methodologies may need to be tested to determine which methodology most accurately captures the effects of large amounts of wind on the network. The Gas Future benchmarked well against Method M1, which was weighted towards on-peak hours. A table of the results, comparing the Gas Future results to the two weighting methodologies can be found in Figure 3.

Fig. 3 Method M1 and M2 PROMOD IV/PAT Ranking

VIII. CONCLUSION In conclusion, through the benchmarking process Midwest

ISO identified that the PAT Transmission Expansion Tool can be used as an economic transmission screening tool. It

produces consistent transmission screening results when compared with PROMOD IV, allowing top-ranked transmission projects to be evaluated further in PROMOD IV. This would enable Midwest ISO to study numerous transmission combinations by screening them initially in PAT, with more detailed analysis to follow in PROMOD IV for those projects or portfolio of projects that show economic benefit to the system. As the number of projects analyzed increases, Midwest ISO hopes to use this process as a way to filter out top projects for further PROMOD IV evaluation.

Midwest ISO identified that the algorithms and raw output from PROMOD IV and PAT are comparable, which means that the only difficulty in obtaining an adequate benchmark between PROMOD IV and PAT lies in the hour-picking methodology. The off-peak weighting methodology worked well for the Reference Future run, while the on-peak weighting methodology worked well for the Gas Future run. Midwest ISO will continue to study whether one hour-picking methodology will work for all futures or different methodologies will need to be developed for each future, as evidenced in the analysis cited above. Once a methodology has been developed for one future, it can be used for futures with similar characteristics.

REFERENCES [1] A. Wood, B. Wollenberg, Power Generation, Operation, and Control.

Mill Valley, CA: University Science, 1996. [2] Midwest ISO Transmission Expansion Plan 2008,

Midwest ISO, Carmel, IN, 2008. [3] Promod Analysis Tool (PAT), Robert Burchett,

Available: burchett@nycap.rr.com Patrick Quinn (M’05) is an Expansion Planning Engineer for the Midwest ISO. His primary duties include transmission planning, and dynamic modeling. He received a BSEE from Iowa State University in 2004 and is currently pursuing a MSEE at Iowa State University. Raja Thappetaobula received his M.S. from Missouri University of Science and Technology in 2001. He has been working as Transmission Planning and Economics Studies Engineer at Midwest ISO since 2001. He is currently Technical Manager Real Time Operations for the Midwest ISO.