This section describes the methods and techniques that we use to analyze our transmission system for this assessment. As part of this assessment, ATC conducted power flow analyses to identify problems or constraints on the transmission system and evaluated the merits of potential reinforcements to address the system limitations that were identified.

To facilitate an understanding of the status of the various future projects, we classify projects into one of three possible categories – Planned, Proposed or Provisional. Each classification has specific criteria based on the status of the project as outlined below:

Planned projects:

• ATC planning is complete;
• if required, we have applied for regulatory approvals, which may be pending or have been issued;
• project may be under construction or in construction planning phase; and
• project typically is included in power flow models used to analyze transmission service and interconnection requests.

Proposed projects:

• ATC planning is not complete;
• ATC has not yet pursued regulatory approvals;
• project represents ATC’s preliminary preferred project alternatives from a system performance perspective; and
• project typically is not included in power flow models used to analyze transmission service and interconnection requests.

Provisional projects:

• ATC planning is not complete;
• ATC has not yet sought regulatory approvals;
• project does not necessarily represent ATC’s preliminary preferred project alternative, but does reflect meeting the need identified; and
• project is not included in power flow models used to analyze transmission service and interconnection requests.

Included in this section is a discussion of which years ATC identified to model to satisfy both the near-term (1 – 5 year horizon) and long-term (5 year and beyond horizon) NERC standards for assessing the transmission system.  Also included in this section is discussion on how ATC built each of the models used in this assessment.  Discussion items include topics such as load forecasting, which reinforcements and new generation to include in models, which system load levels, import levels and system bias scenarios to evaluate.   

During the assessment of the transmission system, ATC performed simulations on a variety of models as discussed below in this section.  ATC not only uses these models to identify where constraints or system limitations may exist, but we also use these models in testing the robustness of potential system reinforcements.  Per ATC’s Planning Criteria, constraints or system limitations are identified for NERC Category A type system conditions when bus voltages drop below 95 percent or exceed 105 percent of their nominal voltage or when any system element exceeds it normal rating for the appropriate seasonal model. For NERC Category A or system intact conditions, ATC’s Planning Criteria also requires for generators to be limited to 90 percent of their net Qmax capability within ATC footprint.

For NERC Category B, C or D contingencies, system limitations or constraints are identified using a slightly different criteria.  For these types of system contingency conditions, ATC’s Planning Criteria identify system limitations when bus voltages drop below 90 percent or exceed 110 percent of their nominal voltage or when any system element exceeds its emergency rating for the appropriate seasonal model. For these three NERC categories, ATC’s Planning Criteria requires generators to be limited to 95 percent of their net Qmax capability within ATC footprint.

The analyses conducted in this transmission system assessment included steady state power flow analyses, stability simulations, multiple outage impacts as well as economic evaluations of transfer capability limitations, generator interconnection impacts and environmental assessment impacts.

In all of the models, normal operating procedures were modeled for the applicable normal system conditions. All existing and planned protection systems, including any backup or redundant systems that would be applicable to a given contingency were simulated in the studies and analyses. All existing and planned control devices that would be applicable to a given contingency were simulated in the studies and analyses. These control devices include transformer automatic tap changers, capacitor bank automatic controls, and Distribution Superconducting Magnetic Energy Storage (DSMES) units. No specific facility outages are modeled in the planning horizon at the demand levels that were studied due to lack of future outage schedules. As the future unfolds and facility outages are scheduled, they will be timed for conditions that provide acceptable reliability.

Assessment development

This 2008 Assessment was developed in a chronological fashion. Planned transmission additions expected to be in service by June 2009 were included in the 2009 model, as listed in Table PF-1. Projects for which we have completed our analysis and are either under construction, have filed an application to construct, or are in the process of preparing an application were included in the 2013 and 2018 models as appropriate based on projected in service dates (See Tables PF-2, PF-3 and PF-4).

• The needs identified in this Assessment were determined by identifying facilities whose normal or emergency ratings or tolerances are exceeded. The criterion we use to determine what these ratings and tolerances should be is provided in Planning criteria).

Steady State Power flow models

Computer simulation model years for the 10-Year Assessment analyses were selected in order to meet NERC requirements for a 1-5 year horizon and beyond the 5 year horizon. The years 2009 and 2013 were selected to meet the 1-5 year horizon. The years 2018 and 2023 meet the beyond 5 year horizon. A range of system conditions and study years were developed and analyzed for the 2008 Assessment. Steady state peak load models for all four years were created. In order to determine how close ATC generators were to their maximum var output, two additional models were created for each year. The one model reduced ATC generator net Qmax by 10 percent for each year studied. These models were utilized to determine generator var output under intact system conditions (TPL-001-0). A second model for each year was created with net Qmax reduced by 5 percent. These models were used for our N-1 (TPL-002-0) analysis.

In addition to the steady state summer peak models, we developed 2009 and 2013 shoulder peak models that reflected load levels at approximately 70 percent of summer peak. The shoulder peak models included a 2000 MW import into ATC. To simulate a steady state reverse east-west bias power flow, models were developed in 2009 and 2013 with 90% load levels, 1700 MW import into ATC, and a 2000 MW transaction from ECAR to MAPP. Finally, in 2013, we developed steady state models for determining the sensitivity of project needs to higher than expected loads (referred to as “90/10” or “hot summer” in this assessment).

The 2009 steady state summer peak model was developed to evaluate near-term needs and to verify findings in the 2008 Assessment. We have taken the approach of evaluating the subsequent summer peak season in each of our annual Assessments to determine the immediacy of needs identified, hence providing a means of prioritization.

The 2013 steady state summer peak model was developed as an intermediate term model to evaluate emerging needs, to confirm that needs identified in 2009 will increase over time, and to test the performance of reinforcements placed in service prior to 2013. The 2013 summer 90/10 (or “hot summer”) model was developed in order to determine in-service date sensitivity to load growth that is higher or weather that is warmer than forecasted. The 2013 shoulder model was developed to identify needs and test the performance of reinforcements placed in service prior to 2013. Shoulder load periods often place as great or greater demand on the transmission system as do peak periods. During these periods, since loads are not at their highest levels, local peaking generation typically is not operating and power transfers into and across our system often are at maximum levels.

The 2018 steady state summer peak model was developed to identify emerging needs in the 2013-2018 timeframe, to confirm that needs identified in 2013 will increase over time and to test the performance of reinforcements to be placed in service prior to 2018. It also reflects a year sufficiently forward in time to determine the need for and assess the performance of larger-scale projects (345-kV lines, for example) that could be expected to be in service in that timeframe.

The 2023 steady state summer peak model was developed to identify emerging needs in the 2018-2023 timeframe, to confirm that needs identified in 2018 will increase over time and to test the performance of reinforcements to be placed in service prior to 2018. It also reflects a year sufficiently forward in time to determine the need for and assess the performance of larger-scale projects (345-kV lines, for example) that could be expected to be in service in that timeframe.

Steady State Power flow model development

We started model development for this Assessment by building a system model that represented 2008 summer peak conditions. This 2008 model is referred to as an “as-built” model because essentially everything in the model is certain to be in service by 2008 summer. This model then was modified to create each of the Assessment study models including the changes listed below for that model.

2009 summer peak model

• We utilized interconnection point load forecasts provided by various distribution companies in 2007 for both real and reactive power components of load. Please refer to the Load Forecast section for further details.
• We revised line and equipment ratings based on updates to our Substation Equipment and Line Database (SELD). As of June 2008, nearly 50 percent of ATC lines and 17 percent of ATC transformers have ratings in SELD that have been validated. Ratings not yet validated in SELD generally are based on the ratings received from the utilities that contributed the facilities to ATC.
• Updated future generation attached to the ATC system was included in the models. The specifics are outlined later in this section (Refer to New generation assumptions). Balancing Authority (Control) area generation was dispatched based on economic dispatch for that Balancing Authority.
• The model for the system external to ATC was taken from the MMWG 2006 Series, 2008 summer model. The external system interchange was adjusted from the 2006 MMWG Series 2008 summer interchange to match the latest ATC members’ firm interchange.
• Included revised system topology based on projects that were placed in service in 2008, or were anticipated to be placed in service by June 2009.
• Refer to Table PF-1 for projects that were included in the 2009 analyses. Please also refer to the Steady State All Project Models section for more discussion about how projects are chosen for inclusion our our models.

2013 summer peak, hot summer and shoulder models

• We utilized interconnection point load forecasts provided by various distribution companies in 2007. Please refer to the Load Forecast section for further details.
• The model for the system external to ATC was taken from the MMWG 2007 Series, 2013 summer model. The external system interchange was adjusted from the 2007 MMWG Series 2013 summer interchange to match latest ATC members’ firm interchange. Updated future generation to be attached to the ATC system was included in the 2013 models. Balancing Authority area generation was dispatched based on economic dispatch for that Balancing Authority.
• In addition to the projects listed above for the 2009 case, the following projects were modeled in 2013 because they were assumed to be completed and  placed in service prior to the summer of 2013. (Refer to Table PF-2.) Please also refer to the Steady State All Project Models section for more discussion about how projects are chosen for inclusion our models.

2018 summer peak model

• We utilized interconnection point load forecasts provided by various distribution companies in 2007. Please refer to the Load Forecast section for further details.
• The model for the system external to ATC was taken from the MMWG 2007 Series, 2018 summer model. The external system interchange was adjusted from the 2007 MMWG Series 2018 summer interchange to match the latest ATC members’ firm interchange.
• In addition to the projects listed above for the 2013 case, the following projects were assumed to be completed and placed in service prior to 2018 (Refer to Table PF-3.) Please also refer to the Steady State All Project Models section for more discussion about how projects are chosen for inclusion our our models.
• Updated future generation to be attached to the ATC system was included in the models. Balancing Authority area generation was dispatched based on economic dispatch for that Balancing Authority.

2023 summer peak model

• Please refer to the Load Forecast section for details about how the load was projected.
• The model for the system external to ATC was taken from the MMWG 2007 Series, 2018 summer model. The external system interchange was adjusted from the 2007 MMWG Series 2018 summer interchange to match the latest ATC members’ firm interchange
• In addition to the projects listed above for the 2018 case, the following projects were assumed to be completed and placed in service prior to 2023 (Refer to Table PF-4) Please also refer to the Steady State All Project Models section for more discussion about how projects are chosen for inclusion our our models.
•  Updated future generation to be attached to the ATC system was included in the models. Balancing Authority area generation was dispatched based on economic dispatch for that Balancing Authority.

Steady State All Project Models

The load flow models described above as built for the 10-Year Assessment are special models built exclusively for system analyses in the 10-Year Assessment. Some projects were purposely left out of these models in order to verify system problems exist and which problems get worse over time. When the analysis portion of the 10-Year Assessment was completed, “All Project” models were built. The “All Project” models were built with all planned and proposed projects in the 2009, 2013 and 2018 models. The later models also include most of the provisional projects. These models are more indicative of the expected system configurations for the three study years. The “All Project” models are more appropriate for internal studies performed by ATC planners throughout the year and for regional models. As part of the 10-Year Assessment, the zone planners perform contingency analyses on each of the “All Project” models. These analyses will verify whether all of the planned, proposed, and provisional projects will resolve issues revealed in the 10-Year Assessment process.      

Dynamic stability/short-circuit assessment models

We conduct transient analyses to evaluate dynamic stability of generators as part of our study of new generation interconnections and voltage stability analysis on portions of the system where severe low voltages are identified. In instances where our stability criteria were not met, remedial projects were devised and included in this Assessment (see Zones & study results). We also conduct short- circuit analyses as part of our study of new generation interconnections to evaluate the adequacy of circuit breakers on the transmission system. In instances where short-circuit duties exceeded existing circuit breaker ratings, plans for circuit breaker replacements have been included in this Assessment.

Load forecast

Summer peak models (2009, 2013, 2018, and 2023)

Steady state summer peak models are built using our customers’ load forecasts (50/50 projections) as a starting point, meaning that there is a 50 percent chance that the load level will either fall below or exceed the customer projection. Customer load forecasts were gathered for all ATC customers through the year 2017 (and in some cases 2018/2023), and our 2009 and 2013 summer peak models were developed using these forecasts. 

Certain ATC customers did not provide an 11th-year load forecast for the year 2018. To obtain a forecast for 2018, certain customer-provided forecasts were extended by growing their load by a fixed growth percentage based upon the previous 3-years’ growth (approximately 1.7% compounded annually). Non-scalable loads were held at their 2017 levels using this methodology.

The 2023 summer peak model was developed utilizing similar methodology. To obtain a projection for 2023, customer-provided forecasts were extended by growing their load by a fixed growth percentage based upon the previous 3-years’ growth (approximately 1.5% compounded annually). Non-scalable loads were once again held at their 2017 (or 2018) load levels. It should be noted that the loads utilized in the 2023 summer peak model do not reflect an actual load forecast, but merely a projection (or “load model”) based upon the best available information. The purpose for the 2023 projection is not to develop projects to address all issues, but to develop a sense for the need(s) for long lead-time projects.

ATC Peak Load Projections (MW) including line losses

*load model, not a load forecast

High load models (2013)

The 2013 high load (or “hot summer”) model was created by increasing load 5 percent above expected summer peak conditions as a proxy for a 90/10 model.  Please refer to the Load forecasting criteria for definition of the 90/10 model.  For purposes of this Assessment, total ATC system load includes transmission and distribution losses, as well as load that could be interrupted for generation emergencies.

ATC worked collaboratively with our customers to determine a reasonable approximation for the hot summer case.  As a result of this customer feedback, the 2013 90/10 proxy model was created by increasing load by 5 percent above expected summer peak conditions.

Shoulder models (2013)

The 2013 shoulder model was created by selectively scaling down loads that generally vary by time-of-day to approximately 70 percent of the summer peak condition. A 70 percent load level was chosen to represent the shoulder model because under this scenario, flows are changing as a result of the Ludington pumping cycle. However, we recognize that loads at individual points will vary under real-time shoulder conditions.

Trends and future plans

Finally, it should be noted that we worked with the distribution companies as much as possible to confirm forecast variations from past trends. In a few cases we revised power factors to reasonable levels to prevent creating expensive transmission projects for voltage support. In most cases these issues would ultimately be solved through distribution system power factor correction. ATC will be in ongoing discussions with our customers to determine the best plan for these situations.

New generation assumptions     

There have been numerous generation projects proposed within ATC’s service territory. Many of these proposed projects have interconnection studies completed and a few have had transmission service facility studies completed. Several have proceeded to or through the licensing phase and several more are under construction. However, there are numerous proposed generation projects that have dropped out of the generation queue (refer to Generation Interconnections), adding considerable uncertainty to the transmission planning process. To address this planning uncertainty, we have adopted a criterion for purposes of this and prior Assessments, to establish which proposed generation projects would be included in the 2008 Assessment models.

Previously (before the advent of the MISO Day 2 market) the criterion was that those generation projects for which, at the time the models were developed,

1. ATC had completed a generation interconnection impact study, a generation interconnection facility study, a transmission service impact study and a transmission service facility study, and
2. the generation developer or a customer of the developer had accepted the transmission service approved by ATC.

In the 2008 10-Year Assessment, the criterion was broken into two time frames, years 1 through 5 and 6+ years.

1. For years 1 through 5, only those generators with FERC approved interconnection agreements will be included in the planning models.
2. Beginning with year 6 and continuing into the future, generators are only required to have a Facility Study completed in order to be included in the 10-Year Assessment models.

A number of wind generators in the ATC footprint have suspended FERC approved interconnection agreements. For the first three years following their requested in-service dates, ATC criterion calls for modeling these facilities but dispatching them at the bottom of the dispatch order. After the three years, the generators will be dispatched in their normal dispatch order. The wind generators with suspended agreements were included in the models built for the 10-Year Assessment analysis. The 2008 and 2009 models showed these generators as out of service. The 2013 and 2018 should have had these generators in-service and dispatched.

Generation Retirement Assumptions

On occasion, generators connected to the ATC transmission system are retired or mothballed. As a result, we developed criteria to determine when generators should no longer be included in our 10-Year Assessment models. If the generator has a completed MISO Attachment Y study, the generator will be disconnected in the appropriate load flow study models. In addition, ATC sent an annual letter to each generation owner. Generating companies were asked to identify generator retirements or mothballing that should be included in ATC’s planning horizon. Generators identified as such by the customer will be modeled off line in the relevant models.

There are generators that have been publicly announced as likely candidates for retirement. However, using the disconnection criteria above, in the 2008 10-Year Assessment models we assumed the following generators were to be out of service:

For model building purposes, we assumed cutoff dates for generation changes to be included in models. In order to include the latest data in the models, cutoff dates correspond to the dates the models were built as follows:

• 2009 models - October 4, 2007
• 2013 models - November 1, 2007
• 2018 models -November 12, 2007, and
• 2023 models - November 26, 2007. 

It was assumed that if the generator was available as of the cutoff date, it was available for dispatch in that grouping of models.

Generation projects schedule

To maintain the schedule needed to complete this Assessment, the models were developed during the month of October and November of 2007. Only those generation projects that qualified to be included in our planning models as of the various cutoff dates, were included in the Assessment models. For generation projects not in service by June 2008, the criterion above resulted in the following proposed generation projects being included in the applicable power flow models:

*wind farm Installed capacity lists is 20% of total installed capacity

A more comprehensive discussion of proposed generation is provided in Generation Interconnections, including a map showing all of the currently active generation interconnection requests that ATC has received (See Figure PR-9.)

Environmental considerations

In addition to the technical and operational factors listed above, environmental considerations associated with alternative solutions identified in the analysis have been taken into account in this Assessment. Screening-level assessments of potential new transmission lines and line rebuilds have been incorporated and are provided in Routing & Siting.

Environmental issues are centered around land use; rivers, streams and wetlands; and threatened, endangered and special concern species. Issues may involve state and federal agencies as well as stakeholder organizations. As planning progresses for specific projects and routes, these considerations will be investigated further to identify potential impacts, alternatives and mitigation measures. We will work with state and federal resource agencies to help identify issues for each specific project.