EPA Clean Power Plan 

Phase I - Preliminary Technical Report

WECC Staff

September 19, 2014

155 North 400 West, Suite 200

Salt Lake City, Utah 84103-1114
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Overview

The recently proposed Environmental Protection Agency (EPA) Clean Power Plan has the potential to 

significantly impact the Western electric generation and transmission industry. This is a highly complex 

rule for an industry that is as dynamic and challenging as ever. WECC’s efforts surrounding the

proposed Clean Power Plan are intended to provide information that can help state officials, public 

utility commissioners, utilities, and other interested stakeholders understand the potential reliability

impacts of the proposed rule. 

WECC, in an effort to fulfil its reliability mission, has embarked on a phased-study process that will 

provide the industry with unbiased and independent analysis of this issue. Since the rule is currently in 

draft form, these initial efforts are preliminary and will have to be revised and updated once EPA 

finalizes the rule and states create compliance plans. Through the initial efforts summarized in this 

document, WECC has performed preliminary analyses in order to provide a starting point for additional 

discussions and more detailed assessments. This analysis leverages WECC’s new cross-functional 

analytical approach where the focus is on a single reliability issue, in this case the proposed Clean 

Power Plan, studying it from a number of different perspectives. In doing so, WECC uncovered some 

interesting observations pertaining to the proposed Clean Power Plan:

• Coal retirements are not mandatory under the EPA’s proposed rule – states have flexibility in 

terms of how they reach the proposed emission goals set forth by the EPA. However, the 

retirement of some amount of coal generation may be an economic and practical reality for 

certain states. Based on this line of thinking, WECC conducted initial frequency response studies 

designed to investigate the reliability impacts of removing ~7,000 MW of coal generation.

Results showed that for the specific condition studied (peak summer condition only), the 

system was able to recover from a large contingency and frequency response was not 

significantly compromised. Importantly, these initial studies focused only on frequency 

response – which is one aspect of the broader system reliability picture. Reliability concerns 

outside this narrow area of study include remedial action schemes (RAS), path ratings, and 

voltage stability. These areas must be addressed in future work by WECC and others. 

• Western states are already taking actions to reduce emissions, prior to Clean Power Plan 

implementation – the effectiveness of these plans in meeting the proposed EPA goals appears 

to vary from state to state. WECC’s preliminary analysis showed that states planning to retire 

carbon intensive generation, implement energy efficiency, and build out robust levels of 

renewable resources could make strides toward achieving the EPA goals in the next 10-years. 

However, some states have emission rates short of the goals in that same timeframe, despite 

their planned activities.

• Although it is still early to consider the impact of state compliance plans, preliminary WECC 

results have shown that decisions made regarding Clean Power Plan compliance in a state can 
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drastically impact grid operations, system resiliency, and potentially, compliance opportunities 

for a neighboring state. As proposed or potential compliance plans are developed, WECC sees 

value in a holistic Interconnection-wide assessment of the plans. While changes in the resource 

mix and forecasted load in one or two states may not drastically impact the system, significant 

changes in four, five, or all western states at once is exponentially more concerning for a

system that is inherently interconnected.

There are countless analytical opportunities surrounding the proposed Clean Power Plan. At this early 

stage, WECC’s work has been limited to analyzing some basic assumptions provided by the EPA – as 

summarized in this document. As states and other stakeholders shift their focus to developing 

compliance scenarios and understanding their potential implications, WECC intends to gather this 

information and provide objective perspectives to the West, striving to assure future reliability of the 

system.

Introduction

On June 2, 2014, the Environmental Protection Agency (EPA) issued its proposed Carbon Pollution 

Emission Guidelines for Existing Stationary Sources: Electric Utility Generating Units, also commonly 

referred to as the Clean Power Plan, which proposes carbon dioxide emission limits for existing electric 

generation facilities. On August 29, WECC posted a short overview document titled “EPA Clean Power 

Plan: An Overview of WECC’s Phased Approach1” that presented WECC’s cross-functional, phased 

approach to evaluating the proposed rule. 

This report presents WECC’s initial Phase I work to evaluate the proposed rule. As WECC continues its 

evaluation of the proposed rule over the coming months, additional reports will follow. 

WECC does not own, operate, or build transmission or generation; therefore, WECC has no direct 

compliance obligation under the proposed rule. However, as the Reliability Assurer for the Western 

Interconnection, WECC does have an interest in the proposed rule’s potential impact on reliability. This

impact will depend on state implementation plans. Knowledge of the contents of the states’ plans will 

be critical to WECC’s evaluation of the reliability impact. However, states are not required to submit 

their implementation plans to the EPA until June 2016 (or later in some cases). There is a role for WECC 

to play in the interim. One way WECC supports Bulk-Power System reliability in the Western 

Interconnection is by providing information about the system under a wide range of plausible futures

that reflect the implications of proposed policy, varied resource mixes, and changes in transmission 

development. WECC will use this information and its analytical tools to evaluate the developing 

 

1 http://www.wecc.biz/library/WECC%20Documents/140829_EPA-111(d)_Overview_Final.pdf.
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reliability and planning issues surrounding the proposed rule and share that information with states as 

they develop their plans.

Goals of this Report

Since the release of the proposed rule, WECC has adapted parts of its existing analyses and data sets to 

provide the information in this initial report. 

The purposes of this initial report are to: 

• Provide insight into how planned generation retirements, load growth, renewable resource

development, and energy efficiency measures might impact carbon dioxide (CO2) emissions, 

and how these emission changes compare to the target-driven assumptions used by the 

EPA;

• Discuss the potential impacts that resource mix changes, such as coal resource 

displacement and/or retirements, could have on regional and West-wide planning reserve 

margins or other reliability criteria; and

• Investigate how coal plant retirements and/or displacement may impact certain aspects of 

system reliability.

Equally as important, this report and its findings are not intended to:

• Promote any specific compliance approach or provide formal comments to any state’s 

regulatory proceedings.

• Advocate any policy position for a utility, generation facility owner or other organization to 

adopt as part of compliance, reliability or planning responsibilities in the Western 

Interconnection.

• Support the policy goals of any particular stakeholder or interests of any particular 

organization.

• Give the impression that WECC’s reliability assessment of the proposed EPA Clean Power 

Plan is complete. Like many others, WECC is just beginning its work and this document 

represents a starting point for additional analyses. 

Overview of the Proposed EPA Clean Power Plan

The purpose of the Clean Power Plan is to reduce CO2 emissions from existing power generation 

sources to 30 percent below 2005 levels by 2030. The proposed rule places the onus for compliance on 

the states but provides flexibility in how states may comply. The rule provides state-specific rate-based

goals for CO2 reduction and guidelines for states to follow in developing their compliance plans. This 

provides states with the flexibility to tailor their plans to their specific circumstances. In addition, states 
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may develop individual plans or work together on multi-state or regional plans, and states may convert 

the EPA’s rate-based goal to a mass-based equivalent goal. 

The Building Blocks

The EPA defined four “Building Blocks” that were used in the development of the proposed rule to 

determine state-specific reduction targets. Table 1 depicts how each building block represents a 

different approach for achieving the proposed targets. According to the EPA, the four building blocks 

represent the “best” system of emissions reduction (BSER) taking into consideration the impacts on 

cost of electricity and system reliability. 2

Table 1: EPA BSER Building Blocks

Building Block 1:

Coal Plant Efficiency

Improving the heat rates of coal-fired power plants by 6 percent would 

reduce the amount of coal needed per MWh of generation, increasing the 

overall combustion efficiency of the coal-fired EGUs. This would result in 

less carbon per MWh of generation.

Building Block 2:

Increased Natural Gas 

Utilization

Increasing the utilization of existing natural gas combined-cycle (NGCC) 

plants up to 70 percent capacity factor would result in a redispatch away 

from higher carbon-intensive fuel sources like coal and oil/gas-fired steam 

boilers. Relying on energy from less carbon-intensive resources would 

reduce the carbon intensity profile for a particular state.

Building Block 3:

Renewable and Nuclear 

Power

Increasing the penetration of renewable (zero carbon) resources (like 

wind, solar, hydro) and nuclear, while also delaying or canceling the 

retirement of nuclear plants currently at risk of being shut down would 

result in more zero-carbon energy and a decreased emission rate for a 

particular state.

Building Block 4:

Energy Efficiency

Increasing energy efficiency programs (or targets under existing programs) 

to reduce energy demand by 1.5 percent per year would avoid CO2

emissions associated with the resulting reduction in generation.

To determine the proposed 2030 target emission rate for each state, the EPA used 2012 emission 

levels as the baseline emission and generation starting points, then applied each of the building blocks 

in a manner tailored to each state’s specific circumstances. This process relied on many assumptions 

 2 Although not listed as a BSER, it is implied by the EPA that the construction of new nuclear units, carbon 

capture and storage, hydro/nuclear uprates, coal co-firing and other emission reduction methods could serve as 

compliance strategies for states. EPA has requested comments on many of these reduction methods. 
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about demand, resources, transmission topology and emissions. Figure 1 depicts how the four building 

blocks were applied individually to each state’s 2012 emission rate, resulting in the 2030 target 

emission rate being proposed by the EPA. The data for this graphic illustration is taken directly from 

the EPA.

Figure 1: EPA 111(d) State Goal Breakdown with Building Blocks (lbs./MWh)

Affected Electric Generating Units (EGU)

The proposed Clean Power Plan is a point-source emission rule, meaning it regulates emissions for a 

specific set of electric generation units (EGUs), not the industry as a whole. In developing the 2030 

target emission rates, the EPA considered generation only from EGUs that met certain criteria, referred 

in the proposed rule as “affected EGUs” or occasionally “covered or qualified EGUs.” Table 2 highlights 

the criteria used to identify the affected EGUs that are subject to the proposed Clean Power Plan.

0

500

1,000

1,500

2,000

2,500

3,000

MT WY CO UT NM AZ WA NV OR CA ID

Total Reduction

Coal Heat Rate

Gas CCs

Nuclear At-Risk

Renewables

Efficiency

Proposed 2030 Goal

2012 Rate

2030 Rate
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Table 2: Criteria for Affected EGUs

Criteria Time Basis Steam Gen 

or IGCC

Stationary 

CT

Construction started on or before January 8, 2014 x x

Base load rating > 73 MW (250 MMBtu/h) x x

Purpose to supply one-third or more of potential 

electric output and more than 219,000 MWh netelectric output to a utility distribution system

Annual Basis x

3-year Rolling 

Average x

Combusts fossil fuel for more than 10% of the heat 

input

3-year Rolling 

Average x

Combusts over 90% natural gas on a heat input 

basis

3-year Rolling 

Average x

State Emission Targets

As described above, the building blocks were used to determine a final goal for 2030, considering only 

the generation coming from the affected EGUs. But, unique to this rule is the fact that both “inside the 

fence” measures, such as Block 1 (heat rate improvements), and “outside the fence” techniques, such 

as Blocks 3 and 4 (renewables/nuclear and energy efficiency), can be used for compliance purposes.

In addition to the final 2030 target emission rate, the EPA created an interim goal for each state, 

effective from 2020 through 2029. Figure 2 shows the proposed final goals for each Western state as 

well as the interim goal that is to be satisfied by the states on an average from 2020–2029. Notably, 

the interim state emission rate goal is only slightly higher than the final emission goal, putting pressure 

on states to make significant progress toward the interim goal in the years prior to the compliance 

period. 
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Figure 2: EPA Proposed State CO2 Emission Goals (lbs/MWh)

Additional Resources

The proposed Clean Power Plan is complex. For additional information, including industry analysis, the 

following documents were particularly useful to WECC and we encourage stakeholders to rely on these 

and other resources for a more thorough briefing: 

• EPA Rule 111(d) webpage: http://www2.epa.gov/carbon-pollution-standards/clean-powerplan-proposed-rule

• EPA Proposal for State Plan Implementationhttp://www2.epa.gov/sites/production/files/2014-

07/documents/webinar_part_3_implementation_presntation_0.pdf

• EPA Rule 111(d) Goal Computation document: http://www2.epa.gov/carbon-pollutionstandards/clean-power-plan-proposed-rule-goal-computation

• EPA Informational Briefing: Carbon Emissions from Power Plants: 

http://www2.epa.gov/carbon-pollution-standards/learn-about-carbon-pollution-powerplants

• Regulatory Assistance Project (RAP) Factsheet: http://westernenergyboard.org/wpcontent/uploads/2014/07/06-27-14RAP_EPA111dSynopsis.pdf

• Duke Nicholas Institute Policy Brief: 

http://nicholasinstitute.duke.edu/sites/default/files/publications/ni_pb_14-01_0.pdf

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

AZ CA CO ID MT NM NV OR UT WA WY

EPA Interim State Goal EPA Final (2030) State Goal
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WECC’s Phase 1 Approach

The draft EPA Clean Power Plan presents numerous

opportunities for technical investigation. WECC looks 

at the draft rule through the reliability lens. However,

as with the development of nearly all energy policy, 

economics and environmental considerations are key 

drivers of the analysis and decision making. Most EPA 

briefing documents circulating through the industry 

focus on EPA data and analysis with efforts to analyze, 

dissect, and in some cases critique the building block 

approach used to establish the goals. To round out 

these viewpoints, WECC’s Phase 1 approach focuses on 

analyzing the potential impacts of the assumptions 

core to the EPA building blocks, leveraging several 

tools and studies in an effort to develop a truly crossfunctional perspective on this issue.

WECC’s cross-functional approach for evaluating the 

potential impacts of the proposed Clean Power Plan relies on three components: 

1. Resource Adequacy Assessment: The resource adequacy perspective is essential to ensuring that 

there will be enough resources to serve peak load under various compliance scenarios. Resource 

adequacy assessments look at forecasted load and expected resources and apply reserve margin 

calculations to determine if resources meet demand.

2. Production Simulation Modeling: Also known as production cost simulation, this perspective gives 

insight into operational, economic, and emission impacts the system might face under a particular 

future. 

3. System Stability Studies: This perspective provides a check on whether potential compliance 

scenarios create stability issues. These studies take a snapshot of the grid and investigate how the 

unexpected loss of equipment could impact grid reliability.

There is merit and value in understanding how the EPA’s proposed BSER could impact resource 

adequacy, system operations, grid stability, and state-level emissions even though states currently do 

not have any obligation to implement the BSER as proposed by the EPA. For each of the EPA building 

blocks, WECC attempted to use these approaches wherever applicable and feasible. These 

perspectives will also be leveraged in future WECC work products surrounding this issue.

Over the next several months, WECC’s objective will shift from providing analysis around the 

foundational assumptions of the proposed rule to evaluating potential impacts of a range of specific 

EPA Clean 

Power 

Plan

Production 

Simulation 

Model

Sysetm 

Stability 

Studies

Resource 

Adequacy
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compliance strategies. This will help states consider the range of compliance options and their impacts, 

helping them to further hone their plans. 

Analysis and Preliminary Results

In the short time since the release of the proposed rule, WECC has used its tools, information, and 

experience to provide useful information appropriate for this stage of the issue. WECC’s analysis is not 

complete and WECC will continue its Phase 1 analysis over the next several months. In an effort to 

release information in a timely manner, WECC will release additional portions of its Phase 1 work as 

the information becomes available.

The remainder of this document reflects the work WECC has completed over the last two months. The 

work is organized by Building Block. The Building Blocks are not given equal treatment in this report. 

This is due to a lack of applicable analyses for a particular Building Block or to the fact that additional 

work will be completed on that Building Block. The results presented below should be read and taken 

as the first in a series of investigations by WECC on this topic. Emphasis on a single building block or 

elements therein should not be construed as WECC recommending any compliance mechanism to 

another.

Block 1: Heat Rate Improvements

Building Block 1 includes the improvement of heat rates of affected coal-fired steam EGUs by 6 percent 

(on average). This would effectively reduce emissions of those EGUs by 6 percent. For the Western 

states, Block 1 accounted for 25 percent of the total emission reductions from 2012 through 2030. The 

Block 1 assumptions impact Wyoming and Montana the most as these two states house a large 

proportion of the West’s coal fleet. 

The EPA has identified several options for improving the heat rates of coal-fired plants, such as:

• Installation of control systems using current technologies

• Operator training

• Smart soot blowing

• Steam turbine upgrades

• Boiler draft fan/driver upgrades

WECC has not collected any data or conducted studies that allow for an assessment of plant-level or 

system impacts of implementing this building block. Although WECC could study a scenario that 

included a reduction in heat rates for coal generation, it may be premature to perform such a study. To 

properly assess which heat rate improvement measures are available for each coal-fired facility, it 

would be necessary to discover which technologies have already been implemented for each affected 
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EGU in the Interconnection. The Generator Owners could best address the improvements that could be 

considered, and the overall heat-rate improvements that might be obtained. Since certain plants may 

already have improved their heat rate, making further enhancements could be technically or 

economically infeasible. At this point, it is difficult to assess the causal impacts of heat-rate 

improvements on resource adequacy, production system and overall reliability of the electric system. 

WECC intends to collaborate with stakeholders in Phase II of our work to better investigate the 

implications of implementing Block 1. 

Block 2: Increased Natural Gas Utilization

The second block of the EPA’s proposed BSER assumes a state-level redispatch from high-carbon EGUs, 

such as coal burning resources, to lower-carbon EGUs, like natural gas combined-cycle generators. By 

providing energy from less carbon-intensive resources, it is presumed by the EPA that a state could 

reduce its emission rate from all EGUs in aggregate. Central to this is the EPA assumption that the

redispatch could occur up to the point where natural gas EGUs were operated at a 70 percent capacity 

factor, as an annual maximum. It must be noted that emission reductions would vary based on current 

dispatch levels and related emissions profile. Since this block assumed that a reduction in coal 

generation would be offset by gas-fired combined-cycle units, states with large fleets of both coal- and 

gas-fired plants were impacted significantly. Building Block 2 was analyzed from three perspectives, as 

detailed below. 

Key Technical Assumptions 

The following assumptions influenced and/or limited the analyses of Building Block 2:

1. Resource adequacy analysis is limited to only peak-hour periods and does not account for 

variations in generation profile due to changes in time (off-peak, winter or shoulder months).

2. The current analysis does not focus on any subregional or locational situations (load pockets) 

or any EGU-specific operational conditions.

3. Frequency response studies were only performed for one “peak” hour for a single scenario. 

Comprehensive analyses, considering RAS, voltage stability, and path rating impacts, among 

other reliability aspects, are still needed for finalize any conclusions around system reliability.

Resource Adequacy 

Planning Reserve Margins

A planning reserve margin is a target that Load-Serving Entities often use in long-term resource 

planning to ensure that adequate generation resources are available for reliable operations of the 

electric system. It is used to capture the uncertainties associated with long-term planning such as load 

forecast errors and unplanned generation and transmission outages. WECC collects electric generation 
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capacity and forecasted demand information from Balancing Authorities annually and, using that 

information, calculates planning reserve margins for the WECC Region and subregions within the 

United States. A 15 percent planning reserve margin is a typical target used in resource planning, and is 

used by NERC as a default margin in the Long-Term Reliability Assessment.

There were 848 MW of coal-fired generating capacity retired in 2013. As reported in WECC’s 2014 

annual Loads and Resources data, which WECC received before the EPA rule was issued, over the next 

10 years at least 2,343 MW of coal-fired generating capacity is expected to be retired (before taking 

the proposed EPA Clean Power Plan into account).

Potential Impacts

The reserve margin for WECC-US is greater than 15 percent through 2020 when generating resources 

that exist or are under construction today are included in the analysis. For 2024, the end of the 10-year 

study period–including resources that exist or are under construction today as well as resources that 

are planned for future addition or retirement–also results in a planning reserve margin greater than 15

percent. 

An analysis of the Anticipated Planning Reserve margins calculated in the NERC 2014 Long-Term 

Reliability Assessment shows that in 2020, the beginning of the enforcement period, all four of the 

WECC subregions have capacity in excess of the 15 percent target. However with the loss of 

approximately 400 MW (or .53 percent) of capacity in the US portion of the NW subregion, 700 MW 

(2.97 percent) in the Southwest subregion, 3,500 MW (35 percent) in the Rocky Mountain subregion, 

or 200 MW (.32 percent) in the US portion of the California/Mexico subregion, the calculated Planning 

Reserve margin for those subregions could drop below the 15 percent target. Note that the surplus 

capacity was calculated with the assumption that any additional generation retirements above that 

reported in the Long-Term Reliability Assessment (LTRA) were not replaced nor were entities able to 

transfer additional excess energy between subregions above that reported in the LTRA.

The electric generation resources in the US portion of the Western Interconnection are a diverse mix 

with hydro capacity located in the Pacific Northwest, solar resources in the Southwest, wind resources 

in the Rocky Mountain area, and geothermal resources in the Great Basin area. By 2024, the resource 

mix could look different as additional wind (6,900 MW) and solar (8,600 MW) generation resources are 

planned to be added, and more natural gas-fired generation is planned to be added to replace 

potential coal-fired generation retirements or displacement in some cases.
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Figure 3: Resource Mix in US Portion of the Western Interconnection

With the current planning reserve margins and the possible resource additions, Load-Serving Entities 

within the United States portion of the Western Interconnection should have adequate reserves 

through 2020. The margin should allow time for Load-Serving Entities to respond to new policies and 

requirements as they continue to reliably serve the changing nature of demand of electric customers. 

Production Simulation

The Transmission Expansion Planning Policy Committee's (TEPPC) 2024 Common Case is a compilation 

of planned generation, load forecasts, and expected transmission that was developed as part of the 

TEPPC planning process. The case represents a trajectory of recent Western Interconnection planning 

information, developments, and policies looking out 10 years. TEPPC stakeholders participated in the 

development of the assumptions that were included in the case. The platform for the various inputs is 

a production cost model that is used to study future transmission needs by simulating the hour-to-hour 

operation of the transmission system. The model is well-suited to study transmission use, generator 

dispatch, variable production costs and effluent emissions from generation resources. The key input 

parameters that are used by the model for generation commitment, dispatch and emissions are listed 

in Table 3.

Coal 32,500 MW

Gas 85,900 MW

Hydro 52,700 

MW

Nuclear 7,300 

MW

Solar 5,700 MW Wind 

19,100 

MW

Other 

Renewable 

3,900 MW

Other Thermal 

1,200 MW
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Table 3: Thermal Generation Parameters

Parameter Units Parameter Units

Capacity (max) MW Startup cost $ per start

Capacity (min) MW Startup fuel MMBtu

Heat rates MMBtu/MWh Emission rates Lbs/MMBtu

Fuel cost $/MMBtu Forced outage rates %

O&M cost $/MWh Planned outages From date to date

Ramp rates MW/hour Must-run status True/False

Coal Retirements and Displacement

The Common Case reflects known and already announced generator retirements – including coal 

plants. The Data Work Group (DWG) leads an effort to review Loads and Resources data submittals, 

Energy Information Administration (EIA) data, and a number of other public data sources with the goal 

of identifying planned retirement dates for generators. The 2024 Common Case assumes 7,016 MW of 

coal-fired EGUs that are retired from 2010 through 2024, as shown in Figure 4. This final total3 includes 

859 MW of retirements in Alberta, which would have no bearing on the EPA draft rule. The remaining 

6,157 MW of coal retirements impact the state emissions results that come out as result of the 

production cost model. The TEPPC Common Case also includes several pre-scheduled retirements and 

conversions of coal-fired generators.

 3 The Common Case has 42,283 MW of coal generation before retirements, of which 7,016 MW are retired (16.6 percent) 

by the end of 2024.
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Figure 4: TEPPC 2024 Common Case Cumulative Coal Retirements by Year (MW)

The proposed EPA BSER framework assumes that there will be a reduction in emissions rate (lbs/MWh) 

associated with coal-fired generation as per the assumptions in Building Block 2. These reductions,

according to EPA, are translated on an energy (MWh) basis. To draw a comparison between the WECC 

2024 Common Case planned coal retirements and the coal reductions in Building Block 2, WECC 

converted the EPA reduction energy to a capacity (MW) parameter by assuming an 85 percent capacity 

factor for coal generation. This interpretation of the EPA coal-based generation reduction creates an 

equivalent level of “interpreted retirements/removals” on a capacity basis rather than energy basis, as 

originally presented by the EPA. This information can then be compared to the “pre-EPA draft rules” 

planned retirements in the 2024 Common Case, as presented in Figure 5. Since the Common Case is a 

2024 data set (i.e., 10-year planning horizon), the Block 2 portion of the EPA 2020-2029 interim goal 

was used for the purposes of this comparison.

4 The goal of this analysis is to gain some insight into how 

the EPA assumptions compare with current plans for generator retirements. 

 4 Note that this comparison fails to account for the operational displacement of coal generation in the 2024 Common Case. 

The Common Case coal displacements due to renewable generation and the California Green House Gas initiative are 

difficult to measure, but would improve the comparison. The disposition of coal plants located on tribal lands is currently 

unclear and these plants are excluded from the comparison.

-10,000

-9,000

-8,000

-7,000

-6,000

-5,000

-4,000

-3,000

-2,000

-1,000

0

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025
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Figure 5: Coal Retirements by State (2010-2024)

The resulting analysis shows that most Western states are already planning to reduce coal generation 

to levels roughly equivalent or beyond WECC’s interpretation of the EPA’s Block 2 compliance option.

5

Arizona and Utah are clearly the outliers. The Common Case has identified about 500 MW of 

retirements in Arizona. This is small in comparison to the nearly 3,500 MW of coal-fired reductions

and/or displacement assumed by the EPA in building Block 2 (WECC's capacity interpretation). Utah 

appears to have a significant gap between its planned retirements and the level assumed by the EPA. 

However, this analysis is based on 2024 data and there are about 1,600 MW of additional Utah coal 

retirements expected in 2025 that would decrease the apparent gap. Units that are expected to retire 

in 2025 are listed in Table 4.

 5 Changes to the coal generation capacity factor assumption significantly impact this comparison.

0

500

1,000

1,500

2,000

2,500

3,000

3,500

AZ CA CO ID MT NM NV OR UT WA WY

2024 Common Case Coal Retirements Estimated EPA Coal Displacement

MW
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Table 4: Coal Retirements expected in 2025

Unit Retired in 2025 Capacity (MW) State

Centralia 2 730 Washington

Intermountain 1 & 2 1640 Utah

Valmy 2 116 Nevada

One of the key elements of the EPA’s building Block 2 assumptions was to increase the dispatch of 

combined-cycle resources (NGCC) such that coal and oil/gas steam resources are displaced, resulting in 

less carbon-intensive energy. The proposed combined-cycle dispatch increase varies from state-tostate impacting those states that have both a large coal fleet and significant gas capacity, like Arizona 

and Colorado. There is a stated expectation to bring the capacity factor of the NGCC generation up to 

70 percent if the resource mix supports that amount of displacement.

The production simulation model performs a security-constrained economic dispatch, so generators 

are dispatched by the model as it minimizes system operation costs, while also observing transmission 

constraints and operational parameters. In an effort to investigate how the West’s combined-cycle gas 

fleet would react in response to any state (or multiple states) effort(s) to adopt policy measures to 

internalize the societal cost of CO2-induced environmental externalities, the following scenarios

featuring the removal of coal generation and CO2 adders ($/Metric Ton) in separate studies. The 

scenarios are described below:

• Common Case – The 2024 “expected future” serves as the base-case business-as-usual 

scenario.

• Common Case with EPA Coal – Maintains all Common Case assumptions except an 

additional 3,900 MW of coal-fired generation are retired to reflect WECC’s capacity 

interpretation of EPA Building Block 2 (as presented in Figure 5). These retired or displaced 

units are replaced by combined-cycle gas generation at the same location.

• Common Case Carbon $40 – Maintains all Common Case assumptions except CO2 emissions

are assigned a $40/Metric Ton cost (no incremental coal retirements beyond Common Case 

assumptions).

• Common Case Carbon $60 – Maintains all Common Case assumptions, except CO2

emissions are assigned a $60/Metric Ton cost (no incremental coal retirements beyond 

Common Case assumptions).

Figure 6 shows resulting annual capacity factors for affected EGU NGCC units, on a state-by-state basis, 

for the studies listed above. As expected, remocving 3,900 MW of coal to reflect WECC’s interpretation 
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of the EPA’s Block 2 assumptions results in increased NGCC operation in every state, especially in 

Arizona and New Mexico. However, the system dispatch the resulted from a carbon price of $40/ton 

(and no coal removals) results in even greater utilization of the NGCC fleet. 

Figure 6: Combined-Cycle Re-dispatch Scenarios

The results of this study validate the EPA’s decision to prioritize considerable flexibility in state’s 

compliance plans. As shown, it is possible to increase the utilization of natural gas resources (and 

reduce carbon emissions) by retiring select coal generation units. It is also possible to increase the 

utilization of the existing gas fleet by introducing a “CO2 adder” that shifts the dispatch away from 

emission-intensive resources like coal and relies more heavily on natural gas resources since the added 

cost of carbon makes gas combined cycle a more economic resource, from the operational perspective. 

System Reliability Considerations 

As the Western resource mix continues to evolve, it is important to track future system reliability. EPA 

Building Block 2 assumes a significant reduction in coal generation, as described above. To evaluate the 

potential impact of this, WECC performed technical planning studies that help to identify important 

system reliability considerations. The studies required high-level assumptions that are necessary due to 

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a lack of more detailed information. An attempt has been made to highlight a number of 

considerations that should be investigated when more detailed information is available. Only system 

frequency response has been investigated in depth.

System Voltage Stability

Implementation of the proposed EPA rule could impact system voltage stability. Voltage stability is the 

ability for the power system to maintain the voltage at every point in the system, within an acceptable 

range. If voltage stability is not maintained, there is the potential for voltage collapse and subsequent 

loss of load or equipment outages. Large base load-generating units, such as large coal-generating 

units, are generally depended on to provide system voltage support. System voltage stability issues can 

occur on both the regional and local levels. 

The extent that voltage stability is impacted by a changing resource mix is heavily dependent on the 

generating resources added or removed. The generator types, locations, and sensitivity of the 

surrounding transmission network to voltage changes can impact voltage stability. Voltage stability will 

most likely need to be addressed case-by-case on a more localized scale where issues can be addressed 

at the sub-transmission-system level. While voltage stability is a highly local issue, the Interconnection 

relies on it to maintain system reliability and it should be considered in a fully comprehensive analysis.

In Phase I and Phase II of this work, WECC will continue to investigate analysis options and coordinate 

with stakeholders around this aspect of system reliability.

Path Rating Impacts

In the Western Interconnection a Path is one or more facilities between transmission systems or 

internal to a system for which schedules and/or actual flows are monitored for reliability purposes. 

Facilities in a path may originate and terminate at the same point (substation or generating station) or 

at different points. Two or more individual paths can be combined into a single path for rating 

purposes, although they may be separate scheduling paths. Paths are often called cut-planes. As 

shown in Figure 7, there are a number of paths throughout the Western Interconnection. Generation 

changes that impact path ratings will be noticed throughout the region. 
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Figure 7: Map of WECC Transmission Paths

Many existing WECC transmission path ratings are dependent on the location, size and type of nearby 

generation. If the generation resources are changed (or removed), path ratings could be impacted and 

studies would need to be conducted to re-evaluate the individual path ratings. Several path ratings 

depend on the ability to trip large amounts of generation to maintain reliability of the system. If this 

generation was removed via implementation of the EPA Clean Power Plan, the path rating could be 

impacted . One example of this is Path 54: Coronado – Silver King 500 kV, which has a rating of 1,494

MW north-to-south. The rating of Path 54 depends on the ability to trip generation at the Coronado

substation. If this generation was retired or offline, the path rating may have to be decreased. 

Remedial Action Schemes Impacts

Remedial Action Schemes (RAS) help to better utilize the transmission system by mitigating reliability 

risks, increasing system transfer capability, and meeting project schedules. RAS are defined as an 

automatic protection system designed to detect abnormal or predetermined system conditions, and 

take corrective actions other than and/or in addition to the isolation of faulted components to 

maintain system reliability. One type of RAS is event based-generator tripping. In this type of RAS, 

transmission equipment trips and RAS actions cause the adjustment or tripping of generation. Changes 

in the generation resources would require evaluation of RAS effectiveness and may require changes to 

the schemes. Many of the WECC RAS are voltage dependent so modifications to the voltage support 

will require investigation and potential changes. RAS arming could become more dynamic, depending 
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on how generation resources are replaced. In general, generation resources that have a large amount 

of variability could significantly change the effectiveness of RAS.

System Frequency Response

According to NERC, frequency response is “a measure of an Interconnection’s ability to stabilize 

frequency immediately following the sudden loss of generation or load.” Frequency response is a 

critical component to the reliable operation of the Western Interconnection, especially during 

disturbances and system restoration. The EPA assumes that Block 2 would reduce the output from coal 

generation, replacing it with generation from less carbon-intensive resources (i.e., gas). Many coal 

units in the West provide critical frequency response support to the system. Given this, WECC 

embarked on an effort to investigate how coal retirements could impact system frequency response. 

The studies were performed on a WECC 2023 Heavy Summer Base Case that was approved by the 

Technical Studies Subcommittee (TSS). The studies were focused on frequency response and did not 

examine any of the other potential issues that could be introduced by changing resources and 

locations of large power generation units, many of which are discussed above. 

To perform the system frequency response analysis, a number of assumptions were made:

• Replacement generation would be located at the same location as the generation 

removed/retired in the study;

• Replacement generation would use generic dynamic models; and

• In an attempt to study a “worst case scenario” in terms of frequency response, all displaced 

generation would be replaced by inverter-based resources that do not provide frequency 

response.

The study relied on generation assumptions from the WECC 2024 Common Case and WECC’s

interpretation of the EPA Building Block 2. These two information sources were used as a basis for 

changing resources in the 2023 Heavy Summer Base Case. The study was formed using the following 

approach:

1. Coal retirements in the 2024 Common Case were compared with those in the 2023 Heavy 

Summer Base Case. With a couple of exceptions, coal units that were assumed to be retired 

by 2024 in the Common Case were retired in the Heavy Summer Base Case. 

2. With the Heavy Summer Base Case and the Common Case now reflecting similar coal 

retirements, WECC interpreted the EPA Building Block 2 into a megawatt “reduction” 

equivalent – as described earlier in this paper. Block 2 is by nature a coal energy reduction 

assumed by the EPA. WECC created an equivalent capacity or megawatt reduction value by 

assuming an 85 percent capacity factor. The difference of Common Case/Base Case 

retirements and EPA “displacements” is reflected below.
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Figure 8: Coal Retirements by State (2010-2024)

3. To reflect the estimation of the EPA’s assumed coal “displacement,” WECC further adapted 

the 2023 Heavy Summer Base Case by removing an additional 3,000 MW of coal– mostly 

located in Arizona and Utah – and replaced it with inverter-based generation, assuming that 

to be the worst case scenario for frequency response.6 WECC selected generation to retire 

within each state based on a reverse economic prioritization of the generation (i.e. the least 

efficient generators are removed first). In doing so, the coal units were ordered by state and 

then by heat rate (taken from the 2024 Common Case). Based on efficiency and location, 

the units were incrementally selected and removed until the EPA assumptions were met. In 

total, 7,107 MW of coal resources were removed from the original 2023 Base Case.

To highlight the difference in the frequency response posed by different resource types, Figure 9 shows

the same unit with three different scenarios. The first model used is a typical large thermal unit at max 

output, the second is an inverter-based generating unit, and the third is a typical large thermal unit 

that is able to respond both short-term and long-term to frequency excursions. The inverter-based unit 

 6 The term inverter-based generation is used to indicate that the generation is not synchronous and will have power 

electronics between the generation and the rest of the interconnected system. The reason that inverter-based renewable 

generation is assumed to be the worst case for frequency response is due to the model being used to simulation the 

generation. The transient stability model for each displaced generator was replaced with a WTG4 model, which is a generic 

inverter-based stability model.

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stays at the same output throughout the simulation, the base-loaded thermal unit gives an initial 

response but returns to the original output, and a responsive thermal unit will respond and keep its 

response based on unit settings. 

Figure 9: Emission Reduction Impacts to Generator Response

To study frequency response, a large amount of generation is tripped during a simulation to investigate 

how the system responds and recovers. WECC has used a two-unit Palo Verde outage for many years 

to examine a number of reliability issues – this practice was continued for this study. In the 2023 Heavy 

Summer Base Case, the loss of the two units resulted in 2756 MW being tripped during the dynamic 

simulation. 

The three scenarios studied were:

1. 2023 Heavy Summer Base Case unmodified;

2. 2023 Heavy Summer Base Case modified by replacing approximately 3000 MW of coal

generation with inverter-based generation (to replicate the TEPPC Common Case 

Assumptions); and

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3. 2023 Heavy Summer Base Case modified by replacing approximately 7000 MW of coal 

generation with inverter-based generation (to replicate the TEPPC Common Case 

Assumptions and EPA Building Block 2 interpretation, taken together).

There were two other scenarios, identical to (2) and (3) in terms of capacity, where the coal generation 

was substituted with a combustion turbine model. However, due to the responsive nature of the gas 

generation used to replace the coal generation, these simulations resulted in little change from the 

original case. Given that, we focus on the other three scenarios.

The plots of the system frequency response are provided for each of the three scenarios. As shown, 

the “worst case” scenario, where 7,000 MW of coal generation was replaced with an inverter-based 

model, did not significantly change the system frequency response for the specific condition studied 

(heavy summer). Similar results were observed in the TEPPC case where 3,000 MW of generation was 

replaced. 

Figure 10: Coal Reduction – System Frequency Response

The study results suggest that removing the specified about of coal generation, given the assumptions, 

would have minimal impact on system frequency response. The minimal change in frequency response 

is not surprising given that 7,000 MW of incremental retirements represents only about 3.5 percent of 

the total generation (198,000 MW) in the heavy summer case. Preliminary analysis suggest that the 

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effects would also be minimal in a typical lightly loaded case as the total generation typically being 

represented is closer to 100,000 MW and still would only result in 7 percent of the generation being 

displaced. While a light-load scenario was not studied at length, some preliminary work was 

performed. This work was different in that “headroom” (i.e., remaining capacity) from existing 

resources dispatched to make up for the loss of coal generation. The findings were similar to the 

summer peak study in that there was not a significant impact on system frequency response. However, 

the light-load scenario needs further investigation and a more robust scenario analyses in order to 

confirm the results. 

As described above, Interconnection-wide frequency response was not significantly impacted by the 

specific scenarios analyzed. That is not to say that there are no other reliability issues that should be 

examined. WECC and other entities need to study more scenarios under various loading and operating 

conditions in order to fully investigate potential reliability concerns. Other potential reliability issues 

that should be considered include:

• Load-serving ability – Changing the location and size of generation will have some effect on 

where energy can be sent throughout the system. In some instances this may make 

delivering power to local load a challenge.

• Resource availability – If replacement generation resources rely on some source of fuel,

such as natural gas or wind, it must be determined whether those resources are available in 

the locations where transmission infrastructure is present. 

• Time required to site and construct any newly required generation to meet load – Siting and 

locating new transmission and generation facilities is a long process.

• Operational issues related to a more dynamic resource mix – Changes in the generation 

being used may require additional operating practices and procedures to be developed. The 

change in resources may also require certain generators to be run to accommodate for 

resources that are difficult to dispatch. 

Block 3: Renewable and Nuclear Generation 

EPA’s Building Block 3 calls for increased energy generation from zero-carbon sources, specifically 

renewables and nuclear power. The block assumes changes in nuclear generation capacity (e.g., 

planned capacity, avoided retirement) and also increasing renewable energy to achieve a regional 

average of current Renewable Portfolio Standard (RPS) mandates. Figure 11 depicts the EPA’s 

construction of the renewable energy portion of Block 3. Here, the EPA assumed that states could 

develop renewable resources to a level similar to that of their neighboring states. Note that states like 

Oregon, Washington, and California that are relatively rich in renewable resource potential, and have 
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made substantial progress in developing these resources, are assumed to be capable of developing 

more renewable energy than states with less development to date, such as Montana and Utah. 

Figure 11: EPA Proposed State Goals and Block 3 (MWh)

One of the key components of the WECC Common Case is that it reflects enacted state policies - RPS 

are assumed to be met within the data set and the assumptions are fully accounted for and vetted 

through a stakeholder process. Comparing the EPA assumptions with state enacted RPS targets 

produces interesting observations. Figure 12 introduces this state-level data. The comparison shows 

that the EPA varied their assumptions across the states. For example, California, Colorado, and Arizona 

are already subject to state RPS requirements, and these requirements are in excess of the RPS 

assumption the EPA used in Building Block 3. In other words, RPS-driven emission reductions of this 

nature are already being planned for due to the statutory requirement. The opposite is true for states 

like Wyoming, where the EPA used Block 3 to develop a target that is beyond Wyoming’s statutory 

requirement. 

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Figure 12: Renewable Energy versus Emissions Targets by State (GWh) in 2024

Block 4: Energy Efficiency

EPA’s Building Block 4 calls for a reduction in demand for carbon-intensive generation through 

demand-side efficiency improvements. Specifically, the EPA seeks to encourage states to consider 

increased investments in energy-efficiency, demand-side management and other market-based 

programs that would enable reduction in energy consumption, thereby reducing CO2 emissions. In the 

development of this building block, the EPA assumed that every state would implement energyefficiency policies, eventually reaching 1.5 percent annual-incremental energy savings. The EPA 

considered that some states would need to “ramp up” their programs to achieve this. Based on this, in 

the EPA’s calculations the timing for when states hit the 1.5 percent target varies.

The load forecasts in the TEPPC 2024 Common Case account for reductions due to energy efficiency. 

The load forecasts submitted to WECC are reviewed by Lawrence Berkeley National Laboratory (LBNL)

and adjusted, if necessary, to account for expected and enacted energy efficiency policies. These 

energy efficiency projections are developed by LBNL and they consider energy efficiency resource 

standards and utility Integrated Resource Plan (IRP) in each state, building the information up from the 

individual utility-level. LBNL has created a number of reports outlining the process used to estimate 

anticipated energy efficiency measures.

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 7 LBNL Report: http://emp.lbl.gov/sites/all/files/lbnl-5803e.pdf

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Comparing the energy efficiency in the 2024 Common Case (LBNL EE) with EPA’s Building Block 4 

interim period (2020-2029) assumptions can provide an understanding of how current Energy 

Efficiency (EE) plans compare with the EPA’s building block assumptions. This comparison is provided 

on a state-by-state basis in Figure 13. The LBNL energy efficiency estimates in 2024 represent the total 

savings from utility-ratepayer-funded EE programs implemented over the ten preceding years. Like the 

EPA building blocks, LBNL does not include certain EE policies, such as federal or state appliance 

efficiency standards or state building efficiency standards, making it possible to directly compare the 

two projections with some degree of accuracy. In most states, the EPA Building Block 4 assumed fewer 

incremental savings from energy efficiency than what is already anticipated, according to the LBNL

data.

Figure 13: Energy Efficiency Comparison using EPA Assumptions (GWh)

To better align the underlying assumptions of this comparison, further information may be needed on 

the EPA’s assumptions regarding timing, the types of programs and sectors covered, and whether EPA 

assumptions are gross or net of free-riders.

WECC has explored policy high-energy-efficiency scenarios through past TEPPC studies and this 

information will serve WECC and other stakeholders well as compliance scenarios featuring EE are 

investigated in the coming months. 

Emissions Analysis 

Although the evaluation of CO2 emissions is not the primary purpose of the TEPPC Common Case, the 

model and study results are highly supportive of this type of work. Over the next few months the data 

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inputs and other assumptions will be reviewed and updated as necessary to improve the quality of the 

emission results. The CO2 emission rates were calculated for the states that are entirely or mostly 

included in the Western Interconnection and compared to the 2012 rates and the EPA interim state 

goals.

8

To evaluate the CO2 emission rates from the TEPPC Common Case, it was necessary to integrate the 

proposed Clean Power Plan calculation methodology into the evaluation process, including the EPA 

criteria for including in-state “affected” generation in the State Goal equation. State emission rates are 

calculated using the equation reserved in the Appendix. WECC used the same technique as the EPA. 

Figure 14: Preliminary Comparison of TEPPC 2024 Common Case Emissions

The comparison shows that emission rate improvement since 2012 varies across the Western states. 

The emission rates from the 2024 Common Case are driven by load-growth projections, planned coal 

retirements, compliance with RPS policies, and ongoing investments in energy efficiency. Even with the 

 8 The proposed EPA Clean Power Plan provides limited guidance on a few issues that will be addressed by the states as their 

plans are developed, leading to the following assumptions.

1. For the most part the CO2 emissions and corresponding energy output were assigned to states based on their 

geographical locations. The EPA has asked for comments on this assumption. 

2. Incremental hydro and biomass additions that may be considered renewable by state definitions were excluded.

3. A few errors related to unit criteria in the EPA calculations were corrected.

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EPA 2020-29 Interim Goal (lb/MWh)
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assumed coal retirements (and other assumptions) in the Common Case, seven of the states are short 

of the proposed interim goal, while others appear to have plans in place that may bring them close to 

the goal. More generally, results vary greatly from state-to-state and the underlying data and 

assumptions need further vetting and review. Looking forward, this type of emission analysis will be a 

valuable resource to WECC, states, and utilities as many models do not have the operational, 

transmission, and generation granularity that is supplied by the Common Case production cost model.

Policy Scenarios - Carbon Price Sensitivities

To exhibit how the valuation of carbon can impact state emission rates, WECC ran the 2024 Common 

Case (August 15th version) with five different combinations of carbon adders to record their effect on 

the system dispatch and the resulting emission rates. The California Greenhouse Gas Initiative ($27.51 

per metric ton) modeling was retained initially, but once the WECC-wide carbon adder became higher

than the California adder, a uniform tax was used for all of WECC. Studies between $0-60/Metric Ton 

were conducted and the state-level emission rate results for the EPA-qualified resources are provided 

in Figure 15, with the carbon adder combinations indicated in the legend.

9 The EPA interim goal is 

provided as well. This allows readers to observe how an increasing cost of carbon impacts the system 

dispatch and, in turn, each state’s emission rate. 

Figure 15: EPA CO2 Goal and 2024 Carbon Price Sensitivities (lbs/MWh)

 9 First value is WECC, second value is California 

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The results of these exploratory sensitivities highlight the interconnected nature of the Western grid. 

As the cost of carbon in the sensitivity studies increases, each state’s emission rate would be expected 

to fall. However, as shown above, that isn’t the case. In some states the expected happens and 

emission rates in that state decrease as carbon costs rise. However, due to the operational 

dependencies inherent to the grid, the emission rate in other states increases with the price of carbon 

because now, that state may house low cost resources. These resources are dispatched more 

frequently, resulting in higher emissions. While this set of sensitivities may not represent a realistic EPA 

Clean Power Plan compliance scenario, it exhibits two critical points: (1) actions taken in state A can 

impact state B; and (2) through this type of analysis the industry has an effective means through which 

to track and identify those impacts.

As states begin to identify potential or proposed compliance plants, it is critical that these plans be 

evaluated jointly in addition to their individual assessments. Individual evaluation may lead to a 

completely different conclusion then joint evaluation when it comes to system reliability and overall 

system impact. Joint evaluation of the plans would (1) ensure that individual and/or regional state 

plans achieve their intended goals and do not unintentionally impact one another; and (2) allow for a 

thorough investigation into any potential reliability impacts that may not be apparent when looking at 

plans on an individual basis. 

Conclusions

Key observations from this technical report are summarized below, with recommendations for future 

assessment of the EPA rules and resulting compliance plans.

• Resource adequacy – WECC’s contribution to the most recent NERC Long-Term Reliability 

Assessment shows that planning reserve margins are currently in excess of their targets

and, based on the current assumptions posed by the EPA Building Blocks, impacts to 

resource adequacy may be minimal. WECC plans to further investigate resource adequacy in 

the coming months as compliance scenarios are identified by states. Like other reliability 

assessments, resource adequacy cannot be properly assessed absent realistic and/or 

proposed compliance scenarios. 

• Emissions – Based on results from the WECC Common Case, current plans for state RPS, 

energy efficiency, and the retirement of carbon-intensive generation reduces 2024 state 

emission rates from 2012 levels. However, the degree of reductions fluctuates state to 

state. The reductions put a handful of states near the interim targets proposed by the EPA, 

while other states have 2024 emission rates higher than the EPA interim goals. 

• Reliability – In a frequency response study where 7,000 MW of coal resources were 

replaced by less responsive “inverter-based” resources, the system responded to a large 

contingency nearly the same as it did before the coal units were replaced. This study 
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represents a strong “first step” reliability assessment, but the results are not conclusive or 

overarching. This area requires further monitoring and review – additional scenarios must 

be studied across varying operational and load conditions in order to draw meaningful 

conclusions about system reliability. More importantly, frequency response is only one 

aspect of reliability and the removal of coal resources could impact other areas such as RAS, 

path ratings, and voltage stability, among others. Central to all of these studies is the 

assumptions on replacement generation posed by potential compliance scenarios. 

Next Steps

While WECC has no role in creating or approving compliance plans, it can support the efforts of the 

states and industry stakeholders in a variety of ways. As WECC fulfills its Phase I work and moves into 

Phase II, potential areas of focus include the following: 

• Work with states and other stakeholders to continue to refine and adjust the underlying 

data sources that provide the analytical foundation for this report and future analyses;

• Provide data and information useful to the development of state compliance plans;

• Investigate potential reliability issues by conducting cross-functional analyses on potential 

or conceptual compliance plans using WECC’s production cost model and powerflow model 

capabilities;

• Compare impacts of emission rate compliance with mass-based emission methods; 

• Analyze possible multistate compliance options; 

• Investigate how proposed state compliance plans could interact and impact one another; 

and

• Convene groups of stakeholders, such as impacted utilities and state officials, to inform 

them of analyses related to any of the above topics and discuss regional impacts of state 

compliance plans.

WECC will continue to communicate with the State-Provincial Steering Committee/Western 

Interconnection Regional Advisory Body, Planning Coordination Committee, TEPPC, individual state 

officials, and other stakeholders, seeking their input and expertise in WECC’s continued efforts. Studies 

and reports created as a part of WECC’s ongoing efforts will be released on an “as completed” basis

over the coming months. 
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Appendix 1

What is The Common Case?

The Transmission Expansion Planning Policy Committee (TEPPC) 2024 Common Case is a compilation of 

generation, load forecasts, and transmission that was developed by WECC as part of the 2014-2015 

TEPPC Study Cycle. It represents an “expected” ten-year horizon outlook of the Western 

Interconnection that is used by WECC and many stakeholders to conduct production cost simulations, 

mostly to address transmission needs. The Common Case includes the trajectory of recent Western 

Interconnection planning information, developments and policies looking out 10 years. TEPPC 

stakeholders assisted WECC in developing the thousands of assumptions that depict the Western 

Interconnection and how it is expected to change over the next 10 years. A simple diagram of the data 

inputs and changes is provided in Figure A-3.

Figure A-3: Summary Diagram of Data Changes

Production 

Cost Database

ADD

• Power Flow Network

• New Transmission Projects

• Transmission ratings

• Load Forecasts

• Load Adjustments (DG/DR/EE)

• Reserve requirements

• Existing and planned generation

• Generator parameters (capacity, 

location, fuel, heat-rates, etc.)

• Generation shapes (hydro, wind, solar)

• Costs for fuel and wheeling

• Emission rates

REMOVE

• Canceled/Delayed transmission projects

• Retired generation
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The GridView production cost model was used to build the 2024 Common Case data set where all of 

the data resides in support of conducting the production cost simulations. The next sub-sections 

describe each of these pieces in more detail.

Building the Load Assumptions

The Common Case load assumptions are derived from the 10-year forecasts for monthly peak 

(megawatts) and energy (gigawatt-hours) loads that are submitted to WECC by the Balancing 

Authorities (BA) each year. An hourly load shape is then created by taking a historical load shape (e.g., 

from 2005) and re-scaling and re-shaping it to match the targeted monthly peak and energy forecasts. 

These values are then assigned to TEPPC Load Areas, shown in Figure A-2. The load distribution to the 

individual electrical buses in each area is based on one or more WECC Technical Studies Subcommittee 

(TSS) power flow base case that reflect seasonal load distribution variations.

Figure A-2: WECC Balancing Authorities

Building the Transmission Network

The transmission network is based on a WECC TSS 10-year power flow base case (e.g., 2023 HS1A1 

Heavy Summer Base Case or 23HS1A1). The power flow base case is then reconciled with the Common 

Case Transmission Assumptions (CCTA), which are recommended by the Regional Planning 

Coordination Group (RPCG) as a list of regionally significant transmission projects that have a high 

expectation of being in-service within a 10-year timeframe. Figure A-3 shows the CCTA for the 2024 
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Common Case. The RPCG also identified projects in the power flow case that are not expected to be 

completed by 2024. Basically, any of the CCTA projects that were not already included in the power 

flow case are added and the canceled or delayed projects are removed.

Figure A-3: Common Case Transmission Assumptions - 2024

In general all branches with ratings of 230 kV or higher are constrained to the provided limits in the 

PCM. The WECC transfer paths that are defined in the WECC Path Rating Catalog are also constrained 

to their limits.

Building the Generation Resource Portfolio

The electrical generating plants that currently provide the electricity to the power grid provide an 

obvious starting point for the generation resource portfolio. Changes to the portfolio to support future 

load growth and replace units that retire are part of the planning process. The development status or 

class is the primary driver behind which generators are included in the resource portfolio. The codes 

are described in Figure A-4. 
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Figure A-4: Description of the Development Status Codes for Generation

Figure A-5: Process for Building the Resource Portfolio
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Overriding Assumptions

The majority of the data inputs are based on information provided by the Balancing Authorities and 

Planning Authorities in WECC; however, there are some issues that require making additional 

assumptions to model a ten-year horizon case. Some of these key assumptions are listed below and a 

complete list of the assumptions can be found in the 2024 Common Case Release Notes.

• It is assumed that the utilities comply with the State Renewable Portfolio Standards (RPS) for 

the loads in the state(s) that they serve. The RPS standards are usually set as a percentage of 

retail sales. For example, a utility with annual retail sales of 100,000 MWh in a state with an RPS 

of 25 percent, would be expected to serve 25,000 MWh with renewable generation. If the 

qualifying renewable generation in a state is deficient, additional resources are selected from 

the generation in the next class of generation.

• It is also assumed that the BAs intend to meet their projected loads and reserve requirements. 

As outlined in Figure A-5, resources are selected from the class portfolios in order of class, until 

the RPS requirement is met and the load and reserve are met.

• Although many of the of the contractual arrangements between Generator Owners and LoadServing Entities are modeled, there is a significant portion that are not modeled.

• Several operating constraints that restrict certain aspects of the transmission system are 

modeled using nomograms.

PCM Solution

The production cost model performs a security constrained economic dispatch to arrive at an overall 

least-cost solution for each hour of the year. For each hour, the loads are met by first applying the 

must-run/must-take resources, and then the least-cost dispatchable resources, subject to the 

transmission and generation constraints. The cost-ranked generation list is called a resource stack. The 

delivered cost of each generator to each load is different depending on factors such as wheeling cost, 

transmission losses, etc. The PCM accounts for all the cost factors and constraints as it converges on a 

solution for each hour.

Some of the key results from a PCM run are listed below. Most results are available on an hourly, 

weekly, monthly, and annual basis.

• Area and Region

o Load served and not served

o Load weighted locational marginal prices

• Generation

o Energy delivered and undelivered (dumped)

o Variable cost
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o Emissions

• Transmission and Network

o Branch and Path utilization

o Bus locational marginal price

o Transmission losses

The results from a scenario or base case can easily be compared to the results from another scenario. 

Some of the results can be compared with historical data to evaluate changes in trends and patterns.

Appendix 2

The state goals are calculated using the following equation with the goal in pounds per megawatt-hour 

(lbs/MWh).

Equation 1: State CO2 Rate

State goal = 

(Coal gen × Coal erate) + (OG gen × OG erate) + (NGCC gen × NGCC erate) + Other emissions

Coal gen + OG gen + NGCC gen + Other gen + Nuclear gen + RE gen + EE gen

Where:

- “erate” is short for emission rate in lbs/MWh

- “OG” is short for oil- or gas-fired steam 

- “Nuclear” is only the nuclear generation that is under construction or deemed at-risk by 

the EPA
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Glossary

Acronym Term Definition

WTG4 Model A generic inverter-based model used to model 

transient stability.

BA Balancing Authority The responsible entity that integrates resource plans 

ahead of time, maintains load-interchange-generation 

balance within a Balancing Authority Area, and 

supports Interconnection frequency in real time.

Base Load The minimum amount of electric power delivered or 

required over a given period at a constant rate.

BSER Best System of Emission 

Reduction

The degree of emission limitation achievable through 

the application of the best system of emission 

reduction that, taking into account the cost of 

achieving such reduction and any non-air quality 

health and environmental impact and energy 

requirements, the EPA determines has been 

adequately demonstrated.

LSE Load-Serving Entity An entity that secures energy and transmission service 

(and related Interconnected Operations Services) to 

serve the electrical demand and energy requirements 

of its end-use customers.

Boiler Draft Fan A fan that maintains pressure differentials in a boiler 

to induce or increase drafts of combustion gases.

Branch or Branch Group A group of transmission branches treated as a single 

entity for purposes of running a congestion 

management market.

BES Bulk Electric System The electrical generation resources, transmission lines, 

interconnections with neighboring systems, and 

associated equipment, generally operated at voltages 

of 100 kV or higher.

BAU Business As Usual The normal execution of standard functional 

operations within an organization in contrast to 

projects or programs which might introduce change.
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Acronym Term Definition

CA Greenhouse Gas Initiative An initiative in California, designed to transition the 

State to a low-carbon future, that requires California 

to reduce its Greenhouse Gas (GHG) emissions to 1990 

levels by 2020 — a reduction of approximately 15 

percent below emissions expected under a “business 

as usual” scenario.

Capacity Factor The ratio of a power plant’s actual output over a 

period of time, to its potential output if it were 

possible for it to operate at full nameplate capacity 

indefinitely.

CC Combined Cycle Any power generator in which a generator generates 

electricity and the waste heat is used to make steam 

to generate additional electricity via a steam turbine.

CT Combustion Turbine A type of internal combustion engine that has an 

upstream rotating compressor coupled to a 

downstream turbine, and a combustion chamber inbetween.

Common Case A compilation of planned generation, load forecasts, 

and expected transmission that represents a trajectory 

of recent Western Interconnection planning 

information, developments, and policies looking out 

10 years.

CCTA Common Case Transmission 

Assumptions

A stakeholder-vetted list of transmission projects that 

have a high expectation of being in service in the 

approaching 10-year timeframe resulted in the 

development of a transparent, repeatable and 

consistently applied process.

Cut-Plane In the context of the Path Rating Process, a path is 

defined as a facility(ies) between systems or internal 

to a system, for which schedules and/or actual flows 

can be monitored for reliability purposes. Facilities in a 

path may originate and terminate at the same point 

(substation or generating station) or at different 

points. Two or more individual paths can be combined 

into a single path for rating purposes, although they 

may be separate scheduling paths. Paths are often 
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Acronym Term Definition

called cut-planes.

DWG Data Work Group A work group of the Transmission Expansion Planning 

Policy Committee's (TEPPC) Technical Advisory 

Subcommittee (TAS), which is responsible for the 

collection and verification of the data in the TEPPC 

data set. The DWG uses publicly available information 

to populate the database.

DSM Demand-Side Management The term for all activities or programs undertaken by a 

Load-Serving Entity or its customers to influence the 

amount or timing of electricity they use.

EGU Electrical Generating Unit A solid fuel-fired steam generating unit that serves a 

generator that produces electricity for sale to the 

electric grid.

EE Energy Efficiency Reducing the amount of energy required to provide 

products and services through technology or operating 

practices.

EIA Energy Information 

Administration

An agency of the United States government that 

collects, analyzes, and disseminates independent and 

impartial energy information to promote sound 

policymaking, efficient markets, and public 

understanding of energy and its interaction with the 

economy and the environment.

Frequency Response The ability of a system or elements of the system to 

react or respond to a change in system frequency.

Heat Rate The amount of energy used by an electrical generator 

or power plant to generate one kilowatthour (kWh) of 

electricity. Heat rate is usually expressed in British 

thermal units (Btu) per net kWh generated.

IGCC Integrated Gasification 

Combined Cycle

A technology that uses a gasifier to turn coal and other 

carbon-based fuels into gas and then uses excess heat 

from the primary combustion generation to power a 

steam cycle, similar to a combined-cycle gas turbine.

IRP Integrated Resource Plan An integrated resource plan (IRP) is a comprehensive 

decision-support tool and road map for meeting a
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Acronym Term Definition

company's objective of providing reliable and leastcost electric service to all of its customers while 

addressing the substantial risks and uncertainties 

inherent in the electric utility business.

Inverter-Based Generation Three-phase generators that output AC current like 

most traditional generators, but that then convert the

current to DC, and then “invert” it back to clean AC 

power that maintains a single phase, pure sine wave at 

the required voltage and frequency.

LBNL Lawrence Berkeley National 

Laboratory

A member of the national laboratory system 

supported by the U.S. Department of Energy through 

its Office of Science. It is managed by the University of 

California (UC) and is charged with conducting 

unclassified research across a wide range of scientific 

disciplines.

LSE Load-Serving Entity An entity that secures energy and transmission service 

(and related Interconnected Operations Services) to 

serve the electrical demand and energy requirements 

of its end-use customers.

LMP Locational Marginal Price The marginal cost of supplying, at least cost, the next 

increment of electric demand at a specific location 

(node) on the electric power network, taking into 

account both supply (generation/import) bids and 

demand (load/export) offers and the physical aspects 

of the transmission system including transmission and 

other operational constraints.

LTRA Long-Term Reliability 

Assessment

An independent assessment of the 10-year reliability 

outlook for the North American BES that identifies 

trends, emerging issues, and potential risks. Additional 

insight will be offered regarding resource adequacy, 

security, and operating reliability, as well as an 

overview of projected electricity demand growth for 

individual assessment areas.

NGCC Natural Gas Combined Cycle An advanced power generation technology in which a

gas turbine generator generates electricity and the 

waste heat is used to make steam to generate 
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Acronym Term Definition

additional electricity via a steam turbine.

NERC North American Electric 

Reliability Corporation

A not-for-profit international regulatory authority 

whose mission is to ensure the reliability of the BulkPower System in North America.

Palo Verde A three-unit nuclear power plant near Phoenix, AZ.

Path One or more facilities between transmission systems 

or internal to a system, for which schedules and/or 

actual flows are monitored for reliability purposes. 

Facilities in a path may originate and terminate at the 

same point (substation or generating station) or at 

different points. Two or more individual paths can be 

combined into a single path for rating purposes, 

although they may be separate scheduling paths.

PCC Planning Coordination 

Committee

A WECC member committee created to advise and 

make recommendations to the WECC Board of 

Directors (Board) on all matters within the jurisdiction 

of WECC that pertain to maintaining the reliability of 

the Western Interconnection through evaluating 

potential future generation and load balance (two 

years or greater time frame); and adequacy of the 

physical infrastructure of the interconnected BulkElectric System.

PRM Planning Reserve Margin The amount of generation capacity available to meet 

expected demand in planning horizon. Planning 

Reserve Margin equals the difference in Deliverable or 

Prospective Resources and Net Internal Demand, 

divided by Net Internal Demand.

PSLF Positive Sequence Load Flow A load flow modeling tool built by GE that describes 

the positive sequence network for the steady-state 

initial condition from which simulations of system 

additions, deletions, status changes and data changes 

can be built.

Powerflow Model A model used in electrical engineering to analyze 

power grids that provides a nonlinear system which 

describes the energy flow through each transmission 
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Acronym Term Definition

line.

PCM Production Cost Model or

Production Simulation Model

A model that captures all the costs of operating a fleet 

of generators and develops an hourly chronological 

security constrained unit commitment and economic 

dispatch simulation.

Redispatch A means to resolve transmission congestion by 

changing generator output levels to reduce 

congestion.

RPCG Regional Planning 

Coordination Group

A group consisting of a member from each TEPPCrecognized Subregional Planning Group. The purpose 

of the group is to coordinate planning activities 

between each other and TEPPC.

RAP Regulatory Assistance Project A global, non-profit team of experts that provides

technical and policy assistance to policymakers and 

regulators on a broad range of energy and 

environmental issues.

Reliability Assurer The entity that coordinates activities of functional 

entities to secure the reliability of the Bulk Electric 

System within a Reliability Assurer area and adjacent 

areas.

RAS Remedial Action Scheme An automatic protection system designed to detect 

abnormal or predetermined system conditions, and 

take corrective actions other than and/or in addition 

to the isolation of faulted components to maintain 

system reliability.

REC Renewable Energy Credit Tradable, non-tangible energy commodities in the 

United States that represent proof that 1 megawatthour (MWh) of electricity was generated from an 

eligible renewable energy resource.

RPS Renewable Portfolio Standard A regulation that requires the increased production of 

energy from renewable energy sources, such as wind, 

solar, biomass, and geothermal.

Resource Adequacy A mandatory planning and procurement process to 

ensure adequate resources to serve all customers in 
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Acronym Term Definition

real time. The program requires that Load-Serving 

Entities (LSE) meet a Planning Reserve Margin for their 

obligations.

SCED Security-Constrained 

Economic Dispatch

A system that evaluates resource and transmission 

system information to produce a least-cost dispatch of 

generation resources that meets current system 

demand while complying with or resolving network 

constraints and resource limits.

Smart Soot Blowing A technology used in coal-fired generation in which 

instrumentation is used to identify the exact location 

of slag accumulation to allow the operator to target 

and adjust clean intensity based on slagging condition, 

thus minimizing tube leakage and clinker formation.

Societal Cost Economic, environmental or other costs that are 

borne by society at large, rather than by the entity 

that caused the cost to be incurred.

SPSC State-Provincial Steering 

Committee

A committee consisting of appointees from each State 

and Province in the Western Interconnection that was 

created to providing input into regional transmission 

planning; improve the efficient use of the existing grid; 

and enable the integrating of large amounts of 

variable generation.

SPG Subregional Planning Group A group that has been organized to address common 

issues within a particular portion of the Western 

Interconnection and have a close relationship with 

smaller Load-Serving Entities such as municipal utilities 

and rural electric cooperatives.

TSS Technical Studies 

Subcommittee

A subcommittee of WECC’s Planning Coordination 

Committee that performs studies, maintains data files, 

evaluates proposed system additions or alterations, 

prepares reports and recommendations, and performs 

such other duties as directed by the Planning 

Coordination Committee.

TEPPC Transmission Expansion 

Planning Policy Committee

A WECC Board of Directors committee that oversees 

and maintains a public database for production cost 

and related analysis; develops and implements 
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Acronym Term Definition

Interconnection-wide expansion planning processes; 

guides and improves the economic analysis and 

modeling of the Western Interconnection and conduct 

transmission studies; and prepare Interconnectionwide transmission plans consistent with applicable 

NERC and WECC reliability standards.

Voltage Stability The ability of a power system to maintain acceptable 

voltage at all buses under normal operating conditions 

and after being subjected to a contingency.

WI Western Interconnection One of the two major alternating current (AC) power 

grids in North America that includes all or parts of 14 

Western states, British Columbia and Alberta in 

Canada and a portion of Baja California in Mexico.

WIRAB Western Interconnection 

Regional Advisory Body

A group created by Western Governors under Section 

215 of the Federal Power Act to advise WECC and 

FERC on whether proposed reliability standards and 

the governance and budgets of WECC are in the public 

interest.