Overview
This executive brief examines the transmission system reliability risks associated with retiring conventional synchronous generators. It explains how transmission planners assess the impact of generator retirements across thermal loading, transient and voltage stability, frequency response, inertia, short-circuit impacts and other grid reliability metrics.
The brief also explores solution options that may serve as alternatives to synchronous generator reliability must-run agreements, helping stakeholders understand how reliability can be maintained as the generation mix changes.
Why it matters
As older coal, gas and nuclear units retire, the grid loses more than capacity and energy. Conventional synchronous generators also provide essential reliability services such as frequency control, voltage support, dynamic reactive power, short-circuit current, system strength and restoration capability.
At the same time, electricity demand is increasing sharply, driven by data centers, artificial intelligence, large industrial loads, transportation electrification, cryptocurrency mining, heat pumps and broader electrification. This combination of retiring conventional generation and growing demand creates a more complex reliability environment for transmission planners.
Key challenges explored
The brief examines several reliability considerations associated with retiring synchronous generation, including:
Thermal overloads
Voltage violations and voltage stability risks
Transient stability impacts
Frequency response and inertia concerns
Short-circuit current reduction
Protection system coordination
System strength
Harmonics and oscillation risks
Resource adequacy and energy assurance concerns
The role of reliability must-run agreements when timely solutions are not available
Questions this resource helps answer
This resource helps stakeholders consider questions such as:
What reliability services do synchronous generators provide?
What can happen when conventional generators retire before replacement solutions are ready?
How do transmission planners study generator retirement impacts?
What types of studies are used to assess thermal, voltage, stability and short-circuit risks?
When might a generator be required to remain online under a reliability must-run agreement?
What alternative solutions can help mitigate reliability risks?
How can utilities and decisionmakers compare conventional and emerging reliability solutions?
What the brief covers
The brief provides a practical overview of how transmission reliability risks are assessed and mitigated when conventional generation retires.
Key sections include:
Purpose and background
Resource adequacy and energy assurance concerns
Generation shifts and demand growth
Essential reliability services provided by generators
Generator retirement processes
Transmission planning studies for generator retirements
Reliability risks and solution options
Multi-pronged approaches to grid reliability
Considerations for deploying different solution options
Key solution options discussed
The brief compares several potential solutions for addressing transmission reliability risks, including:
Battery energy storage systems
Grid-forming BESS
Shunt capacitors
STATCOM / SVC
Synchronous condensers
Series compensation
Grid-enhancing technologies
Reconductoring and advanced conductors
Raising transmission towers
New transmission infrastructure
The comparison table on page 5 is useful because it shows how different options perform across reliability risks such as thermal overloads, voltage control, dynamic reactive power, inertial response, transient stability, system strength and short-circuit current.
Who this resource is for
This brief may be useful for:
Transmission planners
Utilities and grid operators
Regulators and policymakers
Generator owners
Reliability coordinators
Energy storage developers
Renewable energy developers
Technical advisors
Stakeholders involved in generator retirement, replacement resources or transmission planning