Why Transmission Congestion Matters in the Energy Transition
As the United States navigates a critical energy transition, regulators across the country are calling for greater transmission system buildout to meet anticipated electricity demand. Increasing the capacity to transport renewable energy across the grid is essential to meet clean energy goals, reduce greenhouse gas emissions, and ensure grid reliability. However, this buildout will come at a significant cost to consumers, underscoring the need for strategic planning and cost monitoring.
In this post, we’ll explore transmission congestion—what it is, why it matters, and the challenges it presents in our journey toward a sustainable energy future.
What Is Transmission Congestion?
Our transmission system moves large amounts of electricity over long distances before it is transferred to the smaller, local distribution system and then finally into homes and businesses. Transmission lines are the middlemen that move sizable amounts of electricity over long distances, while distribution lines provide the last-mile delivery from the substation to the final consumer. Congestion occurs on the transmission system when the power volumes exceed the physical capacity limits of the existing transmission infrastructure. Lines can only carry so much power before they become over-heated and potentially dangerous to the surrounding area.
Congestion is a bottleneck in the system. When congestion on the transmission system prevents electricity from the least-cost resources from reaching consumers, higher-cost resources must fill in the gap. This results in a higher locational marginal price (LMP) in regions that are import-constrained. The cost difference between the most economic resource and the one able to serve the import-congested region is the cost of congestion. Conversely, regions with ample low-cost generation that cannot serve demand due to congestion will be able to supply the local area with low-cost energy but will be unable to export power to higher-demand areas. This creates a negative congestion price and decreases the LMP, and it could result in potential curtailment of renewable resources.
Key Factors Limiting Transmission Capacity
The three factors that determine the maximum capacity of a transmission system are stability, voltage, and thermal limits. These limits protect power lines from damage, support the stability of the transmission grid, and maintain the safety of transmission corridors.
- Stability limits are the maximum or minimum power that can be transferred through a point while maintaining synchronism with the whole system. Systems specify acceptable frequency ranges during normal operation, which determined as 60 Hertz in the United States. Deviations occur when the frequency exits a very narrowly defined range, typically when there are unexpected declines or rises in generation or load. Stability limits ensure that the power flow does not reach a level that increases the risk of a system disturbance which could have catastrophic consequences such as power failures and equipment damage.
- Voltage limits are set by the physical limits of the substation. To maintain the proper function of substation equipment, operators place limits on transmission to keep voltage within the permissible range of the substation.
- Thermal limits prevent overheating of transmission lines and metal annealing which compromises the metallic structure of the lines. Transmission lines also expand and sag when heated, posing a risk if the sag allows contact with items within the transmission corridor. This could result in arching and potentially devastating forest fires. Thermal limits consider both the heating due to power flows and the ambient air temperature to manage how much power can flow over a given line.
Addressing Congestion: The Challenges
Transmission congestion is not only a matter of infrastructure capacity; time and location also factor in. For one, with the exception of battery storage located at strategic positions on the grid, transmission capacity cannot be stored at off-peak times for use during peak times. New renewable generators compound locational and temporal transmission constraints because they are built in locations that maximize potential generation but may be far from existing grid infrastructure. Additionally, renewables generate electricity when conditions are right, not necessarily when power is needed most or prices are highest. While there are efforts to reuse retired fossil plant's points of interconnection to reduce the need for new rights of way, this does little to help congestion in rural areas where new renewable generation is expected. So, how do we move forward?
Stay tuned for the next post in this series, where we’ll delve into what New England has done to limit congestion, the challenges the region faces, and planning for New England’s transmission future.