Dynamic Line Ratings

Building new transmission lines can be prohibitively expensive. It's not simply a matter of erecting poles and wires; obtaining essential environmental and land use permits from communities and localities can be especially challenging. Transmission lines dictate how much bulk power flows through the system. In situations where the transmission system is constrained and building new lines is not an option, how can we maximize the use of existing transmission lines? This is where Dynamic Line Ratings (DLR) come into play. As discussed below, DLR represents a new paradigm for assessing transmission line capacity and utilizing them to their maximum potential while staying within safety limits.

Physics of Dynamic Line Ratings
The amount of electricity that transmission lines can conduct is limited by several factors, including conductor type, size, voltage level, distance, and environmental conditions. For existing transmission lines, size, type, distance, and voltage are fixed parameters. One major environmental factor affecting transmission lines is temperature. Each transmission line has a maximum temperature limit; when it heats up, it can sag, reaching a maximum sag limit as well. Environmental temperature fluctuates with time and season. DLR adjusts the capacity of transmission lines based on real-time ambient conditions and actual field measurements. This approach contrasts with previous static line ratings, which were fixed and only varied seasonally without real-time measurements.

While the concept seems straightforward, DLR can be complex because actual conditions depend on multiple factors, including wind, ambient temperature, solar irradiation, and others, all of which influence the line rating calculation. Among these factors, wind has the greatest cooling effect on the lines.

Source- ISGAN- https://www.youtube.com/watch?v=xzWoQkVVhFc

As shown in the image above, the transmission capacity doubles from 1000 A to 2000 A as wind speed increases from 0.5 m/s to 5 m/s. The actual calculation of DLR can be intricate. IEEE 738 provides a thermal model for the application and calculation of real-time monitoring and modeling of transmission lines, used in practical applications. The model considers factors such as clearance, temperature, current, and weather parameters to calculate the gains and limits, providing the dynamic ampacity of the transmission line.

These parameters can be based on field measurements or model-based simulations; however, field-based monitoring generally offers higher accuracy.

Sensor installation technologies can be classified into four types, ranked chronologically:

1. Line-mounted sag sensor: Measures sag, wind, and temperature to calculate conductor temperature and clearances. This method provides the highest accuracy for DLR.
2. Line-mounted clearance sensor: Measures clearances, inclination, and vibration to infer sag and conductor temperature.
3. Line-mounted conductor temperature sensor: Measures conductor temperature and inclination to infer sag.
4. Indirect sensors: Do not measure parameters directly on the transmission lines but use information from weather stations and modeling. This is the least accurate method.

Uses

1. Underground: While DLR is primarily used for overhead lines, there are techniques for applying it to underground lines as well; however, the benefits are greater for overhead lines.
2. AC vs DC: Currently, DC circuits cannot use DLR sensors because these sensors are powered by induction from AC lines. In static DC lines, these sensors cannot be powered with existing methods.
3. Voltage applications: DLR can be applied from 20 kV to 800 kV.

Questions and Challenges

Although implementing DLR technologies seems straightforward, there are market and technical challenges to widespread adoption:

• Rethinking market incentives: In market-based systems, a key question is whether the incentives are aligned for transmission system owners to implement DLR. Revenues for transmission system owners vary by region, market, or product. In some cases, higher transmission constraints lead to higher transmission capacity prices. Can this be balanced through increased electricity throughput? Are there other direct incentives for transmission owners to adopt DLR technologies?
• Rethinking grid operations: Traditional grid operation methods use static capacities for transmission lines. Transitioning to a dynamic method with field sensors complicates system operations. Do system operators have the capacity to monitor, control, and settle markets at this level?
• Forecasting methodology: Transmission operations depend on load forecasting, capacity, and generation to meet demand. How does this process evolve with DLR implementation?

DLR is a proven technology already used in places like Germany and Belgium. In the US, FERC is actively looking to implement DLR in the wholesale market. While in 2021, the FERC order 881 required the transmission companies to require dynamic line ratings that accurately reflect the system, that is just the first step. As mentioned above, implementation of DLR has several challenges. FERC is now looking to reform the market rules such that DLR can be effectively implemented in the whole system. With the intent FERC, it is just a matter of time when DLR becomes a norm in the US market, which is a logical step forward for the energy market.