Dynamic Line Rating on Overhead Lines

	Pierre Van Dyke, Chair SC B2		George Watt, Convenor WG B2.79

 

The main themes of the two 2025 CIGRE symposia, “Changes Needed in the System for the Energy Transition” (Trondheim, May) and “Grid Enhancement, Strategic Planning, Technological Innovation, and Climatic Adaptation for a Resilient Future Energy System” (Montreal, September), clearly highlight the critical importance of technological innovation and energy transition in the electrical sector.

 

It is now widely accepted that electricity demand is growing, driven by the increasing number of electric vehicles, the expansion of larger data centres, and the shift from fossil fuel technologies to electric alternatives. Consequently, unprecedented strain is placed on electric grids. Existing transmission lines must carry significantly more power from generation centres to consumers. However, building new transmission lines is time-consuming, costly, and often faces contentious process.

 

Renewable energy sources like wind and solar, often located remotely, are also dramatically altering power flows across the transmission system, meaning existing lines may be forced to operate beyond their original thermal ratings.

 

Climate change adds further complexity, hotter summers not only increase demand for air conditioning but also reduce the capacity of older lines, some of which were designed in the 60’s with the maximum allowable operating temperatures as low as 49^oC.

 

For years, various strategies have been explored to increase transmission line capacity, resulting in numerous CIGRE Technical Brochures, including:

 

• TB 244: Conductors for the Uprating of Overhead Lines
• TB 294: How OH Lines Are Re-designed for Uprating/Upgrading
• TB 299: Guide for Selection of Weather Parameters for Bare Overhead Conductor Ratings
• TB 353: Guidelines for Increased Utilization of Existing Overhead Transmission Lines
• TB 425: Increasing Capacity of Overhead Transmission Lines – Needs and Solutions
• TB 426: Guide for Qualifying High-Temperature Conductors for Use on Overhead Transmission Lines
• TB 498: Guide for Application of Direct Real-Time Monitoring Systems
• TB 583: Guide to the Conversion of Existing AC Lines to DC Operation
• TB 601: Guide for Thermal Rating Calculations of Overhead Lines
• TB 643: Guide to the Operation of Conventional Conductor Systems Above 100°C

 

Most recently, Working Group B2.59 completed TB 969: Forecasting Dynamic Thermal Line Ratings.

 

Dynamic Line Rating (DLR) offers a significant advantage over other capacity-increasing solutions: it can be implemented quickly and at relatively low cost. Typically, DLR can increase transmission capacity by about 10% over Static Line Rating for 90% of the time. DLR optimises power flow by continuously assessing real-time weather data to determine a maximum safe operating temperature of the line.

 

The effectiveness of DLR relies on accurate, line-specific monitoring and robust forecasting. Short-term DLR forecasts (1–6 hours ahead) can be generated using statistical models, while longer-term forecasts (6–48 hours) typically depend on commercial weather data. Overestimating DLR can negatively impact grid operations, leading to system operating interventions like adjusting network topology, redispatching generation, or using power flow control devices like phase-shifting transformers. These actions can incur economic penalties and lead to suboptimal generation dispatch. Therefore, when high forecast reliability (e.g., 98%) is needed, an economic optimum must be defined.

 

DLR is not only a powerful solution but also a valuable interim measure before uprating an existing line or constructing a new one. Work continues in this area with WG B2.79: Enhancing OHL Rating Prediction by Improving Weather Parameter Measurements, which is expected to publish a Technical Brochure in early 2026.