Forecasting Dynamic Thermal Line Ratings

Introduction

The benefits of calculating variable line ratings based on real-time weather measurements within the line corridor have been studied and recognized for decades. These variable ratings, known as Dynamic Line Ratings (DLR), are typically well above static line ratings. However, the limited availability of forecasting methods, both short-term (up to 4 hours) and medium-term (up to 48 hours) for power market, maintenance, and operations planning, has hindered the broader adoption of DLR.

Significant work has been done by CIGRE Working Groups (e.g., Technical Brochures 244, 299, 498, 601, 643, 763) to calculate thermal ratings and identify relevant weather parameters and uprating methods. This brochure describes practices and methods used to increase the thermal rating of overhead lines to relieve constraints on renewable generation and bulk power flow. It also explores forecasting methods for both short- and medium-term applications in system operations. Two key approaches are discussed: DLR - Ambient Adjusted (DLR-AA) and DLR - Real-Time Monitoring (DLR-RTM), the latter involving real-time monitoring systems to better manage grid constraints.

Conductor Thermal Limitations

The temperature of overhead conductors is limited to prevent:

• Violation of minimum electrical clearances due to excessive sag,
• Loss of tensile strength from annealing of aluminum strands,
• Premature aging of connectors and hardware.

The frequency and duration of high-temperature events are also limited by specifying the duration of emergency ratings (e.g. 2-hour emergency rating) or by restricting the number of such events over the line’s lifespan.

Variable Line Ratings

Thermal line ratings vary with ambient conditions. The table below illustrates how line ratings respond to changes in wind speed, solar heating, and air temperature for different TCMAX (maximum allowable conductor temperature) values.

Key conclusions:

• Modest changes in air temperature and solar heating affect line ratings about three times more at T_CMAX = 75°C than at 125°C.
• Higher wind speeds significantly increase ratings but are too variable for static line rating (SLR) use.
• Increasing T_CMAX by 50°C results in a >50% increase in rating - more than any other weather parameter change.
• At T_CMAX = 75 ºC, it might be tempting to rate lines higher, however, wind speed usually drops off at night depending on local conditions.

Line Corridor Weather Implications

Wind speeds in a line corridor vary by geography and time of day - typically higher in the afternoon and lower at night. Effective wind speeds can be estimated statistically using long-term anemometer data. However, airport weather data is generally unsuitable for most line corridors. Weather analysis should also consider correlations between wind, air temperature, and solar heating.

Challenges in predicting effective wind speeds include:

• Short thermal time constants of bare conductors (5–20 minutes),
• High variability and randomness of wind at speeds <2 m/s,
• Strong sensitivity of line ratings to wind speed.

Terrain and nearby structures can cause turbulence and random wind direction. Wind speed and direction vary both along the line and over time. The concept of Effective Perpendicular (EP) Wind, the perpendicular constant wind speed that produces equivalent cooling, is used to address this variability.

Dynamic Line Rating Methods

Instead of using conservative weather assumptions, DLR calculates thermal ratings based on real-time weather conditions in the line corridor. These ratings are typically higher but require continuous monitoring.

DLR methods include:

• DLR-AA (Ambient-Adjusted): Considers air temperature (and sometimes solar heating). It’s simple and widely used in North America, relying on real-time and forecasted regional air temperatures.
• DLR-RTM (Real-Time Monitoring): Uses EP wind speed, air temperature, and solar heating at multiple locations. It provides higher ratings but requires remote sensors, real-time data communication, and software to calculate ratings every 5-10 minutes.

DLR Forecasting

Forecasts of Dynamic Line Ratings (DLR) for the next 1 to 6 hours can be generated using various statistical methods. However, forecasts extending from 6 to 48 hours require more sophisticated weather prediction techniques.

Mesoscale atmospheric conditions are typically forecast using Numerical Weather Prediction (NWP) models, which involve intensive computations over a global grid, corrected by satellite and surface weather measurements. Due to the computational demands, the 3D grid resolution is limited—most models use cell dimensions of around 10 km, though newer models are approaching 1 km resolution.

DLR forecasting cannot rely solely on standard NWP data. Forecasted wind speeds and directions must be adjusted using real-time monitoring to account for line corridor sheltering and the height of overhead conductors. To achieve sufficient accuracy, statistical and/or dynamic downscaling must be applied to the raw weather model outputs.