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Glossary · Energy industry & energy transition

Dynamic Line Rating

Dynamic Line Rating electricity grid grid operation line capacity overhead line Grid enhancing technologies

Dynamic Line Rating (DLR) is the time-varying determination of the permissible current-carrying capacity of an electrical line. It is based on current or forecast ambient and operating conditions. It almost always refers to the overhead line: how much current may it safely carry at a given point in time without the conductor becoming too hot, the prescribed ground clearances being undercut, or other operational safety requirements being violated? In German-language grid operation the method is usually called weather-dependent overhead line operation (witterungsabhängiger Freileitungsbetrieb, WAFB) or overhead line monitoring (Freileitungsmonitoring, FLM).

Physical background

The relevant quantity is the current in amperes, that is, the current-carrying capacity (ampacity). The power in megawatts that results from it depends, among other things, on the voltage level and the load flow. For an overhead line, the loading capacity is limited mainly by the highest permissible operating temperature of the conductor. Flowing current heats the conductor through ohmic losses, and solar radiation also heats it up. Wind, convection and thermal radiation, by contrast, cool it. The warmer the conductor becomes, the more it expands and the lower it sags. This can reduce clearances to the ground, vegetation, buildings or crossing objects to an impermissible degree. High temperatures also make conductors and connections age faster. Current-carrying capacity is therefore not only an electrical quantity, but also a thermal and mechanical one.

An illustrative picture: a conductor behaves like a heating wire outdoors. On a cool, windy day heat is carried away quickly, and the conductor tolerates considerably more current. On a hot, windless summer day with full sun, the margin is small. Wind has the strongest effect, especially when it blows across the line. Even a few metres per second cool the conductor noticeably.

Static, seasonal, ambient-adjusted and dynamic rating

A static thermal rating sets a fixed limit that is meant to be safe even under unfavourable conditions. Typical assumptions are a high air temperature, strong solar radiation and hardly any wind. This caution makes sense. However, it means that in many hours of the year the line is loaded less than would be physically possible. Between static and fully dynamic rating there are intermediate stages. Seasonal ratings use different assumptions for the seasons. Ambient-Adjusted Ratings (AAR) take into account the actual or forecast air temperature and, under US rules, also the difference between day and night in solar radiation. A dynamic rating goes further and additionally includes wind speed and wind direction, solar irradiance or directly measured states of the conductor.

It is important to note that a dynamic rating can also be below the static value, for example in heat, calm conditions and strong sun. DLR is therefore not only a way of sending more current through the line. It makes the limit more accurate, in both directions.

Measurement, models and forecasts

There are several technical approaches to DLR. Some use weather data from measuring stations or from weather models. Others measure directly on the line, for example the conductor temperature, the current, the sag or the conductor tension. There are also thermomechanical models and evaluations of synchrophasor measurement data. An ENTSO-E survey among transmission system operators in Continental Europe found that many operators use DLR in operation or in trials. There, the results often serve as information or as an alarm for the switching personnel, so they do not control operation automatically. ENTSO-E also points to a key point: the loading capacity changes along the route, and what matters is the most strongly limited section or the most strongly limited component.

This also shows the main weakness of the method. An overhead line runs through valleys, forests and the edges of settlements. The wind at a sheltered span can differ greatly from the value at a distant weather station. Where measurements are taken and how measured values are transferred to entire line sections is therefore decisive. Forecast errors are added to this. For next-day operational planning, what counts is not the current measured value but a reliable forecast including safety margins. And because sensors, communication paths and control-system interfaces create additional attack surfaces, cybersecurity is also one of the requirements.

Benefits in grid operation

In practice, DLR is mainly useful in congestion management. If a lot of electricity is generated from wind and sun and the grid capacity is not sufficient to carry it away, grid operators have to intervene with redispatch: they curtail plants upstream of the bottleneck and ramp up power plants behind it. If a line can demonstrably be loaded more heavily in favourable weather, part of these interventions can be avoided. This is particularly interesting for wind energy: strong wind often brings high feed-in and better cooling of the lines at the same time. In heat and light wind, for example in high-load situations in summer, the additional benefit is by contrast small.

The benefit has limits. DLR only helps where the heating of the overhead line is actually the limiting factor. Often other components in the circuit are limiting, such as transformers, disconnectors, switchgear or protection settings. They must be designed for higher currents or be retrofitted. In the meshed AC grid, power flows are also distributed according to the impedances of the lines. If a line is thermally relieved, a voltage or stability limit may take effect elsewhere. Any additional loading capacity must also be included in the security calculation under the n-1 criterion: even if an important item of equipment fails, no impermissible states may arise. A technology overview by the German Federal Ministry for Economic Affairs points out that overhead line monitoring tends to reduce existing operating reserves and requires more effort in operational management.

Delimitation

  • Not grid expansion: DLR creates no new line and no new route. ENTSO-E expressly describes dynamic thermal rating as a complement to grid development and not as a replacement for grid expansion.

  • Part of the grid enhancing technologies: DLR is one of the grid enhancing technologies, which make existing grids more usable. Further examples are load flow control with phase-shifting transformers or high-temperature conductors. The mode of action differs in each case, and high-temperature conductors require a rebuild of the line.

  • No permanent transport commitment: A higher loading capacity for certain hours is not sufficient for long-term grid connection commitments or as the sole basis of investment planning. It can, however, make planning more precise.

  • Different physics for cables: For underground cables, heat is dissipated through the soil. Soil type, moisture, laying depth and neighbouring cables are decisive. Dynamic assessments are possible there as well, but follow different measurement and modelling approaches. The explanation based on wind cooling only applies to overhead lines.

Use and regulatory framework in Germany

In Germany, according to the technology overview of the Federal Ministry for Economic Affairs, weather-dependent overhead line operation is state of the art, above all on 220 kV and 380 kV lines. According to it, TenneT already converted more than 900 km of extra-high-voltage lines to weather-controlled operation in 2010. The transmission system operators assign the method to the NOVA principle: grid optimisation takes precedence over reinforcement and expansion. Required modifications to lines are described in the Netzentwicklungsplan Strom (electricity grid development plan) and confirmed by the Bundesnetzagentur (Federal Network Agency). Technically, the procedure is described in the VDE application rule VDE-AR-N 4210-5 "Witterungsabhängiger Freileitungsbetrieb" (weather-dependent overhead line operation).

European Union and USA

At EU level, recital 83 of the Electricity Market Directive (EU) 2019/944 names the ability of transmission and distribution system operators to operate lines with Dynamic Line Rating as a possible indicator. On the basis of such indicators, regulatory authorities are to monitor the performance of grid operators. A recital is, however, not an independent obligation to use DLR. ACER and CEER have developed proposals for key indicators from this.

In the USA, FERC Order 881 of December 2021 obliges transmission providers to use Ambient-Adjusted Ratings for short-term transmission services. It does not prescribe full DLR. Organised markets must, however, be technically able to accept dynamic ratings. In June 2024, FERC published an advance notice of proposed rulemaking (ANOPR, docket RM24-6). According to it, ratings are to take solar radiation into account based on the position of the sun and forecast cloud cover and, for certain lines that are particularly relevant to congestion, also wind forecasts. According to the available sources, this rule had not yet been conclusively adopted. For questions of grid planning, Order 1920 of 2024 is also relevant. It requires that, among other things, DLR be considered in regional transmission planning.

Economic assessment

Whether DLR pays off cannot simply be read off from the sensor price and the amperes gained. On the benefit side are avoided redispatch costs, less curtailment of renewable energy and, under some circumstances, more targeted grid reinforcement. On the cost side are sensors, communication, software, maintenance, IT security requirements and more complexity in operation. Regulation also plays a role: costs arise at the grid operator, while the benefit is spread across the whole system. In a heavily loaded grid, even a little additional transmission capability in critical hours can be worth a lot. In a lightly loaded grid, the same approach achieves hardly anything.

Overall, DLR shows that grid capacity is determined not only by built infrastructure, but also by assumptions, measurability, safety rules and operational management. The method does not create a new line. It can, however, prevent an existing line from being permanently operated as if the worst possible weather always prevailed.

Sources

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Edited by eCMS on 11.11.2026 19:42

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