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Glossary · Energy industry & energy transition

Overhead line monitoring

overhead line monitoring electricity grid overhead line Dynamic Line Rating grid operation sensor technology

Overhead line monitoring (Freileitungsmonitoring, FLM) refers to the surveillance of overhead lines in the power grid. It combines sensors, current measurements, weather data and thermal calculation models. Quantities that are recorded or estimated include current flow, conductor temperature, sag, wind speed and direction, ambient temperature and global radiation. Depending on the system, icing, vibrations and the condition of towers, insulators and fittings may be added. The term therefore does not denote a single technology but a mode of operation: the line is not assessed solely on the basis of fixed worst-case assumptions, but according to its actual or more accurately estimated condition. In Germany, the grid development plans use the term largely synonymously with weather-dependent overhead line operation (WAFB).

Physical background

When current flows through a conductor, electrical losses occur and the conductor heats up. As the temperature rises, the material expands and the sag increases. The clearance to the ground, vegetation, buildings or crossing objects decreases, and the material is subjected to greater stress. The conductor temperature is therefore the decisive factor. For many lines in the German transmission grid, the permissible operating temperature is 80 °C. This is a common value for conventional conductors, but not a universally valid constant: high-temperature conductors and older lines may have different design limits. ENTSO-E also stresses that the design temperature of a line is the temperature at which no safety clearances are violated in any span. It may be lower than the temperature the conductor material itself could withstand.

The current-carrying capacity in amperes follows from the permissible conductor temperature. Until now it has often been set statically, for unfavourable conditions: high outside temperature, strong solar radiation, hardly any wind. In reality, the weather along a route is often more favourable. Wind carries heat away, cool air increases heat dissipation, and cloud cover reduces heat input from the sun. A simple illustration: a conductor that is already at its limit in heat and calm conditions can carry considerably more current on a cold, windy winter day without becoming warmer. The transmissible active power in megawatts additionally depends on voltage, reactive power flows and switching state. A line limit is therefore not a fixed megawatt figure.

Measurement methods and data quality

There are two basic ways to infer the permissible current from the conductor temperature. Direct methods measure at the conductor with spatial resolution, for example temperature, sag or conductor tension. Indirect methods calculate the conductor state from meteorological variables such as wind speed, wind direction, global radiation and ambient temperature. In practice, both approaches are often combined. ENTSO-E points out that a pure temperature measurement is not sufficient for a dynamic rating: the instantaneous load and the weather situation are not correlated. For a reliable limit, and above all for forecasts, the atmospheric conditions must therefore be taken into account. Closely related is line temperature monitoring, which is limited to the conductor temperature.

A line can only carry as much as its weakest section. A windy ridge helps little if a sheltered valley or a forested section cools the conductor less. Other components in the circuit, such as switchgear, transformers or clamps, can also be limiting. A monitoring system therefore needs sufficient spatial and temporal resolution, and it must not overlook the most limiting element. Sensors can fail or measure incorrectly, and weather models are uncertain. Operation therefore requires plausibility checks and fallback values, and it must function reliably even when measurements are lost.

Distinctions: Dynamic Line Rating, WAFB, grid reinforcement

Dynamic Line Rating (DLR) refers to deriving a variable operating limit from current or forecast conditions. Overhead line monitoring provides the data basis for this. Strictly speaking, monitoring thus describes the observation and evaluation, while DLR or WAFB describes the operating limit derived from it. In German usage, however, FLM and WAFB are often equated. Internationally, the method is counted among the Grid Enhancing Technologies.

German grid planning follows the NOVA principle: grid optimisation before grid reinforcement before grid expansion. In the current draft of the Grid Development Plan 2037/2045 (Version 2025), grid optimisation includes WAFB or FLM. It also includes high-temperature conductors and the conversion of suitably equipped lines from 220 kV to 380 kV. The earlier version of this entry had assigned re-conductoring and higher voltage levels across the board to grid reinforcement or grid expansion. According to the NEP classification, this is not correct. There, reinforcement includes, for example, adding conductors or replacing lines, while expansion covers new routes. Overhead line monitoring itself hardly changes the line physically. It improves knowledge of the line's condition and thereby makes existing reserves usable. As the technical basis for the gradual introduction of WAFB, the NEP cites the VDE application rule VDE-AR-N 4210-5.

Significance for grid congestion

Particularly in strong winds, a favourable correlation arises: wind energy plants feed in a lot of power, and the same wind cools the conductors. A static limit does not reflect this. Weather-dependent operation can allow higher currents under suitable conditions. According to 50Hertz, certain lines in its control area were in individual cases loaded up to 28 percent higher without exceeding the operating temperature of 80 °C. This value applies to specific lines and winter operation and cannot be generalised. A higher current-carrying capacity can reduce interventions in congestion management, such as redispatch. In Germany, the former feed-in management was absorbed into Redispatch 2.0 in October 2021. System requirements such as the (n-1) criterion must still be observed.

However, overhead line monitoring does not eliminate congestion in general. Some congestion arises from missing lines, transformer limits, stability requirements or protection concepts. Issues of voltage control, short-circuit power and system stability cannot be solved through higher thermal loading. Monitoring only becomes effective once the data flow into control systems, load flow calculation and grid security analysis. Sensors alone do not create additional capacity.

Legal framework in Germany: temporary higher utilisation under Section 49b EnWG

Since October 2022, the Energy Industry Act (Energiewirtschaftsgesetz) has contained a temporary special rule in Section 49b. In the version in force since 9 February 2024, it allows operators of transmission grids, until the end of 31 March 2027, to operate the extra-high-voltage grid at higher loading without prior permit. The rule thus applies only to transmission system operators and the extra-high-voltage grid, not to distribution grids. Higher utilisation here means more current-carrying capacity without a higher operating voltage. The towers remain in place; structural changes are permissible at most to a minor extent and at isolated points. Before starting, the higher utilisation must be notified to the competent immission control authority. It must be demonstrated that the limit values of the Ordinance on Electromagnetic Fields for magnetic flux density are complied with. The measure is published in the Federal Gazette (Bundesanzeiger), and the municipalities concerned are informed. Operators of other technical infrastructure, such as pipelines, must tolerate additional electromagnetic interference. The transmission system operator reimburses them for the necessary costs of protective measures. Permanent higher utilisation remains subject to permitting under the regular provisions. Technically, the approach is based on WAFB.

Common misconceptions

  • Monitoring is not a blanket capacity increase, but a more accurate determination of the currently permissible load. In heat, calm conditions or icing, the reliably permissible value can become tight. How operators then handle the static value as a lower bound is governed by their operating rules.

  • Measured values are not automatically relevant for operation. What matters is the value that a grid operator can safely rely on even in the event of data loss.

  • Monitoring is not the same as maintenance. Condition data can improve asset management, for example through early detection of vibrations or ageing. The benefit for congestion, however, only arises through integration into grid operation management.

  • Monitoring does not replace grid expansion. Where large amounts of energy must be transported permanently over long distances, reinforcement and expansion often remain necessary. The NEP already factors in WAFB and nevertheless identifies additional grid needs.

Economic and institutional aspects

Monitoring costs money: for sensors, communication, data processing, model maintenance, integration into control systems and personnel. The benefits show up in avoided redispatch costs, better utilisation and more targeted maintenance, but do not always arise where the costs are incurred. How attractive such methods are for a grid operator therefore also depends on whether regulation adequately takes operating costs and efficiency gains into account. Liability questions also arise: anyone who loads a line dynamically higher must be able to justify the chosen limit technically and control it operationally. Grid capacity is thus not a rigid property of a wire. It results from physics, weather, safety clearances, measurement quality and operating rules.

Sources

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Edited by eCMS on 12.11.2026 05:05

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