1. Principles
Overhead lines are subject to thermal limitations because conductors heat up due to ohmic losses, which in turn increases the sag. The WFB takes these physical relationships into account and, in favourable weather conditions, allows higher currents without exceeding the permissible conductor temperature or safety clearances. In practice, this can unlock capacity reserves of between 10 and over 50 per cent compared with the static rated current.
The result, however, is a variable operating state with current values that fluctuate over time. It is precisely these fluctuations that affect neighbouring steel pipe lines, as the alternating magnetic field of the overhead line changes with the conductor current. Long parallel runs, small clearances and inadequately designed earthing and protection systems are particularly relevant.
2. Effects on pipelines
Steel pipelines can be affected by capacitive, inductive and galvanic coupling, although inductive coupling usually dominates in the ground. The alternating magnetic field induces a longitudinal voltage along the pipeline, which increases with the length of the parallel section, the distance and the geometry. When conductor currents increase due to weather conditions, this induced voltage also increases proportionally.
The practical relevance lies not only in personal safety but also in plant operation: if permissible touch voltages are exceeded, this creates a safety risk for maintenance staff and third parties. At the same time, an increased alternating voltage at coating defects can alter the electrochemical conditions in such a way as to promote alternating current corrosion.
3. Alternating current corrosion
Alternating current corrosion is an electrochemical damage mechanism affecting metal surfaces in soil acting as an electrolyte. At defects in the coating, alternating current can flow into the ground and trigger corrosion reactions there. Although anodic and cathodic half-waves appear to cancel each other out, electrochemical asymmetries lead to measurable material loss.
The assessment focuses in particular on alternating current density and local soil conditions. The thresholds of 30 A/m² and 100 A/m² specified in the documentation indicate an elevated or high risk, respectively. A key factor is that, under WFB conditions, the impact may not only be short-lived but may also recur and intensify during peak loads.
4. Standards and Assessment
The assessment of alternating current interference and corrosion risk is based on relevant standards such as DIN EN 15280 and its successor standards, as well as on the requirements for cathodic corrosion protection set out in DIN EN ISO 15589-1. These standards specify limit values, measurement procedures and protection concepts for underground pipework. For practical planning, it is important to take into account not only steady-state operation but also the maximum achievable WFB currents.
In addition, the documentation highlights the need to provide suitable isolation units and discharge systems in the event of high-voltage interference. This is technically sound, as protection and measurement concepts are only robust if they reflect the actual operational dynamics of the overhead line. The assessment of interference must be re-evaluated, particularly in the event of subsequent changes to the operational concept.
5. Protective measures
A key measure is improved earthing of the pipeline, ideally in combination with AC isolation units. These systems specifically divert alternating currents into the ground without interrupting the direct current component of the cathodic corrosion protection. In addition, insulated joints, appropriate route planning and greater distances from overhead lines can significantly reduce the impact.
Another key focus is continuous monitoring. Measurement points using test plates, data loggers or DFI systems enable the assessment of AC and DC potentials under real operating conditions. This is particularly important because conventional measurements taken whilst the system is switched off can be distorted by equalising currents and superimposed AC components.
6. Conclusion
Weather-dependent overhead line operation is an effective means of making better use of existing electricity grids. However, in the case of steel pipelines running in parallel, it exacerbates electromagnetic interference and thus increases the risk of contact voltages and AC corrosion. For this reason, planning and operational decisions by grid and pipeline operators must be considered in conjunction with one another.
Safe parallel operation is technically feasible only with standard-compliant interference calculations, suitable earthing, AC isolation units, continuous monitoring and a KKS verification under real operating conditions. Where these elements are consistently implemented, the flexibility of overhead line operation can be utilised without neglecting the protection of the steel pipelines.
Sources:
Vulkan Verlag GmbH, Essen, 3R (2026) Nr. 1-2
https://www.dinmedia.de/de/norm/din-en-15280/178469404
https://www.dinmedia.de/de/norm/din-en-iso-15589-1/279496886
https://www.dke.de/de/normen-standards/dokument?id=7142842&type=dke|dokument
https://www.tugraz.at/fileadmin/user_upload/Events/Eninnov2010/files/pr/PR_Braunstein.pdf
https://www.vde.com/de/fnn/dokumente/alle-vde-anwendungsregeln
https://www.pp-engineering.com/wechselstromkorrosion/
