Overhead line conductors ACSR/AAAC/AAC per IEC 61089 — construction, sag and corona

The guide on electrodynamic forces during busbar short-circuits (IEC 60865-1) and the guide on cable-screen cross-bonding for high-voltage cables cover mechanical and electrical aspects of underground and rail-bound conductor systems. This article covers the bare, uninsulated conductor that hangs overhead: the round wire, concentric-lay stranded line conductor, standardised in IEC 61089 ("Round wire concentric lay overhead electrical stranded conductors").

Three main types under IEC 61089

IEC 61089 describes the electrical and mechanical properties of several construction variants, of which three are most common in practice:

AAC (All Aluminium Conductor): exclusively hard-drawn aluminium wires, concentrically stranded around a central wire, with no steel core or alloy addition. Highest conductivity per cross-section, but the lowest tensile strength of the three — mainly suited to short spans with limited mechanical loading.
ACSR (Aluminium Conductor Steel Reinforced): an outer envelope of hard-drawn aluminium wires concentrically stranded around a core of galvanized (or aluminium-clad) steel. The steel core carries most of the mechanical tension, allowing longer spans and higher sag loads (wind, ice) than AAC at the same aluminium cross-section.
AAAC (All Aluminium Alloy Conductor): exclusively wires of an aluminium-magnesium-silicon alloy, also concentrically stranded around a central wire, without a steel core. This alloy combines higher tensile strength than pure aluminium with a better weight-to-strength ratio than ACSR and — unlike ACSR — carries no galvanic-corrosion risk between aluminium and steel.

Note: the standard also allows zinc- or aluminium-clad steel wires and different aluminium alloy classes (A1, A2, A3) to be combined; the exact material codes, unit weights and permitted tensile strengths per cross-section are laid down in the standard's own tables and must be checked against the actual project specification.

Sag and the catenary

An overhead line conductor does not hang in a straight line between two towers but approximately follows a catenary curve, determined by the conductor's own weight, the tension applied at installation and the span length. Sag increases at higher ambient temperature (thermal expansion of the metal) and under additional wind or ice loading, and decreases with higher installation tension. Designers use a ruling span (a representative, weighted average span for a series of consecutive towers) to capture the sag-temperature relationship for an entire tower run in a single calculation, instead of modelling every individual span separately.

Note: the exact tension and sag calculation (including the temperature and ice-loading limits that apply in the Netherlands) is project-specific and must be worked through with the actual tower spacing, conductor data from the standard and the applicable local climatic loading — this article only presents the underlying principle.

Corona: the effect of field strength at the conductor surface

At sufficiently high voltage, a localised, partial ionisation of the surrounding air occurs around an overhead conductor as soon as the electric field strength at the conductor surface exceeds a critical value — corona. Corona is audible (a hissing or crackling sound, especially in humid weather), causes radio and television interference in a certain frequency band, and is accompanied by a (generally limited) energy loss. Because the surface field strength decreases with a larger conductor diameter (at equal voltage and current, the charge is spread over a larger surface), high-voltage lines sometimes deliberately use a conductor with a larger diameter than would be needed for current-carrying capacity alone, or use bundle conductors (two or more parallel sub-conductors per phase) to reduce the effective surface curvature — and thereby corona.

Why this matters

When assessing an overhead line connection (for example a small-scale wind turbine grid connection, see the guide on grid connection of small-scale wind turbines) it is important to recognise that the choice between AAC, ACSR and AAAC is not purely a matter of conductivity: mechanical span length, expected ice and wind loading, corona sensitivity at higher voltage levels and — for ACSR — the galvanic corrosion risk between aluminium and steel (see also the guide on galvanic corrosion at earth electrodes for the underlying bimetallic-corrosion principle) all factor into the final choice.

Common mistakes

Choosing aluminium cross-section based on current-carrying capacity alone, without weighing the mechanical span length and expected wind/ice loading — on a long span this can lead to unacceptable sag or overloading of the conductor.
Applying ACSR without attention to galvanic corrosion between the aluminium outer envelope and the steel core at locations where moisture can penetrate (damaged wire ends, clamp connections) — this can weaken the steel core over years without being visible from the outside.
Ignoring corona effects when selecting a smaller conductor diameter for a high-voltage line — this can lead to unexpected radio/TV interference complaints and higher-than-expected energy loss, even if the conductor is amply sufficient for current-carrying capacity.
Assuming a single fixed sag value instead of performing the temperature-dependent sag calculation with the ruling-span method — sag under winter ice loading can differ substantially from sag at maximum summer operating temperature.