Transmission Tower Types

Their efficient and safe operation depends largely on correctly selected support structures, namely electrical transmission towers. These tower structures support the phase conductors and ground wire, define the line geometry, and bear the mechanical loads resulting from conductor tension, wind action, or icing. 

Poland operates several hundred thousand transmission tower design types of varying transmission voltage levels. They range from concrete utility poles in low-voltage networks, through steel lattice towers for 110 kV lines, to high voltage towers several dozen metres tall used in 400 kV networks. Each of these types of transmission towers serves a different function and must meet precisely defined mechanical and electrical requirements. This article examines the different types of transmission towers, how they are classified, and the key criteria for selecting them.

Ways of categorising power transmission towers

Transmission towers can be classified according to several different criteria. The two most fundamental are classification by voltage and functional classification based on the role a given structure plays in the line.

Classification by voltage 

This is closely linked to the voltage levels of the power system. The transmission voltage of the line determines not only the dimensions of the transmission tower and the required insulation distances, but also the materials used – from concrete utility poles in low-voltage networks to massive steel lattice towers or tubular structures used in extra-high-voltage overhead transmission lines.

Functional classification 

This addresses how a transmission tower behaves mechanically in relation to conductor tension and what line deviations it can handle. Two main categories are distinguished: suspension towers (lightweight tangent towers that suspend the transmission conductors without bearing their longitudinal tension) and anchor towers (robust tension towers designed for one-sided or full conductor tension). Each of these categories comprises several specific tower types, discussed below.

What transmission tower types are used in Poland?

The following sections examine how this classification applies in practice to the different types of transmission towers found on Polish overhead power lines.

Medium-voltage (MV) transmission towers

These transmission towers are used on power lines operating at 10, 15, 20, or 30 kV. The most common forms are spun concrete utility poles (type E) and steel lattice towers, the latter performing particularly well in mountainous areas where heavy icing on the overhead wire generates significant mechanical loads. MV towers additionally equipped with mounting structures for devices such as surge arresters, disconnectors, distribution transformers, and reclosers form pole-mounted units or complete pole-mounted substations, depending on the installed equipment.

High-voltage (HV) transmission towers

These high voltage transmission towers carry 110 kV lines, which form the primary level of power transmission over distances of several dozen kilometres. Steel lattice towers predominate in this category; tubular towers are also becoming increasingly common, as they are more aesthetically pleasing and easier to install. High voltage power lines must also be equipped with a ground wire for lightning protection to shield the line from the effects of lightning strikes, and each transmission tower must have a functional earth connection with the appropriate resistance.

Extra-high voltage (EHV) transmission towers

These electrical transmission towers are found on 220 kV and 400 kV high voltage transmission lines, which transport bulk electric power from generating stations to distribution substations. They are imposing structures – often steel lattice towers several dozen metres tall – designed for very long spans and extreme loads. Their dimensions and foundations must simultaneously account for the tensile forces of large conductor bundles, wind loads, and dynamic vibrations.

Suspension towers (P)

Suspension towers are the most common type of transmission tower. The transmission conductors are carried through them in a continuous run – the tower therefore bears no longitudinal tension, only the weight of the conductors (including ice loading) and horizontal wind forces. The permissible deviation angle of the overhead line at such a tower is typically no more than 2°. Thanks to their lightweight construction, tangent towers of this type are relatively inexpensive and quick to install, making them the economical choice for long sections of straight overhead transmission line.

Guyed towers

Guyed towers are installed at intervals of every few spans to limit the forces acting along the line. Stabilised by guy wires anchored to the ground, they transfer conductor tension from both sides, thereby containing any damage to a single section in the event of a fault or conductor break.

Angle towers (N)

Angle towers, also known as deviation towers, enable a change in the direction of the line route. The lighter through-type is used for deviations of up to approximately 5–10°, while heavy-duty angle towers handle the full resultant tension forces at larger route deviations. Selecting the appropriate variant always requires calculating the deviation angle and the resulting forces on the tower body.

Section towers (O)

Sometimes called section towers, these are the primary representatives of the anchor tower group. Designed to carry one-sided conductor tension, they act as structural anchor points along a long transmission line – particularly important for meeting load-bearing and safety requirements on extended line sections.

Crossing towers (S)

Crossing towers are a special category of robust tower structure used wherever an overhead transmission line crosses a road, river, another power line, or other infrastructure. Minimum ground clearance and load-bearing requirements are particularly stringent at such locations. In Polish design practice, crossing towers are most often classified within the anchor tower group.

Terminal towers (K)

Terminal towers are positioned at both ends of the line. They operate under full one-sided conductor tension and provide safe termination of the overhead transmission line route – typically in the immediate vicinity of a substation or at the transition point to an underground cable section.

Branch towers (R)

Branch towers allow a tap-off to be taken from the main line route. Depending on the configuration, they can simultaneously serve as through or terminal towers for each of the branches.

In operational and design practice, multifunctional transmission towers are also frequently encountered, combining several of the functions described above. The most common examples are:

• Anchor-angle (ON) – combine full conductor tension-bearing capacity with the ability to route the line at an angle.
• Branching-resistance-end (ROK) – serve simultaneously as a section, terminal, and branch tower, which proves especially useful in complex terrain or where a transmission line enters a substation.

Prestressed concrete lattice (hybrid) towers

One of the more recent developments in high voltage transmission tower design is the prestressed concrete lattice tower. Also known as a hybrid tower, it combines a prestressed concrete core with a spun steel shaft and a lattice head, typically made of galvanised steel. This tower design delivers high structural strength through the concrete core while retaining the flexibility and optimal mechanical properties of the steel lattice head.

Design and regulatory criteria for transmission tower selection

Selecting the correct transmission tower type is not simply a matter of choosing from a catalogue – it requires analysing many interrelated parameters. The overhead line designer must simultaneously satisfy mechanical, electrical, and environmental requirements.

On the mechanical side, the key factors are the forces generated by the tension of phase conductors and the ground wire, ice loading, wind pressure, and temperature-dependent sag variations across spans. Foundations must transfer these forces safely to the ground level – their design accounts for both soil bearing capacity and possible seismic or high-humidity conditions. Underestimating mechanical loads or foundation requirements leads to geometric deformation and, in extreme cases, to the collapse of an entire line section.

On the electrical side, the designer must ensure the required insulation clearances – between phase conductors, between conductors and the tower body, and between conductors and the ground or objects beneath the line. Maintaining the minimum permissible ground clearance is a fundamental safety requirement that varies significantly with the line’s rated voltage and is specified, among other standards, in PN-EN 50341.

Environmental conditions are also important: mountainous areas require structures with a higher load-bearing capacity due to heavy icing, urbanised areas often necessitate the use of more aesthetically discreet tubular poles, and the proximity of airports requires aviation warning markings on tower structures and ground wires. Soil conditions are equally significant – the type and depth of foundations depend on load-bearing capacity.

The arrangement of transmission conductors on the tower – flat (via horizontal cross arms), triangular, or vertical – directly affects both the electrical symmetry of the line and the actual transmission tower height. A flat cross arm arrangement requires less height but creates phase asymmetry; an equilateral triangle configuration ensures electrical symmetry but typically demands a taller structure.

Transmission towers and Protektel surge protection

Even the most carefully designed overhead transmission line remains vulnerable to surges – both atmospheric (lightning strikes) and switching surges arising during network switching operations. In MV and HV power lines, surge arresters provide effective protection against overvoltages, provided they are correctly selected and installed.

The position of the arrester relative to the protected equipment is critical: the further it is located from the transformer or other asset, the greater the risk that the overvoltage at its terminals will exceed the protection level set by the arrester – a phenomenon known as the distance effect. For this reason, surge arresters are installed as close as possible to the protected equipment – directly on a transmission tower, at the substation entrance, or at the end of an underground cable section.

Protektel’s range includes the PROXAR family of surge arresters in versions suitable for both line applications (installation on MV transmission towers in outdoor conditions) and substation applications. The PROXAR data sheets contain full selection data – TOV/UTOV parameters, energy (Qrs/Wth), short-circuit capacity, and mechanical parameters (SLL/SSL) – enabling the arrester to be matched to a specific installation site without repeated requests for quotation.

If you are planning to build or upgrade an overhead power line and need guidance on selecting surge protection devices, the Protektel team is available to support you at every stage of the project. Expertise begins before the purchase, and it is at this stage that it has the greatest impact on the reliability of the entire installation.