The engineering behind Kenya’s 132kV underground transmission links

Last Updated 1 hour ago by Kenya Engineer

For most transmission projects, the image of a high-voltage line is familiar: steel towers carrying conductors across a defined wayleave, with insulators, earth wires and optical fibre forming part of the overhead system.

The Nanyuki–Rumuruti and Nanyuki–Isiolo transmission projects introduce a different engineering arrangement. For selected sections of the routes, the 132kV circuits leave the towers, enter underground cable systems and later return to overhead transmission.

It is the transition between these two systems that makes the projects particularly interesting.

Speaking about the works, KETRACO Senior Resident Engineer Eng. Dennis Busolo has described the engineering involved in moving from overhead transmission to underground cable and then back to overhead transmission. The project incorporates terminal facilities where the two technologies meet, with cable sealing ends providing the high-voltage interface between the underground cable and the overhead line.

The principle may sound straightforward. In practice, the transition is one of the most technically demanding parts of a hybrid overhead-underground transmission circuit.

Why the underground sections were introduced

The decision to underground sections of the two 132kV transmission projects was driven principally by the operating environment around Nanyuki.

The Nanyuki–Isiolo line was energised by KETRACO in August 2026. The 72 km single-circuit line includes approximately 4.2 km of underground cable near Nanyuki, where the route was redesigned because of security restrictions associated with Laikipia Air Base. The project also includes a new 132kV substation at Isiolo and works at the existing Nanyuki substation.

The Nanyuki–Rumuruti project forms another part of the same broader transmission reinforcement programme. KETRACO lists the project as a 79 km, 132kV single-circuit line constructed on double-circuit towers. The underground component was introduced because the section of the route passing near Laikipia Air Base could not be constructed as a conventional overhead line.

The African Development Bank’s supplementary financing documentation describes the underground section as a response to the safety and security requirements associated with the airbase. The original design had envisaged an overhead line, but the section passing near the airbase was subsequently redesigned as underground cable.

This makes the project an instructive example of why underground transmission is sometimes used selectively rather than along an entire transmission route.

The terminal is where two transmission technologies meet

An overhead line and an underground cable do not simply connect by joining the conductor to a cable.

The underground cable has a multilayer insulation system, metallic screen or sheath and its own electrical and thermal characteristics. The overhead conductor, by contrast, relies on air as its primary external insulation and operates within an environment where lightning, weather, pollution and mechanical loading have to be considered.

The point where the two systems meet therefore has to manage several changes simultaneously.

KETRACO’s procurement documentation for the Nanyuki–Rumuruti underground system identifies the terminal points and cable sealing ends as specific components of the works. It also requires cable riser structures and surge arresters, while the connections between the overhead conductors and the cable sealing-end compounds form part of the installation.

Internationally, these facilities are commonly referred to as sealing-end compounds or transition facilities. CIGRE, the international organisation for electrical power systems, identifies the transition facility between an overhead line and underground cable as a specialised part of a hybrid transmission circuit requiring coordinated design between the overhead-line and cable disciplines.

At the Nanyuki project, the terminal therefore does considerably more than provide a physical connection.

It provides the controlled electrical and mechanical transition from one transmission environment to another.

What happens at a cable sealing end?

The cable sealing end is one of the most important pieces of equipment at the transition.

A 132kV XLPE cable, for example, contains a conductor surrounded by insulation and layers of semiconductive material and metallic screening. At the end of the cable, these layers cannot simply be stripped back and connected to an overhead conductor because doing so would create severe electrical stress at the end of the insulation.

The termination has to control that electrical field.

This is where the stress cone becomes important. A modern cable termination uses stress-control technology to gradually redistribute the electrical field around the end of the cable insulation. Without appropriate stress control, the concentration of electrical stress at the point where the semiconductor screen ends could contribute to partial discharge, insulation degradation and eventual failure.

The termination also provides the physical connection between the cable conductor and the overhead line, while maintaining insulation, environmental sealing and appropriate earthing and bonding arrangements.

For an outdoor 132kV installation, the sealing end is exposed to the same external environment as the overhead equipment. The termination therefore has to withstand electrical, mechanical and environmental stresses while maintaining the integrity of the cable system.

This is one reason the cable termination is treated as a critical component rather than simply an accessory.

Research into high-voltage cable sealing ends has shown that termination components can themselves become points of electrical stress and ageing, making installation quality and subsequent condition monitoring important to long-term cable reliability.

The transition structure is more than a support

At the end of an underground section, the buried cable has to rise from the ground and connect to the overhead transmission circuit.

This requires a suitable cable riser arrangement and supporting structures for the termination equipment. The overhead conductors are brought down from the terminal tower or connected through the appropriate gantry and conductor arrangement to the sealing ends.

Surge arresters are also important at the transition because the underground cable and overhead line have different electrical characteristics and because the overhead section is directly exposed to lightning.

KETRACO’s procurement documents specifically identify surge arresters, cable riser structures, OPGW termination and the connections between the transmission line and the cable sealing ends as part of the terminal works.

The result is effectively a small piece of substation-type infrastructure inserted into the transmission line.

Underground transmission changes the thermal problem

One of the fundamental differences between overhead conductors and underground cables is how heat is dissipated.

An overhead conductor is surrounded by air and can lose heat directly to the atmosphere. A buried cable is surrounded by soil and other materials whose thermal properties affect the ability of heat to escape.

This matters because the current-carrying capability, or ampacity, of a cable depends partly on its ability to dissipate the heat generated by electrical losses.

Cable installation therefore requires consideration of parameters such as burial depth, soil thermal resistivity, spacing between phases, grouping, ambient and soil temperatures and the arrangement of the cable system.

CIGRE notes that underground AC cable systems have particular design challenges associated with current rating, capacitance and dielectric losses. These considerations become increasingly important as underground cable sections become longer.

The Nanyuki projects are relatively short underground sections within much longer overhead circuits, which limits some of the system-level effects associated with long underground AC transmission links. But the cable section still has to be designed as a complete electrical system rather than simply as a buried conductor.

Cable installation is a civil engineering project as much as an electrical one

An overhead line concentrates much of its civil engineering effort at tower foundations. Once erected, the conductor system is largely above ground.

An underground transmission line spreads the civil engineering requirement along the route.

Trenching, excavation, bedding, cable protection, thermal conditions, crossing arrangements, drainage, reinstatement and protection against third-party interference all become part of the transmission system.

The cable also has strict requirements on bending radius and installation tension. Excessive mechanical stress during installation can damage a cable even though the damage may not immediately be visible.

Once installed, the cable route has to remain identifiable and protected from activities that could damage it.

KETRACO’s procurement documentation for the Nanyuki–Rumuruti system goes further by incorporating visual surveillance of the underground cable system. The specifications provide for cameras and monitoring of the cable sealing-end stations, with the system capable of transmitting alarms and fault information.

This reflects an important difference between underground and overhead assets: putting a transmission line underground does not make it disappear from the asset-management system.

Safety changes rather than simply disappearing

The safety comparison between overhead and underground transmission is therefore not simply a question of which technology is safer.

The two systems present different hazards.

Overhead lines make the electrical infrastructure visible and generally accessible for inspection, but their exposed position introduces hazards associated with working at height, tower climbing, conductor proximity, construction plant, cranes and other tall equipment. Flashover can occur without direct physical contact if equipment approaches sufficiently close to an energised overhead line.

Weather and environmental exposure can also affect overhead lines. Wind, lightning, falling trees, vegetation and other external events can contribute to faults or outages. Undergrounding can reduce exposure to some of these hazards, particularly those associated with wind and physical contact with overhead conductors. The IEA notes that underground infrastructure can reduce vulnerability to some climate-related hazards, although at higher upfront cost.

The underground system introduces a different category of risk.

A buried high-voltage cable is largely invisible once installed. Excavation by third parties, road works or construction can damage a cable without the person operating the equipment necessarily knowing what is below ground. Damage to an underground electrical cable can produce electric shock, arcing and severe burns.

For this reason, cable route records, markers, route inspections and communication with contractors working near the route become important parts of the safety system. CIGRE identifies route inspections, warning signs and providing cable-route information to contractors as established measures for preventing third-party damage to land cable systems.

The two technologies therefore redistribute risk rather than simply eliminating it.

Which one is harder to maintain?

This is perhaps where the distinction between overhead and underground transmission becomes most pronounced.

Routine inspection of an overhead transmission line can be relatively direct. Towers, conductors, insulators, fittings and other components are physically visible and can be inspected from the ground, by climbing, using specialised vehicles, drones or other inspection technologies.

A fault may also leave visible evidence.

With an underground cable, much of the asset cannot be visually inspected while in service.

Modern cable systems compensate for this through electrical testing, diagnostics and monitoring. CIGRE identifies techniques including sheath testing, partial-discharge measurements, temperature monitoring and other diagnostic methods as components of modern HV cable maintenance.

KETRACO’s Nanyuki–Rumuruti specifications provide an interesting example of this philosophy. The project includes a surveillance system for the underground cable route and cable sealing-end stations, with provision for alarm information and fault-location information.

The real maintenance challenge emerges when something actually fails.

An overhead fault can often be located relatively quickly through inspection and system protection information. A damaged underground cable, by contrast, may require specialised fault-location equipment to determine the precise location before excavation and repair can begin.

The US Department of Energy has similarly noted that underground transmission maintenance can be more difficult and time-consuming because faults are hidden, requiring specialised fault-location methods before the damaged cable can be exposed and repaired.

CIGRE’s work on land-cable fault location identifies techniques including time-domain and frequency-domain reflectometry, fibre-optic methods, acoustic methods, step-voltage methods and magnetic-field techniques.

Once the fault has been located, the physical repair is another challenge. A buried cable must first be exposed, the damaged section assessed and, where necessary, a joint or replacement section installed. High-voltage cable jointing is a specialised operation in which workmanship, cleanliness, environmental conditions and electrical testing are critical.

For that reason, an underground cable may experience fewer externally induced faults in some environments while presenting a more demanding repair operation when a major fault occurs.

That distinction is important.

Lower exposure to some causes of failure does not automatically mean easier maintenance.

The terminal may become the critical interface

The Nanyuki projects therefore demonstrate an important principle in modern transmission engineering.

A hybrid line is not simply an overhead line with a buried section inserted into it.

It is a system comprising overhead conductors, underground cables, transition structures, cable sealing ends, surge protection, earthing and bonding, optical communications and monitoring equipment, all of which have to operate together.

The transition point is particularly important because it brings together the characteristics of two different transmission technologies.

CIGRE has specifically identified transition facilities between overhead lines and underground cables as an area requiring coordinated engineering between overhead-line and cable specialists. Its guidance on HV cable accessories also covers transitions from overhead lines to underground cables and the interfaces between cables and substations.

That is what makes the Nanyuki projects a useful case study for Kenya’s transmission engineers.

A selective tool for transmission planning

The Nanyuki experience also illustrates why the choice between overhead and underground transmission is rarely an ideological one.

Overhead transmission generally offers advantages in construction cost, accessibility and ease of inspection and repair. Underground transmission can be attractive where aviation safety, land-use constraints, environmental considerations, urban development or other route restrictions make an overhead line impractical.

The engineering question is therefore not whether underground transmission should replace overhead transmission.

It is where each technology is technically and economically appropriate.

In Nanyuki, the answer has been a hybrid system: overhead transmission for the majority of the route, with underground cable deployed where the operating environment required it.

The result is a transmission line that changes physical form along its route but remains a single electrical system.

And at the point where that change takes place, the cable sealing end and terminal facility become much more than a connection between two pieces of equipment. They are the point at which two different transmission technologies have to behave as one.