Designing electrical powerlines is a critical stage in the deployment of reliable power distribution and transmission systems in Kenya. This chapter focuses on the technical, economic, and safety aspects that must be considered to ensure efficient power delivery from generation points to consumers across diverse terrains. Engineers working in this field must carefully evaluate project requirements, select appropriate transmission lines, and optimize conductor and tower designs to minimize losses and ensure system longevity. The design process integrates practical considerations relevant to Kenya’s power infrastructure, including voltage standards, power demand forecasts, and environmental factors.

2.1 Power lines design

Designing a power line project is a systematic process that turns a need for electricity at a load centre into a line that is safe, efficient, reliable and affordable. The designer first fixes the requirements (voltage, power, distance and current type), then selects the type of line, and finally works out the detailed design of conductors, spacing, supports and losses. Every decision must also meet Kenya Power and KETRACO standards, the Energy Act 2019 and the regulations of the Energy and Petroleum Regulatory Authority (EPRA).

Example: a county rural electrification scheme in Kitui designs an 11 kV overhead line on concrete poles to connect a cluster of trading centres to the nearest 33/11 kV substation.

2.1.1 Determination of requirements: voltage levels, power outputs, transmission distance, and current type

Determining the requirements is the foundation of the design, because voltage, power, distance and current type together decide conductor size, insulation, clearances and support structures. These requirements are fixed from load surveys, demand forecasts, the location of the source and load, and the standards of the utility that will own the line.

Voltage Levels

The voltage is chosen to balance efficiency against equipment cost. For a given power, a higher voltage means a lower current, which reduces I²R losses and allows smaller conductors, but it needs more insulation, bigger clearances and stronger supports. Kenya's standard levels are 400 kV, 220 kV and 132 kV for transmission, 66 kV and 33 kV for sub-transmission and primary distribution, 11 kV for distribution, and 415/240 V for consumers.

Power Outputs

Power output is the maximum power the line must carry, found from the connected load, diversity, peak demand forecasts and allowance for future growth. Underestimating it leads to overheated conductors and overloaded transformers; overestimating it wastes money. For example, a SACCO office complex in Eldoret with a maximum demand of about 400 kW at 0.8 power factor needs at least a 500 kVA transformer, which in turn sets the rating of the 11 kV supply line.

Transmission Distance

The distance between source and load sets the voltage drop and losses. As a rule, longer lines need higher voltages: short 11 kV feeders run a few kilometres, 33 kV lines tens of kilometres, while bulk power over hundreds of kilometres uses 132 kV to 400 kV. The 400 kV line from the Lake Turkana wind farm at Loiyangalani to Suswa is an example of a long-distance transmission requirement.

Voltage drop = current × resistance per km × length
For 100 A over 10 km with 0.15 Ω/km (one conductor):
Voltage drop = 100 × 0.15 × 10
= 150 V

Current Type

Alternating current (AC) is used throughout Kenya's grid because transformers can easily step it up and down. Direct current (DC) is used for special cases: high voltage DC (HVDC) for very long distances, undersea cables and links between grids, as in the 500 kV Ethiopia to Kenya HVDC line ending at Suswa. The current type decides the converter equipment, insulation and protection.

2.1.2 Selection of Transmission Lines

Selection means choosing between overhead, underground and hybrid lines for each section of the route. The choice weighs cost, reliability, safety, maintenance, terrain, land use and environmental impact.

Overhead Lines

Overhead lines carry bare conductors on poles or towers. They are the most common in Kenya because they are cheapest to build, faults are easy to find and repair, and they can be uprated. Their drawbacks are exposure to lightning, wind, trees and vandalism, visual impact and the need for a wayleave corridor. Rural electrification in counties such as Machakos and Meru relies mainly on overhead lines.

Underground Cables

Underground cables are insulated conductors buried in trenches or ducts. They are used in dense urban areas, at airports and across sensitive land where overhead lines are impractical. They are safer for the public and unaffected by weather, but cost several times more than overhead lines, faults take longer to locate and repair, and their capacitance limits length at high voltage. Nairobi's central business district is supplied largely by underground cables.

Hybrid Lines

Hybrid (composite) lines combine overhead and underground sections in one circuit, for example overhead across farmland and underground into a town centre or under a road crossing. Cable termination structures with surge arresters are needed at each transition point.

Line Voltage Ratings

The line type is chosen together with its voltage rating (11, 33, 66, 132, 220 or 400 kV). Higher ratings need larger clearances, longer insulator strings and stronger supports, but carry more power over longer distances.

2.1.3 Design considerations: conductor selection and spacing, tower design and support structures, efficiency and power losses

Detailed design combines electrical and mechanical engineering to make the line carry its load safely for its whole life. The designer must consider weather, terrain, vegetation and access while keeping costs down.

Conductor Selection

The conductor must have low resistance, high tensile strength, light weight, resistance to corrosion and reasonable cost. Aluminium conductor steel reinforced (ACSR) is the most widely used because the steel core gives strength and the aluminium strands give conductivity; all aluminium alloy conductor (AAAC) is used in corrosive coastal areas. The cross-sectional area is chosen for the maximum current, allowable temperature rise, voltage drop and short-circuit rating.

Conductor Spacing

Phase conductors must be spaced far enough apart that they do not clash in wind or flash over, and far enough from the ground, buildings, roads and trees to protect the public. Spacing increases with voltage and span length and also affects inductance and corona. Minimum clearances are taken from the utility's standards; for example, a 66 kV line needs much larger phase and ground clearances than an 11 kV line.

Tower Design and Support Structures

Supports must carry the weight of conductors and insulators, wind loads and conductor tension, especially at angles and terminations. Wooden and concrete poles are used for low and medium voltage lines, and lattice steel towers for high voltage lines. Tower types include suspension (straight line), tension or angle, and terminal (dead-end) towers. In windy or rugged areas such as parts of the Rift Valley, extra bracing, stays and stronger foundations are used.

Efficiency and Power Losses

Losses come mainly from conductor resistance (I²R losses), with smaller losses from corona and insulator leakage. They are reduced by using higher voltages, larger or better conductors, shorter routes, balanced loading and good power factor. Lower losses improve efficiency and reduce operating costs; using 132 kV and 220 kV lines on Kenya's long routes keeps losses manageable.

Power loss = I² × R
For 100 A in a conductor of 1.5 Ω:
Power loss = 100² × 1.5
= 15,000 W = 15 kW per conductor

Chapter Summary

This chapter covered the comprehensive process of designing electrical powerlines, starting with the determination of project requirements such as voltage levels, power outputs, transmission distance, and current type. It then explored the selection of appropriate transmission lines and the critical design considerations including conductor selection, spacing, tower design, and efficiency to minimize power losses. Various types of electrical transmission line designs were examined, including distinctions between DC and AC, short, medium, or long distances, single phase or multiphase configurations, single or multi-circuit setups, as well as underground versus overhead lines. The chapter detailed the essential components of transmission lines, ranging from conductors and insulators to transformers, lightning arrestors, and safety devices like fuses and circuit breakers. Important line parameters such as resistance, inductance, and capacitance were explained for their impact on line performance. The phenomenon of corona was analyzed, including its formation, influencing factors, effects, and mitigation methods alongside critical voltage concepts and associated power losses. Finally, the chapter concluded with guidance on preparing electrical transmission line drawings, Bill of Quantities (BOQ), and contract documents necessary for project execution.