Which are the key cost components of an underground cable power distribution system and how do these compare with overhead lines over the lifecycle?

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A power distribution system, whether with overhead lines or through underground cables requires an upfront investment in equipment, materials and installation and a recurring budget over its lifetime to cover operation, maintenance and energy loss costs.

Investment costs for an underground system include those for:

• The procurement of equipment and materials mainly MV / LV cables, distribution transformers and switchgear.
• Civil works comprising of the digging of cable ways, the construction of ducts and inspection covers, the construction of sub-stations ,the restoration of the constructed area as well as surroundings and improvement of the landscape.
• Electrical works comprising of the dismantling of existing overhead lines (if it is a replacement project), cable laying and termination, the installation and commissioning of transformers and switchgear, and last-mile connections to users.

The type of installation method chosen for the project influences the investment costs significantly. The direct burial of cables in the ground has lower construction costs than laying these in concrete encased ducts, while pipe jacking is the most expensive approach. This applies to the relative cable costs as well, since their ampacity varies with the available heat dissipation capability of the encasement. Another influencing factor is the network layout, i.e. the number and the locations of sub-stations relative to users which should be designed so as to minimize the more expensive MV / LV distribution feeder lengths compared with sub-transmission lengths.

Recurring expenses of underground cabling include:

• Electricity losses – these depend on the efficiency of the network layout and its loading but are in any case lower than for overhead lines as inductive loads are compensated by the cable capacitance thus reducing the current, and consequently the I²R losses.
• Repairs – these are much lower than for overhead lines as the cables are protected from damage being underground.
• Preventive maintenance – this is less frequently required than in the case of overhead lines.
• Lost revenue due to outages – these are lesser in frequency than in the case of overhead lines.

In summary, while the upfront investment required for an underground cable distribution system is higher, the recurring expenses are lower and therefore a fair financial comparison with overhead lines should consider the NPV of both types of costs over a long planning period e.g. 25 to 30 years. In most cases, underground cabling turns out to be cost effective in a financial sense, besides offering environmental and societal benefits which are more difficult to quantify.

Reference: Power Cables Handbook, Volume 3, Conversion of Overhead Lines to Underground Cables, Copyright@2014, International Copper Association South East Asia Ltd. And Provincial Electricity Authority, Thailand