Overhead lines vs underground cables - full comparison

It is a frequent question for electrical engineers (but not only) - what type of technology should be used for their project - overhead line or underground cable. The purpose of this article is to provide information about the technical, economical merits and challenges associated with both options.


You can find below a comprehensive comparison of these two technologies based on real scenarios taking into account following:

    Factors:
  • Cost of supply and assembly,
  • Cost of electrical energy losses,
  • Cost of operation and maintenance,
  • Cost of reactive power compensation,
  • Cost of decommissioning,
  • Cost of easement and land purchase requirement,
  • Reliability issues,
  • Fault detection and repair,
  • Harmonics,
  • Electromagnetic field,
  • Area for construction and O&M,
  • Environmental issues,
  • Life expectancy.
    High voltage levels:
  • 110 kV,
  • 220 kV,
  • 400 kV.

We will first go through general description of factors listed above and then present detailed calculation which will compare overhead lines versus underground cables for each voltage level.

Cost of supply and assembly

Generally, overhead lines have provided the lowest cost method in agricultural environments, while underground cables often provided the only practical transmission option in urban areas. There are a number of issues that make the undergrounding option more technically challenging and expensive. However, despite the costs and technical challenges, there are circumstances in which underground cables are a more preferable option than overhead cables. Also the increasing interest in environmental issues results in increasing interest in underground cables. The costs of new electrical infrastructure depends on many factors, including the transmission capacity, type of terrain, metal prices, labour cost, competitivness of contractors market, size of a transmission project, type of contract for execution of works, costs of plots. Comparative analyzes of the costs of both technologies for a given project should not be based only on cost factors, but should be based on calculations taking into account the specificity and conditions of the analyzed case. No one technology can cover, or is appropriate in, every circumstance, and thus financial cost cannot be used as the only factor in the choice of one technology over another in a given application. A decision-maker should take into account other factors listed in sections below. We recommend to use our calculation tool (contact us for support) to make a comprehensive comparison between overhead lines and underground cables.

Cost of electrical energy losses

Each transmission technology has its own characteristic losses. Some of these losses vary with the voltage of the transmission conductor (for example, cable dielectric losses), whilst others vary with the flowing current, such as conductor resistive losses. Power losses are generally higher for overhead lines compared to equivalent underground cables (in terms of ampacity). The main reason is that Joule losses are much lower for underground cables due to their thicker total conductor cross section area. Other power losses component like dielectric, sheath losses for underground cables and isolators, corona losses for overhead lines don't play a big role. Except in foul weather corona losses are of a lower order than resistive losses. Another issue to consider regarding cable line losses are losses due to capacitive current, which always exists when the cable line is operating. If the cable line is compensated, losses in reactive power compensation devices should also be taken into account.

Cost of reactive power compensation

When comparing the costs associated with overhead and cable lines, one cannot forget about the cost of reactive power compensation, i.e. the cost of purchasing shunt reactors or capacitor banks. The cost of compensation is much higher for cable lines.

Cost of operation and maintenance

Cable systems (i.e. cables and cable accessories) are designed and manufactured for a life cycle of 40 years, while the warranty period usually does not exceed 10 years. During this period, maintenance activities are necessary, which can be divided into:

  • emergency repairs,
  • planned service activities,
  • periodic tour.
Planned service activities include checking the oversheath continuity, checking grounding boxes, cross-bonding boxes with surge arresters, and whether the cable posts have been damaged or removed. To prevent damage associated with third-party activity, regular patrols are carried out to verify that no construction work is taking place nearby. This is particularly important when the cable lines are laid alongside existing infrastructure, e.g. a road. Continuous monitoring of the condition of cable lines is also possible through temperature monitoring by fibre optic.

Insulation

Overhead lines are insulated by air, while underground cable conductors are wrapped in layers of insulating material. Air is the simplest and cheapest insulation and the heat produced by the electricity flowing through the bare overhead conductors is removed by the flow of air over the conductors. When conductors are buried underground, robust insulation is needed to withstand the high voltage.

Impact on land and agriculture

Land which is occupied by overhead lines can be used for agriculture, while underground cables can cause soil to dry out meaning difficulties in growing plants. Growing trees over underground cables is forbidden in order to allow access for service and repair crew.

Reliability

There are various positions regarding failure rates for overhead and cable lines presented by network operators and cable manufacturers. The literature on the subject specifies that the average annual duration of emergency shutdown is longer for cable technology (the number of shutdowns is similar for both technologies, while the time to remove damage for a cable line is longer). Overhead lines have more intermittent faults, which can be cleared almost immediately by auto-reclosing. Permanent faults like insulator breakdown can be repaired within 24 hours. Faults for underground cables are usually permanent and can be caused mainly by imperfections during manufacturing process, insulation degradation, mechanical damage due to third party, unprofessional assembly or ageing. It takes more time to locate and repair faulted high voltage undeground cables. The whole operation can take minimum two weeks for buried cables. The time for putting underground cables back into operation will be longer when no spare cables and accessories will be foreseen on stock. In such case the unavailability of circuit can be prolonged to months. Determining the reliability of HV and EHV cable lines is problematic given the small amount of statistical data on their damage and the reluctance of manufacturers and network operators to disclose failures and their causes. It is also worth considering the fact that when using overhead technology, much more power is transmitted in one path compared to cable technology, where equivalent power can be transmitted through several cable paths. In the event of a failure of one overhead line track, the transmission capacity of approximately 1800 MW (estimated value for 400 kV, which may be different depending on the adopted technical assumptions) of the transmitted power is lost. If one 400 kV cable line path fails, the possibility of transmitting approximately 750 MW of power may be lost, while it is possible to transfer this potentially lost power through the remaining, undamaged cable paths. Cable lines are characterized by high thermal inertia, which is why it may turn out that in the event of a failure of one cable track, the transmission can be transferred to the other tracks for the duration of repairs. Of course, the case where the entire pole and all overhead lines are damaged is even more problematic. Additionally, service experience indicates that those circuits which are subjected to frequent switching operations have higher failure rate. Failure rates increase also for underground cables with wide temperature changes, i.e. 40 Celcius in summer and below 0 Celcius in winter. Weather may restrict access to the cable in very wet spells or prolonged periods of snow. Repairs to cable require excavations of the cable and the provision of clean and dry conditions for jointing. Either of these may affect cable repair times.

Harmonics

High voltage cables cause much more problems related to harmonics compared to overhead lines. It is caused by their much higher capacitance which leads to resonance phenomenon and in result to amplification of certain harmonics. The longer cable the bigger problem with harmonics exists. It is usually required to use harmonic filters for longer cable lines (contact us for harmonic analysis for your system).

Life cycle

High voltage underground cables are designed and produced for 40 year operation compared to 80 years life cycle for overhead lines. The shorter life cycle of underground cables is caused by insulation deterioration. Overhead lines are characterized by longer life expectancy due to maintanence service (isolators replacement, additional corrosion protection for steel towers etc.). It is also recognised that, although many modern XLPE cables are expected to last or service well beyond the 40 year design life under normal circumstances [37], in situations where water/moisture is present, those designs without special protection have a life expectancy of only 15–25 years.

Electromagnetic field

Overhead line is a source of electrical and magnetic field, where underground cables have the magnetic fields screened so only electrical fields might be an issue in this case. The electric field of the cable lines is fully limited by the metallic screen. The magnetic field of the cable lines used on the ground surface meets the applicable requirements by appropriate selection of the cable arrangement (triangular or flat arrangement, laying depth). The electric and magnetic field of overhead lines reaches normalized values in and outside the technological lane, and requires a much wider lane than a cable line.

Environmental issues

Overhead lines have long-term negative visual impact. A number of pole structures have been created for overhead lines that are designed to improve the appearance of the columns, but so far these solutions have not been adopted on a large scale. Therefore, to increase transmission capacity, multi-track overhead lines are used or, as in some countries (e.g. Denmark), the construction of overhead lines is completely abandoned in favor of cable lines. However, cable lines also have a negative impact on the environment, which is particularly associated with the excavation of large amounts of soil, heating of the earth, deforestation of the area above the cable line and agricultural restrictions However, undergrounding has other impacts that should be considered, such as environmental and socio-economic factors. In rural areas, disturbance to flora and fauna, land use and archaeological sites must be assessed. In this respect overhead lines are normally less disruptive than underground cables and cause fewer disturbances. In both urban and rural environments land disruption is greater when laying underground cables than when erecting overhead line towers. Construction works for underground cables leads to high amount of soil being moved or replaced, compared to lower amounts of soil structure changed only for towers' foundations. Heat generated by high voltage cables can cause surrounding soil to dry out completely. If native soil is poorly compacted, it acts like an insulating blanket and a cable can fail prematurely. That's why a corrective backfill (minimum 20 cm below and 30 cm above a cable) should be used to reduce the heat flux experienced by the native soil so that it will not dry out. The choice of proper backfill is a crucial issue, but it will be not discussed in the article (you can find some valuable information in...). The volume of soil excavated for an underground cable, where two cables per phase are installed, is some 14 times more than for an equivalent overhead line route. Vegetation has to be cleared along and to the side of trenches to allow for construction and associated access for vehicles. In addition underground joint bays, which are concrete lined and wider than the trenches themselves, have to be built every 500–1,000m. Social and environmental costs may also be attributed to transmission but, whilst acknowledging the potential generic impacts of transmission in these areas, it is not the purpose of this study to examine or evaluate them.