Comparison of overhead and underground

transmission linesName: Ahmed Usama

Overhead TransmissionO An overhead power line is a

structure used in electric power transmission and distribution to transmit electrical energy along large distances. It consists of one or more conductors(commonly multiples of three) suspended by towers or poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy.

Classification by operating voltage

O Overhead power transmission lines are classified in the electrical power industry by the range of voltages:

O Low voltage (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.

O Medium voltage (MV; distribution) – between 1000 volts (1 kV) and to 69 kV, used for distribution in urban and rural areas.

O High voltage (HV; sub transmission less than 100 kV; sub transmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.

O Extra high voltage (EHV; transmission) – over 230 kV, up to about 800 kV, used for long distance, very high power transmission.

O Ultra high voltage (UHV) – higher than 800 kV.

Circuits

Advantages of overhead transmission line

O Electrical power generated at power plants must be transmitted to its ultimate users. This process requires transmission lines to carry the heavy power loads over long distances. These transmission lines perform the heavy duty work and require stronger supports and larger wires. There are several advantages to placing the transmission lines overhead as opposed to underground.

Advantages of overhead transmission line

O Installation CostsWhen deciding whether to put transmission lines overhead or underground, installation costs become a primary concern for power companies. The advantage in initial capital costs goes to overhead lines.O Repair CostsUnderground power lines are more costly to repair. In addition, it generally takes longer to repair than overhead lines due to the difficulty in reaching the underground problem.O Environmental ConcernsWhile overhead transmission lines may be considered unsightly, they do not impact the environment in the manner underground lines would when crossing large expanses of land. 

UndergroundingO Undergrounding is the replacement of 

overhead cables providing electrical power or telecommunications, with underground cables. This is typically performed foraesthetic purposes, but also serves the additional significant purpose of making the power lines less susceptible to outages during high wind thunderstorms or heavy snow or ice storms. Undergrounding can increase the initial costs of electric power transmission and distribution but may decrease operational costs over the lifetime of the cables.

Undergrounding Cable

AdvantagesO Less subject to damage from severe weather conditions (mainly 

lightning, wind and freezing)O Reduced range of electromagnetic fields (EMF) emission, into the

surrounding area. However depending on the depth of the underground cable, greater emf may be experienced.  The electric current in the cable conductor produces a magnetic field, but the closer grouping of underground power cables reduces the resultant external magnetic field and further magnetic shielding may be provided. 

O Underground cables need a narrower surrounding strip of about 1–10 meters to install (up to 30 m for 400 kV cables during construction), whereas an overhead line requires a surrounding strip of about 20–200 meters wide to be kept permanently clear for safety, maintenance and repair.

O Underground cables pose no hazard to low flying aircraft or to wildlife.O Much less subject to conductor theft, illegal connections, sabotage,

and damage from armed conflict.O Burying utility lines makes room for more large trees on sidewalks,

which convey environmental benefits and increase property values

Disadvantages- higher cost of construction - higher repair cost - potential risk from careless digging - Underground cables are more subject to damage by ground movement.- underground cables cannot be uprated and must be supplemented or replaced to increase capacity.

A comparison of Overhead and Underground Transmission

Cables & conventional types

comparison

comparison

AestheticsO Overhead - Poles, wires and support anchors are visible - Some vegetation can remain under or beside the lines O Underground- No poles, wires or support anchors visible- Steel termination poles are visible- Vegetation must be completely removed on and around the route

Outage ImpactsO Overhead- Outages are more frequent during

normal and severe weather- Repairs can typically be made within a day O Underground - Outages are less frequent- Repairs can take up to six weeks

CostsO Overhead $500,000 to $1 million per mile to construct Costs more to operate and maintain Costs less to repair, upgrade and relocate Costs from car-pole accidents, tree trimming, and transmission line losses are higher Costs are covered by all ratepayers; no additional area costs to local customers is necessaryO Underground $4 million to $10 million per mile to construct Costs more to repair, upgrade or relocate Typical cross section of underground 115 kilovolt transmission lines Costs less to operate and maintain

Construction ImpactsO Overhead Temporary traffic impacts, if work is in or along street rights-of-way Construction entails setting poles and stringing wire Construction equipment used includes line trucks, bucket trucks and tree removal equipment Environmental impacts include removing trees and removing 120 cubic yards of dirt per mile.O Underground Temporary traffic impacts, if work is in or along street rights-of-way Construction equipment used includes line trucks, bucket trucks, backhoes and tree removal equipment Environmental impacts include removing trees and removing 530 cubic yards of dirt per mile