Electrical power is produced at the power generating plants which are located far away from the load centers. Therefore, we need to transport the electrical power generated at the power generating plant to the points of utilization or load points. For this purpose, two major systems of conductors are developed called transmission system and distribution system.
Both transmission and distribution systems are further classified into two types namely,
In the overhead system of transmission and distribution, the overhead line wires supported by poles are used for power transportation; while in the underground system, insulated underground cables are employed for the transportation of electric power.
Read through this article to understand the differences between overhead lines and underground cables used for electric power transmission and distribution. Let's start with the basics of overhead lines and underground cables so it becomes easier to understand the differences between them.
Overhead lines or overhead transmission lines are the bare conductors supported on poles and towers. In overhead system, the line conductors are hanged in air with the help of transmission line supports installed at a certain distance.
An overhead line consists of conductors, supports, line insulators, cross arms, and many other auxiliary components. The conductors are the bare metallic conductors that carry electric power from the sending end to the utilization end. The supports may be poles or steel towers that keep the line conductors at a suitable level above the ground. Since the conductor and support both are conductors of electricity, therefore insulators are used to insulate the line conductors from the support and hence from the ground which may otherwise cause fault and danger for living beings.
The cross-arm is the part of an overhead line which provides housing and support to the line insulators. Other auxiliary component of an overhead line are lightning arrestors, anti-climbing wires, phase and danger plates, stay rod, etc.
Due to some technical and economic reasons, the overhead lines are extensively used in power transmission and distribution.
Underground cables or undergrounds lines are the insulated electric cables which are buried under the earth surface for the transmission and distribution of electric power. However, the power transmission using underground cables is relatively more expensive method, hence it is less frequently used method.
Mostly, the underground cables are used in such cases where it is possible to install overhead lines. A typical underground cable consists of one or more (two, three or four) central cores of stranded conductors (copper or aluminium) and these are insulated from each other by paper or varnished cambric or impregnated paper. In order to protect this insulation against moisture and mechanical damages, a metallic sheath of lead or alloy of aluminium is provided around it. Finally, an insulation cover is provided.
The type of underground cable used for power transmission and distribution depends on the voltage and service requirements.
Since overhead lines and underground cables both are used in the electric power transmission and distribution. However, there are several differences between overhead lines and underground cables that are given in the following table −
Basis of Difference Overhead Lines Underground Cables Definition An overhead line is the one that uses bare conductors supported on a pole or tower for power transmission and distribution. An underground cable is an insulated cable which is buried under the earth surface for power transmission and distribution. Type of conductor Overhead lines use bare conductor. Undergrounds cables use insulated conductor. Size of conductor The size of conductor used in overhead lines is comparatively smaller. The size of conductor used in underground cables is relatively larger. Voltage rating Overhead lines have higher voltage rating, near about 765 kV. Undergrounds cables have comparatively lower voltage rating. It is typically limited to 66 kV. Current carrying capacity The current carrying capacity of overhead lines is comparatively more. Underground cables have lesser current carrying capacity. Insulation Overhead lines require less insulation. Usually, at the contact points of the support. Underground cables need more insulation. It is because, the whole conductor is required coated with the insulation sheath. Interference with communication lines Overhead lines causes interference to the nearby communication lines. Underground cables do not cause interference to the communication lines as these are buried under the earth surface. Safety for living being Overhead lines are not much safer for the living beings. Underground cables are safest for living beings. Fault detection andclearance In case of overhead lines, the detection and clearance of fault is easy. It is very difficult to detect and clear the fault in underground cables. Line cost Overhead lines are relatively less expensive. Underground cables are comparatively expensive. Maintenance cost Overhead lines have high maintenance cost. For the underground cables, the maintenance cost is comparatively low. Extra connection In case of overhead lines, it is very to join an extra connection. It is not easy to make an extra connection in case of underground cables. Reliability Overhead lines are less reliable. The reliability of underground cables is high. Risk of explosion In case of overhead line, the risk of explosion or fire is more. Underground cables reduce the risk of explosion or fire. Layout flexibility The layout of overhead lines is highly flexible. The underground cables possess less flexibility in the layout. Effect environmental conditions Overhead lines get easily affected by the adverse environmental conditions like lightning, thunderstorm, wind storm, moisture, etc. Underground cables are less affected by the environmental attacks. Corona discharge The corona discharge occurs in the overhead lines. No corona discharge takes place in the underground cables. Proximity effect Overhead lines remain unaffected from the proximity effect. The proximity effect influences the underground cables. Applications Overhead lines are used in electric power transmission and distribution over large distances. Underground cables are used in power transmission and distribution over short and moderate distances.The most significant difference is that the overhead lines use bare conductor, whereas underground cables use insulated conductor for power transmission and distribution.
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