Blog

What is overhead power lines

The overhead power line (also known as overhead transmission line, or OHTL) is a compete instructions of towers and electrical conductors to, transmit the electrical energy from generation plants to distribution substations and then to the consumer.

Overhead power lines transmit electricity to very large distances.

Power is transmitted through OHTL because, it’s one of the least costly methods for power transmission.

The construction is very simple. depending upon the phase system, It consists of multiple not insulated conductors suspended by poles or towers.

Classification of overhead power lines

We discussed above that the overhead power line is the type of transmission line through which electrical energy transfers from electrical grids.

We can further classify it in two categories. i.e. Voltage and length. There are different types of voltage-based transmission lines. We are including a short detail about each.

Low Voltage overhead transmission line

This type of overhead power line has a voltage rating below 1000 volts. It’s mostly used for commercial and residential usage.

Medium Voltage OHTL

Medium voltage power line voltage is rated between 1 kV to 69 KV. It is used for power distribution between rural and urban areas.

High Voltage overhead power line

This type of transmission line is also called sub-transmission. The rated voltage ranges from 115 KV to 138 KV.

Extra-High and Ultra High Voltage

Extra high voltage ranges from 345 to 800 KV, while Ultra-high voltage is the transmission line capable to transmit voltage above 800 KV.

Classification by length

Based on the length these are further divided into three classes.

• Short overhead power line: It is the type of transmission line having a length less than 60 km.
• Medium overhead power line: This type of transmission line having a length ranges from 80 Km to 250 km.
• Long overhead transmission line: The type of transmission line having a length of more than 250 km.

Structure of overhead transmission line

In different regions of the world different structures of overhead transmission line are in use.

The simple structure consists of a wooden pole having more than one arm. The arms are used for hanging the wires.

Other structures that are common in use are tabular steel poles and concrete poles.

The wooden structure has three cross arms that are extended on one or different sides. The lattice tower consists of two forms.

The first one is a pyramidal structure having cross arms extended outside, the second one is a pyramidal base having four support points extended outside.

Maintenance and inspection for OHTL

OHTL contains 3 important components, Insulators, tower and conductors. Making regular electrical inspection for the OHTL network makes difference in faults and power reliability.

Hereunder some inspections for overhead transmission lines.

I have written a detailed article about OHTL maintenance, you can find it here for more information.

Insulators maintenance

Insulators get dirty due to the environment pollution and dusty weather.

Visual inspection could be done without power isolation because, you could inspect them from safe distance.

Cleaning insulators is one of the most important preventive maintenance.

As you know that dirty insulators may pass electrical current as a flash-over.

This flash-over can destroy the insulator as well as cause earth leakage fault.

Cleaning insulators process takes two forms. One is by direct contact with it and using a piece of dry and smooth clothes, in this case power must be isolated.

The second way is to clean with water jet without power isolation. Only well trained persons can do this dangerous job with special precautions.

Sometimes the insulators get broken due to many reasons. In this case the broken one should be replaced.

Insulators replacement could be done without power isolation but, with great precautions and special isolated tools.

The company I work for hires third party specialists to do hot washing and live maintenance for OHTL.

Conductors inspection and maintenance

Due to aging and windy weather besides short circuits and lightning strikes the overhead power line conductors are cut.

In this case the damaged conductor could be replaced or repaired with proper connecting devices.

This image is taken by me, in my workplace we used this splicing device to connect a cut OHTL wire, we found the splice it self broken during visual inspection.

Towers or poles inspection

Poles and towers are the less affected by faults and weather. But with age it may get corroded.

This is most likely to happen in high humidity environment. Visual inspection can help detecting any corrosion in its early stage.

OTHL vs underground cables

Electrical power transmission is done through OHTL or underground cables. The difference between them is listed below.