What are Overhead Power Lines

Discuss about the Reliability Centered Maintenance Optimization.

In this modern era, there is a huge leap in technology. The technology is improving with each passing day with the invention of new devices and improvements of existing ones which are making human life easier, comfortable and healthy day by day. As the number of electrical gadgets are increasing the demand for electricity is also increasing. In order to meet the increasing demand of electricity the production of electric current needs to be raised. As the production increases there is a need to have sophisticated infrastructure to transmit the produced current to the targeted areas (Lazaropoulos, 2014). Transmitting the current requires overhead lines to be in place. These overhead lines can be of any type and can be located anywhere. They carry currents ranging from high to low voltage. The overhead lines which carry very high voltage current that is there the current directly comes to them from the power stations and are basically placed in the fields. They carry currents which can be in thousand volts. The overhead lines which carry current of low voltage that is in hundreds of volts are mainly located inside cities or towns or villages.

Overhead lines are those structures which are used to transmit electrical power and distribute them over a long distance. The overhead power lines consists of more than one conductor which are suspended by the poles or towers. As the losses in transmission is less thorough overhead wires due to the insulation that is provided by the air. As air provides insulation to the transmitting current, so the transmission of current with the help of overhead wires is a lot cost method of power transmission and is used to carry and distribute power over long distances. Overhead lines needs support as they can’t hang in the air. The towers are there to provide them the required support. Overhead wires are mainly carried by towers which can be made from wood, steel, aluminium, concrete structures and sometimes are also made from reinforced plastics. The transmission wires which are used to transmit current are generally made from aluminium. These wires can be plain wires or can be of composite materials such as carbon and glass fibre or can be reinforced with steel. Sometimes copper wires are also used to transmit medium and low voltage currents to the customers. The overhead wires are mainly designed to have enough clearance between the ground and the high current carrying wires so that there is no contact with the line. Clearance between the wires are also required so that they wires doesn’t come in contact with each other during storms. They are designed keeping in mind the load that can be imposed to them by earthquakes and ice deposits. In these days overhead wires are widely used to transmit and distribute current. These overhead wires are capable of carrying currents above 765,000 volts (Kiessling et. al,. 2014).

Classification of Overhead Power Lines Based on Voltage

The overhead power lines can be classified into different groups depending upon the ranges of voltage they carry:

Low voltage (LV) – These generally carry voltage which are less than 1kV. These are used for connection in homes or in small utility services.

Medium voltage (MV) – They generally carry voltage between 1kV and 69 kV. They are used to distribute current in rural and urban areas.

High voltage (HV) – They are used to carry current at lower voltage which is less than 100 kV. They are also used for sub transmission or transmission at voltages such as 115 kV and 138 kV. It is used to transmit power in bulk quantity.

Extra high voltage (EHV) – They carry voltages from 345 kV to about 800 kV. They are used to transmit power to very long distances. The power is transmitted at very high frequency.

Ultra high voltage (UHV) – it carries power which is more than 800 kV.

Key design aspects of overhead lines regarding construction

The main purpose of transmission line or distribution line is to carry power from one point to another (Gönen, 2014). Some of the characteristics which a transmission line should possess are as follows:

• Voltage should be constant over the entire length of the line.
• The losses should be as minimal as possible. Decreasing the amount of loss will increase the transmission efficiency of the overhead line.
• There should not be any heating due to the copper losses.

There are many components in a high voltage transmission line. Some of them are as follows:

• Conductors are always kept bare as they have natural insulation which is provided by air.
• Conductors provide vital link in the transmission and distribution system.
• They are designed to meet the specified voltage level.
• The conductor should be built according to the voltage at which the current is required to be transmitted. The tension of the line, maximum allowable losses, and the tension of the line and the maximum thermal capacity of the line should also be considered.
• Factors such as the geographical location, climatic and atmospheric conditions along with the vibration in the line should also be kept in mind.

Different types of conductors are used to transmit power of different intensities. Some of the conductors are as follows:

1. Aluminium conductor steel reinforced (ACSR) – This conductor is widely used in these days due to its low cost and high material strength. This type of conductors mainly have aluminium wires that are wrapped around a steel cylinder. The most common ACSR that are used in these days are 26/7, 6/1, 54/7 (Douglass et. al,. 2016).
2. ACSR conductor with aluminium clad steel reinforced core (ACSR/AW) – These conductors are widely used in those environments which have high humidity content as they don’t corrode.

• ACSR conductor that is self-damping (ACSR/SD) – These conductors are more expensive than the ACSR conductors. It has a steel core with two trapezoidal layers of conductor around it. The wires are made of #6201 aluminium. The structure of the conductor makes it damp proof against Aeolian vibrations. These have the ability to be strung at very high tensions.

1. Aluminium conductor alloy reinforced (ACAR) – It has wire of #1350 aluminium which is wrapped around a core of #6201 aluminium. These are lighter in weight than the ACSRs. They are more expensive but possess the same strength as the ACSR does. These are mainly used in corrosive environments.
2. Aluminium conductor made of #1350 strands (AAC-1350) – These are used when there is a requirement to transmit electricity to a short distance with a very high conductivity and minimal loss.
3. Conductor composed of #6201 aluminium alloy (AAAC-6201) – These conductors are very stronger. They are lighter in weight than the ACSRs but are predominantly more expensive than them. These are mainly used when the environment is corrosive in nature and the transmission is required over long distances.

Factors that are needed to be considered before selecting the type of conductor that is required for an overhead transmission line are as follows:

• Amount of span and sag that is required
• The tension that can be applied on the conductor
• Nature of the atmosphere that is corrosive or not
• If there is any kind of vibration in the line or not
• Permissible limit of power loss
• Permissible limit of voltage loss
• The climatic conditions at the site

In order to transmit power with minimal losses it is important to choose the right size of the conductor. The factors which are taken into consideration while choosing the size of the conductors are as follows:

Voltage drop consideration: The conductor meets the minimum size requirement but transmits power with an acceptable loss. The loss is expressed in terms of maximum voltage drop of 5% (Wang et. al., 2016). The impedance of the total series connection is equal to the maximum voltage drop that is allowed and then dividing it with the maximum load current. Therefore the equation can be written as

Z = R + jX = V/ I max

Thermal capacity: The conductor should have the ability to transmit the power without being getting overheated during long period of time. It is assumed that the conductor can withstand temperatures up to 75 degree Celsius (Qin et. al,. 2015). This should not decrease the strength of the conductor. As the temperature crosses this range the strength of the conductor is assumed to be getting decreased.

Design Aspects of Overhead Lines Regarding Construction

Economic considerations: The conductors are rarely sized to meet the minimum requirement. The total cost per kilometre should be taken into account. There are also energy losses during the transmission of the power. Compensations are needed to be provided for load growth.

There are mainly two type of insulator which are used. Pin type and suspension type. Tension type insulator is also there but are rarely used. Insulators are mainly used to support and anchor the conductors. They also insulate the conductors from the ground. These are mainly made up glass or porcelain and in some cases ceramic is also used.

These are the structures which are used to carry the overhead lines and keeps the conductors at a safer height from the ground. They also helps to maintain distance between the conductors. The support structures are constructed depending upon the budget that has been allocated. The budget takes into consideration labour costs, transportation costs as well as design and material cost. Generally galvanised steel frame towers and wooden H-frame and K-frame towers are commonly used (Wang et. al., 2015). It is very important to place the towers correctly. Erecting the towers is a challenging as well as one of the important task of transmission line construction. The way the towers will be erected depends on the following conditions:

• Workspace
• Availability and experience of the labours
• Time that is given to complete the project
• Terrain
• Space available for work

In addition to all these factors there are also some factors which are important to be considered while choosing the method of construction. The factors are:

• The type of structure that is needed to be erected
• The natural divisions that are available
• The dimensions of the natural divisions
• The conditions to access the right of way

The above factors also helps to decide whether maximum labour minimum equipment or minimum labour maximum equipment would be favourable and feasible.

Location of poles and structures: Poles and structures have to be located in observance of the right way. It is very important to draw a plan profile drawing before starting work on the field. These drawings shows topographical contour maps of the terrains along with the right of way. These drawings helps to decide the elevation of the tower. The drawing works as an pre-planning of the works and helps to decide what needed to be done afterwards. They also help to find the way of certain challenges that are posed during the phase of the construction. These are used to complete the work with respect to structure spotting (Xu et. al,. 2014). The following steps should be taken into consideration while spotting structures:

• Drawing the plane profile drawing on a certain fixed scale
• Establishing the sag template on the same drawing along with the same scale
• Table showing the conductor clearance to the ground as well as with the other overhead lines
• Deciding vertical and horizontal span limitations due to strength requirements and clearances

It is important that the towers are needed to be buried to a certain specified depth so that they doesn’t collapse. The depth depends upon certain conditions and can range from 6 feet up to certain hundred feet’s (Thrash et. al., 2014).

Types of Conductors Usable in Overhead Lines

There are basically two type of towers are used. The types are as follows:

1. Towers used for straight runs
2. Towers used when bends have to be made in the path of the line (Deviation towers)

Putting the deviation towers at their specified place is very important and a risky job too. Guyed wires and blocks are used to balance the tensile forces on the tower. If two forces act on the tower a resultant force gets generated and the guyed wire is used to counteract the resultant force so that the tower doesn’t collapse.

Maintaining the overhead power lines for the next thirty years (including OHS issues and ongoing operating regimes)

In order to ensure their full efficiency and longer shelf life it is required that the established overhead line gets their periodical maintenance time to time. High and ultra-high voltage infrastructure are the future of electrical grids as they have very large capacity, can transmit power to a longer distance and has very high amount of efficiency. As public health, transportation, water supply and other energy related assets, transmission lines are also a part of so called “critical infrastructures” as they provide essential services for the economy and functioning of a society. Therefore maintaining the overhead power line is very important along with keeping in mind the OHS (Occupational Health and Safety) issues (Kiessling et. al., 2018).

There are various ways for preventive and predictive maintenance of the overhead transmission lines. Patrolling the lines thoroughly. Digital photography of the lines are also carried out. In order to detect the hot spots in the transmission lines thermo-graphic scanning is done. Generation of hot spots in a transmission line can be very dangerous as it reduces the strength of the conductor and compromise the safety of the people living there. In order to find out if the insulator and the insulation is properly working or not electric field measurements are taken. These punctured insulators can also be detected by voltage measurement techniques. In order to ensure that the current in the transmission line is flowing with least resistance, resistances are measured in the electric joints in live line condition (CRM) (Phillips et. al., 2014). Corona detection by décor camera is also done. Tower footing earth resistance measurements are also checked.

Maintaining and managing the Extra High Voltage (EHV) lines for the next thirty years along with keeping people safe with the overhead transmission lines is a challenging task. There are various techniques that can help in maintaining the EHV lines. As the EHV towers are mainly made up of steel frames there is a high tendency that they get rusted and lose their strength. So it will be required to strengthen them by changing the rusted parts including nuts and bolts and painting them so that they can last long. If this is not done there is a chance of mishap as the rusted tower can collapse bringing the overhead EHV line to the ground which can kill thousands. Proper tightening of the nuts and bolts will also be required so that the steel frames can achieve their desired target strength (Adabo et. al., 2014). Live line maintenance will also be required in order to check various parameters of the overhead power lines. Live line maintenance can be done by various techniques such as hot stick method and hot line washing. Ground patrolling is carried out by inspection of the line, by walking down survey and through patrolling by climbing on the tower with the help of internal body of the tower and observing the line components maintaining safe body clearance. During visual inspection thermo scanning, digital photograph of the parts and associated components along with corona detection by Daycor camera are also carried out.  In order to maintain the overhead power lines for the next thirty years there is a need to carry out routine inspection and maintenance. During inspection various factors are needed to be checked such as the foundation of the tower, the earthing of the tower, nuts and bolts, tower members, anti-climbing devices, number plate, and danger board and phase plate. The clearances are also required to be checked in compliance with the ongoing operating regime (Zhang et. al,. 2014). Various other things are also needed to be checked and repaired if found defected such as jumper condition, sag and ground clearances, members of the towers that are missing should be replaced, armouring, conductor spacer, bird guard, vibration dampers for conductor and earth wire along with the insulator and insulator fittings, hangers, eye bolts and yoke plates (Suresh et. al., 2017).

Factors to be Considered before Selecting Type of Conductor for Transmission Line

The evolution in the technological sector has also contributed to this field by providing many modern equipment’s which has made the maintenance work easier and efficient reducing the labour cost and saving precious time by detecting problems at an earlier stage. These modern high end equipment’s has made long term maintenance possible and feasible. Thermo vision scanning is one of them. This technique is used to detect hot spots on the live line. The resistance of the electric joints are measured in the live line by a technique known as CRM. This sophisticated instrument is directly attached to an energised EHV line with the help of a hot stick. It directly reads the resistance in micro ohm. These instruments can store up to nine readings. The resistance is calculated with the help of calculated voltage drop and line current. The punctured insulators are detected with PID instrument (Douglass et. al., 2016). The instrument detects the electric field across the insulator. If the electric field is more that means the insulator is punctured. Punctured insulator can also be measured by Ritz’s instrument. It works by detecting the voltage across the insulator. If the instrument shows no reading that means that the insulator is punctured as the punctured insulator has zero voltage drop. Situation arises when the voltage on an electrical conductor exceeds the threshold value. At that time the air around the conductor begins to ionise and forms a purple and blue glow with loud noises. Corona is the polarisation of air molecules due to energy on high voltages. This is detected at an earlier stage by Daycor camera (Yssaad et. al,. 2014).

Hot line or live line maintenance techniques are highly preferable as they are done without interruption in the line and save the company form revenue losses. Contaminated disc insulators can also be cleaned by hot line washing along with the detection of the punctured insulators (Song et. al,. 2014). The hot spots that are detected by thermo graphic scanning can also be rectified by the hot stick method or even by the bare hand technique as a preventive measure. Very less man power and very less time is required. 

As the overhead transmission line carry huge amount of charge with them, they are fatal to living being. History also shows that there are large number of incidents that has happened due to transmission lines. In order to minimise the incidents it is important to follow some specific set of rules and regulations in place to bring the number down. OHS act and OHS regulation are in place to ensure the safety, health and welfare of the residents living nearer to overheard lines (Letzter et. al,. 2014). Present operating regime states that the transmission lines carrying high power within the residential area should maintain proper height from the building and should have proper insulation. This will prevent the people living there from the high energy charge that is being carried out by the transmission line. Transmission lines carrying very high charge should not pass through a heavily populated area as any mishap may risk the life of the people living there.

Support Structures

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