Overhead power lines: Answers for Beginners

Overhead power lines: Answers for Beginners

I see overhead power lines every where while traveling, As an electrical engineer I can’t get my eyes away of them.

I also know, there are tons of questions about overhead power lines. So in this article I will answer 6 common questions for beginners. Let’s start.

Are Overhead Power Lines AC Or DC?

All the distribution grid is AC – 3 phase system. I can say that, all power lines you come through are almost AC not DC, however DC is also used in transmitting high and extra high voltages i.e 500KV and more. Let’s discuss it in detailed.

 

Why are AC power lines are better than DC ones? In majority of the overhead transmission systems, AC transmission lines are favored over DC transmission owing to various reasons. This preference could be due to less complexity in design, cost effectiveness and compatibility to the load.

  • Cost Efficiency: It is more complected to step up the DC voltages for long distances transmission lines, this is why DC power lines require very large size conductors due to high values of current compared to AC lines, which in turns increases the cost on overhead system. Even though the transmission efficiency improves a lot, it is still unwanted for short distance transmission systems.

  • Less complexity: Additionally, two converters (AC-DC and DC-AC) will be required at generating and receiving end of the transmission line for stepping down the voltage, as it is very difficult to step down DC voltages. This process decreases the transmission efficiency while putting extra cost on the system. In comparison to that, AC transmission lines are economically viable and has no complexity in design.

High voltage lines wouldn’t require large size conducting cables, and will have no issues in stepping up or stepping down the voltage at both ends.

  • Compatibility: with few exceptions, almost all kinds of loads at user end run on AC voltage. Ultimately, this negates the requirement of using converters, making the system very cost effective with less conversion losses.

Therefore, in overhead lines, it is still preferred to use AC power lines instead of DC power lines.

Why are Overhead Power Lines Not Insulated?

There is really no need to insulate an entire transmission line in overhead systems, simply because of :

  • Its excessive cost
  • Insulation is useless due to the conductors high distance from ground
  • Increased weight of the conductors
  • Inverse Impact of Di-Electric Strength due to high voltage levels
  • Poor cooling due to insulated conductors

Why Insulation is Useless?

In overhead transmission lines, the distance from the ground is kept very high, whereas two cables on a pole are also kept away from each other to avoid any chances of contact.

This means in normal cases, there is no requirement of any insulation material to provide protection against physical contact. Instead of spending money on useless insulation, companies spend it on tower, insulation and accessories of the power lines.

Inverse Impact of Di-Electric Strength:

AC transmission lines carry high values of voltage, which has inverse impact on the di-electric strength (ability to withstand maximum value of voltage before experiencing discharge of current through the insulating material). 

Therefore, instead of spending money on a thing that has little or no return value, it is preferred to rely on natural Air’s di-electric properties to act as an insulator without any cost.

Low Weight and Heat Resistance:

Insulating material can increase the weight of transmission cables, putting extra pressure on towers, which in turns means extra cost in designing towers and conductors carrying accessories.

Insulation will also slow the process of cooling down the conductors, as insulating material won’t allow the free air to pass through it.

Considering all these problems, it is generally preferred to not use any insulation in overhead lines.

What Are Voltage Levels of Overhead Power Lines?

The voltage levels of overhead transmission lines are classified on the basis of the nature of area and loads. These are classified as low voltage, Medium Voltage, High Voltage, extra high voltage and Ultra High Voltage lines.

  • Low Voltage lines (70 to 600 volts): These overhead lines are used in connections involving residential units or small scaled commercial areas. Typical voltage of these lines remains below 1000 volts and mostly remains between 70 to 600 volts depending on the single or three phase connections.
  • Medium Voltage Lines (750 V to 69 KV): This classification of overhead lines normally remains between the voltage levels of 750 V to 69 KV. However, for economic feasibility, the maximum voltage in these transmission lines remains below 35KV threshold.

These types of transmission lines are used for feeding power to Low voltage lines and require step down transformers. However, in case of synchronous motors, that typically run above 1KV level, the load can be directly connected to these lines.

  • High Voltage Lines (69KV to 220 KV): For supply of electric power from generating stations to substations, High voltage transmission lines are used.

These types of overheard lines are economically feasible for long distance transmission as the transmission losses remains minimal due to low value of current passing through it.

To avoid any accidental human contact, these transmission lines are installed away from the populated areas. Voltage level of these lines typically remain between 69KV to 220 KV.

  • Extra High Voltage Lines (above 300KV and up to 800 KV): For very high-power transmission systems, above 300KV and up to 800 KV, Extra high voltage lines are preferred to reduce the cost and transmission losses.
  • Ultra-High Voltage Lines: For energy resources and generating units far away from load areas, it is preferred to used to Ultra High voltage transmission lines. These lines can carry DC and AC power and has different rating depending upon the distance and nature of supply. Economically recommended voltage levels for these lines are at least 800 KV for DC transmission and 1000KV for AC transmission.

Why Do Overhead Power Lines Have 3 Wires?

Why do power lines have 3 wires

Transmission lines doesn’t typically connect directly to the load, and hence requires no neutral wire to provide added safety factor.

 

Furthermore, in long distance transmission lines, the cost efficiency is an important factor, and therefore instead of transmitting power on a Star connection, companies do prefer Delta connection due to the fact that it has no neutral wire which saves up the cost.

 

The logic behind this phenomenon is the difference of phase angle in a 3-Phase delta system. Every phase angle leads or lags the other by 120 degree, and if we add these differences together, the net value becomes zero.

It means that for 3 phase high voltage systems, there is no requirement of a neutral wire, and because of this reason we see only 3 wires in an overhead power line.

Why Do Overhead Power Lines Never Cross Each Other?

In transmission systems, safety factor is very important and requires necessary ground clearance to avoid any human contact. Overhead power lines have their potential differences, and in cases any of the two lines cross each other, it will compromise the recommended level of safety. This is due to the reason of di-electric nature of the air in between two cables.

When two cables cross, having difference in potential means the air between them acts as a natural capacitor and keeps on charging/ discharging.

This results in heavy transmission losses which is undesirable in transmissions systems. Therefore, it is always ensured that overhead power lines never cross each other.

Why Is Aluminum Conductor Steel Reinforced (ACSR)?

The overhead power lines are feeding power over long distances, and in high voltage. In commercial sectors, it is always desired to keep the cost at minimum while providing maximum efficiency.

 

Typically, Copper is a preferred in household connections, because of its better conductance. However, in overhead cables, use of copper could be costly with increased weight and potential safety risk in case of heavy storms and earthquakes.

Why is Aluminum Conductor is  Preferred in OHTL?

In comparison to copper, Aluminum has considerably less conductance (33% less), and may not be an ideal choice for household connection. However, for transmission lines, it is a preferred option due to various reasons.

 

  • Lighter weight: Aluminum conductors has lower density and are lighter in weight than copper, two times lighter, which makes it easier in installation on high distances.
  • Compression stress: When we are talking about OHTL we can’t neglect compression stress, Aluminum is less prompt to this stress, this makes it better than copper in OHTL .
  • Economic viability: Aluminum is cheaper and lower in price than copper, and proves an economical solution rather than installing new transmission lines.

Why Using a Steel Core?

Even though Aluminum conductors are bulky in size, their weight is much less than a copper conductor. Now for transmission lines having high span lengths (tower to tower distance), the vulnerability associated with aluminum’s weak strength is compensated by using a steel core in the conductor. As a result, the overall strength of power cable will be increased with high durability and resistance against storms and environmental factors.

What should be the distance between the conductors of OHTL?

There is not a particular formula for the calculation of the distance of a conductor from a medium voltage transmission line. The reason is that so many factors affect this function of distance.

The relation of those factors and their empirical effect in determining the distance makes the formulae really complex. So, normally 3-4 meters distance is kept from the medium voltage transmission lines.

This distance is horizontal if the conductors are to be placed horizontally and vertical if the conductor is to be placed vertically with respect to the position of the transmission line. So, some general information that can be useful in this topic is as if the transmission lines are carrying the voltage less than a 11 kV then a 4-5 meters distance is appreciated.

More than 11kV should be 5-6 meters apart. The height of these transmission lines should be at least 6-6.5 meters. In southern Asia, this is general practice because the transmission lines are normally overhead transmission lines. In the United States and Europe, transmission lines are normally underground.

The distance of a transmission line with the potential of 66kV from a conductor should be 6.1 meters from the ground and 8 meters over a highway.

The distance of a transmission line with the potential of 132kV from a conductor should be 6.1 meters from the ground and 8.6 meters over a highway.

The distance of a transmission line with the potential of 220kV from a conductor should be 7 meters from the ground and 9.8 meters over a highway.

The distance of a transmission line with the potential of 400kV from a conductor should be 8.8 meters from the ground and 10.8 meters over a highway.

These distances may vary from country to country depending upon their policies.

Why are ACSR conductors used in transmission lines?

ACSR conductors in the case of transmission lines are the aluminum cables twisted around a steel cable.

The function of this steel cable is to provide the strength for the cable to be hung on the poles. As the current flows on the outer part of the cable that is aluminum that is why they are pronounced as hollow. There are several reasons to use hollow conductors for the transmission lines. Such as.

  1. 1: As the diameter of these cables is larger and so is the weight of these lines, they have more inertia to the wind. Storms and winds have a lesser effect on these transmission lines.
  2. 2: As the diameter of hollow conductors is large, it has more surface area for the same amount of current as of the other conductors such as bundled conductors.
  3. The corona losses are reduced.
  4. The reactance of the electric transmission lines is reduced.
  5. The skin effect of the hollow conductors is lesser as compared to other conductors when used in the transmission lines.
  6. The inductance of the transmission line is also reduced.

What is minimum distance between the conductors of OHTL?

The phase-to-phase clearance of the transmission lines is as:

For 11kV the minimum phase-to-phase clearance must be 787.5 mm.

For 110kV the phase-to-phase clearance must be 990.6 mm.

For 132kV the spacing should be 1219 mm.

For 220kV there should be a distance of 2057.4 mm.

The distance between two transmission lines carrying 765 kV is 45 meters. This is the horizontal distance between these transmission lines.

Why are hollow conductors used in transmission lines?

Hollow conductors are analogous to garden pipes. Say, the larger the pipe is in the diameter the larger will be the water flow. So, larger is the diameter of the current carrying conductor larger will be the current flow through it. Now as the hollow conductors have larger cross-sectional area, they offer larger current flow through them by this function.

Current flow α diameter of the Conductor.

The surface area of the hollow conductors is also greater than other conductors. So, the charge density per unit surface area decreases. As the charge density decreases, the electric field also decreases. Due to this the corona losses also decrease.

Surface Area α Corona losses

Such as.

  1. As the diameter of hollow conductors is large, it has more surface area for the same amount of current as of the other conductors such as bundled conductors. That result is better transmission with lesser losses.
  2. The corona losses are reduced.
  3. The reactance of the electric transmission lines is reduced.
  4. The skin effect of the hollow conductors is lesser as compared to other conductors when used in the transmission lines.
  5. The inductance of the transmission line is also reduced.
  6. The hollow conductors are effective when used as bus bar conductors