Overhead Line Design
Overhead Line Design: The supply of electrical power comes down from the generating stations, where they are being generated and stepped up for transmission and distribution purposes. Well, the whole stress is to ensure that electricity is being brought to the consumers.
Meanwhile, the transmission of electrical power is classified into two segments. There we have the primary transmission of the electrical power at 330/132KV. Also, the secondary transmission at 132/33KV(KiloVolt). The distribution of electrical power is also laid into two segments. That’s the primary distribution at 33/11KV and the secondary distribution at 11/0.415KV.
Well, this article covers the steps required to carry out during the overhead line route design for 33KV, 11KV, and 415V. The word transmission and distribution imply that something is being moved from one point to another. Exactly, but here what we are moving are electrons(Current) and again, with the basis of physics. You will understand that current need a path or medium to flow through.
Moreover, this path provided for this current to flow through is what we call conductors. However, these conductors are not magically held in the air, rather they need an electric pole to hold them. The idea of ensuring that this electrical power gets to users has to follow a line design. So, let’s take a look at the steps required for this overhead line design.
Steps to carry out during overhead line route design
1. Line Route Survey:
You will agree with me that, electric poles can’t be placed in the middle of the road, to obstruct the movement of vehicles. Well, the reason that it isn’t idle because that’s not the right of way for an electric pole to be mounted on. So, with this simple analogy, which implies that Electric pole has its own right of way. But how can we know where to place them.
There comes the idea of carrying-out line route survey, to solve these questions. However, the line route survey is carried out by the surveyors. Meanwhile, carrying out a line route survey is factored on the perimeters of the area. So, they must know the length of every street and bend at every point. After this is done by the surveyors, a Report is being submitted to the engineer before moving to the next step.
2. Line Route Design:
Technically, this aspect of the overhead line design is software-based. This implies that the use of software has a role to play here. Meanwhile, after the line surveyor has finished with the line route survey. This finding is being drafted out, using the assigned software, like the use of AutoCAD Computer software.
This design shows where each of the electric poles is being placed with the help of the information gotten from the line survey. As regards the land area and its landscape.
3. Route Clearing:
However, after carrying out the software design as regards how and where these electric poles will be placed. That is, preparing a perfect route for electrical power to flow. When this is done, the step that follows is to carry out the clearing of any impediment on the route.
These obstacles remover include the clearing of trees, unauthorized structure on that path. Well, you can literally say that the route clearing is done, in other to get a proper sight. Although, in comparison to a pole with that of towers, route clearing for towers costs more.
This goes before pole erection. Meanwhile, the whole essence of sighting is to ensure that all electric poles are aligned on the same horizontal line. Although one person can’t do sighting because it involves someone holding a pole(smaller) at every span. While someone else will have to stay at the first pole to ensure that all pole at that spur is well aligned. Meanwhile, a spur comprises ten(10) span and this is practically the distance between two electric poles.
5. Pole pegging/Tower spotting:
Like that of pole sighting, but here it doesn’t require anyone standing at every span. Although, for regular poles, you practically don’t require carrying out soul test but carrying out soul test is very necessary for towers. Again, it’s expected to know that route clearing costs a lot for tower spot.
6. Digging of hole:
Certainly, as the name implies, this very stage involves digging holes to house these electric poles. The hole for either Reinforced concert pole or wooden pole has to be six(6) feet deep. Although, the regular shovel use for digging is 4ft. This implies that you can’t use it to achieve a hole of 6ft. One of the perfect ways this is done is by stepping down by 2ft. This two(2) feet dug, is to ensure a perfect standing point to dig the remaining 4ft.
The question now is, How do you know the direction to step the 2ft to? Well, to solve this question, one needs to know the direction of the line. Meanwhile, your step shouldn’t be in the direction of the line, rather 90 degrees to the line direction. In other words, the step needs to be adjacent to the direction of the line.
Although, for cases when you can’t dig 6ft to the ground, maybe as a result of the nature of the soil. In some areas, at 6ft, you might start seeing water. So, as such, you can’t dig a hole of 6ft, in such case, you need to set up a concrete beam. Literally, it’s just like a square box built with cement, round the base of the pole.
But is there any specific height for the beam? Not really, the height of the beam is dependent on the level of dig to the ground. Let’s say you were able to dig feet of 3ft, the total height from the top beam to the very level beneath the ground is 8ft. Then, the height of the beam to the ground level will be (8ft-3ft)= 5ft. Well, to erect electric poles inside water, you will need a drum to immerse inside the water.
7. Pole Erection/Tower construction:
Practically, for overhead line design, it involves placing the poles inside the dug hole, in other for it to stand vertical. Meanwhile, when carrying out the erection of pole, stability should be the main consideration. While, for tower construction, when the beam has been laid, the tower is being assembled. However, poles should be erected on a concert foundation, else this might pose an issue with time if the foundation is not strong.
8. Pole Dressing:
Pole dressing is done after pole erection has been made. It involves the placement of cross arm, pot insulators, spindles, tie strap e.t.c. Well, you can read more about Electrical pole dressing: Electrical Pole Dressing: Materials For 33KV, 11KV and 0.415KV
So, let’s just take a brief look at the required items for dressing various poles.
- Cross arm
- Tie strap
- Pot insulators for 11KV/33KV and charcoal insulators for 415V.
- Bolt, nut, and washer
- Aluminum conductor
- Channel or angle iron
- Three(3) J-hook
- Clevis adaptor or socket
- Disc insulators( 3 discs for 11KV and 9 disc for 33KV)
- Six(6) bolt clamp
- stay wire(two stays)
- Aluminum conductor
- Jumper spindle for each line
- Line rap for each line
- Disc insulators( 6 discs for 11KV and 18 disc for 33KV)
- Socket or clevis adaptor, two(2) on each line
- Six(6) bolt clamp, two(2) on each line
- Aluminum conductor for the line
- Stay wire( four stays): You need it, so that, during stringing the pole won’t deviate from its position.
The distance between two vertical electric poles is known as span. One(1) spur equals 10 spans. Meanwhile, for every spur, the pole there has to be sectionalized and this is done in line for maintenance, testing and troubleshooting purposes.
Although, sectional H-pole can also be placed even when the distance between poles is not up to a Spur. Well, this is usually done when the total span on the route is not even.
9. Erection of stay:
During overhead line design, poles that require stay usually carry steel cross arms. This is because if the high resultant force on the pole and this resultant force to pull the pole out of alignment with the overhead line.
The stay assembles for sectionalized pole are used for tensioning the pole and conductor. After this, they can be left In case of damage caused by accident on the pole. The stay assembly on a particular pole acts as a balance for the other poles.
10. The string of line:
Stringing of a line is done to attain a minimum sag level. Also, when stringing a line, minimum clearance should be considered too. Meanwhile, it’s not advisable to string the three(3) lines at the same time. When you string the three-line at the same time, it will shift the electric pole from stability.
To avoid this, firstly you need to string the middle line( usually the yellow phase ) before any of the other phases.
Please note that you don’t string conductors on the insulator, rather you string(tension) the conductor by placing it on the cross arm. Meanwhile, doing otherwise will distort the insulator.
After stringing, you raise the conductor and place it on the groove of the pot insulator. Thereafter, you bind the line. Binding of a line is the next step to take during overhead line design.
11. Binding of Lines:
This aspect of the overhead line design happens to be the penultimate to this practical process. Practically, it’s the use of a wire to tire the conductor on the pot insulator. Meanwhile, you usually do binding on pot insulators because of its groove and not on disc insulators.
12. Line testing:
There are two type of line test done during an overhead line design. Moreover, this is done at the end of the process. The two line test carried out are:
- Insulation testing
- Pressure testing
This test is done using a testing instrument known as the megger meter. This is a high voltage insulation tester, used to check the resistance value. Meanwhile, the black clip of the megger meter is placed on the earth. While The red clip is placed on the line. The insulation tester works with the principal of V=IR. For good insulation, the resistance value read should be very high(Giga value)
The line is insulated from the support structure. However, ever angle pole shouldn’t have a fiber cross arm rather you use steel, channel or angle iron. Meanwhile, every point with this type of cross arm will need to be earthed to an earth mat or rod. Mainly at sectionalized and terminal point. Also, the earth rod for the line has to be galvanized.
The length of an electric pole varies, you don’t use the same length for all cases. Well, for power distribution, a pole length of 28ft is used. While for sub-transmission( 11KV/33KV ) an Electric pole length of 34ft is used. However, for cases of obstacles or trying to cross another electric line, at this point, you use a pole length of 40ft.
Here, a device is known as a Hipotronic tester is used. This is a signal generator to generate high voltage and current. Although, it’s a DC device and it’s used to inject about 20KV or 25KV into an 11KV line. Meanwhile, this is usually twice the line rating. However, the duration of this device on the line shouldn’t be more than 30seconds. Since it’s injecting its own voltage on the line. Well, if the line fails( any insulator ), then you can tell that something is wrong and call for a check.
Finally, after carrying out the overhead line design, it’s expected to be commissioned by the commissioning bodies.
Well, that’s all the necessary steps you need to take when carrying out an overhead line design. Meanwhile, these steps were progressively explained and it’s expected not to avoid any because they are necessary for overhead line design. Also, we got some other interesting articles on Electrical Engineering that will be of interest to you. So, feel free to check them out: Electrical Archive.