Electrical overhead line materials and functions (33kv and 11kv)

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Today we’ll be taking a look at 33kv and 11kv  line materials and functions.

A line which is short for power lines is mostly used to refer to overhead power lines. Its function is to carry electrical power for transmission. Most times, these lines carry either 33kv or 11kv as the case may be.

Without wasting much time at what a line is and all that, let’s cut to the chase.

 

Cross arm

This is the crossing member at the top of a pole. It makes the whole structure look like a crucifix. The usual length of a cross arm is 6ft for 11kv overhead lines and 9ft for 33kv overhead lines. The length of the cross arm determines the line to line clearance. It has holes drilled through it for installation of spindles and for attachment unto the pole.

Cross arms may come in wooden, fibre or galvanized steel material. Wooden cross arms are cheaper but less durable. Fibre cross arms offer good insulation, aesthetics but are quite expensive. Steel cross arms which are usually galvanized come at intermediate prices. However, steel cross arms demands earthing once chosen; which is a challenge. This increases the overall cost and labour. The cost of earthing materials {Bare copper conductor + earth rod} makes it less opted for despite its superior strength among the three materials.

However, steel cross arms are unavoidable at angle points, sectional points, terminal points, and for installation of CSP transformers {point load transformers}, isolators, lightning arrestors and D-fuse. So, if you have the option of choosing cross arms, one must consider cost, aesthetics, strength and durability.

 

Spindles

These are special “bolts” used to suspend the pot insulators. One end is slotted through a hole drilled into the cross arm and held firmly with a bolt. While the other end which is threaded projects vertically for pot insulators to be screwed onto it. There are spindles for 11kv and 33kv lines. They are structurally the same but they differ in sizes. While selecting spindles, you must ensure that they are galvanized and that the bolt end of it fits properly into the hole drilled in the cross arm.

 

Jumper spindles

It performs almost the same role as a normal spindle. However, they are longer and are used at sectional points or H poles carrying transformers. It acts to support the jumpers and prevent them from swinging freely. Jumpers are wires that carry electric power from one section of the pole to another. Without jumper spindles plus pot insulators, a large jumper could easily be swayed by the wind to touch a part of the support structure. This can lead to earth faults. Just like a normal spindle, ensure that they are galvanized while selecting.

 

Pot insulators

These are also known as pin insulators. They are screwed onto the upper end of spindles. The overhead conductors are attached to the pot insulator by a process called “binding”. The 33kv pot insulator is almost 3times bigger in size than that of the 11kv pot insulators. The rated breakdown voltage of any selected insulator must be higher than the rated voltage of the line.

For example, an 11kv pot insulator can have breakdown voltage as high as 24kv. This will ensure that the pot insulator does not shatter under voltage surges below 24kv. Pot insulators may be different shapes or designs depending on the manufacturers. However, they must all serve the purpose of insulating the overhead line from the support structures.

So, when selecting insulators, be sure to know its rated kV as against the voltage level of the line it will be used to string. Never make use of cracked insulators because their breakdown voltage drops drastically when cracked.

 

Disc insulators

Disc insulators are mainly used at sectional points and terminal points. After about 10 spans {1 spur} of overhead line, it is ideal for the line to be sectionalized. This will save you a whole lot of stress.
1. When you are tracing faults on the line.
2. If for any reason you want to isolate a section of the overhead line.

Aside from saving stress, it gives balance to the line structurally. 2 sets of disc {i.e 6pcs for 11kv or 18pcs for 33kv line} is required at terminal points. Remember, an H pole is used to start and end an overhead line. For 132kv and 330kv line which is above the scope of this article, string insulators are stringed together to form suspension insulators.

You can get up to 16 discs per phase for 132kv or 25discs per phase for 330kv line. Each disc has a breakdown value rated in kv. So, when they are strung together, the resultant insulation breakdown value becomes much higher. Disc insulators are either made of glass or porcelain. Either type is good and maybe selected without preference.

 

Socket or clevis adapter

As the name implies, it is an adapter. It connects a disc insulator to a 6 bolt clamp. It is made of galvanized steel. One end of this blob-like adapter is attached to the 6 bolt clamp while the other is attached to the disc insulator. It is also used to string multiple discs together especially for 33kv lines, 132kv lines and 330kv lines. There is no hard and fast rule in selecting the component. Just ensure that the steel is galvanized.

 

6 bolt clamp

The 6 bolt clamp is used to clamp the wire in place. It is connected to the clevis adapter.

 

J hook

The name “J-hook” is actually gotten from its similarity to the shape of letter J of the English alphabet. The hook end of the J hook is used to attach a string of insulator to a supporting channel or angle iron. A hole must be drilled or slotted into the channel or angle iron through which the hook will be passed. J-hook is either made of galvanized steel or stainless. The stainless steel type is usually preferred to the galvanized steel type because of its resistance to corrosion.

 

Stay assembly

It is commonly referred to as stay. It is referred to as stay assembly because it comprises of many materials. These materials act together as the stay. The components are

1. Stay rod
2. Stay bolt
3. Stay drum
4. Stay wire
5. Stay insulator
6. Stay block
7. Stay plate

When you see a complex drawing of the stay, these components are well labelled. Stay is literally the most important material behind a standing overhead line. Stays are required to start and end a line. They are also needed at T-off points {i.e where a line branches off an existing one} and at angle points.

If we are required to string a line along a curved route, stays will be required at almost every pole. All poles that require stays usually carry steel cross arms. This is because of the high resultant force on the pole.   The resultant force tends to pull the pole out of alignment with the overhead line. Fibre and wooden cross arms are not designed to withstand high bending forces because of their low strength.

So, stay serve the basic purpose of balancing the resultant force on a pole or group of poles. You can choose to have multiple stays on a single pole. It all depends on the resultant force acting on the pole. The direction of installation of a stay must be such that it balances or oppose the force pulling the pole out of alignment. You may encounter situations on site where there is no space to place a stay and balance the forces on the pole on one side of the road. This brings us to the flying stay.

 

Flying stay

What is done is to erect a pole directly opposite the “unbalanced” pole on the opposite side of the road, then tie a stay wire between the two poles {across the road} and then “stay” the newly erected pole. This actually transfers the forces on the unbalanced pole to the new pole across the road and the stay will balance it. The type of stay is referred to as a flying stay. Technically, the stay did fly across the road.

See how to effectively install a stay

 

Bolts, nuts and washers

These are small but powerful components of overhead line construction. They come in different sizes such as 5/8 by 2”, 5/8 by 5”, 5/8 by 7” etc. They are used to connect materials to support structures. In the case of towers and trusses, they are used to connect the iron members that form the towers or structures themselves. 5/8 describes the thickness of the bolt while the inches tell the length.

While a 5” bolt may be sufficient to hold a  tie strap to a cross arm, you may require up to 10” bolt for the fibre cross arm to the pole depending on the thickness of the fibre cross arm and the pole at the point of attachment. Washers must be set in place before screwing the nuts to fasten it. There are different types of washers commonly used in overhead line construction.

1. Flat washers
2. Spring washers

Flat washers

It serves two main purposes

1. To distribute the pressure of the nut evenly over its surface thereby preventing the nut from damaging the surface.
2. Since the nut is pressing against the smooth washer surface, Chance that the nut will gradually loosen is greatly reduced.

Another rare but important use of a washer is for packaging. This means that if a bolt is relatively too long such that when screwed completely it still doesn’t hold the object firmly in place, then several flat washers can be used to pack the bolt to fill the space so that the nut is able to hold the object within the length of the threaded part of the bolt.

Spring washers

Usually not flat, must have a split offset that flattens under pressure but spring back to position on withdrawal of pressure. They are used to ensure that the nuts are held with a high pressure thereby preventing it from loosening under vibration.

 

Danger plate

This is simply a flat metallic plate attached to the pole at about 2m above the ground carrying the danger sign or inscription. It helps to tell people to steer clear of the overhead lines. While selecting danger plate, be sure it has bright red inscription or sign and not the type that the paint can easily peel off or fade away.

 

Pole identifier

It is required that every pole used in overhead lines be numbered for easy identification. For instance, if there was a report that a pole with number 348 has been hit by a runaway vehicle, it will be easier to locate where the particular pole was erected by simply looking at the drawings of the line.

Pole identification can be done by hanging number plates on each pole or by simply painting the number on the pole. Painting is actually better. Reasons being that hanged number plate can dangle about, fall off or get tossed around by the wind. However, the paint must be water-resistant and durable.

 

Barbed wire

This is not different from the type used along fences for security purpose. In fact, it serves the same purpose here. It is used to discourage vandals from climbing the poles and carrying the overhead lines. The barbed wire span around the pole at a height of about 3m from the ground. There are no criteria for selecting the barbed wire.

 

Angle iron/Channel iron

In most cases, angle and channel iron serve the same purpose. But in some special cases, channel irons prove superior to angle irons like in the case of pole-mounted transformer installation. They are used as braces and bases for supporting transformers, surge arrestors, line isolators etc on H-poles.

They are also used as cross arms at angle points. At any point an angle /channel iron is put to use, that point must be earthed. In selecting any type of iron, you must consider its thickness. The thicker the iron, the higher its strength and also cost. The weight that is intended to be carried must be put into consideration while selecting irons. Also, a galvanized type is much better than ordinary iron because of its resistance to corrosion (rust) and thus more durable.

 

Line tap

This item is used to connect conductors together. It is the material designed for wire interconnections instead of twisting the wire over another as seen in overhead lines.

They are of two types:

1. All aluminium line tap: It is used to connect aluminium to aluminium. The two-section of the line tap are made of aluminium. At sectional H-poles, jumpers are installed for the connection and continuity of the line. All aluminium line taps are used at such points to connect the line (which is commonly aluminium) to the jumper (also aluminium). It makes the work neat and eliminates projected points that could facilitate corona.
2. Bi-metal line tap: It is actually made of metal but has one of its section lined with copper. It is not advisable to connect aluminium and copper together directly because of the chemical reactions that occur between both metals. In elementary chemistry, aluminium is higher than copper in the activity series, so aluminium tarnishes quickly when in contact with copper. Hence the use of bi-metallic line tap saves the day. It is not uncommon to find points where aluminium may need to make contact with copper in overhead lines. If the line is to feed a mini substation or a pole-mounted transformer; the bi-metal line tap must then be used to make the connection; between the line conductor (aluminium) and the transformer terminal leads which is copper. This arrangement settles the aluminium copper feud.

 

Earth mat

This is actually like a mat woven with copper strips or bare copper wires. Common sizes in the Nigeria market are 4ft\times 4ft and 2ft\times 2ft. However, you can weave any size of earth mat to suit your need. The longer the mat, the more effective is its earthing properties, but the costlier too. Earth mat is needed in overhead lines for earthing mini substation and pole-mounted transformers. They are commonly used in conjunction with earth rods in a hole called earth pit. Construction of earth will be treated in relative details when we discuss substation earthing.

 

Earth rod

This could be made of copper, aluminium or steel. They are used to channel fault current into the earth. Earth rod maybe 4ft or 6ft long. Some earth rods could be screwed into one another to obtain longer length where needed. Earth rods can be bolt into to the edges of earth mat in substation earthing or earth pits to enhance the system.

In order of increasing performance and cost, earth rod is arranged from steel-aluminium-brass- copper. This is the order needed to select earth rods. In very sensitive and delicate installations, copper must invariably be chosen. Otherwise, cost becomes the major factor determining the selection of earth rods.

 

Bare copper conductors

Bare copper conductors are mostly used for earthing be it grid earthing done in a substation or spot earthing done in lines. It comes in various sizes and the selected thickness depends on the anticipated fault current.