Overhead Lines design - Conductor Materials and Main Components Of Overhead Lines

Overhead Lines design:

Now allow us to discuss the about overhead lines and their design. Overhead lines have more advantages than underground lines. The underground cables are rarely used for power transmission thanks to two main reasons. Firstly, power is usually transmitted over long distances to load centers. Obviously, the installation costs for underground transmission lines are going to be very heavy in comparison to overhead lines. Secondly, electrical power has got to be transmitted at high voltages for economic reasons. It's very difficult to supply proper insulation to the cables to face up to such higher pressures.

An overhead line is subjected to uncertain weather and other external interferences. This involves the utilization of proper mechanical factors of safety so as to make sure the continuity of operation within the line. Generally, the strength of the road should be such so on provide against the worst probable weather. In this, we shall focus our attention on the varied aspects of the mechanical design of overhead lines.

Main components of Overhead lines:

Sorts of electric poles in overhead transmission lines:

An overhead line could also be wont to transmit or distribute electrical power. The successful operation of an overhead line depends to an excellent extent upon the mechanical design of the road. While constructing an overhead line, it should be ensured that mechanical strength of the road is such so on provide against the foremost probable weather. Generally, the most components of an overhead line are:

(i) Conductors which carry electrical power from the sending end station to the receiving end station.

(ii) Supports which can be poles or towers and keep the conductors at an appropriate level above the bottom.

(iii) Insulators which are attached to supports and insulate the conductors from the bottom.

(iv) Cross arms which give support to the insulators.

(v) Miscellaneous items like phase plates, danger plates, lightning arrestors, anti-climbing wires etc.

The continuity of operation in the overhead line depends upon the judicious choice of above components. Therefore, it is profitable to have detailed discussion on them.

Conductor materials utilized in Overhead lines:

The conductor is one among the important items as most of the capital outlay is invested for it. Therefore, proper choice of fabric and size of the conductor is of considerable importance. The conductor material used for transmission and distribution of electrical power should have the subsequent properties:

(i) high electrical conductivity.

(ii) high lastingness so as to face up to mechanical stresses.

(iii) low cost in order that it are often used for long distances.

(iv) low relative density in order that weight per unit volume is little .

All above requirements aren't found during a single material. Therefore, while selecting a conductor material for a specific case, a compromise is formed between the value and therefore the required electrical and mechanical properties.

Commonly used conductor materials. The foremost commonly used conductor materials for overhead lines are copper, aluminum, steel-cored aluminum, galvanized steel and cadmium copper. The selection of a specific material will depend on the value, the specified electrical and mechanical properties and therefore the local conditions.

All conductors used for overhead lines are preferably stranded so as to extend the pliability. In stranded conductors, there's generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ... wires. Thus, if there are n layers, the entire number of individual wires is 3n(n + 1) + 1. within the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiraled in opposite directions in order that layers are bound together.

1. Copper: Copper is a perfect material for overhead lines due to its high electrical conductivity and greater lastingness. It's always utilized in the hard-drawn from as stranded conductor. Although hard drawing decreases the electrical conductivity slightly yet it increases the lastingness considerably.

Copper has high current density i.e., the present carrying capacity of copper per unit of X-sectional area is sort of large. This results in two advantages. Firstly, the smaller X-sectional area of the conductor is required and secondly, the world offered by the conductor to wind loads is reduced. Moreover, this metal is sort of homogeneous, durable and has high scrap value.

There's hardly any doubt that copper is a perfect material for transmission and distribution of electrical power. However, thanks to its higher cost and non-availability, it's rarely used for these purposes. Now-a-days the trend is to use aluminum in situ of copper.

2. Aluminum: Aluminum is reasonable and lightweight as compared to copper but it's much smaller conductivity and lastingness. The relative comparison of the 2 materials is briefed below:

(i) The conductivity of aluminum is 60% that of copper. The smaller conductivity of aluminum means for any particular transmission efficiency, the X-sectional area of the conductor must be larger in aluminum than in copper. For an equivalent resistance, the diameter of the aluminum conductor is about 1·26 times the diameter of the copper conductor. The increased X-section of aluminum exposes a greater surface to wind pressure and, therefore, supporting towers must be designed for greater transverse strength. This often requires the utilization of upper towers with the consequence of greater sag.

(ii) The precise gravity of aluminum (2·71 gm/cc) is less than that of copper (8·9 gm/cc). Therefore, an aluminum conductor has almost one-half the load of the equivalent copper conductor. For this reason, the supporting structures for aluminum needn't be made so strong as that of copper conductor.

(iii) Aluminum conductor being light, is susceptible to greater swings and hence larger cross-arms are required.

(iv) Due to lower tensile strength and higher co-efficient of linear expansion of aluminum, the sag is greater in aluminum conductors.

Considering the combined properties of cost, conductivity, tensile strength, weight etc., aluminum has a foothold over copper. Therefore, it's being widely used as a conductor material. It's particularly profitable to use aluminum for heavy-current transmission overhead line design where the conductor size is large and its cost forms a serious proportion of the entire cost of complete installation.

3. Steel cored aluminum: thanks to low lastingness, aluminum conductors produce greater sag. This prohibits their use for larger spans and makes them unsuitable for long distance transmission. So as to extend the lastingness, the aluminum conductor is reinforced with a core of galvanized steel wires. The composite conductor thus obtained is understood as steel cored aluminum and is abbreviated as ACSR(aluminum conductor steel reinforced).

Steel-cored aluminum conductor consists of a central core of galvanized steel wires surrounded by variety of aluminum strands. Usually the diameter of both steel and aluminum wires is that the same. The X-section of the 2 metals are generally within the ratio of 1 : 6 but are often modified to 1 : 4 so as to urge more lastingness for the conductor. The figure below shows steel cored aluminum conductor having one steel wire surrounded by six wires of aluminum. The results of this composite conductor is that steel core takes a greater percentage of mechanical strength while aluminum strands carry the majority of current. The steel cored aluminum conductors have the subsequent advantages:

(i) The reinforcement with steel increases the lastingness but at an equivalent time keeps the composite conductor light. Therefore, steel cored aluminum conductors will produce smaller sag and hence longer spans are often used.

(ii) Thanks to smaller sag with steel cored aluminum conductors, towers of smaller heights are often utilized in overhead line design.

4. Galvanized steel: Steel has very high lastingness. Therefore, galvanized steel conductors are often used for very long spans or for brief line sections exposed to abnormally high stresses thanks to climate. They need been found very suitable in rural areas where cheapness is that the main consideration. Thanks to poor conductivity and high resistance of steel, such conductors aren't suitable for transmitting large power over an extended distance. However, they will be wont to advantage for transmitting a little power over a little distance where the dimensions of the copper conductor desirable from economic considerations would be too small and thus unsuitable to be used due to poor mechanical strength.

5. Cadmium copper: The conductor material now being employed in certain cases is copper alloyed with cadmium. An addition of 1% or 2% cadmium to copper increases the lastingness by about 50% and therefore the conductivity is merely reduced by 15% below that of pure copper. Therefore, cadmium copper conductor are often used for exceptionally long span overhead lines. However, thanks to the high cost of cadmium, such conductors are going to be economical just for lines of small X-section i.e., where the value of conductor material is relatively small compared with the value of supports.