Types of Line Conductors

Types of Line Conductors

When the conductors are used in transmission system for bulk power transfer, then they should fulfill following requirements.

1. They should have low weight.

2. They should have high tensile and fatigue strength. 

3. They must have high conductivity.

4. They should have low co-efficient of expansion, low corona loss.

5. They should have less resistance and low cost.

Thus based on conductivity, tensile strength, fatigue strength, corona loss, local conditions and cost, conductors are selected for a particular line. The conductors used in practice are made up from the materials such as copper, aluminium and their alloys.

The advantages of using aluminium conductors over copper conductors are given below.

1. They have low cost.

2. Less resistance and corona loss.

3. Less weight.

But aluminium has less tensile strength, high co-efficient of expansion and large area which restricts its use alone as a conductor.

In order to increase the tensile strength of a conductor, one or more central conductors of different materials are used. These materials give high tensile strength. The different types of aluminium conductors used in power systems with full forms of their abbreviations are as given below.

AAC : - All aluminium conductor.

AAAC : - All aluminium alloy conductor.

ACSR : - Aluminium conductor with steel reinforcement.

ACAR : - Aluminium conductor with alloy reinforcement.

Normally the conductors are stranded as it possess greater flexibility and mechanical strength as compared to single wires of same cross sectional area. In stranded conductors, a central wire is surrounded by successive layers of wires containing 6, 12, 18, 24 ... wires. The consecutive layers are spiralled in opposite directions so as to avoid unwinding. This also makes outer radius of one layer coincide with inner radius of the next.

The stranded conductors are electrically in parallel and spiralled together. Due to use of stranded conductors the skin effect is reduced.

The conductor size is decided based on its current carrying ability and voltage level on which it is working. The total number of conductors in a strand of n layers are given by

Total number of conductors = 1 + 3n² (1 + n) = 3n2 + 3n + 1

Overall diameter of stranded conductor with n layer D = (1 + 2n) d 

Here d is diameter of each strand. 7 strand conductor will have one central strand with 6 outer strands. While 19 strand conductor has a central strand and two layers of 6 and 12 strands each. The size of conductor is specified by its equivalent copper cross sectional area and the number of strands with the diameter of each strand.

Now we will discuss in brief the commonly used conductors such as hard drawn copper, hard drawn aluminium and steel cored aluminium conductors. The other materials are also used for making conductors.

 

1. Hard Drawn Copper Conductor

The hard drawn copper conductors are used for overhead lines which provides high tensile strength. These conductors have relatively higher conductivity, long life and high scrap value. The copper conductors are used for distribution network where length of line is short and there are more tappings.

 

2. Steel Cored Copper Conductor (SCC)

The steel cored copper conductors are made by surrounding a steel core with one or more layers of copper strands. Due to addition of steel core tensile strength of conductor is increased.

 

3. Cadmium Copper Conductor

With addition of cadmium there is increase in the tensile strength of copper at the cost of decrease in the conductivity. Thus these conductors can be used for longer spans. As tensile strength is increased, longer spans with same sag is possible. The other advantages include easiness in jointing, more resistance to atmospheric corrosion, better resistance to wear and easy machinability. These conductors are carried by smaller supports and are subjected to low wind and ice loadings due to their smaller diameter.

 

4. Copper Weld Conductor

In this type of conductor, copper is welded on to a steel wire by hot rolling and cold drawing a billet of steel coated with copper. The uniform thickness of copper is welded. The conductivity of this conductor lies in the range of 30 to 60 % of that of solid copper conductor having same diameter. These are used for longer spans such as river crossings. 

 

5. All Aluminium Conductor (AAC)

Due to increasing cost of copper, aluminium is used in transmission system. Electrolytically refined aluminium is rolled and drawn hard for use as conductor. For a specific resistance, cross sectional area of aluminium conductor is greater than that of copper while its weight is about 50 % of that of copper conductor. This makes transportation and erection of such conductors economical. Corona effect is reduced due to increased diameter of conductor. These conductors are more used in distribution where transmission lines are short and voltage are lower. There are chances of inter phase faults due to swing if these conductors are employed in the areas where there are high winds. This is because aluminium conductors are lighter, with large conductor area and more sag.

 

6. Aluminium Conductor with Steel Reinforcement (ACSR)

The mechanical strength that is obtained from conductors made up from all aluminium. This difficulty can be overcome by adding steel core to the conductor. The cross section of this conductor is as shown in the Fig. 1.4.2 (a).

As shown in the Fig. 1.4.2 (a) there are 7 steel strands which forms central core. This is surrounded by two layers of around 30 aluminium strands. For a given resistance conductors of different strengths can be made by taking different properties of steel and aluminium areas. The steel core does not contribute to conduction of current practically. The current carrying capacity and resistance of this conductor is dependent on conductivity of aluminium.

The ACSR conductors are more commonly used as they have following advantages.

1. Due to high mechanical strength and tensile strength, the line span can be increased. The sag is small. So shorter supports are required for line. It is also possible to have longer spans for a given sag. Due to smaller supports, breakdown possibility is low. Insulator and other fittings needed are also less.

2. They have low corona loss.

3. Skin effect is less.

4. These conductors are inexpensive as compared to copper conductors having equal resistance without reduction in efficiency, useful life span and durability. 

The disadvantage with ACSR conductor is difficult to make splices and dead ends. There is also possibility of corrosion due to electrochemical action between aluminium and steel core. The service conditions decide corrosion rate. This is higher in industrial and coastal areas.

The compacted ACSR conductor or smooth body ACSR conductor is made by pressing conventional ACSR conductor through dies to flatten the aluminium strands into segmental shape. The spaces within the strands are filled while diameter of conductor is reduced. This does not affect electrical and mechanical properties of this conductor. Thus with same aluminium area, diameter of steel core is increased which increases mechanical  strength. These conductors can be used for larger span lengths. This is shown in the Fig. 1.4.3

The expanded conductors are made by adding a plastic or fibrous material between steel core and aluminium strands. This increases diameter of conductor which reduces extra high voltages. This type of conductor is shown in the Fig. 1.4.4

The filler material such as paper separates the inner steel strands from outer aluminium strands.

 

7. All Aluminium Alloy Conductor (AAAC)

The conductors made from aluminium alloys are suitable in urban areas as they provide better tensile strength and conductivity. These alloys are known with different names in various countries. Some of these alloys are costly as they are heat treated. One of the alloys of aluminium is known as silmalec which contains 0.5 % of silicon, 0.5 % of magnesium and rest of aluminium. Due to this there is improvement in conductivity and mechanical strength. 

 

8. ACSR Conductor

In such conductor, the central core is made up from aluminium alloy which is surrounded by layers of aluminium conductors. The conductivity is better and strength to weight ratio is equal to ACSR conductor having same diameter. As compared to ACSR conductor, ACAR conductor is smaller in size and lower in weight for the same electrical capacity.

 

9. Phosphor Bronze Conductor

This type of conductor is strong than copper conductor and may be used for longer line spans. The conductivity of such conductor is low which can be improved by use of cadmium-copper core. Phosphor bronze is found to be suitable for atmospheres containing harmful gases.

 

10. Alumoweld Conductor

In this type of conductor, aluminium is welded on a high strength steel wire. This is costlier as compared to steel cored aluminium (SCA) or ACSR conductor. Around 75 % conductor area is covered by aluminium. This is used in earth wires.

 

11. Galvanized Steel Conductors

This type of conductor is suitable for large length of line span or in rural areas where load requirement is comparatively smaller. This type of conductor has high strength. The conductor has large resistance, inductance and voltage drop. The disadvantage with this conductor is it has shorter life.

 

12. Thermal Resistant Aluminium Alloy Conductor (TACSR)

This conductor is similar in construction to ACSR conductor. The change is that EC grade aluminium wires are replaced by hard drawn aluminium wires of heat resistant aluminium alloy known as TAL. These wires are doped with zirconium which makes these conductors well suited for high temperatures. The various features of thermal resistant aluminium alloy conductor are,

1. Very high current carrying capacity.

2. Good mechanical and electrical properties at elevated temperatures.

3. Very stable at high temperatures.

4. Cost effective design.

5. Suited for raising the existing line capacity. 

 

13. Super Thermal Resistant Aluminium Alloy Conductor Invar Reinforced (STACIR or ZTAI)

Super thermal alloy is manufactured from aluminium - zirconium (A1 - Zr) alloy rods. The arrangement is shown in the Fig. 1.4.5. The outer layers are made up of super thermal resistant aluminium alloy wires.

These are concentrically arranged over inner core of aluminium clad INVAR (36 % Ni in steel). The current capacity of this conductor is twice that of light aluminium conductor. The load capacity of system can be increased simply by replacing existing conductors by STACIR conductors without changing the steel towers. Thus it is very cost effective and stable at high temperatures.

 

14. Gap Type Thermal Resistant Aluminium Alloy Conductor Steel Reinforced (GTACSR)

It has a unique construction having a small gap between the steel core and super thermal resistant aluminium alloy layer.

The Fig. 1.4.6 shows the construction of GTACSR. The central core is made up of extra high strength steel core. The conductor part arranged around the core is made up of thermal resistant aluminium alloy. There is a gap between inner layer of aluminium alloy and the steel core which is filled with grease to avoid the friction. The inner layer of aluminium alloy is trapezoidal in shape to maintain the gap.

This conductor offers excellent sag and current carrying characteristics. It can carry 1.6 times higher current than ACSR conductor of same size. Its cost is low and construction period is short. To increase the existing capacity, the existing conductors can be simply replaced by GTACSR conductors without changing the towers. This construction allows low sag properties and good mechanical strength 

 

15. Gap Type Super Thermal Resistant Aluminium Alloy Conductor Steel Reinforced (GZTACSR)

The construction is similar to GTACSR but the outer aluminium conductors are made up of heat resistant zirconium aluminium alloy. This makes the conductor well suited for the continuous operation at elevated temperature upto 210°C, without affecting its mechanical and electrical properties. Thus this conductor provides very stable operation at high temperatures.

The various features of GZTACSR are,

1. Can carry two times higher current than ACSR conductor of same size

2. Suitable for the continuous operation at high temperatures.

3. Low sag at high temperatures.

4. Very good mechanical and electrical properties at high temperatures.

5. Low thermal knee point.

6. Economical for increasing the overall capacity of lines, simply by replacing existing lines without changing the towers.

The Fig. 1.4.7 shows the comparison of current capacities of ACSR, GTACSR and GZTASCR conductors of various sizes.

 

16. Bundled Conductors

For high voltage transmission more than 220 kV, two or more conductors are used per phase in close proximity but not touching to each other. Such conductors are called bundled conductors. All such conductors belonging to one phase are grouped together by a metallic structure called spacers. The spacers are used to maintain constant distance between the conductors throughout the length, avoiding touching of conductors amongest themselves. 

The Fig. 1.4.8 shows bundled conductors with two conductors used per phase.

Each conducter joined by the spacer belongs to the same phase. There are three such groups of conductors in single circuit as shown in the Fig. 1.4.8 (b). In double circuit transmission, there are six groups of conductors.

a. Advantages of Bundled Conductors

1. Reduced reactance of the line due to increase in self geometric mean distance.

2. The maximum power transfer capability of the line increases.

3. There is increase in the surge impedance loading.

4. Increase in the capacitance of the line.

5. Reduces the effect of corona and radio interference.

6. Reduces voltage gradient at conductor surface.

7. The current carrying capacity of the line increases due to bundled conductors.

8. It increases effective surface area exposed to air hence it has better and efficient cooling.

9. It has reduced influence of skin effect.

The only limitation is that bundled conductors experience greater wind loading than single conductors.

Review Questions

1. Explain the various types of line conductors used for overhead transmission lines.

2. Write a note on : i) TACSR conductors ii) STACIR conductors, iii) GTACSR conductors iv) GZTACSR conductors.

3. What are bundled conductors ? State its advantages.