Type of Cable

Type of Cable

The type of a cable is basically decided based on the voltage level for which it is manufactured and the material used for the insulation such as paper, cotton, rubber etc. The classification of cables according to the voltage levels is,

1. Low tension cables (L.T. cables) : These are used for the voltage levels upto 6.6 kV. The electrostatic stresses in L.T. cables are not severe hence no special construction is used for L.T. cables. The paper is used as an insulation in these cables. Sometimes resin is also used which increases the viscosity and helps to prevent drainage.

The Fig. 6.4.1 shows the cross-section of a single core L.T. cable. It consists of a circular core of stranded copper or aluminium. The conductor is insulated by impregnated paper. Over the paper insulation, the lead sheath is provided. Then a layer of compounded fibrous material is provided. Then armouring is provided and finally covered again with a layer of fibrous compounded material. 

Many a times, L.T. cables are not provided with armouring, to avoid excessive sheath losses. The simple construction and the availability of more copper section are the advantages of L.T. single core cable.

2. Medium and High Tension Cables (H.T. cables) : The three phase medium and H.T. cables are three core cables. For voltages upto 66 kV, the three core cables i.e. multicore cables are used. These cables are classified as,

a. H.T. cables upto 11 kV level which are belted type.

b. Super Tension (S.T.) cables for 22 kV and 33 kV levels which are screened cables.

c. Extra High Tension (E.H.T.) cables for voltage levels from 33 kV to 66 kV which are pressure cables.

Let us see the constructional features of these types of three core cables.

 

1. Belted Cables

As mentioned earlier, these are used construction of belted cable is shown in the Fig. 6.4.2.

The cores are not circular in shape. The cores are insulated from each other by use of impregnated paper. The three cores are grouped together and belted with the help of a paper belt. The gaps are filled with fibrous material like jute. This gives circular cross-sectional shape to the cable. The belt is covered with lead sheath which protects cable from moisture and also gives mechanical strength. The lead sheath is finally covered by jute like fibrous compounded material.

The electric field in single core cable is radial while it is tangential in case of three core cables. Hence the insulation is subjected to tangential electrical stresses rather than radial one. The paper has good radial strength but not tangential strength. Similarly paper resistance along the radius is much larger than resistance along tangential path. The same is true for dielectric strength also. The fibrous material is also subjected to the tangential electrical stresses, for which, the material is weak. Hence under high voltage cases, the cumulative effect of tangential electrical stresses is to form spaces inside the cable due to leakage currents. Such air spaces formed inside the insulation is called void formation. This void formation is dangerous because under high voltage, spaces are ionized which deteriorates the insulation which may lead to the breakdown of the insulation. Hence the belted cables are not used for the high voltage levels. Another disadvantage of the belted cable is large diameter of paper belt. Due to this, wrinkles are formed and gaps may be developed if the cable is bended. To overcome all these difficulties, the screened type cables are used.

 

2. Screened Type Cables

These cables are used for the voltage levels of 22 kV and 33 kV. The two types of screened cables are 1. H type cables and 2. S.L. type cables.

a. H-Type Cables

The cable is designed by M. Hochstadter and hence the name given to it is H-type cable. There is no paper belt in this type of cable. Each conductor in this cable is insulated with a paper, covered with a metallic screen which is generally an aluminium foil. The construction is shown in the Fig. 6.4.3.

The metallic screen touches each other. Instead of paper belt, the three cores are wrapped with a conducting belt which is usually copper woven fabric tape. Then there is lead sheath. The conducting belt is in electrical contact with the metallic screen and lead sheath. After lead sheath there are layers of bedding, armouring and serving. The metallic screen helps to completely impregnate the cable which avoids the possibility of formation of voids and spaces. The conducting belt, the three metallic screens and lead sheath are at earth potential, due to which electrical stresses are radial in nature. This keeps the dielectric losses to minimum. Another advantage of metallic screens is increase in the heat dissipation which reduces the sheath losses. Due to these advantages, current carrying capacity of these cables increases. In special cases, the use of these cables can be extended upto the 66 kV level.

b. S.L. Cables

The name S.L. stands for separate lead screened cables. In this cable, each core is insulated with an impregnated paper and each one is then covered by separate lead sheath. Then there is a cotton tape covering the three cores together using a proper filler material. Then there are the layers of armouring and serving. The difference between H-type and S.L. type cable is that in S.L. type a common lead sheath covering all the three cores is absent while each core is provided with separate lead sheath. This allows bending of the cables as per the requirement. The construction of S.L. type cable is shown in the Fig. 6.4.4.

The three cores in this type of cable are as good as three separate single core cables.

The various advantages of S.L. type cable are,

1. Due to individual lead sheath, core to core fault possibility gets minimised.

2. The electrical stresses are radial in nature.

3. Due to absence of overall lead sheath, bending of cable is easy.

4. The dielectric which gets subjected to electric stresses is paper which is homogeneous hence there is no possibility of formation of voids.

5. Metal sheath increases the heat dissipation which increases the current carrying capacity.

A combination of H-type and S.L. type cable called H.S.L. cable also can be used.

The limitations of screened cables which are also called solid type cables are,

1. It uses solid insulation only like paper. When the conductor temperature increases, the paper gets expanded. This eventually stretches the lead sheath.

2. When the load on the cable decreases, it cools down and there is contraction of lead sheath. Due to this air may be drawn into the cable forming voids. This deteriorates the cable insulation.

3. Moisture may be drawn in alongwith the air which deteriorates the dielectric strength of dielectric.

4. Mechanical shock can cause voids. The breakdown strength of voids is much less than insulation. Hence voids can cause permanent damage to the cables.

 

3. Super Tension (S.T.) Cables

In solid type cables separate arrangement for avoiding void formation and increasing dielectric strength is not provided. Hence those cables are used maximum upto 66 kV level. The S.T. cables are intended for 132 kV to 275 kV voltage levels.

In such cables, the following methods are specially used to eliminate the possibility of void formation : 

1. Instead of solid type insulation, low viscosity oils under pressure is used for impregnation. The channels are used for oil circulation and oil is always kept under pressure. The pressure eliminates completely, the formation of voids.

2. Using inert gas at high pressure in between the lead sheath and dielectric.

Such cables using oil or gas under pressure are called pressure cables and are of two types,

a. Oil filled cables b. Gas pressure cables

 

4. Oil Filled Cables

In case of oil filled cables, the channels or ducts are provided within or adjacent to the cores, through which oil under pressure is circulated.

The Fig. 6.4.5 shows the construction of single core oil filled cable. It consists of concentric stranded conductor but built around a hollow cylindrical steel spiral core. This hollow core acts as a channel for the oil. The oil channel is filled in a factory and the pressure is maintained in the oil by connecting the oil channel to the tanks which are placed at the suitable distances along the path of the cable.

The oil pressure compresses the paper insulation, eliminating the possibility of formation of voids. When cable is heated, the oil expands but expanded oil is collected in the tank. While when cable is cooled, extra oil is supplied by the tank to maintain the oil pressure. In this type of cable the oil channel is within the conductor, hence it is called single core conductor channel oil filled cables. The other construction of the cable is similar to that of solid type cables.

Another type of single core oil filled cable is the sheath channel oil filled cable. In this type, the conductor is solid with a paper insulation. While the oil ducts are provided between the dielectric and the lead sheath. 

The construction of sheath channel oil filled cable is shown in the Fig. 6.4.6. The laying of such cables must be done very carefully.

The three core oil filled cables use the shielded type construction. The oil channels are located in the spaces which are normally occupied by the filler material. The three oil channels are of perforated metal ribbon tubing.

All the channels are at earth potential. The construction is shown in the Fig. 6.4.7. As the pressure tanks are required all along the route of these cables, the lengths of these cables are limited. Leakage of oil is another serious problem associated with these cables. Automatic signalling units are located to indicate the fall in oil pressure in any of the phases.

a. Advantages

The various advantages of oil filled cables are,

1. The thickness of insulation required is less hence smaller in size and weight.

2. The thermal resistance is less hence current carrying capacity is more.

3. The possibility of voids is completely eliminated.

4. The allowable temperature range is more than solid type cables.

5. Reduced possibility of earth fault. This is because in case of any defect in lead sheath, oil leakage starts, which can be noticed before earth fault occurs.

6. Perfect impregnation is possible.

b. Disadvantages

The disadvantages of oil filled cables are,

1. The initial cost is very high.

2. The long lengths are not possible.

3. The oil leakage is serious problem hence automatic signalling equipment is necessary.

4. The laying of cable is difficult and must be done very carefully.

5. Maintenance of the cables is also complicated.

 

5. Gas Pressure Cables

In case of gas pressure cables, an inert gas like nitrogen at high pressure is introduced lead sheath and dielectric. The pressure is about 12 to 15 atmospheres. Due to such a high pressure there is a radial compression due to which the ionization is totally eliminated. The working power factors of such cables is also high.

The Fig. 6.4.8 shows the section of a gas pressure cable. The cable is triangular in shape and installed in the steel pipe. The pipe is filled with the nitrogen at 12 to 15 atmospheric pressure. The remaining construction is similar to that of solid type cable but the thickness of lead sheath is 75 % of that of solid type cable. There is no bedding and serving. The pressure cable was firstly designed by Hochstadter, Vogel and Bowden.

The triangular shape lead sheath acts as a pressure membrane. The shape reduces the weight and provides the low thermal resistance. The high pressure creates the radial compression to close any voids. The steel pipe is coated with a paint to avoid corrosion.

During heating, the cable compound expands and a sheath which acts as a membrane becomes circular in such a case. When cable cools down the gas pressure acting via sheath forces compound to come back to the noncircular normal shape. Due to good thermal characteristics, fire quenching property and high dielectric strength, the gas SF6 is also used in such cables.

a. Advantages

The various advantages of gas pressure cables are,

1. Gas pressure cables can carry 1.5 times the normal load current and can withstand double the voltage. Hence such cables can be used for Ultra High Voltage (UHV) levels.

2. Maintenance cost is small.

3. The nitrogen in the steel tube, helps in quenching any fire or flame.

4. No reservoirs or tanks required.

5. The power factor is improved.

6. The steel tubes used make the cable laying easy.

7. The ionization and possibility of voids is completely eliminated.

The only disadvantage of this type of cables is very high initial cost.

Review Question

1. With neat diagrams explain constructional features of various types of cables.

AU : Dec,-11, Marks 16