Super Tension Cables (Screened cables)

 Super Tension Cables (Screened cables)

These Cables are used for Voltages up to 33Kv. These are mainly available in two types, namely

a)  H-Type           

b)  SL type

1. H – Type Cables: The constructional details of a 3-Core, H-Type cable. It was first designed by H-Hochstadter and hence it is named as H-type cable.

Each core is insulated by layers of impregnated paper. This insulation on each core is covered with a metallic screen (Aluminium Foils). The core are laid in such away that metallic screens make contact with each other.

An additional conducting belt is wrapped round the three cores. It has no belt type insulating but it is provided with lead sheath, bedding, armouring and serving.

All the four screens (3-core screens and one conducting belt) and the lead sheath are at earth potential hence the electrical stresses are radial therefore the dielectric Losses are reduced.

Advantages:                                                   

1.      The perforations in the metallic screen will gives complete implete impregnation to the cable hence it avoids the the entry of air.

2.      Metallic screens increases the heat dissipating capacity of the cable.

2. SL Type Cable: The constructional details of a 3-core S. L. (Separate Lead) type cable. This type of cable is basically a H-type Cable but the screen round each core is replaced by its own lead sheath. Hence, three Cores can be treated as three separate single core cables.

In this type of cable there is no overall lead sheath but only armouring and serving are provided. This type of cable can be Used Upto 66kv.

Advantages of S.L Cable over H-Type:

1.      Bending of cables becomes easy due to the elimination of overall lead sheath.

2.      It reduces the core-to-core breakdown due to the use of separate sheaths.

3.      Oil leakages’ are less on hilly due to elimination of filler spaces containing compound.

Disadvantages:

1.      Manufacturing of thinner lead sheaths is very difficult.

3. H.S.L Type Cables: This type of cable is a combination of both H and S.L Type cables. The Construction of a H.S.L. Cable.

In this type of cables. Each core is insulated with impregnated paper, metalized paper layers and then it is sheathed with lead.

The three cores are then laid up and provided with filler, braided, amoured and finally served.

Advantages of Screened Cables (Super Tension Cables) Over Belted Cables (H.T. Cables)

1.      Screened cables have more core-to-core thickness for a given Overall diameter Compared to H.T. cables. Hence, possibility of core-to-core faults are reduced.

2.      Electrical stresses developed in the screened cables are uniformly radial in all sections of the dielectric.

3.      The current capacity is more because metal sheaths help in dissipation of heat.

4. The electric stresses developed is confined only to paper which is homogeneous and therefore possibility of formation of voids with in electric field is not there.