UNDER GROUND CABLES

Present by:

LAXMI INSTITUTION OF TECHNOLOGY

Sr. no. Name Enrollment No.

1 Kulkarni Ajay A. 150863109004

1 Nakum Dharmesh M. 150863109005

2 Nayakvade Ragini B. 150863109006

3 Parmar Ashish V. 150863109007

Sub: Electrical Power Systems

A Power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed.

Flexible power cables are used for portable devices, mobile tools and machinery.

Introduction

General Constructions of Cable. Types of Cables

◦ Belted Cables◦ Screened Type Cables

H-Type cables S.L. cables

◦ Super Tension Cables◦ Oil Filled Cables◦ Gas Pressure Cables

Advantages Disadvantages

Contents

1. Conductor or Core2. Insulation3. Metallic Sheath4. Bedding5. Armoring6. Serving

General construction

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

◦ Low Tension Cables (L.T. Cables)◦ Medium and High tension Cables◦ H.T. Cables)

Types of cables

These cables are used for the voltage level up to 11 kV. The construction of belted cable is shown in fig.

◦ The cores are not in circular shape.◦ The cores are insulated from each

other by use of impregnated paper.◦ The gaps are filled with fibrous

material like jute.◦ The belt is covered with lead sheath.

Belted cables

These cables are used for the voltage levels of 22 kV and 33 kV. The two types of screened cables are:◦ H-Type cables◦ S.L. Type cables

Screened type cables

H- Type cables :◦ Designed by M.

Hochstetler and hence named H-type cable.

◦ No paper belt in this type of cable.

◦ Each conductor is insulated with a paper, covered with a metallic screen.

S.L. Cables :◦ S.L. leads for Separate

Lead Screened Cables.

◦ Each lead is insulated with an impregnated paper than covered by separate sheath lead.

Advantages :◦ Due to individual lead sheath, core to core fault

possibility gets minimized.◦ Electrical stresses are radial in nature.◦ Bending of cable is easy.

Disadvantages :◦ Uses solid insulation only like paper.◦ Mechanical shock can cause voids.

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:◦ Instead of solid type insulation, low

viscosity oils under pressure is used for impregnation.

◦ Using inert gas at high pressure in b/w the lead sheath & dielectric.

Super tension (S.t.) 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.◦ It consists of concentric

standard conductor but built around a hollow cylindrical steel spiral core, which acts as a channel for oil.

Oil filled cables

Advantages :◦ Thickness of insulation required is less.◦ Thermal resistance is less.◦ Possibility of voids is completely

eliminated.◦ Temperature range is high.◦ Perfect impregnation is possible.

Disadvantages :◦ Initial cost is very high.◦ Long lengths are not possible.◦ Oil leakage is serious problem.◦ Complicated maintenance.

An inert gas like N at high pressure is introduced lead sheath and dielectric.

Gas like SF6 is also used in cables.

Pressure is about 12-15 atmosphere.

Working power factors is also high.

Gas pressure cables

Advantages :◦ These can carry 1.5 times of the normal load current.◦ Maintenance cost is small.◦ No reservoirs or tanks required.◦ Power factor is improved.◦ Ionization and possible void completely eliminated.

Disadvantages :◦ The only disadvantage is that the initial cost is

extremely high.

The end