different types of power cable and there construction

1. Power Cables
Pramod S Patil
1DB11EE029

2. INTRODUCTION
• 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.

3. CONSTRUCTION OF CABLES
• Core or Conductor: A cable may have one or more
than one core depending upon the type of service
for which it is intended. The conductor could be of
aluminum or copper and is stranded in order to
provide flexibility to the cable.
• Insulation: The core is provided with suitable
thickness of insulation, depending upon the voltage
to be withstood by the cable. The commonly used
material for insulation are impregnated paper,
varnished cambric or rubber mineral compound.
• Metallic Sheath: A metallic sheath of lead or
aluminum is provided over the insulation to protect
the cable from moisture, gases or others damaging
liquids Core Belted paper Lead sheath Bedding
Single wire armoring Overall Serving.

4. • Bedding: Bedding is provided to
protect the metallic sheath from
corrosion and from mechanical
damage due to armoring. It is a
fibrous material like jute or hessian
tape.
• Armouring: Its purpose is to
protect the cable from mechanical
injury while laying it or during the
course of handling. It consists of
one or two layers of galvanized
steel wire or steel tape.
• Serving: To protect armouring from
atmospheric conditions, a layer of
fibrous material is provided.

5. CLASSIFICATION OF CABLES
Classification Of Cables
 Based on Voltage Handle Capability
• Low tension (L.T) ----- up to 1000V
• High tension (H.T) ----- up to 11, 000V
• Super tension (S.T) ---- from 22KV to 33KV
• Extra high tension (E.H.T) cables ------- from 33KV to 66KV
• Extra super voltage cables ----- beyond 132KV
 Based on Construction
• Single Core
• Two Core
• Three Core
• Four Core

6. SINGLE-CORE LOW TENSION CABLE
• A cable may have one or more than one core depending upon
the type of service for which it is intended. It may be
(i) single-core
(ii) two-core
(iii) three-core
(iv) four-core etc.
Fig. shows the constructional details of a single-core low
tension cable. The cable has ordinary construction because
the stresses developed in the cable for low voltages (upto
6600 V) are generally small.
• It consists of one circular core of tinned stranded copper (or
aluminium) insulated by layers of impregnated paper.
• The insulation is surrounded by a lead sheath which prevents
the entry of moisture into the inner parts.
• In order to protect the lead sheath from corrosion, an overall
serving of compounded fibrous material (jute etc.) is
provided.

7. CABLES FOR 3-PHASE SERVICE
• To deliver 3-phase power either three-core cable or three single core
cables may be used.
For voltages up to 66 kV, 3-core cable (i.e., multi-core construction) is
preferred due to economic reasons. For voltages beyond 66 kV, 3-
core-cables become too large and bulky and, therefore, single-core
cables are used. The following types of cables are generally used for
3- phase service :
 Belted cables — upto 11 kV
 Screened cables — from 22 kV to 66 kV
 Pressure cables — beyond 66 kV

8. BELTED TYPE CABLES
• In these cables the conductors are wrapped with oil
impregnated paper, and then cores are assembled with
filler material. The assembly is enclosed by paper
insulating belt.
• These can be used for voltages up to 11KV or in some
cases can be used up to 33KV.
• High voltages beyond 33KV, the tangential stresses
becomes an important consideration.
• As the insulation resistance of paper is quite small
along the layer, therefore tangential stress set up,
hence, leakage current along the layer of the paper
insulation.
• This leakage current causes local heating, resulting
breaking of insulation at any moment.

9. SCREENED TYPE CABLES
• These can be used up to 33kv but in certain cases can be extended up to 66kv.
• These are mainly of two types
o H-type and
o S.L type cables
H-TYPE Cables:
• Designed by H. Hochstadter.
• Each core is insulated by layer of impregnated paper.
• The insulation on each core is covered with a metallic screen which is usually of
perforated aluminum foil.
• The cores are laid in such a way that metallic screen make contact with one
another.
• Basic advantage of H-TYPE is that the perforation in the metallic screen assists
in the complete impregnation of the cable with the compound and thus the
possibility of air pockets or voids in the dielectric is eliminated.
• The metallic screen increase the heat dissipation power of the cable.

10. S.L - Type: (Separate Lead) :
• Each core insulation is covered by its own lead sheath.
• It has two main advantages, firstly the separate sheath minimize the
possibility of core-to-core breakdown. Secondly the, bending of
cables become easy due to the elimination of over all sheath.
• The disadvantage is that the lead sheaths of S.L is much thinner as
compared to H-Type cables, therefore for greater care is required in
manufacturing.

11. SUPER TENSSION CABLES
• 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:
oInstead of solid type insulation, low viscosity oils under pressure is
used for impregnation.
oUsing inert gas at high pressure in b/w the lead sheath & dielectric

12. LIMITATIONS OF SOLID TYPE CABLES
Above cables are referred to as solid type cables because solid insulation is used
and no gas or oil circulates in the cable sheath. The voltage limit for solid type
cables is 66 kV due to the following reasons :
(a) As a solid cable carries the load, its conductor temperature increases and the
cable compound (i.e., insulating compound over paper) expands. This action
stretches the lead sheath which may be damaged.
(b) When the load on the cable decreases, the conductor cools and a partial
vacuum is formed within the cable sheath. If the pinholes are present in the lead
sheath, moist air may be drawn into the cable.
(c) In practice, voids are always present in the insulation of a cable. Under
operating conditions, the voids are formed as a result of the differential expansion
and contraction of the sheath and impregnated compound.
The breakdown strength of voids is considerably less than that of the insulation. If
the void is small enough, the electrostatic stress across it may cause its breakdown.
The voids nearest to the conductor are the first to break down, the chemical and
thermal effects of ionisation causing permanent damage to the paper insulation.

13. PRESSURE CABLE
• In these cables pressure is maintained above atmosphere either by oil
or by gas.
• Gas pressure cables are used up to 275KV.
• Oil filled cables are used up to 500KV.

14. OIL FILLED CABLES
• Low viscosity oil is kept under pressure and fills the voids in oil
impregnated paper under all conditions of varying load.
There are three main types of oil filled cables
oSelf-contained circular type
oSelf-contained flat type
oPipe Type cables

15. Pipe type oil cable

16. ADVANTAGE OF OIL FILLED CABLES
• The pressure in the cable is sustained by connecting the oil channel of the
cable to the oil tank. For maintaining the pressure, the oil channel is placed
far away from the oil reservoir. The oil pressure reduces the formation of
voids in the insulator. Oil filled cables have the following advantages over
solid cables:
• The oil filled cables have greater operating dielectric stress.
• Such type of cable has greater working temperature and greater current
carrying capacity.
• Oil filled cable has better impregnation as compared to solid cable.
• In oil-filled cable, impregnation is possible, even after sheathing.
• In such type of cable, there is no void formation.
• The size of oil-filled cable is small as compared to solid filled cables because
their dielectric thickness is less.
• In oil-filled cable, the defect can easily be detected by oil leakage.

17. GAS PRESSURE CABLES
• In these cables an inert gas like nitrogen is used to exert pressure on paper
dielectric to prevent void formation.
• These are also termed as Compression cables
• They insulated cores similar to solid type
• The cable is inserted in a pressure vessel which may be a rigid steel pipe,
commonly known as pipe line compression cable.
• The nitrogen gas is filled in vessel at nominal pressure of 1.38 * 10 exp 6 N/
square meter with a maximum pressure of 1.725 * 10 exp 6 N/ square meter.

18. ADVANTAGE OF GAS PRESSURE CABLES
• 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.
• Maintenance cost is small.
• The nitrogen in the steel tube, helps in quenching any fire or flame.
• No reservoir or tanks required.
• The power factor is improved.
• The steel tubes used make the cable laying easy.
• The ionization and possibility of voids is completely eliminated.
• The only disadvantages of this type of cables is very high initial cost.