CABEL

Different types cables used in transmission & distribution system:

(a) For low & medium voltage lines PVC cables are used, (b) for high voltage lines XLPE and PILC cables are used (c) for extra high voltage lines XLPE, oil filled, gas filled and gas pressure or compression cables are used.

Classification of underground cables according to the voltage range:

a) Low tension cables: (i) 250-400V, (ii) 650-1100 V,

b) High tension cables: (i) up to 3.3kV, (ii) jp to 6.6kV, (iii) up to 11kV

c) Super tension cables: (i) 22kV, (ii) 33kV

d) Extra high tension cables: (i) 66kV, (ii) 132kV

e) Extra super voltage power cables: Above 132kV

P.V.C: Poly Vinyl Chloride insulated cable

P.I.L.C.: Paper Insulated Lead Covered cable

P.I.L.C.S.W.A.: Paper Insulated Lead Covered Steel Wire Armored cable

P.I.L.C.D.S.T.A: Paper Insulated Lead Covered Double Steel Tape Armored cable

XLPE: Cross Linked Poly Ethylene insulated cable.

M.I.N.D: Mass Impregnated Non Draining cable

The nominal cross section: The nominal cross section is the area of the cross section of one conductor in a plane perpendicular to its length multiplied by the number of conductors.

The actual cross section: It is area of the oblique cross section of a conductor produced by carrying the stranded conductor by a plane perpendicular to the core of the cable multiplied by the number of conductors, which is also greater than the nominal cross section.

The equivalent cross section: It is the cross section of a solid conductor of the same length as the cable and having the same resistance at the same temperature, which is less than the nominal cross section because of the increase in resistance due to spiraling.

Advantages of PVC cables in service and installation: The PVC cables are mechanically strong, flexible, light, fire resisting, non hygroscopic, non-draining, corrosion resisting, acid and alkaline resisting and have good ageing properties. It has a little effect to temporary contact with oil and liquid fuels. These cables are easy workable and can be used in any gradients.

XLPE cable and its advantages: XLPE cable means cross linked poly Ethylene insulated aluminum conductor armored cable. In XLPE cable stranded aluminum conductor is first screened in the form of a semi conducting extrusion which provides a smooth conductor surface and prevents formation of cavities at the surface of the conductor when the cable is subjected to bending. The screened conductor is insulated with extruded XLPE compound. The insulation is further screened with layer of nonmetallic semiconducting material and over that a non magnetic metallic screen in the form of copper or aluminum tape is applied. In case of multi core cables cores are laid together with suitable filler in the interstices and wrapped with PVC tapes or extruded PVC. For mechanical protection non magnetic aluminum wire/strip/tape armor for single core cables and steel wire armor for multi core cables are provided over the inner sheath. A layer of PVC/PE is extruded as outer sheath usually in black color to prevent ingress of moisture.

The special feature of the XLPE insulation is that long molecular chains of polyethylene are cross linked to each other by means of a process similar to vulcanization of rubber and thus forming a three dimensional network structure with strong bond. Pure polyethylene is a thermoplastic material .e. it becomes soft and plastic on heating and hard on cooling. Pure polyethylene is converted into thermosetting XLPE i.e. it sets permanently when heated. By cross linking process the polyethylene insulation of the cable is made thermally stable and also the melting point is greatly increased. So XLPE insulated cable has better resistivity to thermal deformation for its highest thermal tolerance property. Due to excellent thermal properties the current carrying capacity of XLP cable is higher than that of conventional cable. XLPE insulation has higher dielectric strength compared to other conventional dielectrics used for cables resulting low dielectric loss.

XLPE cable is made suitable for high voltage and extra high voltage application up to 132kV. This cable can be used for long cable routes in high voltage transmission where the dielectric losses play a major role. This cable can easily be handled due to lighter in weight than any other high voltage cable. As there is no case of oil migration from insulation it can be installed for inclined or vertical runs without any hesitation XLP insulation is highly resistant to moisture for which no special precaution is needed a the time of jointing and termination. The jointing of XLP cable needs less time than that of any other conventional cable of similar grade. These cables can safely be used on any vibrating layouts like bridges. These cables have the better resistance to salinity of water in ground, chemicals, oils and corrosive fumes. These are free from fire risk. These cables are not prone to failure due to ageing characteristics and have longer life.

Factors on which the insulation resistance of cable depends:

(i) The materials of the insulation

(ii) The thickness of the insulation and

(iii) The length of the cable.

Permissible voltage drop in cable as per I.E. rule: Permissible voltage drop for L.T. cables should be within + 6% of the declared voltage and that of H.T. cables should be within + 9% of the declared voltage.

Capacitance of a single core cable depends on (i) the relative permittivity of insulating materials in between the core and lead sheath, (ii) the diameter of the core and (iii) the inner diameter of lead sheath which is at earth potential.

The capacitance of a cable is more important than that of an overhead line due to (i) narrow spacing between the conductors and between conductors and earthed sheath and (ii) their separation by a dielectric medium of higher permittivity as compared to air.

Sheath losses: The sheath loss of a cable is a power loss occurs in the lead sheath as a result of the induced e.m.f. produced by the flux linkage with the lead sheath. When single core paper insulated lead covered and served power cables are used on a.c. system the presence of a lead sheath around each insulated conductor has an important effect on the electrical characteristics and current rating of the cable. If the lead sheaths are bonded together or earthed a voltage will be induced in the open circuited sheaths by transformer action between the conductors and the sheaths , the value of which depends on the flux interlinked with the lead sheaths and increases voltages are short circuited through the bonding resulting a flow of current along the sheath and thus dissipating energy as heat through the cable serving and surrounding earth or air. The eddy currents are also generated in the lead sheaths due to the lack of perfect symmetry in the lead sheath circuit relative to the magnetic field. Therefore the heating losses which occur in the lead sheaths are of two types e.g. (i) sheath circuit loss for bonded sheaths only and (ii) sheath eddy loss. The sheath eddy loss is generally very small compared with the sheath circuit loss and can be neglected in many cases.

Dielectric losses: The dielectric loss is a power loss which occurs in the paper insulating (dielectric) of a cable as a result of the electric field developed due to the leakage current and the reversal of current in case of a.c. supply. This loss increases with temperature. It has a direct bearing on the operation of a cable. A specific cable has a definite heat dissipating ability at its operating temperature. Therefore, any increase in dielectric loss will result in a decreased in load carrying ability. Dielectric losses are usually negligible for cables operating up to 33 kV.

Cares are to be taken for storage of cable drum: For storing of a cable drum visual inspection should be made to detect any damage suffered during transport. Manufacturer’s seals on both end of the cable should be inspected by removing the battens partially. The condition of armoring, serving, sheathing of the upper layers and the projecting end of the cable should be inspected for any signs of scoring scratches, dents, mechanical injuries or any leakage of impregnation oils. If the above visual inspection proves satisfactory the following special cares should be taken:

(i) Both ends of the cable should be protected from moisture by means of plumbed lead caps in case of PILC cables and sealed plastic caps in case of PVC and XLPE cables.

(ii) The cable drum should be stored on well drained, hard packed soil or preferably having a concrete surface which will not cause the drum to sink and so lead to flange fot and extreme difficulties in moving the drums.

(iii) The cable should be protected from direct rays of the sun.

(iv) All cables drums should be stored leaving sufficient space between them for air circulation.

(v) During storage the drum should be rolled to an angle of 90⁰ once every three months.

(vi) The cable drums must stand on battens placed directly under flanges. The drums should not be stored on the flat i.e. with flanges horizontal.

Minimum depth of a cable trench specified as follows:

Operating voltage	Depth of trench
Up to 1.1 kV	0.75 M
3.3 to 11kV	0.9 M
22 and 33kV	1.05 M
66 and 132 kv	1.2M

Proximity effect: Proximity effect means the interaction of magnetic field associated with adjacent current carrying conductors parallel to each other causing a further redistribution of the current which likewise increases the ohmic DC resistance of the conductor. This effect increases with the conductor diameter and becomes maximum when the cables are practically in contact. Therefore, this effect can also become important under certain condition of cable installation. When cables are laid parallel to metal beams, walls etc as frequently happens in buildings or ships this effect increases the apparent impedance of these cables appreciably. Ultimately proximity effect exerts influence on the current carrying capacity of the cables.

Minimum bending radius of different cables:

Voltage Rating (kV)	PILC cables		PVC & XLPE cables
	Single core	Multi core	Single core	Multi core
Upto 1.1 kV	20 D	15 D	15 D	15 D
Above 1.1to 11kV	20 D	15 D	15 D	15 D
Above 11 kV	25 D	20 D	20 D	215 D
22 & 33 kV

Where D = Outer diameter of the cable

Flux: It is an auxiliary chloride material whose purpose is to dissolve and remove oxides from the conductor surface, protect he metal surface and molten solder against oxidation and improve the ability of the solder to spread over and wet the surface.

Bitumen compound: It is a bitumen base compound of solid or semi solid material obtained by refining crude petroleum. It is (i) good adhesive, (ii) low percentage of contraction of cooling, (iii) high softening point, (iv) high flash point (250⁰C), (v) highly soluble in mineral and synthetic oil, (vi) acid and alkali resistant, (vii) easily oxidized and (viii) water resistant.

Planning cable runs

·         Use the shortest possible motor cable lengths.

·         Use a single length of cable to a star junction point to feed multiple motors.

·         Keep electrically noisy and sensitive cables apart.

·         Keep electrically noisy and sensitive parallel cable runs to a minimum. Separate parallel cable runs by at least 0.25 meters. For runs longer than 10 meters, separation should be increased proportionally. For example if the parallel runs were 50m, then the separation would be (50/10)x0.25m.

·         Sensitive cables should cross noisy cables a 90⁰.

·         Never run sensitive cables close or parallel to the motor, dc link and braking chopper circuit for any distance.

·         Never run supply, dc link or motor cables in the same bundle as the single/control and feedback cables, even if they are screened.

·         Ensure EMC filter input and output cables are separately routed and do not couple across the filter.

INCREASING MOTOR CABLE LENGTH

Because cable capacitance and hence conducted emission increase with motor cable length, conformance to EMC limits is only guaranteed with the specified ac supply filter option using a maximum cable length as specified .

This maximum cable length can be improved using he specified external input or output filters.

Screened/armored cable has significant capacitance between the conductors and screen which increases linearly with cable length (typically 200pF/m but varies with cable type and current rating).

LONG CABLE LENGTHS MAY HAVE THE FOLLOWING UNDESIRABLE EFFECTS;

Ø  Tripping on ‘Over current’ as the cable capacitance is charged and discharged at the switching frequency.

Ø  Producing increased conducted emissions which degrade the performance of the EMC filter due to saturation.

Ø  Causing RCDs (Residual Current Devices) to trip due to increased high frequency earth current.

Ø  Producing increased heating inside the EMC ac supply filter from the increased conducted emissions.

These effects can be overcome by adding chokes or output filters at the output of the VSD.

STAR POINT EARTHING:

A star-point earthing policy separates ‘noisy’ and ‘clean’ earths. Four separate earth bus bars (three are insulated from the mounting panel) connect to a single earth point (star point) near the incoming safety earth from the main supply. Flexible, large cross-section cable is used to ensure low HF impedance. Bus bars are arranged so that connection to the single earth point is as short as possible.

XLPE (CROSS LINK POLLY ETHYLENE)

ADVANTAGE

1.      Conductor operating temperature : 90⁰C

2.      Short ckt withstand temperature    :250⁰C

3.      High current rating

4.      High di-electric strength

5.      Very low di-electric losses

6.      Due to high di-electric strength, size is less compared to other.

7.      Low weight, easy for laying, easy bending ie. 20⁰.

8.      Low water absorption.

9.      Low charging current

10.  Easy jointing procedure

11.  Fire retardant

COMPARISION BETWEEN XLPE AND PILCA

SN	XLPE	PILC
1	On load 900 C Temperature	On load 700 C temperature
2	Low weight	Heavy weight
3	Defuses moisture	Absorbs moisture
4	Short circuit temperature 2500C	Short circuit temperature is 1600C
5	Fire resistance - Fair	Fire resistance - poor

CABLE CODE:

1.      Copper conductor                               Cu

2.      Aluminum conductor                          A

3.      PVC sheath & insulation                     Y

4.      Steel round wire armor                      W

5.      Steel strip armor                                 F

6.      Steel double round wire armor          WW

7.      Steel double strip armor                     FF

GENERAL TECHNICAL DATAS FOR 33KV(E) 400 sq mm XLPE CABLE

SN	DETAILS	Data
1	Name of manufacturer & address of work	Polycab wires Pvt. Ltd, HICCL, 1st fl, 731, Pandit Satwalkar marg, Mahim, Mumbai-400016
2	Applicable standard	IS:7-98(para II)1985 with latest amendments.
3	Voltage grade	33KV(E)
4	Conductor temperature at rated current	900C
5	Max. temperature during short circuit under hot condition	2500C
6	No. of cores	3
7	Cross section area of conductor	400 sq mm
8	Conductor material, hardness & conductor flexibility class	Aluminum H2/H4 grade, class II IS:8130/84
9	No. & diameter of conductor stands	57 strands, 3.06 dia
10	Shape of conductor	Compact, circular
11	Material & min. thickness of conductor screening	Extruded semi conducting compound of 0.30 mm thickness
12	Insulation material composition	Extruded XLPE
13	Min. & normal thickness of insulation	7.82mm (minimum)
14	Max. electric stress on the conductor screen	30 – 35 KV/mm
15	Material, min. thickness type of non-metallic insulation screening	Free strippable with stripper extruded, semi conductor compound of 0.30 mm thickness.
16	Material, min. thickness & width of metallic screen	Copper type 30 x 0.05 mm
17	Filler material	Suitable PVC compound
18	Min, thickness & material of inner sheath	0.70 mm, extruded PVC compound type ST-2

COMPARISION BETWEEN XLPE, PVC, PILC

SN	DATA	XLPE	PVC	PLIC
1	Specific Gravity	0.93	1.35,1.46	1.1
2	Electric loss factor(Temp. 200C)	0.0004	0.07	0.003
3	Volume resisting area 200C Aluminum & Copper	1117	1010-1015	1015
4	Max permissible operating continue temperature	900C	700C	650C
5	Max permissible short circuit temperature	2500C	1600C	1600C
6	Short time O/L temperature	1300C	1200C	800C
7	Dielectric constant at 200C	2.35	6-8	3-5
8	Power Factor at max. conducting temperature	0.008	0.1	0.01
9	Impulse level volts/min	2000	1100	1800
10	Thermal resistivity  0C/Con/Watt	350	650	550
11	Partical discharge picl con/m3	5	40	--

If = Ish/√t             Where It – Short ckt rating for 1 sec  t – directional in sec