UNDERGROUND CABLES

 Introduction Insulating materials Types of cables Laying of cables Electrostatic stress in a single core cable Dielectric loss and loss tangent of a cable Cables for D.C transmission Heating of cables Testing Underground Cables –  Extracts from Indian Standards OBJECTIVE TYPE QUESTIONS

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UNDERGROUND CABLES- OBJECTIVE TYPE QUESTIONS

1. Void formation occurs in
1. XLPE cables
2. Oil-filled cables
3. Oil-impregnated paper cables

Ans: (c)

2.        Insulation resistance of a cable 20 km long is 1 Meg-ohm. Two cable lengths, 20 km and 10 km, are connected in parallel. The insulation resistance of the parallel combination is

1. 1 Meg-ohm
2. 0.5 Meg-ohm
3. 0.666 Meg-ohm

Ans: (c)

3.        If the voltage applied to the core and sheath of a cable is halved , the reactive power generated by the cable will be

a.        Halved

b.       1/4 th of the original value

c.        doubled

Ans. b

4.        The dielectric field intensity at a point within the dielectric of a cable

a.        Is constant

b.       Increases with increase of distance of the point from the centre of the cable

c.        decreases with increase of distance of the point from the centre of the cable

Ans. c

5.        Three insulating materials with identical maximum working stress and permittivities of 2.5, 3 and 4 are used in a single-core cable. The location of the materials with respect to the cable core will be

a.        2.5,3,4

b.       3,2.5,4

c.        4,3,2.5

d.       4,2.5,3

Ans. c

6.        Three insulating materials with breakdown strengths of 2.5,3 and 3.5 are used in a single-core cable. If the factor of safety for the materials is 5, the location of  the materials with respect to the core of the cables will be

a.        2.5,3,3.5

b.       3,2.5,3.5

c.        3.5,3,2.5

d.       3.5,2.5,3

Ans. c

7.        If   d is the loss angle of a cable , its power factor is

a.        sind

b.       cosd

c.         p .f. is independent of  d

d.       p. f depends on d but not as in ' a' or 'b'

Ans. a

8.        Match List A with List B.

List A                                                                                  List B

Voltage range                                                   Critical design factor for cable

I.                     up to 33 kV                                         p. Thermal instability

II.                   33-132 kV                                          q. Ionization

III.                 above 132 kV                                      r. Impulse strength

The correct matching is

a.        Ip  IIq  IIIr

b.       Iq IIp IIIr

c.        Iq IIr  IIIp

Ans. c

9.        If C1 is the capacitance between any two cores of  a 3-core cable, and C2  is the capacitance between any core and the sheath, then the measured value of the capacitance between any two cores with the third core isolated is equal to

a.        C1C2/(2C1+C2)

b.       0.5(3C1+C2)

c.        3C2

Ans.  b

10.     Sheaths are provided in cables to

a.        Provide proper insulation

b.       Provide mechanical strength

c.        Prevent ingress of moisture

TOP

A considerable amount of transmission & distribution, especially in urban areas is carried out by means of underground cables. In order to preserve amenities of both town and countryside the electricity supply authorities resort to underground transmission & distribution. Underground transmission is more expensive than the overhead alternative.

Dielectric properties of cable insulation:

1.        high insulation resistance

2.        high dielectric strength

3.        good mechanical properties

4.        immune to attacks by acids & alkalies

5.        non-hygroscopic

Commonly used insulating materials are:

a)       Oil-impregnated paper

b)       Vulcanized India rubber(V.I.R)

c)       Polyvinyl chloride(P.V.C)

d)       SF6 gas

Void formation

Voids (small pockets of air or gas) are formed in the insulation where constituent parts of the cable are expanded and contracted to different extents with heat evolved on load cycles. The stress across the voids is high and breakdown results.

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1.        Single-core cable

2.        Three-core cable

(a)     Belted -type construction

(b)     H-type construction

3.        Oil-filled cable

4.        Gas-filled cable- consisting of a conductor supported in a rigid external pipe which is filled with a gas under pressure- usually SF6 at 3* atmospheric pressure

5.        XLPE cables

a)       Direct in the soil

b)       In ducts or troughs

c)       In circular ducts or pipes

d)       In air

The potential gradient is maximum at the surface of the conductor. Potential gradient art any point at a distance x from the centre of the conductor is

G= V/ [x ln (R/r)]

Where R is the inner radius of the sheath and, and r is the radius of the conductor.

The dielectric loss, due to leakage and hysterisis effects in the dielectric, is usually expressed in terms of the loss angle,d:

d = 90- f

where f is the dielectric power factor angle.

Dielectric loss  = w C V2  tan d,

Where

C= capacitance to neutral

V= phase voltage

A typical value of tan d lies in the range 0.002 to 0.003. In low voltage cables the dielectric loss is negligible, but is appreciable in EHV cables.

Owing to the absence of periodic charging currents with direct voltage, high voltage cables will play an increasingly important role in D.C transmission links. In A.C cable a power factor of 0.003 can be represented by a loss resistance of 3*10 12 ohm -cm. The D.C resistivity of the same dielectric would be greater than 10 14  ohm-cm. Hence the loss in the dielectric on D.C. is only about 3 % of that on A.C. Whereas the electric stress distribution in A.C cables is determined by the dielectric capacitance, in D.C cables it is determined by the electric resistance of the dielectric. The electric resistivity of the conventional dielectrics is very temperature dependent; for oil-impregnated cable, for example, the resistivity at 20 deg. C is 100 times that at 60 deg. C. In D.C cable, thermal considerations not only determine the rating but also influence the electric stress distribution in the dielectric. The electrical resistivity also varies with the electric stress. Instead of electric stress decreasing through the dielectric from the conductor to the sheath, in D.C cables the stress increases and can be larger at the sheath. This is known as stress inversion and can lead to troubles at terminations and joints where the longitudinal stresses are created.

Explain the phenomenon of void formation in cables? Why is the void subjected to excessive potential gradient?

Void formation does not take place in oil-filled cable -why?

The temperature rise of cable depends on the following factors:

1.        The production of heat within the external periphery of the cable.

2.        The conveyance of the heat as far as the periphery - that is, up to the boundary of the surrounding medium

3.        The conveyance of the heat through this medium, and therefore away from the cable.

4.        The current rating of the cables.

5.        The nature of the load, i.e. whether continuous or intermittent; not infrequently the rating under short-circuit conditions has to be considered.

Heat production

Within the cable, there are three sources of heat:

1.        I2 R loss in  conductors

2.        Dielectric loss

3.        Sheath & armour loss

What is the equivalent circuit for calculating sheath losses?

Why is cross bonding of sheaths done?

The limiting factor in current rating is the temperature to which the insulation nearest the conductor can be raised without suffering deterioration.

The allowable values of temperature rise for different types of cables may be obtained from the manufacturer's data books.

The current rating I of a cable neglecting dielectric losses is given by

I = SQRT[ ( q-qa)/{nR {S1 + (1+l)(S2 +G)}}]   , Amp.

Where

q= Core temperature

qa = ambient temperature

n = number of conductors

R = Resistance of each conductor

l = Sheath loss/core loss

S1 = thermal resistance of the dielectric

S2 = thermal resistance of the protective covering

G = thermal resistance of the ground

·         Write down the expressions for computing the various thermal resistance components.

·         What is the effect of temperature on dielectric loss? Modify the formula for current rating considering the effect of dielectric loss

·         Discuss the factors affecting the short-circuit rating of an underground cable

·         Name the types of cable used for different voltage levels

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Type Tests – Tests carried out to prove conformity with the specifications. These are intended to prove the general qualities and design of a given type of manufactured item.

Routine Tests-Tests carried out on each part/item manufactured to check parameters (as per requirements0, which are likely to vary during production.

Acceptance Tests- Tests carried out on samples taken at random from offered lot of manufactured item for the purpose of acceptance of lot.

PVC INSULATED CABLES up to and including 1.1 kV  [IS: 1554(part-1)-1988]

# TYPE TESTS

 No. Type test Purpose a Tests on conductor 1.        Annealing test (for copper) 2.        Tensile test (for Aluminium) 3.        Wrapping test  (for Aluminium) 4.        Resistance test To check softness of wire To check strength of Al wire To check hardness of Al wire   To check cross-section of the conductor b Tests for armouring wires/strips To check electrical , mechanical and chemical properties of armouring wire/strip c Test for thickness of insulation and sheath To check capability of insulation  to withstand voltage and its mechanical strength d Physical test for insulation & sheath 1. Tensile strength & elongation at break To check mechanical stress and strain during manufacturing and bending 2. Ageing in air oven To check physical & chemical changes  in insulation  due to heat with age 3.  Shrinkage test To prevent problem in termination 4.  Hot deformation To check resistance against deformation due to heat & mechanical pressure 5.  Loss of mass in air oven To check physical & chemical changes  in insulation due to heat  and time 6.  Heat shock test To check ability of cable against overheating 7. Thermal stability To check thermal effect e Insulation resistance test To check uniformities of  insulation in dielectric f High voltage test(Water immersion test) To check ability of cable in water during service g High voltage test at room temperature To check ability of cable against high voltage  during service h Flammability test To check flame retardant properties

OPTIONAL TYPE TESTS

 No. Optional type test Purpose a Cold bend test To check effect of low temperature  during  bending b Cold impact test To check effect of low temperature on outer sheath in terms of hardness & softness c Armour resistance test (or other than mining cables To check  electrical properties of armouring  wire/strip

# ROUTINE TESTS

 No. Test Purpose a Resistance test To check cross-section of the conductor b High voltage test at room temperature To check ability of cable against high voltage  during service c Armour resistance test (for mining cables) To check conductivity of  armouring materials

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# ACCEPTANCE TESTS

The following type tests are taken as acceptance tests: Type test Nos. ,  a1, a2, a3, a4,c,d1, e, and g

## SCALE OF SAMPLING

 No. of Drums in a Lot No. of Drums  to be taken as sample Permissible No. of Defectives Up to 50 2 0 51 to 100 5 0 101 to 300 13 0 301 to 500 20 1 501 and above 32 2

CROSS –LINKED POLYETHYLENE INSULATED PVC SHEATHED CABLES [XLPE from 66 to 220 kV][IS: 7098 (Part-3)-1988]

TYPE TESTS:

 No. Type Test Purpose a Tests on conductor 1. Annealing Test (for Cu) To check softness of wire 2. Resistance Test To check cross-section of the conductor b Physical tests on insulation 1. Test  for thickness & dimensions of insulation To check capability of insulation to withstand voltage and its mechanical strength 2. Tensile strength & elongation at break To check mechanical stress & strain during manufacturing & bending 3. Thermal ageing in oven To check physical & chemical changes  in insulation  due to heat with age 4. Hot set test To check cross-linking of insulating material 5. Shrinkage test To prevent problem in termination 6.Void & contaminants test To check voids & contaminants c Resistivity test for semi conducting layers To check resistance of semi-conducting layer d Test for concentric metallic screen i)         Test for concentric  metallic screen ii)       Test for concentric copper tape To check capacity against short circuit e Thickness of  metallic sheath To check capability of insulation  to withstand voltage and its mechanical strength f Tests for armouring material 1. Dimensions To check that dimensions are within limits 2.Tensile strength & elongation at break To check mechanical stress and strain during manufacturing and bending 3. Wrapping test To check mechanical strength during bending 4. Resistivity test To check resistance of armouring material g Physical tests for outer sheath 1 Measurement of thickness To check mechanical strength 2 PVC Sheath To know the material used 1. Tensile strength & elongation at break To check mechanical stress and strain during manufacturing and bending 2.  Thermal Ageing in air oven To check physical & chemical changes  in sheath  due to heat with age 3. Loss of mass To check physical & chemical changes  in insulation due to heat and time 4. Heat shock test To check ability of cable against overheating 5. Hot deformation test To check resistance against deformation due to heat & mechanical pressure 6. Shrinkage test To prevent problem in termination 7. Thermal Stability To check thermal effect 3. PE SHEATH To know the material used 1. Carbon black content To know the % of carbon 2. Tensile strength & elongation at break before & after ageing To check mechanical stress and strain during manufacturing and bending 3. Hot deformation To check resistance against deformation due to heat & mechanical pressure h Flammability test (for PVC outer sheathed cable only) To check flame retardant  properties j Water tightness test To check penetration of water in cable k 1. Thermal ageing on complete cable sample To check physical &chemical changes  in cable due to heat with age 2. Tensile strength & elongation at break for insulation & outer sheath To check mechanical stress and strain during manufacturing and bending 3. Resistivity test for semi- conducting layers To know resistance of semi-conducting layer m Bending test followed by P. D. test To check bending radius during bending while installation & handling n Dielectric power factor measurement at ambient temperature To check rupturing capacity & voids p Dielectric power factor measurement at elevated temperature To check impurities & voids q Load cycle test followed by P.D measurement To check capacity of cable  under loading conditions r Impulse withstand test followed by HV test To check ability of insulating material to withstand  lightning voltage NOTES: Tests from  (n) to  ( r ) shall be performed  successively  on the same test sample of complete cable, not less than 10 m length between test accessories Tests at (p) and (q)  may be carried out on different samples.

OPTIONAL TYPE TEST:

 No. Type  Test Purpose 1 Cold impact test for outer sheath To check effect of low temperature  on outer sheath  in terms of hardness & softness

ROUTINE TESTS

 No. Routine  Test Purpose a Conductor resistance test To check cross-section of the conductor b P. D. test To check small voids and cavities in insulation c HV test To check ability of cable in service

## ACCEPTANCE TESTS-

No.

Acceptance   Test

Purpose

1

Measurement of capacitance

To check impurities  & voids

## The following Type Tests will be used as Acceptance Tests:

a1,a2, b1, b4,b6,e, g1

b, c

## SCALE OF SAMPLING

 No. of Drums in a Lot No. of Drums  to be taken as sample Permissible No. of Defectives Up to 25 3 0 26 to 50 5 0 51 to 100 8 0 101 to 300 13 1 301 and above 20 1

### DRUMS FOR EECTRIC CABLES [IS: 10418-1982]

The tests under TYPE, ROUTINE and ACCEPTANCE categories are not specified in the Indian Standards. However, the following checks shall be made on DRUMS & their components.

CHECKS FOR CONSTRUCTION OF DRUM:

 S. No. Description Purpose 1 Mechanical strength (a)     Transverse loading test (b)     Impact test (c)      Barrel batten test 2 Flange & outside surface Free from protruding materials or Projections or unevenness capable of damaging the cable/hands 3 Flanges (Main Discs)  construction a)       For dia. Up to 1600mm- 2 ply OR 3 ply construction. b)       For dia. Above 1600 mm- 2 full ply OR 3 full ply plus  1 segmental layer construction (Segments shall not be less than six) Width of middle plank (Minimum) For flange dia up to 700mm-             100mm Dia 701 mm-1600mm-                          150 mm Dia above 1600 mm-                            200 mm 4 Barrel end- supports Shall be complete circular discs or of various segments. Securely fixed to inside of flanges by nailing 5 Barrel middle-supports Shall be complete circular construction of single/two ply layers (at 90 0) OR of various segments (Only for drums having transverse above 1000 mm). 6 Stretchers (Core carrier planks) To be provided for drum sizes of 1206 mm and above 7 Tolerances in mm mm Drum flange dia ,up to & including 1600 mm Above 1600 mm +/- 20 +/-30 Flange thickness up to & including 1600 mm Above 1600 mm +/- 06 +/-09 Barrel dia. up to & including 1600 mm Above 1600 mm +/- 20 +/-30 Overall & transverse widths +/- 25 Barrel battens thickness +/-3 Stretchers thickness +/-3 Centre hole dia. with bush 0 to+2 Centre hole dia. without  bush 0 to +5

Other standards are:

CROSS –LINKED POLYETHYLENE INSULATED PVC SHEATHED CABLES[XLPE up to 3.3 kV][IS:7098 (Part-1)-1988]

CROSS –LINKED POLYETHYLENE INSULATED PVC SHEATHED CABLES[XLPE from 3.3to 33 kV][IS:7098 (Part-2)-1988]