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UNDERGROUND CABLES- OBJECTIVE TYPE QUESTIONS
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
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
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
e) Cross-linked polythene(XLPC)
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.
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
Electrostatic
stress in a single core cable
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.
Grading of cables
a) Capacitance grading
b) Intersheath grading
Dielectric
loss and loss tangent of a cable
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?
Current
carrying capacity of cables
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
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]
|
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 |
|
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 |
The following type tests are taken as acceptance tests: Type test Nos. , a1, a2, a3, a4,c,d1, e, and g
|
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 |
|
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 |
||
The following Routine Tests will be used as Acceptance Tests: b, c |
||
Partial discharge test shall be carried out on full drum length |
||
|
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 |
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]