Underground Cables and Electrical Transmission System.

1. WHY???
CONSTRUCTION
INSULATION
CLASSIFICATION
LAYING
SINGLE-CORE CABLES
GRADING OF CABLES
3-CORE CABLES
TYPES OF FAULTS
TESTING OF CABLES

3.  Affected by ice, snow,
rain, wind, dust,smoke
or fog.
 Affected by Ice storms,
Tornadoes,Hurricanes.
 High maintenance
costs.
 Value of land and
buildings gets affected.
 Greater faults, bumps
on power system.
 Higher voltage drops.
 Greater security of supply,
particularly in bad weather.
 Lower transmission losses,
Virtually no maintenance.
 No noise or air pollution
(due to “corona
discharge”).
 No electric field and lower
magnetic field outside the
right of way.
 Landscape can be
reinstated.
 Long planning delays can
by avoided.
MAIN

4. 33KV XLPE CABLE FROM ELAND CABLES

5. Conductor : Class 2 stranded plain copper conductor to BS
Conductor Screen : Semi-conducting material
Insulation : XLPE (Cross-Linked Polyethylene)
Insulation Screen : Semi-conducting material
Metallic Screen : Individual and overall copper tape screen to
BS6622
Filler : PETP (Polyethylene Terephthalate) fibres
Separator : Binding tape
Bedding : PVC (Polyvinyl Chloride) Type TM1 to BS7655
Armouring :Galvanised steel wire.
Single Core : AWA (Aluminium Wire Armoured)
Multi-Core: SWA (Steel Wire Armoured)
Sheath : PVC (Polyvinyl Chloride) Type TM1 to BS7655
Voltage Rating : 19000/33000V
Temperature Rating : 0°C to +90°C

6.  TheVarious Parts Are:-
Cores or Conductors: The conductors are made of tinned copper or aluminium
and are usually stranded in order to provided flexibility of the cables.
Insulation: Thickness of insulation depends on the voltage to be withstood by
the cable.
Metallic sheath: It protects the cable from moisture, gases or other damaging
liquids in soil & atmosphere, a metallic sheath of lead or aluminium is provided.
Bedding: It protects the metallic sheath against corrosion and from mechanical
injuries due to armouring.
Armouring : It protects the cable from mechanical injury while laying.
Serving : It protects armouring from atmospheric conditions, a layer of fibrous
material similar to bedding is provided over the armouring.
MAIN

7. CHARACTERISTICS
• High insulation resistance
• High dielectric strength
• Non Hygroscopic
• High mechanical strength
• Non inflammable
• Non affected by acids and
alkalies
• Low cost
Types of insulating
materials
• Rubber
• Vulcanised India rubber
• Impregnated paper
• Varnished Cambric
• Polyvinyl chloride (PVC)
• Crosslinked polyethylene
(XLPE)
MAIN

8. 1. Low voltage (L.T.) cable (operating
Voltage up to 1 KV
3.Super tension (S.T)
Cable (operating voltage
Up to 33 KV.)
5.Extra super voltage
Cable (operating voltage
up to 132 KV.
4. Extra super tension
(E.H.T.) cable (operating
Voltage up to 66KV.
2. High voltage (H.T)
Cable (operating voltage
Up to 11 KV)
On the basis of insulating material
On the basis of Operating Voltage.

9. Belted cables -> upto 11 kV
-The cores are generally stranded and may be of non-
circular shape to make better use of available space.
-Suitable for low and medium voltages as electrostatic
stresses developed are mostly radial.
-At high voltages-tangential stresses set up leakage
current-that cause
heating-breakdown
of insulation.

10. Screened Cables ->from 22 to 66 kV
 Each conductor insulated with
impregnated paper, covered
with a metallic screen of
aluminium foil.
 Instead of paper belt, the three
cores are wrapped with a
conducting belt(copper woven
fabric tape) then lead sheath at
earth potential , due to which
electrical stresses are radial in
nature keeping the dielectric
losses to minimum.
 The metallic screen avoids the
formation of voids & increase
heat dissipation.

11. Screened Cables ->from 22 to 66 kV
 Each core is insulated with
impregnated paper and
each one is then covered
by separate lead sheath.
 Due to individual lead
sheath, core to core fault
possibility gets minimised.
 Due to absence of overall
lead sheath, bending of
cable is easy.
 Thinner-require greater
care.

12. Advantages:
1. Formation of voids and
ionisation are avoided
2. Allowable temperature
range and dielectric
strength are increased
3. If there is leakage, the
defect in lead sheath is
at once indicated and
the possibility of earth
faults is decreased.
Disadvantages:
1. High initial cost
2. Complicated system of
laying
OIL DUCT
Oil Pressure is 0.2
Kg/cm2
They can range up to
345KV
Pressure cables

13. Pressure cables
-The cable is laid in gas-tight steel
pipe which is filled with
dry nitrogen gas at 12 to 15 atmospheres.
-The thickness of lead sheath is 75% of that
of solid type cable.
Advantages:
1. They carry more load current.
2. They operate at higher voltages than a
normal cable.
3. Maintenance cost is small.
4. Nitrogen gas helps in quenching any
flame.
5. No reservoir or tanks required.
6. The power factor is improved.
Disadvantage:
1. Overall cost is very high.
MAIN

14. The reliability of underground cable network
depends upon the proper laying.
A trench of about 1.5 metres deep and 45 cm wide
is dug. A cable is laid over a sand bed (10 cm) and is
covered with concrete material and bricks in order
to protect it from mechanical injury.
Gives the best dissipating conditions beneath the
earth. It is clean and safe method.
Localisation of fault is difficult. It can be costlier in
congested areas where excavation is inconvenient.
The maintenance cost is high.

15. -In this conduit or duct of concrete Is laid in ground with main holes at
suitable positions along the cable route.
-The cables are then pulled into positions from main holes.
This method is suitable for congested areas where excavation is expensive
and inconvenient.
-Less chance of fault due to strong
mechanical protection.
-Initial cost is very high.
-Heat dissipation condition are
not good.

16.  In this system the cable is laid in open pipes or troughs
dug out in earth along the cable route.
 The troughing is of cast iron or treated wood.
 Troughing is filled with a bituminous after cable is laid.
-Provides good mechanical strength.
-Has poor heat dissipation conditions.
-Require skilled labour and favorable weather conditions.
-Very much expensive.
MAIN

17. In these cables, the leakage current flows
radially from centre towards the surface.
The resistance offered by cable to path of
the leakage current is called an insulation
resistance. So to calculate the insulation
resistance consider an elementary section
of the cylindrical cable of radius x and the
thickness dx.
Let d =Diameter of the core / conductor
r = Radius of the core
D= Diameter of the sheath
R= = Radius of cable with sheath
As the leakage current flows radially outwards, the length along which the
current flows in an elementary ring is dx. While the crossectional area
perpendicular to the flow of current depends on the length of cable=(2πx)* l

18. Hence the resistance of this elementary cylinder shell is :
The total insulation resistance of the cable can be obtained by integrating the
resistance of an elementary ring from inner radius up to outer radius i.e. r to R.
The value of Ri is always very high. The expression shows that the insulation
resistance is inversely proportional to its length. So as the cable length increases,
the insulation resistance decreases.

19. A single core cable is equivalent to two long co-axial cylinders.The inner
cylinder is the conductor itself while the outer cylinder is the lead sheath.
The lead sheath is always at earth potential.
Let d = Conductor diameter, D =Total diameter with sheath
Let Q = Charge per meter length of conductor in coulombs
ε = Permittivity of material between core and sheath
Now ε = εo εr
Now, electric flux density at a
distance x meters from the axis,
D= C/m2
The value of electric intensity,
E = = V/m

20. dV = E. dx Volts
V = = = = V/m
Therefore the capacitance of the cable is given by C= F/m
Thus substituting the values of known constants we get C=

21. The electrical stress in insulation is the electric
field intensity acting at any point P in
insulation.
The stress is maximum at the surface of the
conductor i.e. when x = r.

22. Similar the minimum stress will be at the
length i.e. x = R hence
The ratio of maximum and minimum stress is,
NEXT

23. Now the value of ∂gmax/∂d must be zero
to get minimum gmax.Therefore..
The value of minimum gmax is,
=>
When the voltageV and sheath diameter D are fixed, the only parameter to be
selected is the core diameter d. So d should be selected for which value is minimum.
The value of will be minimum when ∂gmax/∂d = 0
Most Economical Conductor Size
MAIN

24. The unequal distribution of stress has two effects,
1. Greater insulation thickness is required, which increases the cost and size.
2. It may lead to the breakdown of insulation.
Hence the grading of cables is done.
There are two methods of grading the cables:
Capacitance Grading
The process of achieving uniformity in the dielectric
stress by using layers of different dielectrics.
Suppose there are three dielectrics of outer
diameter d1 d2 and D and relative
permeability ἐ1 > ἐ2 > ἐ3 and they are worked
at the same maximum stress.Then-
ἐ1d = ἐ2d1 = ἐ3d2

25. Potential difference at inner layer is-

26. Intersheath Grading
In this method of grading, in between the
core and the lead sheath number of metallic
sheaths are placed which are called
intersheaths.
Consider a cable with core diameter d and
overall diameter with lead sheath as D. Let
two intersheaths are used having diameter
d1 and d2 which are kept at the potentials
V1 andV2respectively.
Maximum stress between
core and the first intersheath
is:
Similarly,

27. Since the dielectric is homogeneous, the maximum stress in each layer
is the same i.e,
g1max = g2max = g3max = gmax (say)
Disadvantages
-Complications in fixing the sheath potentials.
-Danger of damage to intersheaths while transporting
and installation.
-Losses due to charging current.
MAIN

28.  In underground cables capacitance becomes much more important than
what it is in over head system because:
Conductors are nearer to each other and to the earthed sheath.
 They are separated by a dielectric of permittivity much greater
than that of air.
The capacitances are shown in the Fig.
Cc Cc
Cc
Cs
Cs
Cs

29. The core to core capacitances are
denoted as Cc while core to sheath
capacitance are denoted as Cs.
The core to core capacitances Cc are
in delta and can be represented in the
equivalent star as shown in the Fig.
The impedance between core 1 and the star point, Z1 can be obtained as,

30. O
CN = C1 + 3Cc
CN = C1 + 3Cc CN = C1 + 3Cc
C1 Cs
Cs
C1
Cs
C1
If star point is assumed to be at earth potential and if sheath is also earthed then
the capacitance of each conductor to neutral is,

31. IfVph is the phase voltage then charging current per phase is,
MAIN

32. The three cores are bunched together and the capacitance is measured between
the bunched cores and the sheath.
It eliminates capacitor Cc leaving Cs in parallel.Therefore:
C1=3Cs
Or Cs=C1/3
In this two cores are bunched with the sheath and the capacitance
is measured between them and the third core.
If C2 is measured capacitance, then
C2= 2Cc + Cs
Cs is found from first test.
C2 can be experimentally , hence Cc can be determined.
C1
C2
Sheath
Cores

33. If value of Cn=(Cs+3Cc) is desired, there’s another test.
The capacitance between the two cores or lines is measured with the third
core free or connected to the sheath.
This eliminates one of the capacitors Cs so that C3 is the measured
capacitance, then
C3 = Cc + Cc/2 + Cs/2
= (Cs + 3Cc)/2
=Cn /2 C3

34. Cables are generally laid directly in the ground or in the ducts , for this
reason there are little chances of faults in underground cables.
Open-circuit fault- Occurs when there is a break in the
conductor of a cable. It can be checked by a megger.
Short-circuit fault- Occurs when two conductors of a multi-core
cable come in electrical contact with each other.
Earth fault- Occurs when conductor of a cable comes in contact
with the earth.