1. POWER SYSTEM-II (III-1)
CABLES
MASARATH UNNISA
ASSISTANT PROFESSOR
EEE

2. ELECTRICAL CHARACTERSTICS OF CABLES ELECTRICAL STRESS
INTRODUCTION
CONDUCTORS
TYPES OF CONDUCTORS
WIRES
TYPES OF WIRES
WIRING APPLIANCES
CABLES
ELECTRICAL CHARACTERSTICS OF CABLES
ELECTRICAL STRESS
GRADING OF CABLES
CAPACITANCE GRADING
DIFFICULTIES OF CAPACITANCE GRADING

3. CONDUCTORS
A conductor has many free electrons so is good at transferring electrical current -
High Resistance
Low Resistance
Good Conductor
Bad Conductor
Conductance is the opposite of resistance
It is measured in ‘Mho’s (ohm backwards) ℧

4. DIFFERENT TYPES OF CONDUCTORS
Material Used
Aluminum
Copper
High conductivity
60% conductivity of copper
Easily soldered
Heavier & more expensive than aluminum
Cheap & lighter than copper
Copper used in house wiring
1mm2, 1.5mm2
4mm2 , 6mm2
Galvanized Iron (GI)
Heavier than aluminum
Lowest conductivity
Used in overhead lines

5. DIFFERENT TYPES OF CONDUCTORS
Respective of their property
Good Conductors
Bad Conductors
Low resistance
Medium resistance
Carry current
Used for converting electrical energy into heat, light & sound
Copper & Aluminum
Tungsten & Nichrome
Non Conductors
Wires & cables use conductors & non conductors to their advantage
High resistance
Insulators
PVC, glass

6. DIFFERENT TYPES OF CONDUCTORS
Physical Appearance
Solid Conductor
Used in cables.
e.g. copper, aluminum, steel
Stranded Conductor
Flexible
1.13 to 3.73 mm diameter
1, 7, 19, 37 stands
Multi stranded Conductor
0.2 or 0.3 mm diameter
14, 22, 24,84 strands
Flexible Conductor
14, 23, 40 strands
<0.2 mm diameter

7. WIRES & CABLES Wires & Cables are purpose built conductors
The size & type of wire/cable must suit the power rating required for their use. The higher the power the thicker the wire/cable
Wires
Domestic & small industry wiring
In appliances
Cables
Small & big industries
Distribution Lines
Transmission lines

8. TYPES OF WIRES Vulcanized India Rubber (VIR)
To protect against corrosion from the VIR
Cotton tape & cotton braiding
tinned copper/ aluminum
suitable for: low & medium voltage supply only
Bitumen
Vulcanized India Rubber (VIR)
Old type: not readily available to purchase

9. TYPES OF WIRE Cabe Tyre Sheath wire (CTS) tinned copper
Thicker Rubber/plastic
Old type: not readily available to purchase
Rubber/plastic .
Don’t absorb moisture
Available in 250/440V only

10. TYPES OF WIRE PVC Wire copper/ aluminum Widely used Long life
Durable against water, heat, oil, UV light
Polyvinyl chloride (PVC)
Available in 600, 660, 1100 Voltage

11. WIRING APPLIANCES Returns current to power source
Neutral
Returns current to power source
What do each of these wires do?
Live
Provides current to appliance
Earth
Takes current to ground if appliance has fault

12. CABLES Larger sized conductors Types of cable are sorted by:
Type of insulation
Type of conducting material
Cotton covered
Silk coated
Asbestos covered
Rubber coated
PVC coated
Copper
Aluminum
Mechanical protection
Voltage Grade
Their shape
Flat
Round
Unarmored
Armored
Low
High

13. ELECTRICAL CHARACTERISTICS OF CABLES
Electric Stress in Single-Core Cables
Capacitance of Single Core Cables
Charging Current
Insulation Resistance of Single- Core Cables
Dielectric Power Factor & Dielectric Losses
Heating of Cables: Core loss ; Dielectric loss and
inter-sheath loss

14. ELECTRICAL CHARACTERISTICS OF CABLES

15. Electric Stress in Single-Core Cables D= q/(2πx) E = D/ε = q/(2πεx) q: Charge on conductor surface (C/m) D: Electric flux density at a radius x (C/m2) E: Electric field (potential gradient), or electric stress, or dielectric stress. ε: Permittivity (ε= ε0. εr) εr: relative permittivity or dielectric constant.

16. r: conductor radius.
R: Outside radius of insulation or inside radius of sheath.
V: potential difference between conductor and sheath (Operating voltage of cable).
Dielectric Strength: Maximum voltage that dielectric can withstand before it breakdown.
Average Stress: Is the amount of voltage across the insulation material divided by the thickness of the insulator

17. Emax = E at x = r = V/(r. lnR/r) Emin = E at x = R = V/(R
Emax = E at x = r = V/(r.lnR/r) Emin = E at x = R = V/(R.lnR/r) For a given V and R, there is a conductor radius that gives the minimum stress at the conductor surface. In order to get the smallest value of Emax: dEmax/dr =0.0 ln(R/r)=1 R/r=e=2.718 Insulation thickness is: R-r = r Emax = V/r (as: ln(R/r)=1) Where r is the optimum conductor radius that satisfies (R/r=2.718

18. GRADING OF CABLES
Grading of cables means the distribution of dielectric stress such that the difference between the maximum and minimum electric stress is reduced.
Therefore, the cable of the same size could be operated at higher voltages or for the same operating voltage, a cable of relatively small size could be used.
TYPES OF GRADING
CAPACITANCE GRADING
INTERSHEATH GRADING

19. CAPACITANCE GRADING
This method involves the use of two or more layers of dielectrics having different permittivities, those with higher permittivity being near the conductor.
Ex =q/(2 πεo.εr .x)
The permittivity can be varied with radius x such that (ideal case):
εr = k/x
Then Ex =q/(2 πεo. k)
Ex is constant throughout the thickness of insulation.

20. In the figure shown At x=r Emax1 =q/(2 πεo. ε1r)
If all the three dielectrics are operated at the same maximum electric stress (Emax1=Emax2=Emax3=Emax) , then:
(1/ ε1r) = (1/ ε2r1) = (1/ ε3r2)
ε1r = ε2r1 = ε3r2, get r1 , r2

21. The operating voltage V is:

22. DIFFICULTIES OF GRADING
CABLE CAPACITANCE
DIFFICULTIES OF GRADING
Capacitance grading :
1- non-availability of materials with widely varying permittivities.
2- The permittivities of materials will be change with time, so the electric field distribution may change and lead to insulation breakdown.