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

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

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) ℧

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

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

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

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

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

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

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

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

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

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

ELECTRICAL CHARACTERISTICS OF CABLES

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.

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

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 = 1.718 r Emax = V/r (as: ln(R/r)=1) Where r is the optimum conductor radius that satisfies (R/r=2.718

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

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.

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

The operating voltage V is:

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.