High voltage underground cables

University of Turkish Aeronautical AssociationElectric and Electronic Engineering Department

EEE 589 SPECIAL TOPICS IN ELECTRIC & ELECTRONIC ENGINEERING

Presented by:Layth Faeq & Mahmood Natiq

Supervisor: Dr. Ibrahim Mahariq

Fall -2016

Contents Introduction

Advantages and disadvantages of cables Construction of cables

Classification of cables Calculation of high voltage cable parameters

Power losses and capacity amperage losses Extension of underground cables

Types of cable faults High voltage cables faults location methods

conclusion

IntroductionUnderground cables is one of the means used for the transmission and distribution of electric power in addition to the overhead lines.

Where these cables are placed?

Directly buried underground Placed inside the underground tracks

Placed inside the underground ducts.

these cables are considered more safer than overhead lines, where the probability of touching people or birds or animals or small metal objects with conductors carrying voltage electrode to be very rare .

Advantages & DisadvantagesAdvantages

Better general appearance

Less liable to damage through storms or lighting

Low maintenance cost Less chances of faults

Small voltage drops

Disadvantages

The major drawback is that they have greater installation cost and

introduce insulation problems at high voltages compared with equivalent

overhead system.

CONSTRUCTION OF CABLES

the general construction of a 3-conductor cable. The various parts of cable are:

1-copper conductor2 -Inner semi-conductive

3-XLPE insulation4-Outer semi-conductive

5-Semi-conductive tape6-Copper tape screen

7-filling8-PVC separation sheath

9-Galvanized double steel tape10-PVC outer sheath

Classification of cables according to-:

1 - number of cores in the cable.

2 - the type of insulating materials used in their manufacturing.

. 3 - the voltage for which they are manufactured

The number of cores in the cable.(1 _)single core cable

(2_)multi-core cable

the type of insulating material used in their manufacture

1 -Paper insulated cables

It has good electrical properties . It needs the experience and accuracy

In performance plug endings.

2-Impregnated Paper cables

3-ethylene propylene Rubber

Easier to install More flexible- Better flame resistanceIncreased thermal stability

4-Polyvinyl chloride (PVC)It is cheap. It considered the best choice until 3.3 kv. It is inappropriate for high voltage.

5 -Cross Linked poly- ethylene (XLPE)

1-This material has temperature range beyond 70-90 2-This material good insulating properties

the voltage for which they are manufactured

Low-tension (L.T.) Cables......upto 1000V Thickness of insulating = 1.5 mm

high-tension (H.T.) Cables.... 11KV Thickness of insulating = (4-5) mm

super-tension (S.T.) Cables.. 33KV Thickness of insulating = (8) mm

Extra-high tension(E.H.T) Cables.. 66KV Thickness of insulating = (16) mm

Extra-super voltage Cables.. 132KV Thickness of insulating = (23) mm

Conductor resistance of A single-core cables

L

A ᵨ

ρ =Material with resistivity

L = Length

A = Cross – sectional area

R = ᵨ L

A(ohm)

Where ᵨcu

(ohm ∕ m)(m)(m2)

= 1,724 *10-8ᵨAl

= 2,803 *10-8

Calculation of high voltage cable parameters

Insulation resistance of A single –core cables

Radius of conductor=r1Radius of sheath=r2Thickness of layer=dx

R = ᵨ2π L

ln r1r2 (ohm)

This shows that insulation resistance of a cable is inversely proportional to its length. In other word, if the cable length increases , its insulation resistance decreases and vice-versa

Capacitance of single core cable

Conductor diameter=d Inner sheath diameter=DThe charge per meter =Q coulombs

the relative permittivity of the insulation=ϵr

Permittivity=ϵ0

Vph=∫ E.dx= ∫

C=

IC= ωcVph (Amper)

Where ϵ=ϵ0 ϵr 2 πϵ ln D

d

QVph

D∕2

d∕2 Q2 πϵ x .dx =

d∕2

D∕2

Q2 πϵ ln d

D (Volt)

= ( F∕m)

Where ω=2π F

Capacitance of 3-core cable

CcCc

Cc

Ce

CeCe

A

N

B C

Ce3Cc

3Cc

Ce 3Cc

Ce

CAN = CBN = CCN = 3Cc+Ce

Electrical field intensity

VphX.ln r2

r1

Emax=Vph

r1.lnr1r2 Emin=

Vph

r2.lnr1r2

E= (volt∕ m)

(volt∕ m)

E=Q

2πϵ0ϵr X(volt∕ m)

Q=2πϵ0ϵr Vph

lnr1r2

(coulombs)

WdI2 R

T2

T3

R.λ1 nl2

R.λ2 nl2 T4

T1

Power losses and capacity amperage losses

Δθ = ( I2 R+0.5wd ) T1+ ( I2R (1+λ1)+ wd ) nT2 + ( I2 R (1+λ1+λ2) + wd) n( T3+T4)

I = { Δθ – wd [ 0.5T1 + n (T2+T3+T4) RT1 + nR ( 1+λ1) T2 + nR (1+λ1+λ2)(T3+T4)

}1/2

where Thermal resistances T1,T2,T3,T4= wd= loss in insulationn= number of conductors

λ1= loss between conductor and metallic sheathλ2= loss between conductor and armour

Extension of underground cables

20 cm20 cm60 cm80 cm

30 cm

10 cm

20cm

5 cm

20 cm

20 cm

30cm

5 cm

110 cm

River sand

Cutting concrete

Soft soil

Natural soil without stones

Warning tape

Natural soil

slabbing

High voltage Cable Joint

1 2 3 4

5 6 7 8

9 10 11 12

Types of cable faultsthe following are the faults most likely to occur underground cables.

1 )open circuit fault

2 )short circuit fault

Time Domain Reflectometer (TDR) MethodTDR method is based upon the measurement of the time that it takes the pulse to reach a fault and reflect back

The distance d of the fault can be obtained as follows

t = time of pulse to travel to the fault and backV = propagation velocityc = the speed of light )3*108 m\s)μr = dielectric materials

ϵr = relative permittivity of the dielectric material

 

 Propagation

Velocity v

(m/μs)

Dielectric

Constant εrInsulation

198 2.3 PE

237 1.6 Paper

134 5 PVC

198 2.3 XLPE

High voltage cables faults location methods

For fully understand the principle, the equivalent circuit of a transmission line

Characteristic Impedance of transmission line

Z o = LR + jωL

G + jωC=

C

ᵨr =Reflection coefficient

ZL = 0 at short circuit

ᵨr = ZL – Z0

ZL + Z0Vr

=-Vs

Arc Reflection Method

The arc-reflection analyzer simultaneously applies high-frequency, low-voltage pulses to the cable to reflect from the low resistance arc.

Surge Generator provides high impulse causes the high-resistance fault to break down, .thus causing a low- resistance arc at the fault

conclusion

Thank you for

listening