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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