Lecture4 1UndergroundCables
Lecture4 1UndergroundCables
Lecture4 1UndergroundCables
MESA, MANILA
UNDERGROUND CABLES
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ELECTRICSTRESSINASINGLECORE CABLE
The cable must designed properly so that the electric field strength or maximum electric stress at the surface of the conductor does not exceed that required a break down the insulation. It has been found out that the optimal ratio p of the radius of the conductor is given by: R1 = e = 2 .718 R2
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ELECTRICSTRESSINASINGLECORE CABLE
r1 = e = 2 .718 r2
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1 r2 = 2 r3
If Emax is the maximum allowable electric field , the operating voltage V of the cable is
r2 r1 V = E max r3 ln + r2 ln r3 r2
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E 3 max
V V1 V V1 = = r2 r3 ln a r3 l ln r3
V1 V1 = = r r2 ln 1 r2 ln a r 2
E 2 max
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a V1 = V 1+ a
Therefore,
E 3 max
V 1 = r3 ln a 1 + a
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E 3 max =
Comparing
V V V = = 2 r3 ln a r1 r3 ln r3 r3 ln ar2 r 2 a
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In finding the capacitance per phase, change the delta connected capacitances C2 to their equivalent wye form
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C n = C1 + 3C 2
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0 r1 L= ln , H/ l H/m r2 2
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Therefore,
2
IR R tan t = = I C CV
2
V P=I R= R
P = CV tan CV
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EXAMPLE1 EXAMPLE 1
A 13.2kV single conductor cable has an outside diameter of 10.0cm. 10 0cm Determine the conductor radius and the electric field strength that must be withstood by the insulating material in the mist economical ratio.
10
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EXAMPLE2 EXAMPLE 2
R1=2.5cm, R3=0.92cm and R2=1.75cm. Find the maximum electric field for an opeining voltage of 13.2kV a. With capacitance grading b. Without capacitance grading
1 .75 2 .5 2 + 1 .75 ln a.)13 .2 10 3 = E max 0 .92 ln 10 0 .92 1 .75 E max = 1085 .80 kV/m b.) b ) E max 13 .2 10 3 = 1435 .3 kV/m = 2 0 .92 ln (2 .5 0 .92 ) 10
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EXAMPLE3 EXAMPLE 3
A single-core cable consisting of a 1cm diameter cable inside a 2.5cm diameter sheath, is 10km long and operates at 13.2kV and 60Hz. The relative permittivity of the dielectric sheath is 5, and the open circuit power factor of the cable is 0.08. calculate the capacitance of the cable and the charging current through the capacitance
= 0 r
EXAMPLE4 EXAMPLE 4
Refer to E3, determine the insulation resistance and dielectric loss.
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