Download G. Surge Impedance Loading (SIL)

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Design and Analysis of 66 kV Transmission Line
(IJSETR)
Htun Yu Aung1,Khin Thuzar Soe2

Abstract— From economical point of view, transmission line
system is very important in the electricity supply system. The
choice of the line voltage is important to do the design of power
transmission line. Different voltage systems with the same power
flow have different carrying current on the line. Higher current
tends to produce the higher losses. To reduce the current on the
line, the line voltage can be increased. The electrical transfer of
energy from one place to another over long distance with
standard regulations is one of the major problems in the field of
electrical power engineering. The parameters of overhead
transmission line are resistance, inductance and capacitance.
For three-phase line, the choice of spacing of conductors is
important and the lines must be transposed to compensate the
mutual inductance. In this paper, design of Chaunggu-Harkhar
66 kV overhead transmission lines and performance analysis of
MV transmission system are mentioned.
Keywords — Transmission line system, Different voltage level,
Overhead line, Line parameters, 66 kV Overhead transmission
line
I. INTRODUCTION
The use of electrical power is more developing in
accordance with the development of the nation and more
development of the living standard of the people.
Transmission line means essential link between generating
station and load point. Transmission lines are a vital part of
the electrical system, as they provide the path to transfer
Power between generation and load. Transmission lines
operate at high voltage levels, and are ideally tightly
interconnected for reliable operation. The length of
transmission lines, the amount of power transmitted, short
circuit levels, stability requirements have also become
paramount importance. Most high voltage transmission
systems are interconnected in a network system of circuit
elements. In order to transmit heavy power efficiently for any
considerable distance, comparatively high voltage is required.
So, economic choice of voltage should be considered for any
power transmission line. The selection of size of the
conductor is also important in order to carry the amount of
enough current that flows on the line due to the transfer of
power. Moreover, the amount of power losses and voltage
drop on the line should be in an acceptable range as in the
standard regulations. In transmission system, there are two
Manuscript received Oct 15, 2011.
Htun Yu Aung, Department of Electrical Power Engineering, Mandalay
Technological University (e-mail: [email protected]). Mandalay ,
Myanmar , Phone/ Mobile No:+959256234756
Khin Thuzar Soe, Department of Electrical Power Engineering,
Mandalay Technological University, Mandalay , Myanmar, Phone/ Mobile
No.+9592058554
kinds of transmission line, namely overhead lines and
underground cables. Overhead line transmission system is
cheaper than underground cable system. But maintenance
cost for overhead line is higher than that of the underground
cables The resistance of an overhead line produces the power
loss and the capacitance will affect the voltage of sending end
an the receiving end. If the lines have very much capacitance
effect, the reactors must be used to compensate the
capacitance effect. The inductance of an overhead
transmission line may interfere to the nearby communication
channel. In highly induction line, the mutual induce voltage in
the telephone line is dangerous for the people who use the
telephone. Therefore for the three-phase line, the choice of
spacing of conductors is very important and the lines must be
transposed to compensate the mutual inductance if needed.
II. Types of Conductor
In the early days of transmission of electric power
conductor were usually copper but aluminum conductors have
completely replaced copper because of the much lower cost
and lighter weight of aluminum conductor compared with a
copper conductor of the same resistance. The fact that an
aluminum conductor has a large diameter than a copper
conductor of the same resistance is also an advantage. With a
larger diameter the line of electric flux originating on the
conductor will be farther apart at the conductor surface for the
same voltage.[47Woo]
Different types of aluminum conductors are as follows;
AAC ; all-aluminum conductors
AAAC; all-aluminum-alloy conductors
ACSR; aluminum conductor steel-reinforced
ACAR; aluminum conductor alloy-reinforced
ACSR (aluminum conductor steel-reinforced) is the most
wildly used conductor material, having particular application
at high voltage. It is made up of galvanized steel core one or
more strands, and one or more outer layers of aluminum wire.
The conductivity is taken to be that of the aluminum alone,
and the strength to be 85 percent of the sum of the steel wire
plus 95 percent of the sum of the aluminum wires.
A. Line Constants
The transmission line is an electric circuit which has four
constants, that is resistance R, inductance L, capacitance C
and leakage admittance y and it is necessary to fully
understand these line constants so as to calculate its electrical
characteristics.
1
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
.
µ= permeability = 1.0 for non-magnetic material
B. Resistance
The resistance of transmission line conductors is the most
important cause of power loss in a transmission line. The term
resistance, unless specifically qualified, means effective
resistance. The effective resistance of a conductor is
Rdc = dc resistance in Ω/mile
R=
power loss in conductor
2
I
Where the power is in watt and I is the rms current in the
conductor in amperes. The effective resistance is equal to dc
resistance of the conductor only if the distribution of current
throughout the conductor is uniform.
Table I. Skin Effect
X
K
X
K
X
K
0.0
0.1
0.2
1.0
1.0
1.0000
1
1.0000
4
1.0001
3
1.0003
2
1.0006
7
1.0012
4
1.0021
2
1.0034
0
1.0
1.1
1.2
1.00519
1.00758
1.01071
2.0
2.1
2.2
1.07816
1.09375
1.11126
1.3
1.01470
2.3
1.13069
1.4
1.01969
2.4
1.15207
1.5
1.02582
2.5
1.17538
1.6
2.6
1.20056
1.7
1.02332
3
1.04205
2.7
1.22752
1.8
1.0524
2.8
1.2562
1.9
1.0644
2.9
1.28644
0.3
0.4
0.5
Direct current resistance is given by the formula.
ρl Ω
R =
0
0.6
A
where ρ = resistivity of conductor
l = length
A = cross-sectional area
The resistance of a conductor at any temperature is
T  T2
R2
 o
R1
To  T1
0.7
0.8
0.9
R1 and R2 are the resistance of conductor at temperature T 1
and T2. T1 and T2 are conductor temperature in degrees
Celsius.
T0 = constant varying with conductor material
= 234.5 for annealed copper
= 241 for hard-drawn copper
= 228 for hard-drawn Aluminum
C. Skin Effects
Uniform distribution of the current throughout the cross
section of a conductor exists only for direct current. As the
frequency of alternating current increase, the non-uniformly
of distribution become more pronounced. An increase in
frequency causes non-uniform current density. This
phenomenon is called skin effect.
D. Inductance
When the conductors of three-phase line are not spaced
equilaterally, the problem of finding the inductance becomes
more difficult. Then the flux linkages and inductance of each
phase results in an unbalanced circuit. Balance of the three
phase can be restored by exchanging the position of
conductors at regular intervals along the line so that each
conductor occupies the original position of every other
conductor over an equal distance .Such an exchange of
conductor position is called ‘transposition’.[85Cot]
The inductance per phase in bundle is
Deq
H
L  2  10 7 ln
m
D
s
In resistance calculation the following formula should be used
to consider the skin effect,
where,
Deq = equivalent GMD
Ds = GMR of the conductor
Rac = K Rdc
where, K is a function of X.
The inductive reactance,
XL = 2 π f L Ohm/km
X = 0.63598
μf
R dc
f = system frequency in Hz
µ= permeability = 1.0 for non-magnetic material
Rdc = dc resistance in Ω/mile
where, K is a function of X.
X = 0.63598
μf
R dc
Table II. Self-GMD or GMR of Stranded Conductors
Solid round conductor
0.779 R
Full stranding:
7-strands
0.726 R
19-strands
0.758 R
37-strands
0.768R
61-strands
0.772R
91-strands
0.774 R
127-strands
0.776 R
Hollow stranded conductors and ASCR (neglecting steel
strands):
30-strands (two-layers)
0.826 R
f = system frequency in Hz
2
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
26-strands (two-layers)
54-strands (two-layers)
0.809 R
0.810 R
economic voltage can be determined by the following
equation.[88Tur]
V  5.5 L 
E. Capacitance
Capacitance to neural is the ratio of the change on a conductor
to the voltage between that conductor and neutral.
Cn=
2k
F / m F/m
Deq
Ln (
)
r
Kelvin’s Law may be represented by the following
formula in case of aluminum conductor.
C  0.013
Deq = equivalent GMD
r = radius of conductor
k = 8.85x10-12F/m
The capacitive reactance, Xc =
1
Ohm/km
2 fc n
F. Surge Impedance
In transmission system, characteristic impedance is
called surge impedance. It is usually reserved for the special
case of a losses line. If a line is lossless, its resistance and
conductance are zero and the characteristic impedance
reduces as,
(Ohms)
where,
XL= series inductance per unit length of the line
XC= shunt capacitance per unit length of the line
These XL and XC are the basic parameter of the transmission
line
G. Surge Impedance Loading (SIL)
Surge Impedance Loading (SIL) of a line is the power
delivered by a line to a purely resistive load equal to its surge
impedance. Under this condition the sending-end and
receiving-end voltages are equal in magnitude but different in
phase position. Surge impedance loading in itself is not a
measure of maximum power that can be delivered over line.
SIL (3ϕ) = 3 V LL I LL
where,
L = Line length in mile
B. Economic Size of Conductor
Where,
Z0 = X L X C
Load in KVA
( KV )
150
(MW)
where,
VLL = line to line voltage (kV, rms)
ႈ ILL= line current at surge impedance loading in ampere
III. DESIGN AND CONSTRUCTION FOR 66KV
TRANSMISSION LINE ( CHAUNGGU-HARKHAR)
A. Selection of Transmission Line Voltage
It is very important to select proper voltage level for a
transmission line because that will lead to many consequences
in operation of power system and if there is an incorrect
decision by a designer or decision marker, it is very difficult
and costly to solve the problems in future. Generally the
supply power and the line length will be given; the most
a.p
(Ampere/mm 2 )
q
Where,
C = most economical density of current (Ampere/mm2)
a = percent annual expense to the construction cost of
conductor
p = price of conductor (kyat/kg)
q = cost of electricity (kyat/kWh)
The current I is work out as follows:
p
I
( Ampere)
3Vpf
where,
µ = utility factor being (0.6 ~ .15)
pf = power factor being 0.85
V = line voltage (kV)
P = Maximum Power (kW)
The most economic size of conductor = A = I/C (mm2)
C. Choice of Conductor by Corona Voltage
Critical voltage of corona formation of conductor is
worked out by the following formula.
Vc  24.3 3m 0 m1δdlog
2D
(kV)
d
where,
VC= disruptive critical voltage in kV (line to line)
m0= factor of irregularly of conductor surface being 0.8
m1= factor of weather being 1 in fair weather and 0.8 in
rainy weather
δ = factor of air density being 1.0
D = spacing of conductor (cm)
d = Diameter of conductor
D. Contamination Design for Porcelain Insulator
The target withstand voltage for contamination design
can be calculated with the following Equation,
Abnormal voltage = normal operation voltage(kV )  3.8
3
The design withstand voltage of each insulator discs is
proportional to the number of insulator strings. The flashover
voltages of insulator discs of 10″ dia. × 5¾″ spacing are
shown in Table.
Table III. Flashover Voltages of 10″ dia. × 5¾″of Insulator
Discs
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Number of Discs
50 cycles flashover
voltage, Wet (kV)
2
3
4
5
90
130
170
215
50 cycles flashover
voltage, Dry (kV)
155
215
270
325
Impulse flashover
voltage (kV)
255
355
440
525
IV. DESIGN AND CALCULATION OF 66KV TRANSMISSION LINE
Line Data,
Total Line Length = 166km (103 mile)
Transmitted Power = 6MW
System Frequency = 50 Hz
Phase Spacing
= 6.5m
Line configuration = Horizontal
Economic voltage selection,
V = 5.5 L+
Load in KVA
kV
6000
kV
150
 65.77 kV
I
(mm2)
C
Most economical density of current,
Economic size of conductor, A =
ap
(Ampere/mm2)
q
a = 20%
p = 2500 kyat/kg
q = 50 kyat/ kWh
At 20˚C, R dc 
Power factor,
pf = 0.85
Utility factor,
µ = 0.75
Load current, I
=
μP
3  V  p.f
3
= 0.75  6  10
3  66
= 46.31A
A
Line constant calculations are as follow;
Line Data
Line length
= 103 miles
Conductor size
= 367.8 MCM
Outside diameter
= 0.755 in
Conductor spacing
= 6.5 m = 21.325 ft
Configuration
= horizontal
Line voltage
= 66 kV
Resistance Calculation,
2
0.2  2500
 0.41 A/mm
50
C  0.013
5
8
10
16
The numbers of insulator discs are decided as follows:
Suspension Insulator
– 5 units
Tension Insulator
– 6 units (Double string of 12
units)
The insulation of the line approaching within a mile from
Power Station or Sub-station would be reduced by decreasing
the number of insulator discs in order to have the insulation co
ordination with the insulation strength of station equipment.
Economic voltage = 65.77 kV
So, selected voltage = 66kV
C = 0.013
Number of insulator
Internal abnormal voltage (kV) = Normal operating voltage
(kV)
× 3.8
66
 3.8  145 kV
3
Table IV .Line voltage and number of insulators used
Line to Line Voltage (kV)
Number of insulators used
66
110
132
230
150
 5.5 103 
Corona critical voltage, VC = 182.811kV (Line to Line)
Thus, 367.8MCM, ACSR conductor should be chosen to
avoid from corona effect.
I 46.31

112.95 mm 2
C 0.41
Economic cable size = 367.8 MCM (185 mm2)
Consideration for choice of conductors by corona
voltage,
2  650
VC  24.3  3  0.8  1.0  1.0  1.9177 log
1.9177
= 182.811 kV
ρ
Ω/mile
A
1  50
 0.8599
0.2735
By using linear interpolation, from Table,
(1.0034  1.00212)
K 1.00212 
 (0.8599  0.8)
(0.9  0.8)
K = 1.002887
Rac = 1.002887 × 0.2735= 0.2743 Ω/mile
Line Inductance Calculation, L
X  0.063598
Deq  3 Dab Dbc Dca
 3 21.325  21.325  42.65
 26.87 ft
For 26-strands,
Ds= 0.809 R
0.755
 0.809 
 0.02545 ft
2  12
D
L  2  10-7 ln eq H/m
Ds
4
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
 2  10-7 ln
in stringing the conductors, sags for different spans with poles
at equal or unequal height.
26.87
H/m
0.02545
1.3924 10-6 H/m
Transmission line tower with fabricated steel members
are used in this MV transmission. The transmission lines are
made more wind- resistant as they are to bear out the wind
pressures during storms and cyclones. Problems
of
transportation and erection arise as the supporting structures
are to be transported over long distances and standard
workman- ship is required for erection of the transmission
lines.
The demand of electric power is increasing throughout
the world and in many countries it is doubling every five to
eight years. The increasing demand of electric power has to be
met; but it is presenting unusual problems to the power
engineers. For developing power systems and transmission
lines, and for the economical transmission of large blocks of
power over long distances, it is necessary to go in for higher
and higher transmission voltages.
Inductive reactance,
XL = 2 π f L
= 2 π × 50 × 1.3924 ×10-6×1609
= 0.7038 Ω/mile
Line Capacitance Calculation, C
2 k
Cn 
D
ln ( eq )
r
k = 8.85×10-12 F/m
Deq = 26.87ft
0.755
R
 0.03146 ft
(2  12)
Cn 
2   8.85  10 12
26.87
ln (
)
0.03146
Capacitive reactance, XC = 1/2 π f C
= 0.24×106Ω/mile
Parallel Admittance, y
y = g + j ω Cn= j 2 π f CnƱ/m ( g is neglected )
= j 2 π × 50 × 8.238 ×10-12 ×1609
= j 4.164 ×10-6Ʊ/mile
VI. ACKNOWLEDGMENT
The author would like to express grateful thanks to his
supervisor Dr.Khin Thuzar Soe, Associate Professor and to all his
teachers from Department of Electrical Power Engineering,
Mandalay Technological University for their encouragement and
helpful suggestions. And the author also would like to express his
gratitude to his parents for their support and encouragement.
X X
L
C
Surge impedance, Z0 =
XL = 0.7038 Ω/mile
XC = 0.24×106 Ω/mile
Z0 =
REFERENCES
[05KEP] KOREA
INTERNATIONAL
COOPERATION
AGENCY,
KOREA
ELECTRIC POWER CORPORATION, KEPCO., “FEASIBILITY STUDY
0.7038  0.24  10  410.99 
6
AND BASIC DESIGNS FOR THE 500 KV TRANSMISSION
SYSTEM IN
MYANMAR”, FINAL REPORT AD APPENDIX. (2005).
Surge Impedance loading:
SIL3Ø =
[00DES]
3 VLL I L W
GRAW.HILL PUBLISHING COMPANY LTD, (2000).
Z = 77.8 ∟68.71 Ω
Y = 0.4288×10-3 ∟90 Ω
IL 

SIL3Ø
[88TUR]
TURAN GONEN, “ELECTRIC POWER TRANSMISSION SYSTEM
ENGINEERING” (ANALYSIS AND DESIGN), JOHN WILEY &SONS,
VLL
3
DESHPANDE, M. V, “ELECTRICAL POWER SYSTEM DESIGN”, MC
INC., (1988).
Z
Y
[85COT]
COTTON H. AND BARBER H., “TRANSMISSION AND DISTRIBUTION
OF
66000
 89.46 A
3  425.54
ELECTRIC ENERGY”, HODDER AND STOUGHTON, (1985).
[47WOO] WOODRUFF,
L.
F.,
“PRINCIPLES
OF
ELECTRIC
POWER
TRANSMISSION”, JOHN WILEY AND SONS, INC. (1947).
 3  66000  89 .46 10 .2267 MW
[MEPE]
MYANMA ELECTRICAL POWER ENTERPRISE, MYANMAR.
[MOEP]
MINISTRY OF ELECTRIC POWER, MYANMAR
V. CONCLUSION
In this thesis, the design and calculation of the
Chaunggu-Harkhar 66 kV overhead transmission line is
presented. Both of the electrical and mechanical designs are
also considered. In the electrical design, choice of voltage,
size of conductors, spacing of conductors, corona losses,
regulation and efficiency of the line were considered. In the
mechanical design, it included types of poles or tower, span to
be used, number of insulators in string, size of the ground
wire, location of ground wire on towers, permissible tension
5
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