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ELECTRICAL DESIGN OF 400 kV D/C OVERHEAD TRANSMISSION LINE
M/S LAKE TURKANA WIND POWER LTD. IN KENYA
1)
PREAMBLE
M/s Turkana Wind Power has appointed the undersigned as a Consultant for the
electrical design. Under the Aegis of the Ministry of Energy of KENYA M/s LAKE
TURKANA WINDPOWER LTD.(LTWL) is planning to construct a 400kV Double
Circuit (D/C) line from Lake Turkana Wind farms to SUSAN Naivsaha (Approx.427
kM). This D/C line is expected to evacuate 400 MVA of Wind Power into the Grid of
M/s Kenya Power Lighting Ltd. (KPLC).
The route survey has been done by the renowned consultant and has described the
terrain, the topography, and hydrology in detail. The consultant has also made a survey
on four alternative routes and has prepared a socio-economic and techno-economic
feasibility report. Based on this survey, the consultant has made a comparative
statement giving the best alternative route amongst the four routes. The consultant has
selected the route no.2. Accordingly the undersigned has made a comprehensive study
of the project report and is pleased to give the details of electrical design in the
presentation which follows:2)
TOWER CONFIGURATION
For the given power evacuation scheme, the D/C towers with two earth wires are most
suited. Each circuit will have three phase and each phase will comprise two conductors
in a bundle. The sub-conductors in a bundle shall be spaced 450 mm. apart. The earth
wire shall also be provided with cross arms which may be either horn type or horizontal
type. Thus, the tower will have on each side, three cross arms for conductors and one
cross arm for earth wire. Therefore the towers will have totally 8 cross arms. The tower
will have a barrel type of configuration and will be a self supporting without any guys
(stay wire). One of the earth wire will be of GSW type and other will be OPGW type. A
typical 400kV D/C tower configuration is indicated in the Appendix- I.
3)
SIZING OF LINE
Since 400MVA power is to be transmitted, the current comes to 580 Amps. per phase at
rated voltage of 400 KV. The current rating of ACSR Moose is about 614 Amps at 65ºC
operating temperature. This will increase to 827 amps if the conductor temperature is
allowed to be raised to 75º C. Similarly the current rating for Zebra at 65º C will be 550
amps and at 75º C it will be 735 amps. However, looking to the length of the line, the
1
inductive reactance and capacitive reactance will be substantial. Besides, the voltage
being 400kV, the corona effect will also be predominant. It is also possible that the line
under reference may also need to carry additional load in future. Therefore, it will be
worthwhile to deploy Twin ACSR Moose Conductor having 54/3.54mm. Aluminum
strands and 7/3.54mm Steel strands or Twin ACSR Zebra conductor having
54/3.15mm Alluminium strands and 7/3.15 Steel strands. It may be important to note
that there is lot of margin available in current carrying capacity even by using Twin
Zebra conductor. At 400 MVA the current per phase comes to 580 Amps (maximum).
If this load is to be carried by two circuits, the current per phase will be 290 amps only.
With Twin conductor, the current per sub-conductor will come down to 145 amps.
Considering the current carrying capacity of ACSR Zebra, the line can be loaded to
almost double of load. However, the selections of the conductor do not depend upon the
ampacity alone. There are other factors such as line losses, voltage regulation, corona
losses etc.
The ground clearances shall be maintained at 9.0 meter or more depending upon the
span. At 500MVA the current will be 720 Amps and twin bundle conductor will be
sufficient. The average span between two towers for the line can be kept at 400 Mtrs,
which is common for 400KV system in most of the countries. However, this can be
fixed based on the meteorological data of the region through which the line is passing.
4)
AMPACITY CALCULATIONS
The ampacity of ACSR Moose & ACSR Zebra conductor has been worked out
considering 40ºC ambient and conductor temperature at 65ºC, 70ºC, 75ºC, 80ºC, 85ºC
& 90ºC. The set of calculations is appended in a tabular form in this report as Appendix
II (a) (Table 1-6) for ACSR Moose and Appendix II (b) (Table 1-6) for ACSR Zebra
conductor. It can be seen that, if higher temperature operations are resorted to,
conductor can be loaded to higher ampacity. Since the project report does not indicate
ambient temperature range, 40ºC has been considered as a basic ambient temperature as
per international practice for the countries around the Equator. The maximum sag of
conductor will depend upon the temperature at which the conductor is recommended for
operation. The tower height will also vary depending upon the maximum temperature
recommended for the conductor. Going by the project report & allowing a margin for
future loadings we recommend to consider 75ºC as final temperature of conductor under
maximum load. This will also result into optimizing the tower designs. The sag and
tension calculations as well as the geometry of the tower will be decided on the strength
of this temperature.
2
5)
RESISTANCE OF CONDUCTOR
The Direct Current (D.C.) resistance of the conductor at 20ºC is obtained from the table
made available by a reputed Conductor Manufacturer. Mostly the resistance indicated in
the manufacturers’ catalogue is based on the actual experimental values which are
proved even during the acceptance tests carried out on each lot. This D.C. resistance is
then re-worked out at various temperatures ranging from 65ºC to 90ºC by the following
equation:
R20 = Rt
1_ __
1 + α (T- 20)
Where, R20 = resistance corrected at 20 º C
Rt = resistance measured at T º C
α = constant – mass temperature coefficient of resistance
T = ambient temperature during measurement.
The DC resistance worked out as above then can be converted to AC resistance by
following relation.
(Rt2 / Rt1) = (M + t2) / (M + t1)
Where,
Rt2 = d.c. resistance at any temperature t2ºc
Rt1 = d.c. resistance at any other temperature t1ºc
M = (a constant for any one type of conductor material)
= 234.5 for annealed 100% conductivity copper
= 241.5 for hard drawn 97.3% conductivity copper
= 228.1 for aluminum
The above formula is useful for evaluating changes in d.c. resistance only, and cannot
be used to give ac resistance variations unless skin effect can be neglected.
Skin Effect in Straight Round Wires has to be worked out precisely to arrive at
the Alternating Current (A..C.) resistance.
The resistance of non-magnetic conductors varies not only with temperature but also
with frequency, due to skin effect. For any given frequency the following formula
should be used.
Rac = KRdc ohms per kM
Where,
Rac = the ac resistance at the desired frequency
3
Rdc = dc resistance at any known temperature
K = Value given in Table-1 below
In Table, K is given as a function of X,
Where,
________
X = 0.063598 √ (µf / Rdc)
f = frequency in cycles per second
µ = permeability = 1.0 for non magnetic materials
Rdc = d-c resistance in ohms per kM.
TABLE - 1
Skin Effect (Value of K corresponding to value of X)
X
K
X
K
0.0
1.00000
2.0
1.07816
0.1
1.00000
2.1
1.09375
0.2
1.00001
2.2
1.11126
0.3
1.00004
2.3
1.13069
0.4
1.00013
2.4
1.15207
0.5
1.00032
2.5
1.17538
0.6
1.00067
2.6
1.20056
0.7
1.00124
2.7
1.22753
0.8
1.00212
2.8
1.25620
0.9
1.00340
2.9
1.28644
1.0
1.00519
3.0
1.31809
1.1
1.00758
3.1
1.35102
1.2
1.01071
3.2
1.38504
1.3
1.01470
3.3
1.41999
1.4
1.01969
3.4
1.45570
1.5
1.02582
3.5
1.49202
1.6
1.03323
3.6
1.52879
4
1.7
1.04205
3.7
1.56587
1.8
1.05240
3.8
1.60314
1.9
1.06440
3.9
1.64051
TABLE – 2
AIR PARAMETERS
Average air film
temp. tf in ºC
Absolute viscosity
of air μf in
kg./m.hr.
Air Density at
see level Pf
kg./cum
Thermal conductivity
of air Kf Wt/Sq.m ºC
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
0.061759
0.06265
0.063545
0.064438
0.06533
0.066074
0.066967
0.06786
0.068604
0.069497
0.07039
0.071134
0.072027
0.072771
0.073515
0.074408
0.075152
0.075896
0.07664
0.077533
0.078277
1.2927
1.2703
1.2478
1.2254
1.2046
1.1854
1.1661
1.1469
1.1277
1.1101
1.0941
1.0764
1.0588
1.0444
1.03
1.0156
1.0044
0.098674
0.97393
0.95951
0.94669
0.024245
0.024606
0.025
0.025361
0.025722
0.026083
0.026476
0.026837
0.027231
0.027592
0.027953
0.028346
0.028707
0.029068
0.029462
0.029823
0.030217
0.030577
0.030938
0.031234
0.031693
It is pertinent to note that the AC resistance of the conductor is frequency dependent.
The A.C. resistance is worked out at different temperature and is exhibited in a form of
statement appended herewith.
The value of A.C. resistance obtained as above will be used for working out the
transmission line losses, voltage regulation and also the ampacity.
5
The ampacity of the conductor is worked out using the following relations.
SYMBOLS
I
D
d
A
a
ε
α
Rdc/20
Rdc/tc
Rac/tc
tc
ta
kc
ka
Tf
V
Pf
mf
Kf
s
q
S
S1
Qs
Qc
Qr
= Conductor current, amps at 50 Hz
= Conductor outer diameter, meters
= Conductor inner diameter, meters
= Projected area of conductor per meter length, Sq. m.
= Coefficient of Solar absorption of conductor
= Coefficient of Emissivity of conductor
= Constant mass temp, coefficient of resistance of conductor per ºC
= D.C resistance of conductor at 20 ºC, Ώ / km.
= D.C resistance of conductor at temperature tc ºC, Ώ / km.
= A.C. resistance of conductor at 50 Hz and temperature tc ºC, Ώ / km.
= Average conductor temperature, ºC
= Average ambient temperature, ºC
= Average conductor temperature, Kelvin = tc + 273
= Average ambient temperature, Kelvin = tc + 273
= Average air film temperature = ( tc + ta ) / 2
= Average velocity of wind, meters/ hours
= Density of air at temp. tf , kg/cu. mtr
= Absolute viscosity of air at temp. tf , Kgf/hr.(m)
= Thermal conductivity of air at temp t, watts/m (ºC)
= Stefan-Baltzman constant = 5.678 x 108 watts/m (ºC)
= Effective angle of incidence of sun’s rays on conductor surface, deg
= Direct Solar irradiation on conductor surface, watts/ Sq. m.
= Sky radiated on conductor surface, watts/ Sq. m.
= Heat gained by conductor by solar radiation per linear meter, Watts/Mtr
= Heat lost by conductor by convection per linear meter, watts/m.
= Heat lost by conductor by radiation per linear meter, watts/m.
 FORMULAE:
 Fundamental Heat balance equation
I2 (R ac / tc ) = Qc + Qr - Qs
____________________
I = √ [(Qc + Qr - Qs) / (Rac / tc)]
Heat lost by conductor by conduction to connected metallic parts is insignificant
and therefore neglected.
6

Heat gained by conductor due to Solar irradiation
Qs = a (S Sin Ø + S1) D Watts / m
Heat gained by sky radiation (S1) is negligible and hence neglected. For worst
condition Sin q = 1. Therefore,
Qs = a SD watts/m. where
a = 0.23 to 0.85 for conductor up to 1 year age and 0.90 to 0.95 for conductor
above 1 year age.

Heat lost by conductor by radiation
Qr = α ε π D (K c4 - Ka4) watts/m.
ε
= 0.17838 x 10 -6 x є x D (K c4 - Ka4) watts/m.
ε = 0.45 for conductor less than 1 years age
0.75 for conductor 1 year to 10 years age
0.85 for conductor over 10 years age

Heat lost by conductor by convection
 Natural Convection loss (wind speed less than 2200 m/hr)
Qc = 3.71272 D 0.75 ( tc - ta )1.25 watts/m. at sea level.
Qc = 3.646 1606 (pf ) 0.5 D 0.75 ( tc - ta )1.25 watts/m. at altitudes above sea level.
 Forced convection loss (wind speed 2200 m/hr and above)
Qc1 = {1.00531 + 1.35088 (D pf V / µ f) 0.52 } kf (tc - ta ) watts/m
Qc2 = { 0.75398 (D pf V / µ f) 0.6 } kf (tc - ta ) watts/m
Whichever is higher of the above two equations is to be considered. The values of
pf, µf and kf at air film temperature, tf are taken from Table – 2.
It may be interesting to note that the ampacity worked out from above relations is
the ultimate load ability of the conductor with reference to the atmospheric
parameters. This can not be taken as a final load ability of the line. There are
number of other design parameters which have to be taken into account before
arriving at precise load ability of the line. These parameters are being discussed in
details in the presentation which follows.
6)
INDUCTANCE OF CONDUCTOR & LINE
7
This is one of the important design parameter for Transmission Line. The
calculations of inductance in milli Henry (mH) per Km. will lead to a calculation
of inductive reactance per km. The voltage regulation depends upon the value of
inductive reactance, which in turn depends upon the inductance. The conductance
of the conductor is worked out using following relations.
The inductance per phase per km is
L = 2 X 10-7 ln (Deq/Ds)
Where Equivalent Mutual GMD
Deq = ( DAB DBC DCA ) 1 / 3
DAB = (Dab Dab’ Dba Dba’)1/ 4
DBC = (Dbc Dbc’ Dcb Dcb’)1/ 4
DCA = (Dca Dca’ Dac Dac’)1/ 4
Where
_______
Dab = √ ad2 + bd2 = Dba
_________
Dab’ = √ ad2 + b’d2 = Dba’
_________
Dbc = √ be2 + ce2 = Dcb
_________
Dbc’ = √ ec’2 + be2 = Dcb’
________
Dca = √ ck2 + ak2 = Dac
_______
Dca’ = √ a’l2 + cl2 = Dac’’
AND Equivalent self GMD or GMR
DS= ( DsA DsB DsC) 1 / 3
DSA = ( Daa Daa’ Da’a Da’a’ d2) 1 / 6
DSB = ( Dbb Dbb’ Db’b Db’b’ d2) 1 / 6
DSC = ( DccDcc’ Dc’c Dc’c’ d2) 1 / 6
Where d is distance between two sub conductors
It may be pertinent to note that the inductance for bundle conductor is worked out
using the above relation. In any case, the reactance per km. of a single conductor
8
will be more than that for bundle conductor. The inductance per km. and for the
entire line having a length of around 430 km have been worked out and appended
with this report as Appendix III (a) & (b) respectively for ACSR Moose & ACSR
Zebra conductor. It can be seen that the inductive reactance for ACSR Zebra
conductor is more than that for ACSR Moose conductor.
7)
CAPACITANCE OF CONDUCTOR & LINE
As the transmission line passes over the long distance, capacitance is developed
between the various conducting mediums. The capacitance has a positive effect
on the voltage regulation when the line is fully loaded. If the line is not loaded or
is poorly loaded, the capacitance has a bad effect on the voltage regulation. To be
precise, the receiving end voltage becomes higher than the standing end. This is
termed as a Ferranti effect. The capacitance for bundle conductor transmission
line can be worked out by the following relation. The capacitance is worked out
and indicated in Micro Farade (µF). The capacitive reactance can be worked out
from the value of capacitance. The capacitance of the bundle conductor line is
worked out using following relation.
It may be noted that for the calculation of capacitance the value of mutual GMD
will be same as that for the calculations of inductance but the value of self GMD
will change.
Capacitance per phase per km
CN = 0.02412 / log ( Deq / Ds )
Where Ds = ( DsA DsB DsC) 1 / 3
________
DsA = √ r x Daa’ x d
__________
DsB = √ r x Dbb’ x d
__________
DsC = √ r x Dcc’ x d
It is recommended in this report to transpose the line at various locations so that
the line is fragmented in six sections. To be precise, the line will be transposed at
five places along the line and last (sixth) transposition will be on the receiving
end gantry, so that the position of phases R, Y & B at sending end matches that
with those at receiving end. Besides, each phase will travel on the position for an
equal distance. Thus the capacitance is considered for 72 km. The voltage
regulation has been calculated accordingly.
9
SENDING END
GANTRY
RECEIVING END
GANTRY
a (R)
b (Y)
c (B)
a (R)
b (Y)
c (B)
72 KM
72 KM
72 KM
72 KM
72 KM
a
b
70 KM
a'
b'
d
c
c'
e k
l
The Capacitance worked out is appended with this report in Appendix III (a) &
(b) respectively for ACSR Moose & ACSR Zebra conductor.
10
8)
VOLTAGE REGULATION
The voltage regulation is the percentage difference in voltage with reference to
the receiving end voltage. It is given by the following relation:
% VR = (Vs – Vr) x 100 / Vr
Where,
VR = Voltage regulation
Vs = Sending end voltage
Vr = Receiving end voltage
The receiving end voltage Vr is a function of voltage drop which will take place
when current is passing through it & the resistance, inductance and capacitance
of the line are accounted for.
Thus the voltage regulation can be found out by the following relation
Here, VS = A x VR + B x IR
IS = C x VR + D x IR
Where A = D = 1 + (ZY/ 2)
B = Z (1 + ZY/ 6 + Z2Y2/ 120)
C = Y (1 + ZY/ 6 + Z2Y2/ 120)
Where Impedance Z = R + j X
Reactance X = 2 x π x f x L
Admittance Y = 2 x π x f x CN
The values of Impedance and Admittance are appended in the Appendix IV (a) &
IV (b) respectively for ACSR Moose & ACSR Zebra conductors. It can be seen
that the impedance for ACSR Moose conductor is less than that for ACSR Zebra
conductor. The voltage regulation of 400 kV line is generally around 20%.
The voltage regulation has been worked out with different percentage of load for
the transmission line under reference and is appended here with in Appendix V
(a) & V (b). It may be interesting to note that the Voltage regulation is calculated
considering 420 KV at the bus of sending end substation, which is actually a
maximum rated voltage for a 400 KV line. If a constant voltage at the bus of
receiving end substation were to maintain, the sending end bus will need a much
higher voltage above the maximum rated voltage of 420 KV (say 440 to 500 KV
depending upon the percentage load). It will be interesting to note that the total
load is shared by two circuits.
11
9) CORONA LOSSES
The lines having a rated voltage of 400 kV and above lines are susceptible to the
occurrence of Corona which is a phenomena causing a disruption of insulation of
air which is associated with a hissing noise and a lighting glow all along the
conductor, this also causes a power loss. It may be important to note that the
corona loss has a relation with the diameter of the conductor and humidity
(density of air). Smaller the diameter, larger is the corona. The calculation for
corona loss is done using the following relation:
δ = 3.86p / (273 + Ө)
For twin (bundle) conductor
Critical Disruptive Voltage
____
-7
Vd = 3 x 10 x (2r / ( 1 + (2r / d)) x δ x m0 x x ln (Deq / √(r d))
____
-6
2
Pc = ( ( 21 x 10 x f x V ) / (log (Deq / √ (r d))) x F
Where F is the factor varies with ratio V / Vd
V is phase to neutral voltage = receiving end voltage
Vd = Critical Disruptive Voltage
The corona losses calculated for the line under reference are appended with this
report in Appendix VI (a) & (b) respectively for ACSR Moose & ACSR Zebra.
As the corona occurs when humidity increases viz., monsoon/fog seasons we
have considered 4 months for corona losses means 120 days.
10)
CALCULATION OF TRANSMISSION LINE LOSSES AND COST AT 10
YEARS OF EXPLOITATION
The losses due to the current and resistance are given by the relation as follows.
Losses in KW = I2 x R
The current has to be considered at 50% load which comes to 290 amps (580/ 2)
As we have proposed D/C system, the current will be half of 290 Amps that is
145 Amps per phase. This current will further reduce by half to 72.5 amps for
each sub conductor. Thus the total losses of two circuits with 50 % load will be
equal to 72.52x2(bundle of two) x3(phase) x 2(D/C) x R per hour. The yearly
losses can be worked out by multiplying this figure by hours per year i.e. 8760
hours. This is exhibited in Appendix VII (a) & VII (b) respectively for ACSR
Moose and ACSR Zebra conductor. In addition to this the corona losses also
contribute to the total losses. As stated above they are calculated for number of
12
hours corresponding to 120 days (2880 hours). The capitalization of losses for 10
years with a cumulative interest is worked out and appended with this report as
Appendix IX (for ACSR Moose & ACSR Zebra). It may be interesting to note
that Twin ACSR Zebra conductors are found to be more than adequate when we
equate the maximum current which is likely to flow through the line under
maximum power generation to the ampacity of conductor. However, if the
capitalization of losses for 10 years is taken into account, the deployment of Twin
ACSR Moose conductor will be a better proposition. This is clear from
Appendix-IX. It is seen that the difference between the capital cost of ACSR
Moose and ACSR Zebra comes to USD 6.2 million. The capitalization of this
differential amount at 7% interest for 10 years, comes to USD 12.17 million. The
capitalization of losses at 10 years exploitation at 7% interest for ACSR Moose
conductor comes to USD 176,116,759 million. Corresponding figures for ACSR
Zebra comes to USD 213,908,004 million. The difference comes to USD
37,791,245 million. It is interesting to note that the break even point is achieved
in the 4th year of exploitation. Thus ACSR Moose conductor will be cost effective
after 4th year of service of the line.
11) EFFICIENCY OF THE LINE
The efficiency of the line is worked out with the basis of losses at different line
loadings in the steps of 25%, 50%, 75% & 100%. The efficiency is worked out
separately for a condition of with and without corona losses. This gives a
comparative idea regarding the maximum power which the line can deliver under
different loading conditions. The efficiency worked out with above options are
exhibited at Appendix VIII (a) & VIII (b) respectively for ACSR Moose and ACSR
Zebra conductor.
12) OTHER ITEMS OF DESIGN
a) The creepage distance required will be 25mm per kV.
b) It is not advisable to do away with the use of shield wires. It is known that the shield
wires are used to provide a protection to the conductor in the mid span. If lightning
arresters are provided at regular interval, they will be able to improve the
performance of transmission line against lightning strokes and reduce the incidences
of insulator failure. It is also a known fact that the lightning arrester is able to
provide a passage (to the earth) to the impulses (switching impulse / lightning
impulse) within a particular radius only, which is much smaller than a span of 400
meters. Thus, the conductor in mid span will be attacked directly by the surges and
there can be a worst back flashover. The provision of earth wire helps in very quick
diffusion of the impulses to the ground by means of number of earth points attached
to the towers.
13
c)
d)
e)
f)
g)
h)
i)
j)
k)
Since the length of the line is very long, it is recommended to provide lightning
arresters approximately at a distance of 25 Kms with solid and independent earth
pits for each lightning arrester.
It is proposed to use I-string which are very versatile and easy to maintain. The
insulators can be ceramic or polymeric on mutual agreement bases. Normally, the
phenomenon of Galloping is found in the region where there is a snow fall. The
Galloping starts when snow melts and creates a temperature stress on the conductor.
In the present case there is no region of snow fall. Further, the swing of the
conductor under high wind velocity will be included in the tower design. The
conductor swing can take place only if there is a whirl wind of high intensity or a
tornado. But for this it is not practical to make provision in the design.
On the suspension towers, we may use single insulator string only, save the locations
of major road crossings, railway crossings, river crossings etc. For such locations,
double suspension insulator strings will be used. All the tension towers are required
to be provided with double tension insulator strings. The single tension insulator
strings shall be provided for the purpose of transposition arrangement only.
Grading rings are recommended for entire line on line side. The critical impulse
flashover voltage of 1700 kV is in order.
Shield angle for conductors shall be kept at 15º.
The uplift or take off angle on towers located in hilly region shall be taken care of by
strengthening the cross arm members.
For a better performance of the line we recommend that the ground resistance should
not be more than 10 ohms for each tower. Necessary measures shall be taken to
improve the earthing system if the earth resistance is found to be high.
The report suggests a depth of foundation to the tune of 15M. however, this may
apply only to pile type or augured foundations. For open type foundations the
maximum depth of 3.5M is recommended.
Since the line is radial with only one source of 400 kV, reactors are not required.
However, if the line is going to work below 25% of the load (less than 100MVA),
we may have to deploy shunt reactor/s at the receiving end. The capacity of the same
can be worked out only after the load pattern is studied.
It is presumed that the wind mills will not draw any reactive power from the grid and
will depend upon themselves for the same.
13)
RECOMMENDATIONS
1. It is seen from the above presentation that we can use ACSR Moose or ACSR Zebra
conductor with substantial spare capacity in double circuit. However, voltage
regulation in both the conductors will be different, so as the losses. The equivalent of
ACSR Moose and Zebra are available in AAAC also. However, there are
reservations. The generation is through wind mills and therefore the terrain is going
14
2.
3.
4.
5.
to be very windy. The AAAC conductor do have high ampere capacity and also the
required tensile strength which is equivalent to ACSR but the AAAC is not suitable
to take the fatigue stresses which are imposed due to high wind velocity and
consequent Aeolian vibrations. As the core is also Alluminium alloy, AAAC can not
withstand fatigue. It is known that the wind mill farms are established where there is
a wind at least for 7 months in a year, out of which 4 months is a minimum peak.
For AAAC this period is fatal. Since ACSR is having a steel core, it has a better
withstand capacity to the fatigue. The AAAC conductor is therefore not
recommended. The Twin ACSR Zebra would be sufficient which is proved
from the technical calculations done above.
The insulators can be ceramic type, toughen glass type or silicon rubber type. It is
reported that the Silicon rubber insulators have performed very well at all voltage
levels. However, the most unfortunate part in their usage is that the birds and
vultures tear apart the fins of the rubber insulator which ultimately results in to their
failure. The line under reference is passing through an area which is full of flora and
fauna may become susceptible to damage the insulators. Since the line is very long,
frequent breakdown maintenance will incur lot of financial loss. Therefore, it is
recommended to use ceramic or toughen glass insulators and design the towers
accordingly.
Out of the two earth wires, one on each side, one should be an OPGW and other can
be GSW 7/3.53 mm, 110 Kgf quality wire. This arrangement will serve the purpose
of communication, tele-protection and SCADA. In addition to this, it will also help
in providing a meter shielding against lightning, surges and switching surges.
Considering the importance of such a long transmission line it is necessary that the
line is provided a best protection against the lightning and switching surges. Even if
each tower is provided with proper earthing, it is likely that with the passage of time
the earthing material may be pilfered / damaged or become ineffective. This may
give rise to increase in potential across the insulator string during the surges. This
may result into damage to the insulators and frequent of strings. If the insulators are
able to sustain the impact of surges, the surges will travel to the sub-station
equipment and damage them. It is recommended that lightning arresters are
provided at a distance of 25 Km to 30 Km. There will be six lightning arresters
at each location for double circuit.
From the initial survey report it is gathered that the route of the line is mostly
passing through the hilly terrain. It is therefore recommended that counterpoise
earthing is deployed where earth resistivity is very high. It may be pertinent to
note that under no circumstances the earth electrode resistance is allowed to be
more than 10 Ohms. However, where soil is very soft or black cotton type,
counterpoise earthing will not be useful and instead pipe type earthing may be
done. If the earth electrode resistance is not found to be within 10 Ohms, it is
recommended to deploy multiple earths for the towers.
15
6. Since wind velocity and consequent Aeolian vibrations are very high, it is
recommended to use a spacer-cum-dampers for mid spans and rigid spacers for
jumpers.
7. The normal span may be kept as 400 Mtrs. However, the towers near Lake Turkana
may be spotted at the span of 370M to 380M (if the tower design is common for the
entire line).
8. The tower deign may be based on a minimum ground clearance of 9 Meters. The
other clearances may be kept as under:
 Clearance between conductor and ground wire at mid span – 9.0 mtrs
 Minimum clearance above communication line – 4.48 mtrs
 Clearance above rail track – 17.9 mtrs
 Clearance from the tower body when the insulator string or the jumper swings
0 deg swing – 3.05 mtrs
20 deg swing (jumper) – 3.05 mtrs
22 deg swing (insulator) – 3.05 mtrs
40 deg swing (jumper) – 1.86 mtrs
44 deg swing (insulator) – 1.86 mtrs
 Minimum clearance above high flood level of rivers and lakes – 6.40 mtrs
 The maximum operating temperature of the conductor may be taken as 75ºC.
9. Since the length of the line is 430 Km, it is recommended that the line may be
transposed at five locations along the line at a mutual distance of
approximately 70 to 72 Km. The last (sixth) transposition will be done on
receiving end gantry.
10.The deployment of ACSR Twin Moose conductor will be a techno-economical
proposition going by the figures of 10 years’ exploitation.
11.Under the unloaded or minimum load condition the voltage at receiving end may
increase. The installation of line reactors at receiving end may remedy the
situation.
12.Since the length of the line is very long, it may be worth while to install static
VAR compensators (SVC). This may comprise fixed capacitance automatically
switched capacitance and thyristor controlled reactors.
13.For above 11 & 12 proper load flow and system study will be required.
16
14 CONCLUSIONS
 The electrical design of 430 km line given above will lead to most reliable and
economical design of transmission line.
 The line losses and voltage regulation are also moderate and do not warrant
any undesired effect.
 The transposition on the line at 5/6 places will increase the efficiency of the
line.
 Since the capacity of conductor provided is higher than the required, there will
not be any problem in transmitting the power with one circuit.
 Shunt inductive compensation is required if the line is going to operate at
much lower (25% or less) loads.
15.
REFERENCES
1. Electrical Transmission and Distribution Reference Book by Westinghouse
Electrical Corporation
2. Electrical Engineering Design Manual by S. Parker Smith & M. G. Say
3. Generation, Transmission & Utilization of electric power by A. T. STAAR
4. Elements of Power System Analysis by William D. Stevenson
5. Transmission line manual – Publication no. 268 of Central Board Of Irrigation
& Power, New Delhi, India.
6. Power System Analysis and Design by B. R. Gupta
7. Modern Power System Analysis by D. P. Kothari & I. J. Nagrath
8. Principles of Power System by V. K. Mehta & Rohit Mehta
9. Electrical Power System by C. L. Wadhava
REPORT SUBMITTED ON 11TH SEPT.2008 PREPARED AT VADODARA.
S.M.TAKALKAR
PROPRIETOR
17
Appendix-I
OPGW
OPGW
GSW
4 0 0 k V DOUBLE CI RCU I T TOWER
Ap p en d i x - I
18
GSW