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Transcript
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