Asst.Professor,EEE Department, SSTC, Chhattisgarh, India, e-mail id: [email protected] Abstract In India, ACSR and AAAC are most commonly used conductors fortransmission of power through overhead lines for transmission.To meet theincreasedload demand either we have to construct the new UHV or EHV T/L for transmissionof bulk power through long distances. Uprating of transmission lines i.e.modifications in the existing transmission line to enable increased currentflow limits.
Making a new transmission lines also have few constraints:ROW constraints (Lack ofavailability of corridors for construction of new transmission lines due to HighPopulation Density, Forest conservation) and Time constraints (due to shorter time schedules, the constructionof transmission lines projects need to match with generation projects). Keywords: Ampacity, Uprating, HTLS,INVAR. IntroductionA rapid increase in electric power consumption iswitnessed which results the increase in demand of the uninterrupted powersupply.
The new generation units are being built with increased installedcapacity, but theexisting transmission lines are reaching their critical limitsof ampacity and there is shortage of corridorsparticularly in dense populatedcities to construct the new overhead lines specially in country like india.Mostof the times it becomes impossible to obtain a right of way for the newtransmission lines and hence present circumstances demands the use of availablelines with cheaper solution than going in for an underground transmission (1 Dae-Dong Leea, 2011) Theconstructions of new line have several disadvantagesIn addition, there is alarge limitation ofconstruction space, ROW issues and construction costs arevery high when rebuildingthe towers, hence the best suited method is to increase the operatingtemperature by adopting heat resistant aluminum alloy conductors .The purposeof developing a new type ACSR conductor was to double the current capacity by restringingconductors on existing steel towers Thus itis unnecessary to either rebuild orconstruct tower with longer in height, the steel towers to enlargethecapabilities of overhead transmission lines. New HTLS requireslowerconstruction costs, has a shorter construction period and does notneedlarger towers, larger conductors, or bundled conductors.
The structure,fittings, and construction methods of HTLS are designed to be the same as thoseof ordinary ACSR conductor (2S. Sakabe N. Mori, 1981).
During last fewdecades the world is going through a phase of rapid industrialization. A thetime the electrification in developingcountries in being carried out at a high rate and due this cumulative effect the power demand is increasing day by day. In response government and private projects fare involvedto increase the power generation, subsequently the transmission and distribution of increased required poweris becoming a great challenging for the utilities in terms of cost andcapacity, where the existing lines have reached their maximum limits. Hence onthe solution to build new lines parallel with existing one but this is not aneconomical solution. One of most cost effective solution is to adopting hightemperature low sag (HTLS) conductor for transmission and distribution.Theseare different from the conventional ACSR conductor in terms of material butsame in size. It can carry approximately 2 to 2.5 times the current thatof conventional ACSR conductors of same size and can withstand highertemperature (>200 °C),due to high current carrying capability the elongationof conductor is less, so the sag is very less.
One of the major advantages ofHTLS over conventional ACSR conductor is to re-conductoring the existing double circuit line with HTLS without disturbing the another circuit.Thepossibility of replacing conventional overhead conductors with new generations highperformance conductor is called high-temperature low-sag (HTLS) conductors, itis attractive choice particularly in those corridors which are thermally limitedand it can operate upto temperatures as high as 210 C, almost doubling theampacity of existing ACSR conductors. (3 Chatterjee, 2016)3, (4 Antonio Gómez Expósito, 2007)4. Characteristics of HTLS (High Temperature Low Sag Conductors): G(Z)TACSR (Gap Thermal Alloy Conductor Steel Reinforced),ZTACIR (Thermal Alloy Conductor Conductor Invar Reinforced) ACSS (Aluminum Conductor Steel Supported) Here G refers to Gap between steel and aluminum conductor and Z refersto trapezoidal same of aluminum coundctor. HTLS conductors are similar toconventional ACSR conductor in terms of electrical conductivity and geometrically.The main difference is that it offers the low coefficient of thermal expansionand as results of this HTLS can operate at a higher temperature with anincreased CCC (current carrying capacity) with maintaining same sag that oftraditional ACSR conductors. A) G(Z)TACSR In G (Z) TACSR type conductor is known as Thermal Resistance AluminumAlloy conductor Steel Reinforcement as shown in Figure 1, where inner core iscomposed of galvanized steel and outer layers are composed of thermal resistantaluminum conductor. A small gap is maintained between the steel core and theinnermost aluminum layer,and the gap is filled with heat-resistant grease toreduce friction between the steel core and the aluminum layer and to prevent ingressof waterand hence its improves corrosion resistance.
(5 G.Filippone, 2014) Figure1. Crossectionalview of G(Z)TACSR Conductor B.
ZTACIR Super thermal alloy (STAL) is made from Al-Zr(Aluminum Zirconium) alloy. The conductor comprises of an inner core of Aluminumclad Invar (36%Ni in steel) and outer layer are made of STAL wires..
Here the Figure 2 shows the cross sectionalview of (Z) TACIR conductor. (5 G.Filippone, 2014)Figure2. Crosssectional view of ZTACIR Conductor Instead of using conventional steel in conventional ACSR conductor, inZTACIR conductor, INVAR is used which is made of an alloy of steel and 36%nickel and as a results the coefficient of expansion practically becomelinearand it is invariable with application of heatand that’s why the name was givenasINVAR. Super thermal alloy contains Zr which deposits over the grain boundary ofAluminium, thus increasing the recrystalisation temperature of Aluminium whichenables STAL tooperate at high temperature without any loss in strength. C.
ACSS ACSS is known as Aluminum Conductor Steel supported as shown in Figure.3.(5 G.Filippone, 2014) In ACSS the core is made of round steel and aluminum strands are made oftrapezoidal shape.The steel wires may either galvanized wires or aluminum clad(aluminum coating). In ACSS conductors the aluminum wires can be the standardroundstrand or it may be trapezoidal aluminum strand. Figure.
3. Crosssection of ACSS Trapezoidal Conductor IN HTLSconductor the main modification is done on aluminum strands which arecompletely annealed wires and steel core which is made of INVAR strand andconductivity of core is enhanced by 14%, where in ACSR conductor theconductivity of core is almost zero. During stringing when tension is applied ontheHTLS conductors, the permanent elongation takes place quickly in aluminum wires,since the core is made of INVAR strands, where the coefficient of linearexpansion is invariable with temperature and as a results the sag of theconductor will be greatly reduced. In operating conditions, the coefficient of expansionof conductors is close to the value provided by the steel core, in the order of(10to 13×10-6°C),which is quite low as compared to conventional ACSRconductors i.e. order of (18 to 22×10-6°C) and results of this reductionin overall sag and therefore an increase in the ground clearance.Table1 Conductor Parameters Description Different Conductor ACSR G(Z)TACSR ZTACIR ACSS Area (mm2) 307.7 308.
4 306.9 307.7 Rated Ultimate Tensile Strength ( kgf) 9945 10960 10065 9900 DC Resistance at 20 °C(ohm/km) 0.108 0.
110 0.1106 0.107 Weight (gm/km) 1067 1097 1082 1067 Coefficient of linear expansion (10-6/°C) 18.8 11.4 16.3 11.
5 Here Table 1shows the parameters of different type of HTLS conductor having approximatelysame cross sectional area. (5 G.Filippone, 2014) The conventional ACSR and AAAC are designed to operatecontinuously at temperature of 85°C and 95 °C respectively. High Temperature Low Sag (HTLS)conductors are designed tooperate continuously at temperature of at least 180 °C.
Some HTLS conductors can beoperated as high as 240 °C. The new material used in HTLS conductor differs fromconventionalsteel reinforced ACSR. The new material includes INVAR steel (Fe-Ni alloy),temperatureresistant Aluminum-Zirconium (Al-Zr) alloys, annealed aluminum, highstrength steel and both metal & polymer composites.
A conductor in generalis a simple combination of core and aluminumand aluminum alloy. HTLS conductoris stranded with combination ofaluminum alloy wires for better conductivity andreinforced by steel core. Figure 4 Different scheme of uprating of Transmissionline Here figure 4 indicates the two different way of upraitng of transmissionline,firstly the uprating can be done by constructing a new transmisison line withtraditional ACSR conductor,by extra HV lines or with bundling of transmissionline or making bigger size conductor diameters.Secondan way to change theconductor with advanced material by increaing therir thermal raing. (Recommendation, 6) Different type of HTLS conductors areZTACIR (with INVAR steel core), GZTACSR (with specified gap between steel coreand inner layer of aluminum wires), ZTACSR (with steelcore), ACSS (with steelcore)The TACSR, GZTACSR, ACSS and ACCR areavailable with both round wire and trapezoidal Al-Zr alloy wires in theoutermost layer. ACCC uses only trapezoidal annealed aluminum wires. GZTACSR,commonly known as Gap type conductor, the Gap is filled with heat resistantgrease (filler material) to prevent water ingress and improves the corrosive resistance,such type of conductors are mainly required in coastal areas. Advantages of HTLSover Conventional ACSR conductor is as shown in Table.
2 Table 2 Build a new Line Replacement of old conductor with HTLS conductor Build period Transmission line approx.30 km 18 month 6 month Construction cost Preliminary work Required Not req. Cost of right of way High cost No cost Tower foundation Required Not req. Conductor cost required 2 to 3 times the ACSR Stringing cost Required Required Here Table 2 shows the comparative analysis of constructionof new transmission line and replacement of old conductor with new HTLSconductor. (upgrading., 7) MaterialsusedSTAL wire containing Zr(Zirconium)element which hashighly improved annealing property, without loss of tensile strength..
HTLS using Al cladinvar has low thermal coefficient of expansion (approx 1/3 rd) of steel at Temp210°C. Figure5. Annealing characteristic of STAL wires Figure 6.Coefficientof thermal expansion The ordinary hard drawn aluminum wires used inconventional ACSR, start losing tensile strength at 90°C and therefore itis not suitable for long term use at temperature above this. Al-Zr aluminumalloys wires have the same conductivity and same tensile strength as ordinary ECGrade aluminum wire but it can operate at higher temperature range upto 150 to 200°C. In India since lastfew years, the need for use ofHTLSconductors in some corridors has been felt.
The power flow in thosecorridors has increasedand congestion has been reduced by using suchconductors. Such conductor would be requiredwhere the power transfer over theline is constrained due to consideration of thermal loading. InIntra-state transmission system, requirement of suchconductor is expected at 220kV, 132kV and66kV level. The requirement of suchconductor may not be much in ISTS, which is dominatedby 400kV and 765kVnetwork. In case of ISTS lines, the HT/ HTLS conductor would be agoodsubstitute to Quad bundle ACSR and AAAC conductor, particularly at 400kVlevel when linelength is short.
Therefore the HTLS conductor canbe considered for reconductoring of existing lines and can also beused in newlines. The cost of such conductor is about 2 to 3 times the cost ofconventional ACSR conductor. (8) Draft guidelines for HTLS Conductors CEA, 20168) Methodology:Power Line System –Computer Aided Design and Drafting (PLS-CADD)is the most powerful and comprehensive program/tools for thestructural and geometric design of overhead lines. It covers all environmentaspects of transmission line design, including terrain modelling, routeselection, manual or automatic minimum cost spotting, sag-tension, clearanceand strength checks, plan & profile drafting and much more. For new transmissionlines projects PLS-CADD will significantly increase capabilities andproductivity of line.reconductoring of existing lines and can also beusedin new lines. The cost of such conductor is about 2 to 3 times the cost ofconventionalACSR conductor.
By placing six nos of 400 KV tower structure in plain terrainplaced approx 400meter apart in by using PLS CADD Tools as shown in Fig.7 Fig.7Model of 400KV Transmission line using PLS Cadd Case Studies:Case 1 :Comparison when maintaining same Current and their operating temperature: Table 3 Description MOOSE (ACSR) ACCC-Moose (HTLS) Calculations are carried out at temp degree 85 76.70 Current to be maintained: 902 902 AC Resistance (ohms/km) 0.
0687 0.0532 Line losses in kW/ckt 168 130 Power Factor 0.85 0.85 Power Transferred in MW/ckt 531 531 Price Loss (in Lacs Rs/KW) 256 198 Fig.
8 Comparisonchart of ACSR Moose and ACCC Moose (HTLS) conductor when operated at samecurrent rating Conclusion of Case 1: The maximum operatingtemperature of ACSR Moose conductor is 850C and maximum currentcarrying capacity is 902 Amps in specified working condition, therefore thecomparison is done at 902 Amps between ACCC-Moose (a type of HTLSconductor)conductor and ACSR conductorand all the calculation is done based this ampere rating.ACSRMoose conductor reaches 902 Amps at 850 C (operated at maximumoperating temperature level) and while ACCC-M achieved this current rating atreduced temperature level i.e. 76.70 C (well below the maximumoperating temperature level i.e. 1800C ).
ForACSR conductor the ac resistance is 0.687 ohm/kms whereas for ACCC-M conductorthe ac resistance is only 0.0532 ohm/kms which is quite lower as compared toACSR Moose conductor and as a results of this the line losses will be lowerside i.e. 130 kw/ckt which is approximately % lower than the ACSR Moose conductor. ForACCC-Moose the price losses will be only 198 (Lacs/kw/ckt) as compared to 256(Lacs/kw/ckt) of ACSR Moose conductor. Case-2 : Comparison whenmaintaining maximum Current in Amp at maximum continuous operating temperature:Table 4 Description MOOSE (ACSR) ACCC-M (HTLS) Calculations are carried out at temp degree 85 180.00 Current to be maintained: 902 1960 AC Resistance (ohms/km) 0.
0687 0.0706 Line losses in kW/ckt 168 814 Power Factor 0.85 0.85 Power Transferred in MW/ckt 531 1154 Price Loss (in Lacs Rs/KW) 256 1242 Fig.9 Comparison chart of ACSR Moose and ACCC Mumbai (HTLS) conductor when operatedat maximum current and maximum ratingConclusion of Case 2: In this case both theconductors are operated at their maximum operating temperature and maximumcurrent carrying capacity. The maximum operating temperature of ACSR Mooseconductor is 850C and maximum current carrying capacity is 902 Ampsin specified working condition, whereas the maximum operating temperature ofACCC-M conductor is 1800C and maximum current carrying capacity is1860 Amps and all the comparisons were done based their maximum operatinglevels.• ACSR Moose conductor islimited to operate upto 850 C maximum while ACCC-M can be operatedupto much higher temperature level i.
e. 1800 C .• The power transfercapability of ACCC-M is 1154 (MW/Circuit) almost doubled the power transfercapability of ACSR conductor which is 531(MW/), it means by for transferringsame amount of power by using ACSRconductor ,we have to construct the another transmission line, that will becomeanother time consuming and costly project. Fig. 10 Current V/s Temperature curve for ACSR and HTLSconductor HereFig. 10 represents the Current V/s Temperature curve, curve shows thatACCC-Moose (HTLS ) can be easily operated upto 200 0C, but themaximum operating temperature of ACSR Moose conductor is 85 0C only(Thermal limit).
v Note: For comparison purpose only, upto 200 0C thereport were calculated for the ACSRconductor, otherwise the ACSR conductor can’t be operated above 85 0C. Fig. 11 Resistance V/s Temperature curve for ACSR andHTLS conductor HereFig.
11 represents the Resistance V/sTemperature curve, curve shows that ACCC-Moose (HTLS Conductor) can beeasily operated upto 200 0C with minimum resistance but the maximumoperating temperature of ACSR Moose conductor is 85 0C only (Thermallimit) and because of less resistance ascompared to ACSR conductor, HTLS conductor offers less (I2R loss). Fig. 12 Temperature V/s Power Transfer Capability curve HereFig. 12represents the Temperature V/s Power Transfer Capability curve, whichshows that ACCC-Moose (HTLS Conductor) can be easily operated up to 200 0Cwith better power transfer capability as compared to ACSR Moose conductor. Fig. 13 Temperature V/s Power Loss curve HereFig. 13 represents the Temperature V/s Power Loss curve, curve shows thatACCC-Moose (HTLS Conductor) can be easily operated upto 200 0C withbetter power transfer capability as compared to traditional ACSR Mooseconductor.
ConclusionsIn present scenario the majordifficulties of construction of new transmission line is to get right of way(ROW) approval from public and local administrations. For this squeezing morepower into existing corridors in becoming quite crucial and for which the HTLSconstitutes a attractive and cheaper solution. These conductors being capableof working at over 200 °C,withdouble the ampacity as compared to conventional ACSR conductor with maintainingapproximately same sag or ground clearance.In growing congestion in existing corridor oftransmission and distribution network, the enhancement of power flow per unit(or meter) of Right of Way and reduction in losses under normal as well as underemergency condition is highly recommended.
High Temperature Low Sag (HTLS)conductors should be considered in those corridors where the power transferover the line is constrained due to consideration of thermal loading ofconductor. In Intra-state transmission system, requirement of such conductor isexpected at 220kV, 132kV and 66kV level. In case of ISTS(Inter State Transmission System) the HTLS conductor would be a excellent substituteto Quad bundle ACSR and dual HTLS conductor, particularly at 400kV level whenline lengths were short.