Australian Journal of Basic and Applied Sciences, 5(1): 090-097, 011 ISSN 1991-8178 Technical and Economic Assessment of Upgrading a Double-circuit 63kV to a Single-circuit 30kV Transmission Line in Iran 1 Reza Sirjani and Badiossadat Hassanpour 1, Science and Research Branch, Islamic Azad University, Tehran, Iran. Abstract: This paper presents a feasibility study of upgrading 63kV double-circuit to 30kV singlecircuit transmission line by minimum changes. The case study in this research is a double-circuit 63kV transmission line in Iran. Different phase coordinations are considered and four alternatives are proposed and compared in mechanical, electrical and structural views. Finally the best alternative is selected and economic evaluation has demonstrated that this upgrading is almost two million dollars cheaper than constructing a new 30kV transmission line near the existent line. Key words: Transmission Line, Upgrading, Line Post Insulators, Phase Coordination. INTRODUCTION Constructing new power transmission lines leads to onerous expenses so engineers have decided to construct new lines on existent line corridor with higher voltages. The main object of feasibility study of transmission lines upgrading is consideration of necessary transformations on existent line to increase transferable electric power by increasing the voltage. In this method, available towers remain on the previous locations and number of circuits, type of insulators, and electrical clearances will change. The case study in this research is a double-circuit 63kV transmission line in Iran which located in Balouchestan and its length is 53km. Although we have 63,13,30 and 400 kv transmission lines in Iran, ever-increasing demands of electrical energy in this zone and high strength towers used in this line caused to do this feasibility study for upgrading this transmission line to the second higher voltage level (30kV). Since we want to keep existent conductors, designing a Single circuit two bundle transmission line" by decreasing the number of circuits is the best and the most economic method to upgrade a double-circuit line. According to our researches for upgrading this line by minimum possible changes, using Braced Line Post Insulators is the most practical solution. In this way we can keep the allowable electrical clearances of conductors from tower body and ground. Moreover the previous Right of Way remains applicable. In this paper, different phase coordinations are considered and four alternatives are introduced. Firstly, mechanical conditions such as: maximum single span, distances between conductors and shield wires in midspan, and clearances are considered and finally the best alternatives are chosen. Secondly, electrical conditions such as: maximum conductor ampacity, maximum transferable electric power, maximum voltage gradient, corona losses and shield angle are evaluated. For this aim, software has been produced to calculate electromagnetic fields around the line and to evaluate Right of Way for these alternatives. The next part is tower loading and structural consideration. In this section the best alternatives are chosen in new structural conditions. Finally we have compared all chosen alternatives in mechanical, electrical and structural views and selected the best one. An economic study has been appraised expenditures of this conversion compared to the cost of constructing of new 30kV transmission line near the existent line. MATERIALS AND METHODS Consideration Of Appropriate Design To Increase Line Voltage: For upgrading a 63kV double-circuit to 30kV single-circuit transmission line by minimum required changes, using Braced Line Post Insulators is the most practical solution (Laforest, 000). These Ceramic or composite insulators divided into two types: Braced and Un-Braced. Also they will be useful because of their more creepage distances and capability to use in longer spans and heavier conductors. (Laforest, 000; IEC 1109,003; IEC 6195,00). In this consideration we can suggest four alternatives by replacement of line post insulators to cross arms in existent towers. The double-circuit 63kV existent tower design is shown in Fig.(1) and four alternatives for using line post insulators to upgrade the existent design to a single-circuit bundle 30kV are shown in Fig (3). Now we will study and compare these alternatives mechanically, electrically and structurally. Corresponding Author: Reza Sirjani, Science and Research Branch, Islamic Azad University, Tehran, Iran. E-mail: reza.sirjani@gmail.com. 090
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 Fig. (1) shows the types and installing methods of these insulators on tower: Fig. 1: Line Post Insulators. Fig. : Existent tower design. 091
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 Fig. 3: Alternatives under consideration. Assessment Of Mechanical Conditions In The Existent And Upgraded Line: Existent line characteristics are as follows: Length of line: 53 km Elevation: 1195 m Average days by thunderstorm: 30 days/ per year Conductor: HAWK/ACSR Shield Wire: LYNX CORE two shield The zone loading conditions are presented in Table (1): Table 1: Loading Conditions. No. of Case Loading Case Wind(m/s) Ice(mm) Temperature ( C) 1 Wind & Ice 0 6.5-10 High Wind 40 0 15 3 Heavy Ice 0 15-5 4 (EDS)Every day stress 0 0 3 5 Minimum Temperature 0 0-10 6 Maximum Temperature 0 0 50 According to weather condition Sag-Tension can be calculated in all spans for conductor and shield wire (Laforest, 000). Phase-Shield Clearance In Mid-Span: According to structure list of existent line, maximum length of spans is 380m. We have considered two cases, EDS (Every Day Stress) and Lightening, and calculate the clearance of phase to shield in these cases for each alternative. Experientially, lightening case conditions is equivalent to 30% of high wind. Fig. 4: Distances between conductor and shield. 09
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 From Fig.(4) : C(actual) =ƒ c + a ƒ s (1) Every Day Stress: ws 0.975 380 f c 1. 6m 8 H 81435.5 0.335 380 f S 7. 11m 8 850.5 Lightening ( 30% high wind): ws 1.77 380 f c 1. 1m 8 H 81739.4 0.4 380 f S 7. 47m 81013.7 In which a is distance between phase and shield on the tower, f c and f s are sag of conductor and sag of shield wire in meter, H is tension in Kg, w is unit weight of wire in(kg/mm ) and S is span in meter. According to NESC (National Electrical Safety Code) the minimum allowed clearance of phase to shield in EDS case is 3.5 m. Moreover this mid-span clearance in lightening case should be more than 8.5 m. Clearances in all alternatives are suitable in EDS but only the second alternative clearance is enough in lightening condition. Maximum Single Span And Phase To Ground Clearance: In the under consideration line, maximum span between tension towers and suspension towers are 380 m and 378 m respectively. We use Thomas formula in good weather condition because of lack of freezing and galloping phenomenon in that zone (Fink Ronald and Wayne Beaty, 1999). C % f D Spacing A w () Spacing: Distance of phases (ft) W: unit weight of conductor (lb/ft) C: experiential factor depends on weather conditions D : diameter of conductor (inch) l : length of suspension insulator string if any (ft) %f : percent of sag of conductor A : flash over distance ( 1ft for 110KV) C for this 63kV transmission line is 1.87 (Fink Ronald and Wayne Beaty,1999). As the weather conditions do not change and the existent line is operating without any problem, we use the same C factor in this converted design (Fink Ronald and Wayne Beaty, 1999). Now we getting allowed vertical clearances for this upgraded 30kV line: % ƒ = 4. Also the spotting parameter of existent line is a=100m and maximum span(s) is: % f S 8 a 405m 380m 100 Therefore existent spans will be suitable in new condition and it is not necessary to add new towers or change conductors' tension force. Assessment Of Electrical Conditions In The Existent And Upgraded Line: Conductor Ampacity And Transferable Electric Power: This line is operating in double-circuit form and its conductor is HAWK/ACSR. Thus maximum conductor ampacity is: 093
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 I Max (75 C) = 39.38 Amp We have calculated total transferable electric power in the suggested alternatives and as indicated in Table(4) the second alternative is the most suitable design. Corona Phenomenon: The main factor in corona phenomenon is the maximum surface gradient on conductor. Maximum surface gradient should be lower than 17 kv/cm in good weather condition. The method to calculate maximum surface gradient according to existent references is : E MAX 18 c V n r (3) In which : V : Phase Voltage n : number of bundles c: capacitance r : radius of conductor The influence of conductor bundling on surface gradient reduction showed all of the alternatives are suitable for upgrading to 30 kv. Ohmic Losses: The following formula is used to calculate power losses in transmission lines (Laforest, 000) : 1 Ploss 3 Rac I I I c Sin I c MW 3 In which: R ac : AC resistance l : legnht of line I φ : Maximum transferable current in conductor (4) I c V 3 X c Sin 0.31 The power losses should be lower than %5, and the results represented in Table(4) show that these losses are acceptable in all alternatives. Corona Loss Calculation: Both corona loss and corona critical voltage depends on air conditions and transmission line characteristics. Corona loss in good air conditions is limited to the following quantities (Laforest, 000): Table : Acceptable limit for corona loss in each voltage. Line Voltage( kv) Acceptable Losses(kW/km/3φ) 13 0.06 30 0.6 400 1 V cr Critical voltage of corona is calculated as: D 3 1.1 m re ln r In which : (5) 094
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 δ is air density factor that given from air pressure and temperature m is constant factor that proportionate to conductor surface. In ACSR coductors this factor supposed 0.83 to 0.88. D is GMD (Geometric Mean Distance of phases) And r e is given from this spinning formula: D ln n. S rc re S D (6) ( n 1) Sin ln r n r e In this way calculate V cr for all alternatives. If Vph/Vcr <1.8 use experimental Peterson formula in good air conditions: V ph 6 Pc 1.1 f F 10 ( kw / km / 3 ) D log (7) r In which : f is frequency of system in Hz V ph is Voltage of phase to ground in KV V cr is Critical voltage of corona KV D is Geometric mean distance in m r is Conductor radius in m F is Constant factor depends on Vph/Vcr The obtained results show that Corona loss in all of alternatives are low enough and no important to determinate the best alternative. Shield Angel: This angel is considered for 4 alternatives from existent formula and curve in (Laforest, 000). Table 3: Shield angle for each alternative. Alternative Alt 1 Alt Alt 3 Alt 4 Shield angel 8.7 18.16 8.7 8.7 Outages per Year 0 1.8 0 0 Only the second alternative has 1.8outage/100miles/year during 30 days. Also the average number of days with storm and lightening in this zone is 30.Thus:: Number of 1000 shielding failuare 1.8 1.15outage /100km / year 1600 The line length is 53km and this number of shielding failure will be 0.59. If we suppose the probability of lightening involved in tower is 60 percent, number of shielding failure will be 0.36 which is lower than allowed failure number in 30kV transmission lines (1.5 outages per year) (Laforest,000), so four alternatives are suitable in this case. Right of Way: We account allowable limit of electric field around transmission lines is kv/m and then calculate Right of Way for each upgraded alternative by prepared software. According to the last approval of Iranian Ministry of Energy, Right of way for 63kV and 30KV transmission lines are 13m and 17m respectively. Our calculations show that electric field resultant in the distance of 13m around this line is lower than kv/m and we can conclude that the previous Right of Way is suitable for each alternative.a sample curve to calculate electric field around the line is shown in Fig.(5). 095
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 Fig. 5: Electric field in different distances from tower. Tower Loading: To upgrade a single-wire double-circuit transmission line to a bundled single-circuit one, in addition to new suggestions for cross arm designing, it is necessary to control tower analysis. There are not important differences between transverse and vertical loads in converted design and the previous one. But lack of vertical load symmetry especially in the first and third alternative will make bending forces to tower members. These cases should check by manufacturer. RESULTS AND DISCUSSION Choosing The Best Alternative By Technical And Economic Considerations: Table (4) indicates the conclusions of comparison in various cases: Table 4: Different alternatives comparison. Cases Alt.1 Alt. Alt. 3 Alt. 4 Transferable Power (MW) 45 56 47 49 Corona Limit(KV/cm) 1.37 13.7 1.54 1.7 Ohmic losses percent 0.815 0.81 0.815 0.813 Bending forces to tower members High Low High Low Shield Angle Suitable Suitable Suitable Suitable Corona Loss 0.055 0.08 0.053 0.061 Vertical Clearances (ph-ph) Suitable for all Spans Suitable for all Spans Suitable for all Spans Suitable for all Spans Clearance (ph-shield) in mid-span Not suitable Suitable Not suitable Not suitable Right of Way Suitable Suitable Suitable Suitable As seen in all cases, the second alternative is preferable and we choose it for this upgrading project. Economic Evaluation: In this part firstly we have calculated expenditures of this conversion performance. Secondly we have explained the benefits of this design for economic saves and thirdly compared the costs of this upgrading to new 30kV transmission line project. Table (5) showed that this upgrading saves almost two million dollars that is 54 percent of a new 30kV transmission line project costs. Conclusion: This paper presents a technical and economic assessment of upgrading a double-circuit 63kV to a single-circuit 30kV electric transmission line in Iran. Four alternatives by replacement of line post insulators to cross arms in existent towers are proposed and compared in mechanical, electrical and structural views. High strength towers, line shortness, suitable conductors, and smooth routes are the most important reasons to make this design practical and we can introduce this project as the first upgrading in transmission lines in Iran. Moreover difference between the cost of this upgrading and a new 30kV transmission line project, proved the benefits of the proposed method. 096
Aust. J. Basic & Appl. Sci., 5(1): 090-097, 011 Table 5: Cost evaluation of performance. Expenditures of suggested design performance Price in Iran ($) Costs($) New fitting for 30KV bundle 886$ per km 15,960 Black out Cost We have accounted 5hours for black out in upgrading projects and supposed the economic lost 0 MW per hour. Also the cost of each KW is 0.3$ per hour. ( The period of performance of project accounted 1 months which is more than real time) 1,083,870 Installing new insulators and conversion on cross arms 36$ for each Tower 538,710 Purchase of Line-Post insulators The mean price of double Line-Post insulators is 500$ and each MS tower needs 3 insulators. (10% of insulators are supposed unutilizable) The mean price of quadruple Line-Post insulators is 1000$ and each HS or T-60 tower needs 3 insulators. (10% of insulators are supposed unutilizable) 180,440 17,450 Purchase of Conductors We have 6 conductors in previous single- wire double-circuit line and can use them for new bundle single-circuit design. Nothing Total Costs,156,070 $ ACKNOWLEDGEMENT The special thank to Miss Ziba Fakheri Darian, the kind manager of Technical Unit of Transmission Line Department in Power Engineering Consultant Office (MOSHANIR). Her supervision and support truly help the progression and smoothness of this research. REFERENCES Fink Ronald, C. and H. Wayne Beaty, 1999. Standard hand book for electrical Engineers, 30th edition. IEC., 1109, 003, Composite Insulator for a.c overhead lines with Nominal definitions > voltage 1000 V, IEC Standard. IEC., 6195, 00, Insulator for overhead lines composite line post Insulator for alternative current with nominal voltage >1000 v, IEC Standard. Laforest, J.J., 000. Transmission Line Reference Book/345 kv and Above, Second Edition. National Electrical Safety Code (NESC)., 199. The institute of Electrical and electronics engineers, Inc. 097