Project of overhead lines
1. A project of overhead lines includes: plan of the site, overhead lines’ profile, climatic conditions from the weather stations and EIR, a cord according to EIR and calculations, typical projects of supports agreed upon with the customer.
2. The permissible tension for the line of used wires could be indicated in typical supports projects. If not, find the permissible stress on supports of the chosen typical supports projects, sort out the permissible tension based on this stress on the wires. Compare the found permissible tension based on the stress affecting the supports with the maximum permissible tension at average annual temperature in the wire according to EIR, National Standard, Technical Conditions or the manufacturer’s data. Based on the lowest value, we sort out the reference maximum permissible tension in the wire and at average annual temperature for our project.
Based on the calculation of voltage losses in the network of 10kV, self-supporting insulated wires СИПЗ(50) with the sectional area of 50mm2 were selected. The supports allow for the stress not higher than 640daN per a phase. This is why the line designer accepts the maximum permissible tension as 640/50 = 12.8daN/mm2 of the wire’s sectional area. The same tension the designer uses for the mode of average operating temperature. The calculations of mounting stresses and sag are performed (including our programs) and the mounting tables for the mounters are prepared.
The experienced mounters could see without any calculations that the sag were considerably understated (the wires will be overdrawn) and were right.
According to EIR, these wires allow for the maximum tension of 11.4 daN/mm2 at the maximum load and lowest temperature; 8.5 daN/mm2 – at average annual temperature.
Two mistakes are obvious:
1. The maximum permissible tension was increased from 11.4 to 12.8daN/mm2 at the maximum load and the lowest temperature.
2. The maximum permissible tensions was increased form 8.5 to 12.8 daN/mm22 at average annual temperature.
The same permissible tension could occur for a wire, cord, self-supporting cable in all modes, but not for the given conditions. Besides, it is never higher that at average annual temperature according to EIR, National Standard, Technical Conditions.
Thus, the mechanical calculation of the wire was not performed, the overall span was not accounted for. It is unknown if the clearances for undercrossing the lines of higher voltage were provided.
This is an obvious violation of EIR, but such a project was done and accepted by the customer.
3. Perform calculation of the wire in LineMech, inputting the reference maximum permissible tension at average annual temperature as well, defined during the previous step. The minimum span, pitch of calculation, and the maxim calculated span are approximated, based on the preliminary estimation of possible spans of the overhead line. If the reference catalog of the program does not contain the data on this wire, input them according to the recommendations given on this page. When entering the climatic conditions, follow the recommendations of EIR and “Help” file for the program
The theory of mechanical calculation and an example of “manual” calculation are given on “Theory and Practice” page.
In all the programs the basis for the calculation by permissible tension method is defining the parameters for mounting the cables, such that the tension in the wire during maintenance in the three modes denoted in EIR – the maximum external load, the lowest temperature, average annual temperature – was never exceeded. The mode of the maximum external load occurs either in case of maximum wind or in case of ice with wind. These are the defining modes calculated in LineMech. The rest of the modes are needed to make decisions or for other calculations.
4. Based on the height of wire attachment on mass-supports, regarding or regardless of suspension insulator strings, define the overall span. The overall span is a span the length of which is defined by the norm vertical distance from the wires to the ground when the supports are placed on an ideally even surface (EIR). To specify the overall span it is convenient to make calculation in LineMech with the pitch of 1 meter in the required diapason of the spans. Depending on climatic conditions and characteristics of the wire, the overall span could be defined either in mode 2 (ice, temperature of ice accumulation, no wind) or in mode 7 (highest temperature, no wind) for the maximum sag. Based on the value of the overall span, the planning of spotting the supports of the overhead line of 0.4kV is performed, sometimes of the overhead line of 10kV, or the supports-spotting pattern for the line profile is prepared. When creating the pattern, it is necessary to make some clearance reserve, see manuals. The preliminary spotting of the supports is done with some clearance reserve at crossings with engineering objects and natural obstacles, using higher or lower supports if necessary.
Defining the sag (modes 2 and 7) inattentively and, correspondingly, making an incorrect decision on the overall span could lead to possible revision of the project. In the programs presented to you, the sag is checked only by LineCross program. There were some inquiries from design organizations: “Why does your program changes the working temperature at crossing from +40 to -5C and (vice versa). This means that the designers were inattentive when analyzing the results of mechanical calculation of the wire and made an incorrect decision on the overall span, and this was not discovered until calculating the crossings clearances. This is sad.
4а. The patterns for spotting the supports on the line profile could be received in LineMechCad - the extension of LineMech program.
The line designer decided to show the sagging curves of wires by LineMachCad at different temperatures on or the supports-spotting profile provided by the customer to provide clearances. This solution for making the line profiles is more vivid that with no sagging curves for wires.
5. After the initial spotting of the supports there is an opportunity to make calculation of equivalent spans in strain sectors and specify the sagging curves of the patterns for different strain sectors. Based on the value of the equivalent span and the calculations by LineMech, new patterns are prepared and the spotting of the supports is controlled with their help. The control over the supports-spotting is not always necessary, it depends on the correlation of spans to critical spans (see manuals).
Treat each strain sector as a separate line. If there is a possibility to decrease the operating permissible tension in the wire of the strain sector, it should be decreased, by making the necessary calculation with the lowered tension parameters as related to the norm. In case of unfavorable climatic conditions, the overall breakdown rate on the entire route will decrease. Provided there are programs available at all times, it would be unacceptable not to calculate each strain sector, especially, in case of considerable difference in equivalent span.Treat each strain sector as a separate line. If there is a possibility to decrease the operating permissible tension in the wire of the strain sector, it should be decreased, by making the necessary calculation with the lowered tension parameters as related to the norm. In case of unfavorable climatic conditions, the overall breakdown rate on the entire route will decrease. Provided there are programs available at all times, it would be unacceptable not to calculate each strain sector, especially, in case of considerable difference in equivalent span
Digression. Using the spotting of the supports showed in the previous step, after the initial spotting of the supports there is some clearance reserve in all the spans. The line designer decided to lower the permissible tension in the wire by several percent, providing higher reliability for the entire line.
5а. The modification of LineMech program LineMechCad provides, besides the support-spotting patterns, modeling of sagging curves of wires, cords at different temperatures. The clearances at different temperatures.
6. For spans of crossings with engineering objects and natural obstacles the calculation should be done in LineCross using the same source data for climate and tension. The required clearances at operating temperatures are input according to EIR. Some shifting or replacing of the supports with heigher or lower supports is possible.
The theory of crossing calculation and an example of manual calculation are given on “Theory and Practice” page..
6a. In modification of LineCross program LineCrossCad, the draft of the crossing is parepared at a particular scale to simplify preparing of the crossing’s layout plan.
7. With the help of LineMechCad it is possible to prepare the sagging curve of the wire at the lowest temperature and define if the calculation for the balancing load is required in case insulator strings are raised up or replacing an intermediate support with a strain one.
8. If all the clearances are provided, the loads on supports are minimal, the following note could be made to the project:
“With the purpose of decreasing the loads on supports, providing the permissible crossings clearances and sag, the permissible tension in the wire (indicate the type) accepted for the project for the denoted sectors is:
1-2, 5-6 - maximum - ... daN/mm2; at average annual temperature - ... daN/mm2;
2-3 - maximum - ... daN/mm2; at average annual temperature - ... даН/мм2;
3-4 - maximum - ... daN/mm2; at average annual temperature - ... daN/mm2;
4-5 - maximum - ... daN/mm2; at average annual temperature - ... daN/mm2."
9. Defining the tension in the cord
The first and foremost task is to provide the required vertical distance between the wire and the cord in the middle of the span. This distance depends on the length of the overall span of the overhead line and is indicated in 2.5.16 table of EIR. The distance between the wire and the cord is defined based on conditions of lightning-proofness at the temperature of +15C.
Some information on this is given on this site on page Ground wire. Optical ground-wire cable. Defining the tension (manually and explanations for the programs) of “Theory and Practice” section.
The second task is to decrease the load on supports and ground wire peaks.
Provided the length of the wire in the sector and its sag in mode 5 of LineMech program, find out what sag is permissible for the cord. It is clear that to provide the distance between the wire and the cord according to table 2.5.16 of EIR, the sag of the cord should be less than that of the wire. Define the sag of the optical ground-wire cable in advance using the formula on page Ground wire. Optical ground-wire cable. Defining the tension (manually and explanations for the programs) of “Theory and Practice” section.
10. Perform calculations of the cord in LineMech program decreasing the permissible tension starting with norm and sort out that the required sag of the cord (mode 5 of LineMech) is provided at definite operating permissible tensions, maximum and at average annual temperature. There is the solution. The value could be different
Make a note to the project:
“When providing the necessary distance between the wire and the cord in the span according to 2.5.16 of EIR, alongside the decrease in the loads on supports, the maximum permissible tension in the cord (indicate the type) accepted for the project is …daN/mm2, at average operating conditions – …daN/mm2”. If the values for some sectors of the route are different, give the data for them as well.
Attention! To exclude the overlapping between the wire and the cord it is recommended to verify the distance between the wire and the cord in case of ice accumulation for the variant when there is ice on the cord and no ice on the wire.
11. Verify the deflection of insulator strings due to the influence of maximum wind and stress at the lowest temperatures. Replacing the intermediate support with a strain support is possible, moving the supports around, recalculation of the equivalent spans, verification of clearances. Calculation of ballasts. Use LineLoad programm.
Upon the requests of line designers and constructors, some sectors could call for decreasing the operating tension in the wire and the cord to lower the load on supports, due to the conditions of fixating the supports in the ground. In this case, to make the decision, the calculation should be done once again from the beginning. Some of the previously provided clearance reserve could be used.
12. The basis for mounting are the calculations according to LineMount program.
The calculation is done for the sectors of the line route with the accepted climatic conditions and permissible operating maximum stresses (tension) justified by previous calculations.
The theory of calculating mounting stresses and sag “manually” and some explanations for the program could be found onе “Theory and Practice” page.
13. When mounting, it is necessary to provide some overdraw.
The calculations (by default) do not take into account the further creep elongation of the wires, cords and self-supporting cables (in maintenance). This is why when designing it is necessary to give recommendations to decrease the mounting sag by 3-5-7-10%. Taking into account the further creep elongation, mounting stresses and sag are calculated in % from the elongation indicated for the wires, cords in reference literature, for cables – in Rules on Mounting Self-Supporting Optical Cables.
To avoid misreadings, it is better to make a note under the table with mounting stresses and sag:
“The table of mounting stresses and sag of the wire and cord are given regardless of further creep elongation. When mounting the wire and cord, decrease the sag by 5-10%”,
“The tables of mounting stresses and sag are comprised considering the further creep elongation provided the time of mounting before fixating the cable clips”.
We do recommend to supply the project with both calculation, regarding (settled stress mode) and regardless of the elongation, so that the mounting organization could control the elongation.
14. The results of mechanical calculation of wires and cords in their final form are justifying materials, they are not attached to the projects, if not otherwise indicated by the customer, they are kept in the archive of the design organization. Only the solution based on the calculations are announced in the project (see above).
The calculation results of the crossings' clearances also could be not attached to the project, except for the final table put on the crossing’s layout plans, in this case the justifying materials for the calculation of crossings should be kept in the project archive on the designing organization. Recently, it has become customary to put the entire calculation results on the plans.
The calculation results for mounting stresses and sag are the task for the mounters for mounting the wires and cords and are a part of project and estimate of cost documents.
15. According to supports-spotting catalog and calculation results by the program, there is an option of preparing a new form of calculation results in AutoCad (LineMountCad program), promising for using in projects of an overhead line – support-by-support scheme of an overhead line.
The received form of the calculation results could be used in the projects of an overhead line, for example, when replacing wires, ground wire, vibration dampers according to new requirements, especially when the line route profile is lost.
This form could replace the customary tables of mounting stresses and sag due to its being visual and easy to create.
Taking the measurements on the existing lines, it is easy to justify possible reconstructions of overhead lines.
16. LineMount (LineMountCad) program with damp module provides calculation of vibration dampers.
Requirements for wires' and cords' characteristics for the reference catalog of the programs (the cables’ characteristics should be asked from their manufacturer):
1. Diameter, mm – the outer diameter of the wire, cord, self-supporting insulated cable including the insulation. Experiences wind and ice loads.
2. Sectional area, mm2 – sectional area of the solid part of the self-supporting cable – (reference sectional area). For calculation sakes the air between the lashing wires, greasing and other similar materials are not taken into account. Experiences stress applied to the wire, cord, the supporting part of self-supporting insulated wire, cable due to its own weight, tension, wind and ice loads, temperature changes.
3. Weight per unit length, kg/m – the weight of 1 meter of the wire, cord, all the self-supporting insulated wire, cable. 1 kg=0,981 daN=9,81 N (approximating these to 1 daN, 10 N practically does not affect the calculation results).
4. Modulus of elasticity of the cable, daN/mm2 - 1,0 hPa=1000000000 Pa=1,0 kN/mm2=100 daN/mm2. For self-supporting cables with solid load-bearing elements of aramid fibers, fiberglass plastic, the modulus of elasticity is low as compared to the metal solid elements.
5. Linear thermal expansion coefficient, 1/K – the change in the length of cable at temperature change by 1 degree. For self-supporting cables with solid load-bearing elements of aramid fibers, fiberglass plastic, the coefficient is low as compared to the metal solid elements.
For inputting to the program reference catalog 0,000002 corresponds to 2х10-6 (2.0Е-6).
Requirements to climatic conditions data asked for at weather stations:
1. Ice, mm – according to weather stations data, if the data are unavailable – based on the charts and recommendations of EIR. Its return period is once in 25 years.
2. Maximum, minimum (not the temperature of the coldest 5 days), average annual temperatures according to weather stations data, those are absolute temperatures, their return period is once in 25 years.
3. Ice-with-wind and ice-without-wind temperatures are taken according to EIR.
4. Maximum wind velocity, m/s – return period is once in 25 years and wind velocity in cases of ice accumulation (wind pressure calculation given in EIR) are taken according to weather stations data, if no such data are available, than according to EIR.
Requirements for Maximum Setting Tension, Stresses
1. Maximum permissible tension, daN/mm2 – tension in the material of the cable, input by the line designer for project calculations and accepted for the project.
Tension – stress, divided by the load-bearing sectional area, mm2, of the cable. The permissible tension is usually not more than 50% of tensile stress of mechanical capacity.
2. Permissible tension at average annual (average operating) conditions, daN/mm2 – the tension permissible at average annual temperature. Usually, it is 60 - 75% from the maximum permissible tension if the manufacturer’ s data, EIR, National Standard or Technical Conditions do not indicate otherwise.
The reliability coefficients are taken as equal 1 if there are no data according to 2.5.11.
If the customer did not give them in the task, they should not be considered.
In some projects a coefficient of 0.5 for ice is thoughtlessly used. It is unspeakable if this is the common approach to designing overhead lines, some software applies this coefficient by default. This is a direct path to wire breaks and supports destruction in case of ice accumulation.