Development of industry and social sphere causes constant growth of power consumption that can be ensured <1> in densely populated areas only due to increase in capacity of existing overhead lines. This problem is solved by creation of more expensive wire design with increased current-carrying section or increased operating temperatures, characteristics of which are higher than characteristics of standard steel-aluminium wires.
Analysis of overhead line element failures shows <2> that troubles associated with fallout of wires and lightning protection cables are from 40 % to 55 % from general amount of all troubles and increased by 3–5 % per year. The main reasons of damage are ice load, wear due to vibration effect, dancing and corrosion. Thus while wire design optimization t is necessary to consider not only capacity and operational losses (wire heating due to their active resistance, costs for magnetization reversal of support strand, losses at corona discharges), but operating strength under various climatic conditions.
In the opinion of the new wire designer <3>, high level of losses in Russian electric mains is determined not only by deterioration of electrical equipment, but also by outdated normative base regulating wire production. Let’s consider retarding action of normative base by the example of new class of wires developed by Energoservice LLC and Severstal JSC with increased strength and capacity, i.e. plastically compressed high-strength steel-aluminium wires <4, 5> having passed certification of interdepartmental commission of Federal Grid Company of Unified Energy System JSC. High-strength steel-aluminium wires have significantly higher strength and current capacity in comparison with steel-aluminium wires that is obtained by increase in design wire density due to sequential plastic compression of strand and current conducting lays after laying <5>. Contrary to wires with high percentage of section filling by use of shaped rods, standard round aluminium wires are used in plastically compressed wires, and higher density of section filling is obtained while plastic compression, then while assembly of shaped rods. Plastic deformation with 8–9 % compression ratio of wire cross section area doesn’t allow wire unwinding and mutual displacement of elements under action of tension forces, causes increase in strength of aluminium rods up to twice due to work hardening, and difference in specific conductivity of soft aluminium and work-hardened solid aluminium doesn’t exceed 1 % <6>. Costs for high-strength steel-aluminium wires and reequipment of overhead lines for them slightly exceed the similar costs while using steel-aluminium wires, but increase in capacity (from several tens to several hundreds of percents) and strength covers additional costs, reduces loads on supports, tangential tension, wind and ice load and finally increases reliability of overhead lines. Smaller diameter of high-strength steel-aluminium wires in comparison with steel-aluminium wires with the same strength allows the following: wire dancing; aerodynamic coefficient; level of internal corrosion in wire; level of metal fatigue in wire; possibility of ice covering and snow deposit on approximately cylindrical external surface of wire. While operation <7> it has been shown by experiments that torsion rigidity of small wires is higher than torsion rigidity of standard ones even at close values of diameters. By many characteristics high-strength steel-aluminium wire is close to operating properties of small wires Aero-Z or ACSS/TW <8> at significantly lower cost.
High lightning resistance of high-strength steel-aluminium wires shall be noted due to extensive contact of large area between aluminium rod loops, the first and the second lays of which are made with linear contact of rods. Designs with linear contact are widely used in lifting cables, their basic advantage over ropes with point contact consists in combination of flexibility with high wear resistance and strength <9>. However according to para. 2.3 of GOST 839-80 “Uninsulated Wires for Aerial Power Lines. Specifications” lays shall be twisted to the opposite sides. In EIC-7 <10> there is no direct indication of laying direction for stranded wires, but according to para. 2.5.78 “in order to reduce electric power losses for magnetization reversal of support strand in steel-aluminium wires… it is recommended to use wires with even number of aluminium wire lays”. Reduction in losses for magnetization reversal at even number of lays is possible only when laying rods to the opposite sides.
Thus, use of wires with laying in one direction with linear contact of rods ensuring combination of flexibility, high wear resistance and strength and, therefore, allowing sudden reduction of wire failure is not recommended only due to the probability of increased losses while magnetization reversal of support strands.
Let’s perform approximate evaluation of the possible values of losses according to the procedure <11>, by which while transferring alternating current surface effect and magnetic losses in support strand shall be considered. Heat losses P released in the wire don’t exceed the following: see Appendix.

In order to determine possible range of losses alternating current passing via section of steel-aluminium wire with 7 steel rods (diameters of non-deformed wires 2.7 and 2.55 mm) and 28 aluminium rods in two lays (diameters 1.95 and 2.8 mm) with the following four various designs (Fig.1) was simulated by finite element method:
undeformed wire, in which laying direction in each next lay is changed to opposite one (variant 1);
undeformed wire with constant direction and laying pitch in all lays (variant 2);
plastically compressed wire, in which laying direction in each next lay is changed to opposite one (variant 3);
plastically compressed wire with constant direction and laying pitch in all lays (variant 4). Variant 4 is the closest to the design of high-strength steel-aluminium wires.
Laying pitch for all lays was taken to be 160 mm. In variants 1 and 2 there was no electric contact between aluminium wires, and in variants 3 and 4 influence of contaminations and oxide films on contact surfaces of aluminium rods on interturn current passage was neglected.
For simulation software package COMSOL Multiphysics with modules Magnetic Fields and Heat Transfer in Solids was used. Due to absent of temperature gradient along wire axis heat transfer in this direction was neglected. Or heating simulation the following dependences were used: see Appendix.

Fig. 2 shows distribution of normal magnetic flux density (T) obtained while simulation in cross section of steel-aluminium wires in all variants.

Plastic compression with formation of high conductivity electric contacts and change in direction of wire laying at chosen pitch slightly caused change in visualization of value and nature for distribution of normal magnetic flux density (Figure 2). Module Magnetic Fields of package Comsol
allows determining value of heat releasing while alternating current passing in wire elements due to electromagnetic processes (Table).

As the Table shows change in laying direction slightly changes value of released heat in elements of steel-aluminium wire, and use of plastically compression with formation of electric contacts with high conductivity between rods causes reduction in hat release by 1 % in aluminium, and by 10 % in iron. Therefore, electric losses in high-strength steel-aluminium wires don’t at least exceed losses in steel-aluminium wires.

CONCLUSIONS
1. Simulation of alternating current passing via steel-aluminium wires with various design performed by finite element method has shown that laying direction of aluminium rods at even number of lays slightly influence heat release in support strand.
2. Forming of electric contacts with high conductivity between rods allows reducing ehat release by 10 % in support strand of high-resistance steel-aluminium wires as a result of plastic compression.

List of References
1. Increase in Overhead Line Capacity: Analysis of Technical Solutions

S. V. Kolosov, S. V. Ryzhov, V. E. Syuksin

Energetik: Industrial Mass-Circulation Magazine. – 2011. – No. 1. – p. 18–22.
2. Yakovlev L. V. Complex of Works and Proposals on Improvement of Overhead Line Reliability at the Stage of Designing and Operation

L. V. Yakovlev, R. S. Kaverina, L. A. Dubinich. Collection of Reports of the Third Russian Research and Training Conference with International Participation “Power Transmission Lines 2008: Designing, Construction, Operating Experience and Scientific-Technical Progress”. Novosibirsk, June 3rd-5th, 2008. – Novosibirsk, 2008. – p. 28–49.
3. Fyodorov N. A. Energy-Efficient Solution with New Generation Wire АССС™ by the Example of Rehabilitation of Overhead Lines 110 kV

Collection of Reports of International Research and Training Conference “Supports for Smart Networks: Deigning and Rehabilitation”. RF, SPb – 2013.
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S. V. Kolosov, V. A. Fokin

Electric Power: Transmission and Distribution. – 2014. –No. 1. – p. 90–92.
5. Printed Material 132241 RF MPK N01V5/08 Steel-Aluminum Wire for Overhead Line

V. A. Fokin, A. K. Vlasov, V. V. Petrovich, A. V. Zvyagintsev, V. I. Frolov. Published: September 10th, 2013. Bulletin No. 25.
6. Manual on Electrotechnical Materials. Volume 3

Under editorship of Yu. V. Koritskogo, V. V. Pasynkova, B. M. Tareeva – L.: Energoatomizdat, 1988 – 728 p.
7. Nazim Ya. V. Investigation of Torsional Rigidity of Wires for Overhead Lines

Ya. V. Nazim

Metal Structures. 2011, Volume 17, No. 3. – p. 199–215.
8. Alekseev B. A. Increase in Capacity of Overhead Lines and Use of Wires with New Grades

B. A. Alekseev

“ELECTRO. Electrical Engineering, Electrical Power Engineering, Electrical Industry”, 2009, No. 3. – p. 45–50.
9. Buzuyev I. I. Assurance of Safe Operation for Lifting Mechanisms of Hoisting Machines. – Samara: Samara State Technical University, 2012. 88 p.
10. ELECTRIC INSTALLATION EIC. Seventh Edition. Approved by the Order of Ministry of Energy of Russia No. 204 dated July 8th, 2002.
11. STO 56947007-29.240.55.143-2013. Procedure for Calculation of Limit Current Loads under the Terms of Keeping Mechanical Strength of Wires and Permissible Overall Dimensions of Overhead Lines. Standard of Organization. Effective Date: February 13th, 2013. Federal Grid Company of Unified Energy System JSC, 2013

authors Gurevich L. M., Danenko V. F., Pronichev D. V., Trunov M. D.