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Abstract

Content

Introduction

Electric overhaed transmisson lines (OTL) are intended for transfer and distribution of electric energy by the wires located in the open air and attached to various supporting frameworks. Overhead lines consist of the following basic constructive elements: piers of various type for a suspension wire`s bracket and lightning-protective cables; wires of various designs and cross-sections for transfer by them electric current; lightning-protective cables for lines protection against lightning discharges;the insulators collected in garlands, for isolation of wires from the grounded parts of a piers; linear fittings for fastening of wires and cables to insulators and piers, and also for their connection; grounding devices for branch of lightning discharges or short circuit currents to the ground.

During OTL construction the most responsible stage is design. From a right choice of wires cross-sections, designs of piers depend not only operating modes of the line, but also safety of objects surrounding it, and also economic costs of its construction and term of its work and payback. Further in this work methods of a choice wire`s cross-sections, economic and technical feasibility in a choice of this or that type of a wire, a pier, etc. will be considered.

1. Choice of wires cross-section

1.1 Method of economic current density

Wire`s cross-section is the most important parameter of transmission lines. Techniques of determination wire`s cross-section and cables are based on search of the economic cross-section corresponding to the minimum expenses. With increase in section costs of a construction of the line, assignment grow by depreciation, repair and service (in fig. 1 З1), but power and electric power losses and the related expenses (in fig. 1 З2) decrease:

Figure 1 – Dependence of the given expenses from cross-section

Figure 1 – Dependence of the given expenses from cross-section

From fig. 1.1 it is visible that there is a point in which З1=З2 and total expenses will be minimum. To this point there corresponds the cross-section which call economic cross-section:

Order of a choice wire`s cross-sections by method of economic current density is the following:

1)Find economic cross-section by a expression.

2)Choose the immediate standard cross-section.

3)Carry out the checking of the chosen standard cross-section:

· by heating in normal and postemergency modes;

· by voltage drops in normal and postemergency modes;

· by mechanical durability.

Now by the economic density of current choose cross-sections of overhead lines with Unom = 35-110 kV, for networks with higher rated voltage this method serves for a choice of initial cross-section for a method of economic intervals.

Disadvantage of a method is that linear dependence К1км on section is accepted, it isn't true [2].

1.2 Method of economic intervals

The method of economic intervals is applied to a choice of cross-sections for networks with rated voltages 35-750 kV. For the standard cross-sections of wires accepted on this rated voltage count given expenses З1км depending on the greatest current of the line. Calculation by means of this method is based on costs of one kilometer of the line without damage:

At a cross-section choice on economic intervals of loading for this section of conductors such interval of loadings which the given expenses on transfer of current (or power) unit per unit length the conductor are minimum in comparison with other sections is called economic:

Figure 2 – Graph of economic intervals (animation: 5 frames, 4 cycles, 13.7 kilobytes)
Figure 2 – Graph of economic intervals
(animation: 5 frames, 4 cycles, 13.7 kilobytes)

Values of current on border of an economic interval is defined by equality by expenses of two cross-sections:

The method of economic intervals considers a real economic situation (the first radicand) the real graph of a power consumption and a changing tariff for energy carriers [3], [7]. If the second radicand is negative, it means that curves aren't crossed and any section at any currents will be economically not expedient.

On the basis of this method nomograms are constructed. Tables of economic intervals of currents and capacities are provided in Shapiro and Faybisovich's reference books transferred on lines with a different rated voltage.

In a method also change of the greatest current by years of operation is considered.

Advantages of this method:

1. The actual nonlinear dependence of an investment on section is considered.

2. The change continuity is considered .

3. Gradualness standard section is considered.

4. The method allows to consider dynamics of growth of loadings.

5. The network chosen on this method doesn't need to be checked by voltage losses [1].

Disadvantage: at change of cost of the high voltage line it is necessary or to reconstruct nomograms or directly to build dependence ЗF (Iнб) [9].

Economic intervals of currents are for sections which are equal minimum admissible under the terms of a crown or more them. Therefore to check under the terms of a crown higher than 1500 m above sea level are necessary only overhead transmission lines 110 kV and above, laid on routes with marks. To check on admissible losses and deviations of voltage and wire`s cross-section of 35 kV and it isn't necessary above as increase of voltage level by increasing wire`s cross-section of such lines is economically inexpedient. Cross-sections of wires of OTL need to be checked on admissible heating in postemergency operation.Comparison of postemergency currenr operation is for this purpose made Iаv with Iper for any type of wire. Emergency current is defined with quantity of chains.

The wire doesn't overheat when performing ratios: Iаv < Iperm

If for any site of a network it appeared, Iаv > Iperm, то it is necessary to increase wire section on this site. Thus, however, it is necessary to check that it didn't exceed the maximum recommended section for the used rated voltage.

2. Piers of OTL

Piers of the overhead transmisson lines are designs which serve for support over a terrestrial surface of energized wires and lightning-protective cables. They happen various forms and the sizes. Piers can be ferroconcrete, wooden, metal or even from composite materials. Basic elements of a OTL piers – racks, the bases, traverses [5].

Distinguish the following types of the support applied in air-lines of an electricity transmission: anchor, intermediate, angular, trailer and special. Anchor support establish on direct sites of the route for OTL transition through natural obstacles or engineering constructions and perceive longitudinal loading from weight of wires and cables. Intermediate support establish on direct sites of the route and are intended only for maintenance of wires and cables and not calculated on loading from weight of wires along the line. The angular support established on angles of rotation of the route OTL, under normal conditions perceive the net force of weight of wires and cables of adjacent flights [6].

Conclusion

This paper considers the problem of choosing optimal TL wire`s cross-sections and viewing the practice of designing power lines in Ukraine and other countries. The main problem of this issue is a choice between rising of transmission lines construction production costs and reducing energy losses during transporting or the minimum cost during the construction and large power losses then.

This master's work is not completed yet. Final completion: December 2014. The full text of the work and materials on the topic can be obtained from the author or his head after this date.

References

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