The theme urgency works consists that in a new wording PUE of Ukraine is absent an economic and theoretical substantiation of a choice of sections of wires.
The scientific importance of work consists in the recommendation of a scope of section of wires.
Practical value of results consists in introduction in educational process, namely in course designing.
Choice of the minimum section of conductors on heating by a current of a steady state condition, on thermal firmness and mechanical durability, and also on admissible voltage loss result proceeding from technical reasons. Comparison of the sections chosen by specified criterias, defines greatest of them on which gets out standard. The economic reason which account results to conductor cross section increase, and consequently also capital investments thus are not taken into account. The conductor cross section increase results to reduction of the electric power losses at its transmission and capital investments cover quickly enough at the expense of electric power loss decrease [1]. Criterion of a choice of economically expedient cross-section is the minimum of the reduced expenses. The cross-section corresponding to the minimum value of resulted expenses is called as economic. Economic cross-sections can choice on rationed values of economic current density or on economic loading intervals [2].
The problem of cross-sections choice on economic density has been solved taking into account such technical limitation:
- Equality of longitudinal active resistance to ohmic:
where p is specific resistance of a material, Om•mm^2/km;
- absence of corona losses in overhead lines and dielectric losses in cable lines;
- absence of a damage from breaks of an electrical supply of consumers;
- directness of dependence of cost 1 km line construction (k0) from cross-section (F):
where k0 ’ is a component k0, which is not depend on cross-section, grn. / km;
k0 "- the component k0, depending on cross-section (the factor defining an inclination of dependence k0 = f (F) concerning a horizontal axis), grn / (km • мм^2).
At these assumptions the reduced expenses on a transmission line with length L calculate as follows:
where - En is standard effectiveness ratio of capital investments;
Клеп - is capital investments in TL construction;
Илеп - is working expenses on a transmission line;
is the expenses connected with the electric power losses.
Annual expenses for lines operation include the amortization charges intended for full replacement based capital after their deterioration, and also expenses on service TL (capital and flowing repairs of TL elements, preventive tests, the general expenses). They are defined on deduction norm pлэп from capital investments.
The expenses connected with electric power losses (variable expenses), are defined under the average electric power rate b ':
where 
- is capacity losses at the maximum loading;
Імах - is current value at the maximum loading;
- is maximum losses time.
Considering that КTL = k0•L і ИTL = pTL•КTL, and also expression (3), expenses on TL construction and operation can be written down in a kind
or taking into account (1) і (2)
In a general form reduced expenses are looked as follows:
For economic cross-section determination the derivative of function (4) on F is necessary equaled to zero:
From here at the account of expressions for А1 and А2 the economic cross-section value is determined
To this cross-section corresponds so-called economic current density (jек, А/мм^2) which is determined under the formula:
Expression (5) shows that value of economic current density is determined the big set of parametres. And consequently demands the differentiated approach to determination of economic current density for various TL types, construction conditions.
Conductor cross-sections are chosen on economic current density for overhead TL with voltage to 500 кV inclusive. For overhead TL above 500 кV voltage and also cable TL 110 кV and above the economic current density are not normalised. Besides, to a choice on economic current density are not subject: networks of the industrial enterprises and constructions of voltage to 1000 V at a usage time of the maximum loading to 4000 – 5000 h; branches to separate consumer with voltage to 1000 V and lighting networks; networks of temporary constructions and constructions with service life to three – five years.
Advantages: the approach to designing is unified. Method of economscal current density (MECD) allows to avoid the defference in economic efficiency estimations (it is the first method in which except capital investments were considered also costs of electric power losses).
The method of a choice of conductor cross-sections on rationed economic current density possesses that lack that these density are determineded at the fixed values of amortization charges for various types of a line in the assumption of linear dependence of specific cost from cross-sections. The named factors result to the big errors at a choice of economic cross-sections, especially in the non-uniform networks which separate sites have different technical and economic indicators. The big ranges of a usage time of the maximum loading cause the essential errors also. For example, for the third range (tab. 1 see) at Тmax=5000 h the error in determination Fec makes 30 %, and at Тmax=8760 h it is equal 20 % in comparison with the cross-section chosen for the middle of a time interval.
Cross-sections chosen on economic current density are checked on heating by currents in posrfault operational mode, network check on voltage losses is carried out also. If the economic cross-section is less than section demanded on other conditions it is necessary to choose the greatest cross-section defined by these conditions.
The method of economic current density was progressive for the time however there is a number of assumptions which considerably reduce an efficiency.
It is possible to carry to lacks:
1. It is supposed that capital investments are directly proportional to cross-section. It is not so. Use of the unified support results to proportionality infringement.
2. It is supposed that the cross-section is changed continuously whereas the cross-section is discrete.
3. It is supposed that the greatest current on which choice cross-section is constant. In real it too changes that it is necessary to consider at a cross-section choice [5].
Therefore the method of economic intervals (МEI) has been developed. It is based on expenses for one kilometre of a line without a damage.
At cross-section choice on economic loading intervals the economic one for the given conductor cross-section is called such loading interval in which limits the reduced expenses on transformation of current unit (or capacities) on length unit of a conductor are minimum in comparison with other cross-sections.
Values of a current on border of an economic interval it is defined by equality of expenses of two cross-sections.
1. Method of economical interval takes into account the real economic conditions (the first radicand), the real power consumption curve and changing rate for energy bearers.
2. If the second radicand appears negative it means that curves are not crossed and any cross-section at any currents will be economically not expedient.
On the basis of this method nomogramms are constructed. In reference books the tables of economic current intervals and capacities transferred on lines with different rated voltage are given.
The change of the greatest current on years of operation is considered in a method.
Advantages of a method:
1. Actual nonlinear dependence of capital investments from cross-section is considered.
2. The continuity of change 
is considered.
3. Stepness of standard cross-section is considered.
4. The method allows take into account the dynamics of loading increase.
5. The network chosen by МEI does not need to be checked on voltage losses.
Lack: at change of TL cost it is necessary or to reconstruct nomogramms or directly to build the dependence ЗF(Inb).
In Russia and Europe the basic method of a choice of wire cross-section at voltage to 500 кV is the method of choice on economic current density, however values of density of a current differ from ours. For example for Russia jec=0,8 А/мм2, and for Europe 0,6 А/мм2. In Ukraine this method was the based also, however since 2006 editing of REE [8] considerably narrows a range of cross-sections: at TL construction with voltage 35 - 500 кV it is recommended to use only on one cross-sections - 120, 240 and 400 мм2. At reconstruction of networks with voltage 35 кV it is possible to use cross-sections 70 and 95 мм2, 110 кV - 120 мм2 (tab. 2).
* Concerns lines of 35 кV voltage, being with branch from existing trunk lines with wire cross-section 70-95 мм^2 or representing continuation of trunk lines.
** Concerns lines of 110 кV voltage for a feed of electroconsumers with capacity to 20 МW or for issuing of power stations capacity with quantity of hours the fixed capacity use to 2500 (wind, gas-turbine peak power stations, etc.)
*** Perspective development of lines 220 and 500 кV is limited.
**** Development of lines 400 кВ is not provided.
Electric calculation of wire cross-sections is absent. Besides, perspective development of networks by voltage 220 and 500 кV is considered limited. In connection with such recommendations of REE there are misunderstanding. For example: why at wires production by cross-section 70 and 95 мм^2 they are not recommended to use at reconstruction of networks with voltage 110 кV. At considerable extent of networks with 220 кV voltage in Ukraine their further development is limited. Earlier at each voltage some sections were used. So, at 35 кV voltage it was possible to use 3 sections (95, 120 and 150 мм^2), at 110 кV voltage- 6, at 220 кV voltage - 5. At pressure 330 and 500 кV the split design of a phase was used: three sections at each voltage. Therefore it is necessary to make the detailed analysis of recommendations of REE in relation of a wire cross-sections choice of overhead TL on an example of operating electric networks.
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[Электронный ресурс].- Сайт режимщиков: - http://www.rza.org.ua/down/o-51.html