1、BRITISH STANDARD BS 7450:1991 IEC 1059:1991 Method for Determination of economic optimization of power cable size (Implementation of CENELECHD558S1)BS7450:1991 This British Standard, having been prepared under the directionof the Cables and Insulation Standards Policy Committee, was published undert
2、he authority of the Standards Board and comesintoeffect on 28June1991 BSI 01-2000 The following BSI references relate to the work on this standard: Committee reference CIL/20 Draft for comment 89/31146 DC ISBN 0 580 19757 3 Committees responsible for this British Standard The preparation of this Bri
3、tish Standard was entrusted by the Cables and Insulation Standards Policy Committee (CIL/-) to Technical Committee CIL/20, upon which the following bodies were represented: Aluminium Federation Association of Consulting Engineers Association of Manufacturers of Domestic Electrical Appliances British
4、 Approvals Service for Cables British Cable Makers Confederation British Plastics Federation British Steel plc British Telecommunications plc Department of the Environment (Property Services Agency) Department of Trade and Industry (Consumer Safety United, CA Division) ERA Technology Ltd. Electricit
5、y Supply Industry in UnitedKingdom Engineering Equipment and Materials Users Association Institution of Electrical Engineers London Regional Transport The following bodies were also represented in the drafting of the standard, through subcommittees and panels: Electrical Contractors Association Inst
6、itution of Incorporated Executive Engineers London Underground Ltd. Amendments issued since publication Amd. No. Date CommentsBS7450:1991 BSI 01-2000 i Contents Page Committees responsible Inside front cover National foreword ii Introduction 1 1 Scope 2 2 Normative references 3 3 Symbols 3 4 Calcula
7、tion of total costs 4 5 Determination of economic conductor sizes 6 Annex A (informative) Examples of calculation of economic conductor sizes 8 Annex B (informative) Mean conductor temperature and resistance 16 Figure A.1 System layout 15 Figure A.2 Economic current ranges 15 Figure A.3 Variation of
8、 cost with conductor size 16 Table A.1 Cable details 13 Table A.2 Economic loading 13 Table A.3 Current-carrying capacity criterion 13 Table A.4 Economic loading, standard conductor size for all sections 14 Publication(s) referred to Inside back coverBS7450:1991 ii BSI 01-2000 National foreword This
9、 British Standard has been prepared under the direction of the Cables and Insulation Standards Policy Committee. It is identical with IEC1059:1991 “Economic optimization of power cable size” published by the International Electrotechnical Commission (IEC), which has been endorsed by CENELEC as HD558
10、 S1. The Technical Committee has reviewed the provisions of IEC228:1978, IEC287:1982 and IEC853, to which reference is made in the text, and has decided that they are acceptable for use in conjunction with this standard. Arelated British Standard to IEC228:1978 is BS6360:1981. A British Standard doe
11、s not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an insi
12、de front cover, pagesi andii, pages1to20, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS7450:1991 BSI 01-2000 1 Introduction 1 General part
13、 The procedure generally used for the selection of a cable size leads to the minimum admissible cross-sectional area, which also minimizes the initial investment cost of the cable. It does not take into account the cost of the losses that will occur during the life of the cable. The increasing cost
14、of energy, together with the high energy losses which follow from the operating temperatures possible with the newer insulating materials (e.g.90 C for XLPE and EPR), now requires that cable size selection be considered on wider economic grounds. Rather than minimizing the initial cost only, the sum
15、 of the initial cost and the cost of the losses over the economic life of the cable should also be minimized. For this latter condition a larger size of conductor than would be chosen based on minimum initial cost will lead to a lower power loss for the same current and will, when considered over it
16、s economic life, be much less expensive. The future costs of energy losses during the economic life of the cable can be calculated by making suitable estimates of load growth and cost of energy. The most economical size of conductor is achieved when the sum of the future costs of energy losses and t
17、he initial cost of purchase and installation are minimized. The saving in overall cost, when a conductor size larger than that determined by thermal constraints, is chosen, is due to the considerable reduction in the cost of the joule losses compared with the increase in cost of purchase. For the va
18、lues of the financial and electrical parameters used in this standard, which are not exceptional, the saving in the combined cost of purchase and operation is of the order of 50% (see clauseA.6 inAnnex A). Calculations for much shorter financial periods can show a similar pattern. A further importan
19、t feature, which is demonstrated by examples, is that the savings possible are not critically dependent on the conductor size when it is in the region of the economic value, seeFigure A.3. This has two implications: a) The impact of errors in financial data, particularly those which determine future
20、 costs, is small. While it is advantageous to seek data having the best practicable accuracy, considerable savings can be achieved using data based on reasonable estimates. b) Other considerations with regard to the choice of conductor size which feature in the overall economics of an installation,
21、such as fault currents, voltage drop and size rationalization, can all be given appropriate emphasis without losing too many of the benefits arising from the choice of an economic size. 2 Economic aspects In order to combine the purchase and installation costs with costs of energy losses arising dur
22、ing the economic life of a cable, it is necessary to express them in comparable economic values, that is values which relate to the same point in time. It is convenient to use the date of purchase of the installation as this point and to refer to it as the “present”. The “future” costs of the energy
23、 losses are then converted to their equivalent “present values”. This is done by the process of discounting, the discounting rate being linked to the cost of borrowing money. In the procedure given here inflation has been omitted on the grounds that it will affect both the cost of borrowing money an
24、d the cost of energy. If these items are considered over the same period of time and the effect of inflation is approximately the same for both, the choice of an economic size can be made satisfactorily without introducing the added complication of inflation. To calculate the present value of the co
25、sts of the losses it is necessary to choose appropriate values for the future development of the load, annual increases inkWh price and annual discounting rates over the economic life of the cable, which could be25 years or more. It is not possible to give guidance on these aspects in this standard
26、because they are dependent on the conditions and financial constraints of individual installations. Only the appropriate formulae are given: it is the responsibility of the designer and the user to agree on the economic factors to be used. The formulae proposed in this standard are straightforward,
27、but in their application due regard should be taken of the assumption that the financial parameters are assumed to remain unchanged during the economic life of the cable. Nevertheless, the above comments on the effect of the accuracy of these parameters is relevant here also.BS7450:1991 2 BSI 01-200
28、0 There are two approaches to the calculation of the economic size, based on the same financial concepts. The first, where a series of conductor sizes is being considered, is to calculate a range of economic currents for each of the conductor sizes envisaged for particular installation conditions an
29、d then to select that size whose economic range contains the required value of the load. This approach is appropriate where several similar installations are under consideration. The second method, which may be more suitable where only one installation is involved, is to calculate the optimum cross-
30、sectional area for the required load and then to select the closest standard conductor size. 3 Other criteria Other criteria, for example short-circuit current and its duration, voltage drop and cable size rationalization, must be considered also. However, a cable chosen to have an economical size o
31、f conductor may well be satisfactory also from these other points of view, so that when sizing a cable the following sequence may be advantageous: a) calculate the economic cross-sectional area; b) check by the methods given in IEC287 and IEC853 that the size indicated bya) is adequate to carry the
32、maximum load expected to occur at the end of the economic period without its conductor temperature exceeding the maximum permitted value; c) check that the size of cable selected can safely withstand the prospective short-circuit and earth fault currents for the corresponding durations; d) check tha
33、t the voltage drop at the end of the cable remains within acceptable limits; e) check against other criteria appropriate to the installation. To complete the field of economic selection, proper weight should be given to the consequences of interruption of supply. It may be necessary to use a larger
34、cross-section of conductor than the normal load conditions require and/or the economic choice would suggest, or to adapt the network accordingly. A further cost component may be recognized in the financial consequence of making a faulty decision weighted by its probability. However, in doing so one
35、enters the field of decision theory which is outside the scope of this standard. Thus, economic cable sizing is only a part of the total economic consideration of a system and may give way to other important economic factors. 1 Scope This International Standard deals solely with the economic choice
36、of conductor size based on joule losses. Voltage dependent losses have not been considered. NOTE 1It is recommended that the method given in this standard should not be used for cables operating on system voltages equal to or greater than the following (see IEC287): NOTE 2Modifications to the method
37、 given in this standard in order to take dielectric losses into account are under consideration. Likewise, matters such as maintenance, energy losses in forced cooling systems and time of day energy costs have not been included in this standard. An example of the application of the method to a hypot
38、hetical supply system is given inAnnex A. Type of cable System voltage U o kV Cables insulated with impregnated paper: solid type 38 oil-filled and gas pressure 63,5 Cables with other types of insulation: butyl rubber 18 EPR 63,5 PVC 6 PE (HD and LD) 127 XLPE (unfilled) 127 XLPE (filled) 63,5BS7450:
39、1991 BSI 01-2000 3 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agree
40、ments based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. IEC 228:1978, Conductors of insulated cables. IEC 287:
41、1982, Calculation of the continuous current rating of cables (100% load factor). IEC 853, Calculation of the cyclic and emergency current rating of cables. 3 Symbols A = variable component of cost per unit length related to conductor size cu/m.mm 2 B = auxiliary quantity defined by equation (16) C =
42、 constant component of cost per unit length related to laying conditions etc. cu/m CT = total cost of a system cu D = demand charge each year cu/W.year F = auxiliary quantity defined by equation (10) cu/W I max = maximum load in first year i.e. the highest hourly mean value A I(t) = load as a functi
43、on of time A l = cable length m CJ = present value of the cost of joule losses during N years cu N = period covered by financial calculations, also referred to as “economic life” year N p = number of phase conductors per circuit N c = number of circuits carrying the same type and value of load P = c
44、ost of one watt-hour at relevant voltage level cu/W.h CI = installed cost of the length of cable being considered cu CI 2 = installed cost of the next larger standard size of conductor cu CI 1 = installed cost of the next smaller standard size of conductor cu CI(S) = installed cost of a cable as a f
45、unction of its cross-sectional area cu Q = auxiliary quantity defined by equation (8) R = cable a.c. resistance per unit length, including the effect of y p , y s , 1 , 2 , (considered to be a constant value at an average operating temperature, seeclause4) 7/m R 2 = a.c. resistance per unit length o
46、f next larger standard conductor size 7/m R 1 = a.c. resistance per unit length of next smaller standard conductor size 7/m R(S) = a.c. resistance per unit length of a conductor as a function of its area 7/m S = cross-sectional area of a cable conductor mm 2 S ec = economic conductor size mm 2 T = o
47、perating time at maximum joule loss h/year a = annual increase in I max % b = annual increase in P, not covered by inflation % i = discounting rate used to compute present values % r = auxiliary quantity defined by equation (9) BS7450:1991 4 BSI 01-2000 4 Calculation of total costs The total cost of
48、 installing and operating a cable during its economic life, expressed in present values, is calculated as follows. Note that all financial quantities are expressed in arbitrary currency units, (cu). where Evaluation of CJ The total cost due to the losses is composed of two parts: a) the energy charg
49、e and b) the charge for the additional supply capacity to provide the losses. a) Cost due to energy charge where As the economic conductor size is usually larger than the size based on thermal considerations (i.e. the size determined by the use of IEC287 and/or IEC853), its temperature will be lower than the maximum permissible value. It is convenient to assume, in the absence of more precise information, that R is constant and has a value corresponding to a temperature of (F F a )/3 + F a . Here F is the maximum rated conductor temperature for
