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1、BRITISH STANDARD BS 7450:1991 IEC 1059:1991 Method for Determination of economic optimization of power cable size (Implementation of CENELEC HD 558 S1) Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:19
2、91 This British Standard, having been prepared under the direction of the Cables and Insulation Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on 28 June 1991 BSI 01-2000 The following BSI references relate to the work on this standard: Com
3、mittee reference CIL/20 Draft for comment 89/31146 DC ISBN 0 580 19757 3 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Cables and Insulation Standards Policy Committee (CIL/-) to Technical Committee CIL/20, upon which the following bod
4、ies were represented: Aluminium Federation Association of Consulting Engineers Association of Manufacturers of Domestic Electrical Appliances British Approvals Service for Cables British Cable Makers Confederation British Plastics Federation British Steel plc British Telecommunications plc Departmen
5、t of the Environment (Property Services Agency) Department of Trade and Industry (Consumer Safety United, CA Division) ERA Technology Ltd. Electricity Supply Industry in United Kingdom Engineering Equipment and Materials Users Association Institution of Electrical Engineers London Regional Transport
6、 The following bodies were also represented in the drafting of the standard, through subcommittees and panels: Electrical Contractors Association Institution of Incorporated Executive Engineers London Underground Ltd. Amendments issued since publication Amd. No.DateComments Licensed Copy: London Sou
7、th Bank University, London South Bank University, Fri Dec 08 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:1991 BSI 01-2000i Contents Page Committees responsibleInside front cover National forewordii Introduction1 1Scope2 2Normative references3 3Symbols3 4Calculation of total costs4 5D
8、etermination of economic conductor sizes6 Annex A (informative) Examples of calculation of economic conductor sizes8 Annex B (informative) Mean conductor temperature and resistance16 Figure A.1 System layout15 Figure A.2 Economic current ranges15 Figure A.3 Variation of cost with conductor size16 Ta
9、ble A.1 Cable details13 Table A.2 Economic loading13 Table A.3 Current-carrying capacity criterion13 Table A.4 Economic loading, standard conductor size for all sections14 Publication(s) referred toInside back cover Licensed Copy: London South Bank University, London South Bank University, Fri Dec 0
10、8 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:1991 ii BSI 01-2000 National foreword This British Standard has been prepared under the direction of the Cables and Insulation Standards Policy Committee. It is identical with IEC 1059:1991 “Economic optimization of power cable size” publ
11、ished by the International Electrotechnical Commission (IEC), which has been endorsed by CENELEC as HD 558 S1. The Technical Committee has reviewed the provisions of IEC 228:1978, IEC 287:1982 and IEC 853, to which reference is made in the text, and has decided that they are acceptable for use in co
12、njunction with this standard. A related British Standard to IEC 228:1978 is BS 6360:1981. A British Standard does 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 o
13、f itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 20, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This wil
14、l be indicated in the amendment table on the inside front cover. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:1991 BSI 01-20001 Introduction 1 General part The procedure generally used for the select
15、ion 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 of energy, together with the high energy los
16、ses 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 of the initial cost and the cost of the lo
17、sses 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 its economic life, be much less expensive. Th
18、e 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 the initial cost of purchase and installatio
19、n 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 values of the financial and electrical parame
20、ters 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 clause A.6 in Annex A). Calculations for much shorter financial periods can show a similar pattern. A further important feature, which is demonstrated by exam
21、ples, is that the savings possible are not critically dependent on the conductor size when it is in the region of the economic value, see Figure A.3. This has two implications: a) The impact of errors in financial data, particularly those which determine future costs, is small. While it is advantage
22、ous 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, such as fault currents, voltage drop an
23、d 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 during the economic life of a cable, it is
24、 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 losses are then converted to their equ
25、ivalent “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 and the cost of energy. If these items ar
26、e 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 costs of the losses it is necessary to ch
27、oose appropriate values for the future development of the load, annual increases in kWh price and annual discounting rates over the economic life of the cable, which could be 25 years or more. It is not possible to give guidance on these aspects in this standard because they are dependent on the con
28、ditions 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, but in their application due regard s
29、hould 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. Licensed Copy: London South Bank University, London South Ba
30、nk University, Fri Dec 08 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:1991 2 BSI 01-2000 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 o
31、f economic currents for each of the conductor sizes envisaged for particular installation conditions and 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,
32、 which may be more suitable where only one installation is involved, is to calculate the optimum cross-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 ca
33、ble size rationalization, must be considered also. However, a cable chosen to have an economical size of 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)
34、 check by the methods given in IEC 287 and IEC 853 that the size indicated by a) is adequate to carry the 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 with
35、stand the prospective short-circuit and earth fault currents for the corresponding durations; d) check that 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
36、weight should be given to the consequences of interruption of supply. It may be necessary to use a larger 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 th
37、e financial consequence of making a faulty decision weighted by its probability. However, in doing so one 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 ot
38、her important economic factors. 1 Scope This International Standard deals solely with the economic choice 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
39、 on system voltages equal to or greater than the following (see IEC 287): NOTE 2Modifications to the method 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 e
40、nergy costs have not been included in this standard. An example of the application of the method to a hypothetical supply system is given in Annex A. Type of cableSystem voltage Uo kV Cables insulated with impregnated paper: solid type38 oil-filled and gas pressure63,5 Cables with other types of ins
41、ulation: butyl rubber18 EPR63,5 PVC6 PE (HD and LD)127 XLPE (unfilled)127 XLPE (filled)63,5 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:1991 BSI 01-20003 2 Normative references The following standar
42、ds 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 agreements based on this International Standard are encouraged to in
43、vestigate 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 287:1982, Calculation of the continuous current rating of cables (1
44、00 % 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 sizecu/m.mm2 B=auxiliary quantity defined by equation (16) C=constant component of cost per unit length related to laying conditio
45、ns etc.cu/m CT=total cost of a systemcu D=demand charge each yearcu/W.year F=auxiliary quantity defined by equation (10)cu/W Imax=maximum load in first year i.e. the highest hourly mean valueA I(t)=load as a function of timeA l=cable lengthm CJ=present value of the cost of joule losses during N year
46、scu N=period covered by financial calculations, also referred to as “economic life”year Np=number of phase conductors per circuit Nc=number of circuits carrying the same type and value of load P=cost of one watt-hour at relevant voltage levelcu/W.h CI=installed cost of the length of cable being cons
47、ideredcu CI2=installed cost of the next larger standard size of conductorcu CI1=installed cost of the next smaller standard size of conductorcu CI(S)=installed cost of a cable as a function of its cross-sectional areacu Q=auxiliary quantity defined by equation (8) R=cable a.c. resistance per unit le
48、ngth, including the effect of yp, ys, 1, 2, (considered to be a constant value at an average operating temperature, see clause 4)7/m R2=a.c. resistance per unit length of next larger standard conductor size7/m R1=a.c. resistance per unit length of next smaller standard conductor size7/m R(S)=a.c. re
49、sistance per unit length of a conductor as a function of its area7/m S=cross-sectional area of a cable conductormm2 Sec=economic conductor sizemm2 T=operating time at maximum joule lossh/year a=annual increase in Imax% b=annual increase in P, not covered by inflation% i=discounting rate used to compute present values% r=auxiliary quantity defined by equation (9) Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:54:21 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7450:1991 4 BSI 01-2000 4 Calculation of total costs