IEC 60287-3-2 pdf download.ELECTRIC CABLES – CALCULATION OF THE CURRENT RATING – Part 3-2: Sections on operating conditions – Economic optimization of power cable size
4 Calculation of total costs
The total cost of installing and operating a cable during its anticipated operational life, expressed in present values, is calculated as follows. Note that all financial quantities are expressed in arbitrary currency units, (cu).The selection of the method of bonding the sheaths, screens or armour of single-core cables will have a significant effect on the losses due to circulating currents in these components. Where the system design permits, the bonding method should be selected to balance the cost of these losses over the life of the installation against the initial cost of installing the equipment and additional earth conductors required for certain bonding arrangements. As the economic conductor size is usually larger than the size based on thermal considerations (i.e. the size determined by the use of IEC 60287-1 -1 , IEC 60287-2-1 and/or the IEC 60853 series), its temperature will be lower than the maximum permissible value. It is convenient to assume, in the absence of more precise information, that R L is constant and has a value corresponding to a temperature of (θ – θ a )/3 + θ a . Here θ is the maximum rated conductor temperature for the type of cable concerned and θ a is the ambient average temperature. Factor 3 is empirical, see Annex B. NOTE 1 If greater precision is required (for example where the calculations do not indicate clearly which nominal conductor size should be chosen or the growth in load is such that its value during the final years is significantly higher than that of the first year) a better estimate of conductor temperature can be made using as a starting point the conductor size obtained from the approximate temperature given above. Methods for making a more refined estimate of conductor temperature and resistance are given in Annex B. The economical size is then redetermined using the revised value of conductor resistance. The effect of conductor resistance on the value of the economical size is small and it is seldom worthwhile to perform the iteration more than once.
5 Determination of economic conductor sizes
5.1 First approach: economic current range for each conductor in a series of sizes All conductor sizes have an economic current range for given installation conditions. The upper and lower limits of the economic range for a given conductor size are given by:The formula for the relationship between CI(S) and conductor size can be derived from known costs of standard cable sizes. In general, if a reasonably linear relationship can be fitted to the costs, possibly over a restricted range of conductor sizes, it should be used. This will cause little error in the results, in view of the possible uncertainties in the assumed financial parameters for the anticipated operational life period chosen.5.2.3 Effect of charging current and dielectric losses Dielectric losses and the losses due to charging current are always present in an a.c. system when the cable is energized and therefore operate at 1 00 % load factor. Both types of losses are significant only at high-voltage levels and are dependent on cable capacitance. Evaluation of transmission cable systems often assumes the placement of shunt reactors at the ends of the cable system to supply the reactive VARs required by the cable. The reactors have losses equal to about 0,8 % of power rating. Those losses should be considered in the evaluation of cable system losses and the cost of the reactors added to the cable purchase cost. For a given voltage level and insulation thickness, an increase in conductor diameter results in an increase in cable capacitance and, as a result of this, an increase in voltage dependent losses. Because of this, when dielectric losses are included in the analysis, these losses will tend to decrease the conductor diameter as opposed to the effect of current dependent losses. The dielectric and charging current losses are sometimes referred to as voltage-dependent losses, in contrast to the joule losses which are referred to as current-dependent losses. The cost of these voltage-dependent losses is included in the calculation by the following modification to Formula (1 1 ).
IEC 60287-3-2 pdf download
