IEC 60287-3-2:2012 pdf download.Electric cables - Calculation of the current rating
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 100 % 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 (11).
Relationships between maximum load during the first year and total cost per unit length for three sizes of cable are given in Figure A.2. It can be seen that each size of cable provides the most economical installation over a range of currents.
The effect of a change in conductor size on the overall costs, when carrying a given load, is shown in Figure A.3. Here the cable and financial parameters of this example have been retained, but a fixed load, /max, of 100 A has been assumed. It can be seen that, in the region of the most economic size, the total costs are not greatly affected by the choice of cable size.However, the reduction in costs, compared with those based on the use of a size chosen from thermal considerations, is very significant.
In order to make a fair comparison with the losses and financial figures calculated for the economic choice of conductor size, it is necessary to assume an appropriate conductor temperature at which to calculate the losses. For the economic choice, it was assumed that the temperature of the conductor would be about 40 °C (see Clause 4). It is proposed here that a comparable assumption for the temperature of conductors chosen on the basis of thermal ratings would be the maximum permissible value of 80 °C.IEC 60287-3-2 pdf download.