The International Electrotechnical Commission Standard IEC 287-3-2 / 1995 puts forward the viewpoint of economic optimization of cable size, that is, conductor section: the selection of cable conductor section should consider not only the initial cost of cable line, but also the power loss cost of AAAC Cable during its economic life, which should comply with the principle of minimizing the sum of the two costs. Expressed mathematically, i.e
CT=CI+CJ
Where CT is the total cost; Ci is the initial investment cost; CJ is the present value of Joule loss during the economic life of N years, that is, it is converted into present value according to the discount rate (I). The economic optimization of cable conductor section is called “economic conductor section”, which can be obtained by calculation.
The section determined according to the maximum allowable temperature and current carrying capacity of cable insulation is actually the minimum allowable conductor section. At this time, only the initial investment is calculated without considering the conductor loss cost during the economic life of the cable. If the conductor section is increased, the line loss cost is reduced and the initial investment is increased. However, the initial investment increased by increasing the conductor section can be compensated from the reduced ACSR Cable loss during long-term operation, so as to reduce the total cost of power supply and improve the economic benefits of the power sector.
Using economic conductor section, the cable operating temperature is much lower than the maximum allowable temperature of cable insulation, which can prolong the service life of cable line and improve the safety of cable power supply.
IEC 287-3-2 / 1995 points out that the temperature difference between the operating temperature of the cable and the average ambient temperature shall be taken as 1 / 3 of the temperature difference between the maximum allowable temperature of the cable and the average ambient temperature, i.e
For example, the maximum allowable temperature of XLPE cable is 90 ℃, and if the average ambient temperature is 24 ℃, its operating temperature should be
℃
The cable section meeting this operating temperature is in line with the principle of economic optimization. According to the calculation, the section of the economic conductor shall be enlarged by 2 grades than the section standard determined by the maximum allowable temperature. For example, if the section determined according to the maximum allowable temperature is 120, the economic conductor section should be 185.