220/20 kV power supply scheme in high load density urban areas

Application of 20 kV in MV distribution system has been proved to be feasible and advisable with the rapid growing load density in urban areas. The decreasing of gap between neighboring voltage value in the existing 220/110/20 kV sequence results in overlapping in power supply area and that the capacity of devices in system cannot be in full use subsequently. Assuming the load in the area is uniformly distributed, this paper proposes a mathematical model with a decision-making index based on power supply cost per unit load and makes a Voronoi diagram based 220 kV substation optimal supply area. A case study of a real distribution system in Jiangsu province is employed to make a comparison of the cost per unit load in the horizontal year in both 220/110/20 kV and 220/20 kV system. Preliminary conclusion about reasonable load density of higher than a certain level is appropriate for voltage sequence in power supply system in special conditions is presented. And it also makes a further suggestion that 110kV be eliminated from the existing voltage sequence in urban distribution system with this load density for avoiding overlapping in voltage stepping down and also getting ready for further HV distribution voltage optimization.