Simple Method for Selecting Overhead Distribution Line Conductor

Distribution Utilities (DUs) when planning for their distribution system, are faced with various options and constraints. One of the DUs’ process in system planning is choosing the right size of overhead distribution conductor for a given load, voltage level and expected load growth just to name a few. Most of the time, DUs have a set of standard sizes of conductors in the stock shop ready to be utilized for meeting additional demand or has a set of policies what size of conductor to be used for level of voltage or type of load. These practices do not follow that given the new conductor installation, DUs comply with various performance regulation.

In the performance regulation of DUs, it is important to meet the following:

• Supply the voltage within allowable limits - normally this is between 0.90 to 1.10 per-unit voltage.
• Minimize losses - to select a conductor is to ensure that as the power is being delivered, lesser line losses are produced.
• Minimize cost of conductor - economic regulation addressed in planning that DUs as much as possible to lower cost and at the same time operate within performance criteria.

In addition, the DUs must maximize power delivered, i.e. - the DU sees to it that the demand will be supplied with the power it needs given any time. Maximizing power transfer enables maximum utilization of the asset.

In this blog, consider a load of 200 amperes at 13.8kV level and selection between conductors ACSR sizes of 1/0 to 4/0 is to be determined for a line length of 1000 meters. The cost of conductors are derived from National Electrification Administration (NEA) website. The variations of voltage versus losses, cost of conductors versus losses and power delivered versus losses are plotted below. The conflicting patterns of voltage, cost of conductors and power delivered versus line losses are evident from the figures. The size of the conductor to be selected must be near the intersection point which minimizes the parameters involved. In this example the appropriate conductor size, the nearest conductor size to the intersection of the curves must be chosen, is ACSR 3/0.