Purpose. Most electric transmission and distribution circuits are three-phase and require three lines to show them schematically in detail. For many functions it is not necessary to have all the detailed information shown on the schematic diagram. Therefore single-line diagrams are utilized to represent three-phase circuits and equipment. A single-line diagram provides a quick method whereby lineman or cableman may analyze the complete facilities in schematic form.

Distribution Unit Substation Single-Line Diagram. A 13,200-4160Y/2400-volt unit-type distribution substation is illustrated by Fig. 38.1 in single-line form. Analyzing the unit substation single-line diagram, it can be determined that the substation has as its source an overhead 13,200-volt three-phase subtransmission electric circuit designated 13-61-T-1. The overhead line connects to the substation transformer through 300-kCMIL copper conductor, paper-insulated, lead-covered cables with pothead terminations on each end of the cable circuit. A dotted line representing the cable circuit indicates that it is installed underground. The power transformer has three potential transformers connected to the primary leads through fuses.

The high-voltage winding of the power transformer is connected delta, has a tap changer, and the low-voltage winding is connected wye with the neutral grounded. The transformer is rated 13,200-4160Y/2400 volts, 5000/6250 kVA. The capacity varies with the type of cooling. Oil-air cooling provides 5000 kVA of capacity, and forced-air cooling provides 6250 kVA of capacity. The tap changer operates under load. The current transformers in series with the 4160-volt bus connections to the power transformer provide a means of measuring the current flow in...