The four major voltage high voltage engineering book pdf are 5, 15, 25, and 35 kV. For example, a 15-kV insulator is suitable for application on any 15-kV class voltage, including 12.
Cables, terminations, insulators, bushings, reclosers, and cutouts all have a voltage class rating. Only voltage-sensitive equipment like surge arresters, capacitors, and transformers have voltage ratings dependent on the actual system voltage. Utilities most widely use the 15-kV voltages as shown by the survey results of North American utilities in Figure 1. The most common 15-kV voltage is 12. 47 kV, which has a line-to-ground voltage of 7. The dividing line between distribution and subtransmission is often gray. Some lines act as both subtransmission and distribution circuits.
5-kV circuit may feed a few 12. 5-kV distribution substations, but it may also serve some load directly. Some utilities would refer to this as subtransmission, others as distribution. The last half of the 20th century saw a move to higher voltage primary distribution systems. The great advantage of higher voltage systems is that they carry more power for a given current. A higher-voltage circuit has less voltage drop for a given power flow.
A higher-voltage system can carry more power for a given ampacity. For a given level of power flow, a higher-voltage system has fewer line losses. With less voltage drop and more capacity, higher voltage circuits can cover a much wider area. Because of longer reach, higher-voltage distribution systems need fewer substations.
An important disadvantage of higher voltages: longer circuits mean more customer interruptions. Crews do not like working on higher-voltage distribution systems. From transformers to cable to insulators, higher-voltage equipment costs more. Information above shows maximum power levels typically supplied by various distribution voltages.