Power – For the same current, power changes linearly with voltage.Ĭurrent – For the same power, increasing the voltage decreases current linearly. Utilities can run much longer distribution circuits at a higher primary voltage, which means fewer distribution substations. Table 1 – Power Supplied by Each Distribution Voltage for a Current of 400 A System Voltage (kV) Utilities can use smaller con-ductors on a higher voltage system or carry more power on the same size conductor. Higher voltage systems need fewer voltage regulators and capacitors for voltage support. Less current means lower voltage drop, fewer losses, and more power-carrying capability. Information above shows maximum power levels typically supplied by various distribution voltages. Reliability – An important disadvantage of higher voltages: longer circuits mean more customer interruptions.Ĭrew safety and acceptance – Crews do not like working on higher-voltage distribution systems.Įquipment cost – From transformers to cable to insulators, higher-voltage equipment costs more. The great advantage of higher voltage systems is that they carry more power for a given current. Higher-voltage distribution systems have advantages and disadvantages (see Advantages and disadvantages of higher voltage distribution below). The last half of the 20th century saw a move to higher voltage primary distribution systems. (Data from IEEE Working Group on Distribution Protection, 1995) Figure 1 – Usage of different distribution voltage classes (n = 107). Some utilities would refer to this as subtransmission, others as distribution. A 34.5-kV circuit may feed a few 12.5-kV distribution substations, but it may also serve some load directly. The dividing line between distribution and subtransmission is often gray. Some lines act as both subtransmission and distribution circuits. The most common 15-kV voltage is 12.47 kV, which has a line-to-ground voltage of 7.2 kV. Utilities most widely use the 15-kV voltages as shown by the survey results of North American utilities in Figure 1. Only voltage-sensitive equipment like surge arresters, capacitors, and transformers have voltage ratings dependent on the actual system voltage. Cables, terminations, insulators, bushings, reclosers, and cutouts all have a voltage class rating. A voltage class is a term applied to a set of distribution voltages and the equipment common to them it is not the actual system voltage.įor example, a 15-kV insulator is suitable for application on any 15-kV class voltage, including 12.47 kV, 13.2 kV, and 13.8 kV. The four major voltage classes are 5, 15, 25, and 35 kV.
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