Transmission and distribution voltage levels

Power systems includes a generating system, a transmission and distribution system and loads. Transmission and distribution system is an important connecting link between the bulk power generating stations and the load centers. The generation plants are normally located in place where the resources are available to generate power economically. The recent trend in power system practice is to locate the generation plants away from the heavily populated areas. The power is then transmitted to the load by transmission lines and distribution networks.

Transmission is made of a high voltage network, generally 110 - 765 kV ac. The higher the voltage level of a transmission line, the lower is the transmission power loss. However, the voltage limits for the transmission line voltages are set by insulation and protective devices.

The standard transmission line voltages depend upon the country and they are: 765, 550, 500, 400, 345, 275, 230, 169, 145, 132, 110, 66, 33 kV. The voltages in the range of 345 - 765 kV are classified as extra high voltage (EHV). The voltages above 765 kV are considered as ultra high voltages (UHV). Currently, the UHV systems, at 1000-, 1500-, 2250- kV voltage levels are in research stage. Point to point transmission is, some time, preferred by HVDC. The voltage levels used for HVDC power transmission are: 250 kV, 400 kV, 500 kV and 550 kV. Higher dcvoltages are being planned.

Distribution system is the connecting link between the transmission system and the loads. The major investment in the power system has been in the generating sector, with distribution system a close second. The maintenance expense and major operation has been in the generating sector, followed by the one for the distribution sector. Thus the economic importance of the distribution system is very high. Further more, the distribution being the system close to the consumer, it plays a significant role in providing good quality and cost effective power supply. Thus, the economical and technical importance of the distribution system dictates careful planning, design, construction, and operation.

Distribution level voltages are: 11kV, 6.6 kV, 4.16 kV, 415/400 V, 240/230 V single phase and in USA 110 V single phase.

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