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what are the advantages of having high voltage in transmission
lines?
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Answer Posted / srajan
AC is used for both transmission and distribution because it is possible to easily transform voltage. Increasing voltage reduces current, thereby reducing the size conductors required to transmit large blocks of power, and also minimizes losses. Lower voltages are desired at the point of consumption because of safety and convenience.
With DC, the advantages of higher voltage (smaller conductors, lower losses) and lower voltages (safety and convenience) are also present, but there is no convenient way to change the voltage with DC. That's the main reason that all-DC systems are not used.
DC does offer two advantages over AC - with AC, the maximum distance that power can be transmitted is limited by two factors. Obviously, one is losses. But a more limiting factor is that the maximum magnitude of power transfer is governed by the phasor angle between the sending and receiving voltages - that angle physically cannot exceed 90 degrees, and as a practical matter can not be allowed to exceed about 30 degrees without jeopardizing the ability to the system to respond to contingencies. With DC, there is no angular limitation, only a limitation based on losses.
That angular concern comes about because of the need to maintain synchronism across an AC system. The second advantage of DC is that there is no need to maintain synchronism, and in fact DC can readily be used to interconnect systems that are not in synchronism (for example, the State of Texas is not in synchronism with anyone else - and that goes beyond politics - and is interconnected with the rest of North America via high voltage DC. It is also possible to interconnect systems that are operating at different frequencies using DC - for example, the Amtrak rail system between Washington DC and New York is a 25 Hz system, and is interconnected to the rest of the grid via either DC or special rotating frequency converters.
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