DC system vs AC?

Question

Explain why a DC system cannot use an efficient stepped up voltage for long distance transmission.

kind of lost here. How come AC can't do it?

sorry..what I meant on the last part was, how come DC can't do it?

Answer 1

Before the answer would make sense, you need to read up on the operation of a transformer. Transformers can not be used with DC. Other methods of stepping up/down DC are quite expensive.

Answer 2

I'll try to keep this simple; when we generate a lot of electricity in one place (a power plant) and want to get it distributed over a wide area, it is desireable to be able to easily change the voltage higher or lower. If the voltage is alternating current (AC) we can simply use a transformer, It is two (or more) coils of wire wound around a laminated iron core. The voltage coming out is determined by the input voltage and the ratio of number of turns of the two (or more) windings there are no moving parts (except, possibly cooling fans), and they are very efficient ~ 95% maybe better. If the electricity is generated as direct current (DC) you can't use transformers, because they require a rapidly changing magnetic field to operate. Why do we want to change the voltage? Because using very high voltages/lower currents on the major transmission lines allows a lot of power through a fairly small cable.

Anyway, the important part of this is that you can't use transformers with DC current; trans formers require a rapidly changing magnetic field to operate. This doesn't exist with DC current.

Answer 3

Faraday's law explains the ideal efficiency of transforming AC. Basically the long distance distribution of DC would be expensive and dangerous. The use of the AC transformer allows each distribution (or branch off to a house) to be the same thus having a stable and standardized source of power for manufacturers of home appliances. If you study Ohms law,
and Faradays law, you may see the complications of load changes in DC circuits vs. AC circuits. Simply stated DC can not be transformed as efficiently as AC.

Answer 4

Actually, when transferring DC power, the only real inefficiency is the resistance of the lines (aka "copper losses")

Resistance of a wire in regards to DC is affected by the material (i.e. copper, aluminum, silver, gold, etc.) the cross-sectional area of the conductor (more is better), and temperature (the colder the better.)

In AC power distribution, along with resistance you start factoring in capacitive and inductive reactance. These things all factor together to formulate "impedance" which is frequency dependent.

AC power is more flexible in that it can be more easily stepped-up and down via transformers (DC power can't utilize power transformers - it simply saturates the core.)

There are actually less losses in DC power distribution since impedance is not a factor, only resistance.

Answer 5

A DC system has certain advantages. It doesn't have peak voltages and currents larger than the average. It need not worry about skin effect. It doesn't lose energy and cause interference due to inductive coupling with its surroundings. But as others stated, there's not a step-up / step-down technology as good as the mutual inductance transformer for cost, simplicity, efficiency, and reliability.

Answer 6

DC is direct current, as you know, do when you talk about being stepped up are you talking about thru a transformer? A transformer requires AC to make the coupling from the primary coils to the secondary coils.

I'm not sure I'm on the right track for you tho.

Answer 7

Megadeth