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Step Down Transformers and Power Distribution - Help!

by mathological
Tags: power loss, transformer loads, transmission
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mathological
#1
Feb15-11, 06:13 AM
P: 10
Hi,

so I have been having sleepless nights about this concept.

From what I understand, say a typical power generating plant that produces 2000MW of energy, for example, will want to transmit as much of 2000 MW of power to the grid as possible and they do so by stepping up the voltages to 500kV or more in order to reduce transmission losses.

Now lets get down to what happens once the distribution company gets this power. Say the voltage is stepped down to 132kV from 500kV.

Now my question is, since there is a huge reduction in voltage, won't that mean that there will be significant loss in power as well due to P = VI?

So what I am saying is that let's say the grid transmits 500 MW of the 2000 MW they receive to one of the distribution companies/utility, then when distribution company distributes at lower voltages, won't that mean they will be distributing much less than 500 MW due to low voltage levels (by low voltage here I mean below 132kV, i.e 33kV, 22kV, 11kV, 415V, etc)? (I am talking about losses due to lower voltage levels due to P = VI, on top of the heat losses)??????

Thanks for your help!
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Jiggy-Ninja
#2
Feb15-11, 07:56 AM
P: 310
The stepping up and stepping down is done by a transformer. The way a transformer works, whatever change is done to the voltage, the inverse is done to the current. Double the voltage, and the current is halved. Halve the voltage, and the current is doubled. This conserves power. There are some small power losses through eddy currents, hysteresis, flux leakage, wire resistance, etc, but overall transformers are very efficient. I believe around 95% or so.
vk6kro
#3
Feb15-11, 09:10 AM
Sci Advisor
P: 4,032
The losses at the lower voltages do not have to be higher than at high voltages. You just have to use thicker wires to carry the current.

The question is how much of this thicker wire you can provide economically.

There are good reasons for having lower voltages at street level and high voltages in the long distance lines from the power station.

How you balance these needs is a matter of design. High voltages use big towers, big insulators and big transformers and all this costs money.

Against this, you need the relative safety of 120 volts (or whatever) for your house, so you have to accept that this will be expensive in copper wire but more economical on towers etc.

So, designing distribution systems is always a matter of economics, among other considerations.

mathological
#4
Feb15-11, 05:40 PM
P: 10
Step Down Transformers and Power Distribution - Help!

Quote Quote by Jiggy-Ninja View Post
The stepping up and stepping down is done by a transformer. The way a transformer works, whatever change is done to the voltage, the inverse is done to the current. Double the voltage, and the current is halved. Halve the voltage, and the current is doubled. This conserves power. There are some small power losses through eddy currents, hysteresis, flux leakage, wire resistance, etc, but overall transformers are very efficient. I believe around 95% or so.
cool! understood! Thanks.
mathological
#5
Feb15-11, 05:43 PM
P: 10
Quote Quote by vk6kro View Post
The losses at the lower voltages do not have to be higher than at high voltages. You just have to use thicker wires to carry the current.

The question is how much of this thicker wire you can provide economically.

There are good reasons for having lower voltages at street level and high voltages in the long distance lines from the power station.

How you balance these needs is a matter of design. High voltages use big towers, big insulators and big transformers and all this costs money.

Against this, you need the relative safety of 120 volts (or whatever) for your house, so you have to accept that this will be expensive in copper wire but more economical on towers etc.

So, designing distribution systems is always a matter of economics, among other considerations.
Right! Ok so when people tell us that a certain power station provides 2000 MW, then not all of this 2000 MW gets to the consumer. Does anyone know how efficient a typical power distribution system is (in terms of what is being output at a generating station say 2000 MW and what the consumers are receiving)?
romeff00
#6
Feb16-11, 09:51 AM
P: 13
short circuit makes u have to go for a higher capacity wire. Sometimes the reason why we step up is because the wire we using will not be able to withstand a short circuit
lindsey33
#7
Sep9-11, 01:55 AM
P: 1
But very heavy critics sometimes makes u give up


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