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Author Topic: Why do electricity companies transmit power at very high voltages?  (Read 13590 times)

Offline erickejah

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What is the reason that utility companies transmit power using very hing voltages?

i know that transmitting electricity at high voltages makes the mediums of transportation to be smaller, but how does the inductance an transformers correlates to this?   ??? ::) ???
« Last Edit: 11/12/2008 09:56:49 by chris »


 

lyner

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For a given amount of power, you can choose your voltage and that determines the amont of current.
Power = Voltage times Current

Choosing a very high voltage means the current is low. If the current is low then there is very little power lost due to the resistance of the supply cables.

There is a certain amount of loss in transformers but less than if you just used lower voltages. The cost of thick enough cables would be prohibitive.
The self inductance of the primary is chosen to be high enough not to limit the  primary current with no load. The mutual inductance between primary and secondary is what produces the secondary voltage (turns ratio is appropriate) and, as long as you have enough Iron, there is no saturation and the losses are low. The load that is presented to the secondary appears as a different resistance to the supply on the primary.

The complete theory of transformers is quite complicated but they work almost 'ideally' if you design them correctly.
« Last Edit: 29/11/2008 00:26:24 by sophiecentaur »
 

Offline erickejah

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There is a certain amount of loss in transformers but less than if you just used lower voltages.
that is what i was looking for tx  :D :D
 

Offline highvoltpower

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High voltage communication lines are used to communicate electric power above long spaces. Selecting high voltage mean the current/power is low. If power is low then actually slight power loss due to the resistance of the supplying wires.
 

Offline chris

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current/power is low

That's not quite right. P (power) = volts (V) x current (I); thus, the power (rate of energy transmission) can be kept the same by increasing the voltage and dropping the current.

Since V = I x R (resistance), you can also express P = V x I (the equation above) as P = (I x R) x I

This is simplified to P = I2R

In other words, the power transmitted is proportional to the current SQUARED. The same will therefore be true of the losses in the cable owing to heating. So, the ideal is to minimise the current to minimise the losses.

From P = V x I (above), the best way to do this is to increase V and decrease I; the outcome is the same rate of energy transmission (P) but a dramatic drop in losses secondary to cable resistance.

The cost of this strategy is that to transmit high voltage safely requires considerably more investment in pylons, insulators, transformers and so on. But over the operating lifetime of the infrastructure this is more than offset by the energy saved.

 

Offline wolfekeeper

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i know that transmitting electricity at high voltages makes the mediums of transportation to be smaller, but how does the inductance an transformers correlates to this?   ??? ::) ???
A transformer has relatively little inductance when it's carrying current; the secondary windings carry, in a sense, the same amount of 'current' around the iron core (specifically the amp-turns is nearly the same) and opposes the input side's inductance, and consequently transformers when carrying current have very low inductance.

When the output side isn't carrying current, the inductance goes really high, since the inductance on the input side isn't being opposed, and this means that the transformer then has high inductance, which means high impedance, and because it's high impedance, the input side ends up carrying relatively little current, so the resistive losses on the input side are small, and transformers are still fairly efficient when nothing is drawing current.

If that didn't happen, every time we turned off all the power in the house, the transformer at the substation would glow red hot!

So, a lot of the efficiency is about what happens when you're not using power as much as when you are.

Still, some power is still lost like that. Those 'fat' power supplies that you plug into the wall, they step the mains down to 12v or 5V. But even if they not carrying any current, if you touch them they're often slightly warm; that's the current that's not being blocked by the transformer's inductance. The modern slimline ones are much more clever, and the transformer inside is much smaller, and they disconnect the transformer from the mains when they're not in use, and so they are cold to the touch when they're not powering anything.
« Last Edit: 13/04/2016 17:31:32 by wolfekeeper »
 
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