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 Author Topic: Why we need High Voltage Transmission Line  (Read 226072 times) 0 Members and 1 Guest are viewing this topic. Click to toggle author information(expand message area).
Fu-Kwun Hwang
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 « Embed this message on: February 08, 2009, 06:21:28 pm »

One of the key concerns in the transmission of electricity is the power loss in transmission lines, dissipated as heat due to the resistance of the conductors.

High-voltage transmission lines are used to transmit electric power over long distances. Normally, high voltage (HV) transmission power lines are made of high voltage (between 138 and 765 kilovolts) conducting lines of copper and/or aluminum.

Assume the power to be transmitted is P, and the resistance of the transmission line is r.
If the power is transmitted with voltage V, then the current flow through the transmission line is I=P/V.

The power loss Ploss=I2*r=(P/V)2*r
Since P and r are fixed conditions, less power will be lost if high voltages V are used.

Some students will raise questions like: From Ohm's law. if the voltage is increased, the current will increase ,too. Why is the current smaller when high voltage is used to transmit the power.

Textbooks forgot to tell students that the transmission line needs a transformer to step down the voltage.
And the transformer does not have a fixed impedance. If higher voltage is used to transmit the power, the ratio of the transformer will also change which will change the impedance of the transformer.
The following applet was developed to help you understand the high power transmission line.

You can change the Power/Voltage V and resistance r in the transmission line with sliders.
I will show current flow through the transmission line.
Z is the total impedance of the transmission line, Zt is the impedance of the transformer.
N:n shows the ratio of the high voltage transformer (Assume user voltage is 100V).
Efficiency of the power line is also shown at the right side.

-*-

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lookang
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http://weelookang.blogspot.com

 « Embed this message Reply #1 on: December 06, 2009, 10:54:55 am »

There is a similar flash (copyrighted not embeded here) Absorb Physics  by David Fairhurst http://www.absorblearning.com/media/item.action?quick=7v, free to use from their website only.

found this
http://www.forcvec.com/powerkids/images/distribution.swf
http://www.forcvec.com/powerkids/images/distribution.swf

http://www.energy.com.au/energy/ea.nsf/AttachmentsByTitle/energy_cycle.swf/\$FILE/energy_cycle.swf

the applet here is explain in detail just the one part of the energy transmission
 « Last Edit: December 06, 2009, 12:35:28 pm by lookang » Logged
ahmedelshfie
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 « Embed this message Reply #2 on: May 25, 2010, 12:46:00 am » posted from:SAO PAULO,SAO PAULO,BRAZIL

prof Hwang problem in XML file of Why we need High Voltage Transmission Line
I think in variable vy could you solve prof
Thanks
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Fu-Kwun Hwang
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 « Embed this message Reply #3 on: July 14, 2010, 09:57:18 am » posted from:Taipei,T\'ai-pei,Taiwan

Sorry! I do not understand your question?
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ahmedelshfie
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 « Embed this message Reply #4 on: July 14, 2010, 05:35:39 pm » posted from:SAO PAULO,SAO PAULO,BRAZIL

long time a go applet not work with me because chines character but in this time
I was not able to discover it my experience in EJS but after learn step by step prof
Wrong was from me.
I find a way to solve my problem about applet i delete chines character and delete evolution page
And run a gain work very well.
Thanks prof for you learn me all what i know
You the hero.
 « Last Edit: July 14, 2010, 06:16:04 pm by ahmedelshfie » Logged
moody
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 « Embed this message Reply #5 on: October 09, 2012, 05:06:48 am » posted from:Newport Beach,California,United States

Dear Prof:
I look at your applet and note that the ratio of Io/I does not follow n1/n2.
Can you pls explain why?
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Fu-Kwun Hwang
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 « Embed this message Reply #6 on: October 10, 2012, 11:09:23 pm » posted from:,,Satellite Provider

The relation you assumed is under the assumption for an idea case: No power loss from the transformer: all the power are transformed. However, the above applet simulate real transformer: Some power are loss due to resistance in the transmission line.
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moody
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 « Embed this message Reply #7 on: October 13, 2012, 10:24:05 am » posted from:Irvine,California,United States

Thanks Prof.
BTW, how did you model a lossy transformer in your applet?
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Fu-Kwun Hwang
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 « Embed this message Reply #8 on: October 14, 2012, 08:23:54 pm » posted from:,,Satellite Provider

Just like the simulation shown: I added a resistor to the transformer circuit.

You can download the simulation and open it with EJS to find out the detail equations being used in the simulation.
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Surath Asia
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 « Embed this message Reply #9 on: October 24, 2012, 08:32:01 pm » posted from:Tirunelveli,Tamil Nadu,India

One of the key concerns in the transmission of electricity is the power loss in transmission lines, dissipated as heat due to the resistance of the conductors.

High-voltage transmission lines are used to transmit electric power over long distances. Normally, high voltage (HV) transmission power lines are made of high voltage (between 138 and 765 kilovolts) conducting lines of copper and/or aluminum.

Assume the power to be transmitted is P, and the resistance of the transmission line is r.
If the power is transmitted with voltage V, then the current flow through the transmission line is I=P/V.

The power loss Ploss=I2*r=(P/V)2*r
Since P and r are fixed conditions, less power will be lost if high voltages V are used.

Some students will raise questions like: From Ohm's law. if the voltage is increased, the current will increase ,too. Why is the current smaller when high voltage is used to transmit the power.

Textbooks forgot to tell students that the transmission line needs a transformer to step down the voltage.
And the transformer does not have a fixed impedance. If higher voltage is used to transmit the power, the ratio of the transformer will also change which will change the impedance of the transformer.
The following applet was developed to help you understand the high power transmission line.

You can change the Power/Voltage V and resistance r in the transmission line with sliders.
I will show current flow through the transmission line.
Z is the total impedance of the transmission line, Zt is the impedance of the transformer.
N:n shows the ratio of the high voltage transformer (Assume user voltage is 100V).
Efficiency of the power line is also shown at the right side.

-*-
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Bellay
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 « Embed this message Reply #10 on: September 29, 2013, 11:51:31 am » posted from:Jinan,Shandong,China

High-tension lines are so dangerous.Every time I flying my -*- helicopter I am very careful.
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 : 1 users think this message is good12 Re: Why we need High Voltage Transmission Line « Embed this message Reply #11 on: May 04, 2014, 08:54:26 pm » posted from:Mumbai,Maharashtra,India

BY equation ,
transmitted power, p= √3VIcosØ
circulating current, I = (P/√3 VcosØ)

(1) For a amount of power transmission at higher voltage ,
If power is constant and voltage increases then current decreases.

Dimension of conductor and weight depend on current density, therefore lower conductor sizes would have lower weight and cost.

(2) Distance between two tower depends on conductor weight , therefore lower weight would have long span between two tower and higher weight  would  have short span between two tower .

therfore , lower weight would have cost advantage due to reduction of a number of tower or pole.

(3) Design and strength of tower should be according to weight and tensile strength of conductor because in limit of elasticity , force stress is directly  proportional to the strain.

therefore , young modulus would have a ratio of force stress and strain.

Y= Force stress / strain  (newton per meter^2)

According to young modulus , cost of higher young modulus tower will be higher than lower young modulus tower in proportional to tensile strength .

(4)  Line losses = I^2 * R  (in watt)

For higher voltage transmission line, line losses depend on circulating current and conductor resistance.
Therefore , lower current would have lower losses and cost and higher current would have higher losses and cost.
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 : 1 users think this message is good13 Re: Why we need High Voltage Transmission Line « Embed this message Reply #12 on: May 08, 2014, 01:17:19 pm » posted from:Mumbai,Maharashtra,India

BY equation ,
transmitted power, p= √3VIcosØ
circulating current, I = (P/√3 VcosØ)

(1) For a amount of power transmission at higher voltage ,
If power is constant and voltage increases then current decreases.

Dimension of conductor and weight depend on current density, therefore lower conductor sizes would have lower weight and cost.

(2) Distance between two tower depends on conductor weight , therefore lower weight would have long span between two tower and higher weight would have short span between two tower .

therfore, lower weight would have cost advantage due to reduction of a number of tower or pole.

(3) Design and strength of tower should be according to weight and tensile strength of conductor because in limit of elasticity , force stress is directly  proportional to the strain.
therefore , young modulus would have a ratio of force stress and strain.

Y= Force stress / strain  (newton per meter^2)

According to young modulus , cost of higher young modulus tower will be higher than lower young modulus tower in proportional to tensile strength .

(4)  Line losses = I^2 * R  (in watt)
For higher voltage transmission line, line losses depend on circulating current and conductor resistance.
Therefore , lower current would have lower losses and cost and higher current would have higher losses and cost.
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Life well spent is long. ..."da Vinci (1452-1519, Italian artist, sculptor, painter, architect, engineer and scientist) "