Power Lines: CR4 Challenge (01/26/10)

And induction losses will be reduced

Regards

GA for good reply.

I would like to add:

Due to smaller dia of conductors in Transmission low cost in material, less problems in INSTALLATIONS & Maintenance & Breakages in Power Lines which extend to hundreds of miles.

So most economical in subsequent costs in running a system.

Yes, you are right.

let me elaborate a little; for a particular power 'P' to be transmitted, the line losses are directly proportional to line resistance 'R' and inversely proportional to square of transmission voltage(of course power factor is not considered). therefore,transmission voltage is kept as high as possible.line resistance is affected by several factors(diameter,length,etc),so it is better to vary voltage which technically is easier(by using transformers).this helps

• to maintain voltage regulation(almost uniform voltage for load points)

also mechanical considerations limit the line diameter and weight for suspension thus limiting the current carrying capacity(which is usually many times smaller than voltage).

I am old enough to remember the switch in autos from 6 volt to 12 volt. The cost savings since the switch is probably in the trillions for both the consumer and the auto maker.

So the answer is very simple, first it is cost savings, and secondly making the transmission practical.

P E Bobimm

That's why automotive might go up to 48V and electric vehicules are using over 500V between power control and engine.

Because the wire used to transmit the energy has a resistance (R) which will result in losses - the basic equation is P(watts) = I(amperes) *I(amperes)*R(ohms) or "I squared R". You can reduce (but not eliminate) the losses if you can make the current (I) as small as possible.

In the example given, if our transmission line had a resistance of 1 ohm then at 1 volt our losses would be 100*100*1 or 10,000 watts (not going to work very well if all you had to start out with was 100 Watts ...), With the "high" voltage the loss would be 1*1*1 or 1 Watt!

This can get really complex with AC power and other factors ( its all imaginary ) but this is the basic rule.

QED

The use of AC here begs the question:
"Why do we (in the USA) transmit AC instead of DC?"
I wonder how many Good Answers we get for this one.
BTW #3 is the Best Answer (simple, direct, correct).

AC or DC??? For long distance high V is obvious for low I and low cost on copper. But for safety you must go to low V when getting to the home. Getting from high V to low V is easy with AC due to transformers. That's why as long as power semicond where not availables and cheap AC was the choice.

Now we might as well go to DC

Regards.

But now DC transmission is not that difficult & costly as it was thought some 20 years back.

And advantage is of thinner wires due to no SKIN_EFFECT in DC.

Have a fine day !

There is no way dc transmission is cheaper than ac. Calculate the cost to transmit 100kw a distance of 1kilometer with ac and with dc. I think you will conclude that the thoughts 20 years ago hold true today.

PAPADOC

"If we all worked on the assumption that what is accepted as true is really true, there would be little hope of advance." -- Orville Wright

The break even point DC x AC was 500 to 1000 km 30 years ago (think of the costs for towers carrying 2 (thinner) cables instead of 3). It must be less nowadays, still more than 1 km...

brgds

Snel

Please read:

Transmission_in_China

Transmission and distribution

On the transmission and distribution side, the country has focused on expanding T&D capacity and reducing losses by:

deploying long-distance Ultra High Voltage AC (UHVAC, referring to 1000 kV) and Ultra High Voltage DC (UHVDC, referring to +/-800 kV) transmission

installing high efficiency amorphous metal transformers

I remember that some 1 years back Transmission at 500KV was under consideration even in our under-developed country like PAKISTAN.

I do not know what is the state of the project.

Regards

Thanks to confirm my reply

So it gets there ... to where ever. (It's like... if you're pushing a wheelbarrow of stones up the hill or over a long distance, your legs (that's the force) can push a lesser, minimal load of stones (that's the increments/parcels of current required...somewhere) faster and easier than weaker legs can push a full barrow of stones.

The higher the current the more the electrons tend to travel on the surface outside the conductor causing a skin effect that increases the resistance, so the higher the voltage the more center conductor is used causes much less voltage drop due to less heating and resistance not to mention the wire size is a big factor.

I think you'll find that "skin effect" relates to higher frequencies, and not to current. Hence the use of Wave Guides for high freq RF signals (this is simply a copper tube with no centre), because basically the current of the high freq signal only uses the outside (skin), and all the copper in the centre is useless and adds cost and weight.

Cheers Tony

Dear,

Ther are certain reasons behind it but the two main reasons are,

01- As you said, at higher voltages,current is reduced and therefore conductor size also decreses and hence it is cost effective.

02- As power is to be transmitted for long distances, voltages are kept very high because of voltage drops with the distance.

If power is transmitted using high current and low voltage, a lot of heat will be generated and this shall amount to power loss. hence in order to minimise loss and increase efficiency, transmission is by high voltage and low current.

Are you kidding? What reasonable response can be provided to this question? For one thing, the power transmitted depends on more than current x voltage; it also depends on the phase angle between the two.

Yes, but more on current x voltage.

Phase angle in DC? the challenge doesn't say it's AC.

Power lines carrying high voltage and correspondingly low current can use thinner and lighter cables which are easier to install as well as being a great deal cheaper. The heat losses are less beacause the I² x R value will be less than in the situation where the current is high and the voltage less. The product in each case being the same i.e. the power being equal. Another problem arises from the extra weight of the conductors possibly neccessitating a reduction in the safe span of the pylons. The increase in wind loading is also a factor to be considered.

UFFARNDAN

The higher the voltage, the farther the current can travel. A low voltage/High current couldnt overcome the resisitance of "miles of cable"

Think of it as a pipe. With high pressure (voltage) you can push a large volume (amps) through a small pipe. With low pressure, it would take a large pipe to carry the same amount. Large pipes cost more than small pipes.

basic I squared R losses of the cable. higher voltages less loss

Higher voltage is required in order to overcome the resistance inherent in the wires that carry the transmitted electric power.

IMHO:

HV transmission is possible because air is an insulator (within certain limits, of course).

HV transmission is economicaly feasible because insulation costs are lower than conductor costs.

Things could be different (dual) in a Bob Sponge world... high currents, low voltages, outlets shorted while not in use...

It is because the higher the voltage for the same amount of power, the smaller the wire to carry it. Therefore, it's cheaper to step up the voltage with transformers and use smaller, less costly wires to transfer the power.

PAPADOC

RMFR

"Love is Service....in coveralls" Mary Ann in Houston

Losses are minimised. I'm sure the previous 30 comments have adequately explained why...

Going to read them now !

line loss

microwave in a wave guide would be better

bcoz for a conductor to carry large current(power being constant) conductor diameter required is proportionally large. And since cost is a prime consideration in engineering design alongwith efficiency,we use conductor which carries smaller current than voltage.

Dear abishek tiwary,

nice that you mention "cost" again!

As we design even higher voltage systems, we take a fact for granted: air is an insulator. This is true to about 1 MV, and this is the practical limit today... ohmic losses would be even lower at 10 MV, but what about corona etc.?

So I think "when electric power is transmitted," "we use high voltage" [hence low current] because our insulating atmosphere makes this possible.

best regards

Snel

Elementary. Transmission involves passing current through lines which have some resistance. Resulting in power losses during transmission, and is proportional to square of current. Hence lower current results in reduced losses. Thing to be noted is that the power being product of voltage and current, we have to increase voltage to reduce current.

Oops!

In a simple conductor (discounting impacts of hysteresis or reactance), the Power Loss due to ohmic heating is given by the simple formula:

P = R x I^2

For electrical power transmission lines, the length of line is long (hundreds of kilometers), Therefore even though the resistance of the conductor appears to be a very small number as conventionally expressed in standard look-up tables, the overall resistance of the transmission line is considerable.

For the sake of argument assume that the resistance is 1 ohm and that the power being generated for transmission is 1 gigawatt.

At a transmission voltage of 1,000,000 volts, the amperage needed to transmit the power is 10E9/10E6 = 1,000 amperes. Therefore from ohmic resistance alone the power loss is P = 1 ohm x 1,000^2 amperes = 1 megawatt. (ie. only 1% line loss).

Drop the transmission voltage down to 100,000 volts and the current must be 10E9/10E5 = 10,000 amperes. The power loss goes up to 100 megawatts (10% line loss).

Hi,

The answer is wrong. It is not a matter of saving energy; but it is a matter of reducing the conductor size. Higher the transmission voltage, lower the size of the conductor needed for transmitting same amount of power.

The answer is incomplete, but your objection more so.

It's an economic decision, and energy loss is significant in the calculations. There are of course additional constraints due to standardisation, but within those the chosen Voltage should be the one at which the trade-off between wiring cost, transformer cost, energy loss and load sensitivity are considered optimum. (Even 11kV transmission lines are capable of carrying several times the currents that are actually used)

As conductors have finite resistance ( usually) it would be better to save power loss by transmitting the power at high voltage and low current as the loss is proportional to current squared times resistance. The resistance is likely to be fixed so if the current can be reduced, the losses would be minimised.

Nick

Current does not flow inside of a conductor. It travels on the surface.

This is not strictly correct for power transmission. Unfortunately, your comment is too cryptic for me to be certain whether you are writing (somewhat misleadingly) about the effects of economic criteria, or are basing your comment on one of two false assumptions.

Error possibility 1:
If the conductor is charged, the net charge will be stored close to the surface - but electronic current flow equally occurs in parts of the conductor where the charge is balanced.

Error possibility 2:
The "skin effect" can limit the depth of conduction, but this is not a significant issue for power transmission. At 60 Hz (the highest frequency generally used for HV power transmission) the skin depth in copper is about 8.5-mm. If no precautions were taken this would mean that the current would largely be carried in the surface if the thickness of the wire became too great. For solid copper conductors this would correspond to a cross-sectional area above about 150-mm².
In practice, however, the conductors are built from strands that are thinner than the skin depth. This suppresses the component of the current that runs orthogonally to the surface, which allows the current to penetrate deep into the conductor (see here for example).

So, the reality is that the current can be carried throughout the bulk of the intended conductor.

This brings us to a real effect, to which you could possibly be referring - the current is often confined to a region near the surface of the cable.

There can be (at least?) two reasons for the cable to be designed in this way:
If we make the wire too thin the electric field at its surface becomes too high for the insulation to withstand; and
With buried cables self-heating can also be a problem.
In either case the difference between the costs of highly-conducting material and bulking materials (or mechanical supports) means that it is more practical and economic to make the core of the cable from poor conductors and restrict the conduction to a region of finite thickness near the surface. In most cases the current density will be relatively uniform through most of the conducting region.

Fyz

Higher voltage across the resistor (power lines.) starts to improve the current capabilities.

V=IR is the answer. as long as the cables carrying the current can handle the power ; W=IV the resistor (power lines.) ; One of the factors in the equation remains constant......The resistor or power lines....do we get the idea ??

forget W=IV for this question....and forget about :

The amount of thermal energy produced by electrons moving inside a wire is the product of the number of electrons per second moving (current) and the resistance of the wire. The more current is transported the more electrical energy is converted to thermal energy and it is lost....as this is to complex for the question....it only tells you when your insulation on cable loses it's ....well insulation or isolation from other conductors.

Regards.

Philip.