Wire and Cable Technology Blog

Guest. It bothers the rest of us as well. At least it's saved me the bother of reading the article.

In the "War of Currents" era (sometimes, "War of the Currents" or "Battle of Currents") in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison's promotion of direct current (DC) for electric power distribution over alternating current (AC) advocated by Westinghouse and Nikola Tesla.

http://en.wikipedia.org/wiki/War_of_Currents

Hi JCase and group -

I'm a subscriber to BBC World TV global news channel. I stream/subscribe via Real Networks -

A few years ago (last 5 years or so), I saw a high-quality documentary aired detailing the rivalry being discussed here (theme was War of the Currents). Much of the battle took place throughout New York State, and included were details of fires caused in New York City by Edison's DC experiments to electrify the city.

I've been trying to locate where to purchase this documentary on DVD, but no joy. Tried contacting BBC, Tesla society, extensive web search, and so on.

Anyone else see this documentary on BBC domestic or international TV news channels?

Thanks in advance for anyone's comments/suggestions.

- Larry

Goodho,

Well said, but a questionable conclusion. First, connecting multiple power sources to a DC network is much more simple than with AC because you only have to match voltage. Most of the protective relaying problems that have caused wide-spread blackouts in the USA were inherent to maintaining the system's frequency. Second, as line length increases, AC transmission line losses increase due to radiative losses. DC lines have no radiative losses, so the long distance lines you mention are DC.

In the USA, HVDC transmission lines are becoming more common, because of greater system-wide reliability and the simplicity of adding nodes or new systems. I believe this is also true in a number of other countries.

--JMM

Goodho

'That is an area where the economics will certainly drive a switch to new technology".

I Agree and have always wondered about this conflict. Since I was a child and had this dynamo on the front of my bike and thought it was good for something.

This "switching" will find its place in many other areas but will supposedly end at some stage. Efficiency is not surprisingly the overruling principle of the evolution of things. Sometimes it just takes a bit longer for it to make sense and money at the same time.

Like we say down here "she'll be right mate". Ky.

Yup, according to historical reports, Tesla said AC, and Edison said DC.

Kinda "ticks me off" that I rushed to correct the wrtier and found so many others beat me to it !@!

I feel your pain, I too rushed to make the correction, Tesla was one of Edison's grinders and was somewhat unsung.

Apologies for the editorial mistake. It has been corrected.

I doubt the advantages would justify the cost of a wholesale change to our entire infrastructure. Think - every power production facility, transmission line, and end-of-wire device is set up to run AC (certainly some with DC battery backup). What would be the cost of replacement for everything? How long would it take? The switch would have to be nearly instantaneous to be successful, and how could that ever be made to happen?

With greater emphasis on small scale generation to take advantage of an increase emphasis on renewables (waste heat microturbines, small wind power and solar); we will see a greater desire to transport DC power from these generators. Having a DC interconnect will save on the synchronization and maintenance problems of trying to make each one of these these AC power at the source.

We may see that regional distribution grids in the West, Midwest and Prairie provinces are converted to DC, with DC/AC rectification at each end user. This would get the best of both worlds. The benefits of the current large scale AC generation and transmission are kept. Small scale distributed generation can tie into the existing infrastructure at the distribution level on a DC system. Only one phase of each AC distribution line would need to be converted to DC to allow simple interconnect from small generators to access the grid at low cost. Small rural substations would need to add a DC/AC rectifier to complete the overall interconnection.

OK, Now I'm really confused. When I worked for a manufacturer of HVDC test equipment in the 50kv to 2 mega-volt DC range, the given was that DC traveled on the external portions of the conductor. Thus flat buss bars instead of round or square. DC, even at high voltages is non-lethal due to this effect. Also the reason for using AC for the electric chair. Personal observations include one worker zapped with 10KVDC @ 5 amps. It blew him 15 feet away but only singed his arm and sholder joints and burned some meat off his heel. (Funny thing is he ejaculated but didn't piss his pants). He regained conciousness in about 5 minutes, and limped out to a car for a trip to the hospital. He was back to work a week later. Had that been AC energy, (50Kw) he would have exploded and probably started a hell of a fire.

Another instance was the boss/owner guaranteed his equipment was non-lethal and so grounded himself with one hand, and had us crank up a new unit to 1 megavolt @ 5ma. with the output lead in his other hand. All went fine for a minute, untill he set down the output and declared the test successful. Trouble was he had built up a charge in his own body (ionization began as he neared a grounded lolly column) and had to be bled off through a series of 1 meg resistors.

I expect that residential elecrical use will be site generated in the future and the debate will begin anew.

Carl

The "given" did not refer to travelling current, but to residual charge. Clearly whoever did the training did not trouble his audience with finer points he did not think they needed to know.
In the limit of true DC (and ignoring second-order effects), any charge unbalance appears at the surface. But that does not mean that current is not carried in the conductor - if you have a copper cable any DC current will flow more-or-less uniformly throughout its cross-section.

Often, the shape of the busbars owes more to convenience than anything else. However, wide bars with the flat surface perpendicular to the field and with rounded edges on the high-field side can give lower electric fields than square or circular bars - but that only delays the onset of sparking; the reason is exactly the same as why HV transmission lines are typically built from groups of four cables each of the four in a group carrying the same Voltage.

Then you write about the worker being zapped with 10kV DC, 5-amps. Your description is consistent with him being zapped by a spark from a 10-kV supply that was capable of supplying 5-Amps - the actual current would be unknown (and either of such short duration that only the total charge was important, or a great deal lower). AC under similar conditions (when the victim is not gripping an electrical conductor) is almost certainly no more (nor less) dangerous.
You say the worker was allowed back to work after a week? Without any retraining? Hopefully such days are long passed.

I doubt your boss ever got as high as 1 MV - the 5-mA limit could leak away through his body without significant discomfort, and he would be charged up to (probably) a few tens of kV (still enough for a most discomforting belt). But the proof of safety would be if he grounded himself with one hand, and then grabbed the already fully-turned-up output lead. There are lower-Voltage insulation testers that are safe like that - but at 1 MV I'd expect you would get a very uncomfortable spark to your hand - and these equipments can never be unconditionally safe in use, as they could always charge up a capacitor in the system being tested.

Enough already

Buried wiring carrying AC is generally easier to identify than is DC - particularly when no current is flowing (this makes the electrician's job easier and safer...) ?

Another significant advantage of AC is

It is easier to break an AC circuit than DC.

For DC:

Synchronisation of Grid is not necessary.

Agreed - the first statement is usually true in practice - although I have seen situations where series resonators are used (which are close to being equivalent to the dreaded DC inductor)

And yes, I seem to remember discussing the synchronisation issues and techniques in another thread.

(Read the entire comment... it is not off topic).

Why are the Brooklyn Dodgers, so named?

In the time of competing AC and DC systems, the Trolleys in Brooklyn were run with DC.

At sporadic times, the trolleys would being to hum and vibrate. Brooklyn residents quickly learned this was a warning to get off and away from the trolley. The vibration and noise was often followed by a large arc of DC which would strike something in the vicinity, often a horse, or a baby carriage, an adult or group of children; usually with fatal consequences.

This behavior of 'dodging' bolts of DC resulted in the naming of the Brooklyn Dodgers.

It is my understanding that extreme High Voltage DC transmission is used over long distances because over long distances ac power is lost through radiation affects like a radio station transmits energy.

The HVDC is fed to rotary converters that put out ac into the local grid.

Jon

You are mistaken about the reasons. The loss due to finite wire diameter dominates, followed (a long way down) by discharge between the wires and induced current in the ground. Even with a balanced pair, radiation would be a long way below that; with three-phase transmission the difference is even larger.

DC has the advantage that it requires smaller wire diameter and separation for the same RMS current and Voltage (this is basically because the peak current and Voltage of AC are √2 of the RMS). For intermediate transmission distances, this additional cost is more than offset by the fact that you can use transformers to change between the Voltages required for distribution and for more local connections.

There are two reasons that you would use DC over large distances, and radiation is not one of them:
One is that additional wiring costs make up a larger proportion of the whole (though this is not really a practicality if the system is distributed),
The other is that the delay in transmission over large distances makes synchronisation of the AC impractical using passive circuits, so the saving is not available.
The first will be the prime cause when the transmission is used to take the power from a low-cost source to a centre of population that is a very long way away (I could be wrong, but don't believe this happens much in western Europe or in the USA). The second will be more significant when generation and usage are geographically distributed.

Phys,

Soooo...

High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids like in Japan where a mixture of 50 and 60 Hz systems are used.
When electrical energy is required to be transmitted over very long distances, it is more economical to transmit using direct current instead of alternating current. For a long transmission line, the lower losses and reduced construction cost of a DC line can offset the additional cost of converter stations at each end. Also, at high AC voltages, significant (although economically acceptable) amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried.

An alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated represents a loss. This is good for radio broadcasting.

Depending on the frequency, different techniques are used to minimize the loss due to radiation.
HVDC links are sometimes used to stabilize against control problems with the AC electricity flow. In other words, to transmit AC power as AC when needed in either direction between Seattle and Boston would require the (highly challenging) continuous real-time adjustment of the relative phase of the two electrical grids. With HVDC instead the interconnection would: (1) Convert AC in Seattle into HVDC. (2) Use HVDC for the three thousand miles of cross country transmission. Then (3) convert the HVDC to locally synchronized AC in Boston, and optionally in other cooperating cities along the transmission route.
One prominent example of such a transmission line is the Pacific DC Intertie located in the Western United States.

An alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated represents a loss. Depending on the frequency, different techniques are used to minimize the loss due to radiation.

For 60 Hz a half wave radio antenna is:

468 468
Length in feet = ---------- = ----------- = 7800000 ft = 1477 miles.
Freq (mhz) 0.00006 (60Hz)

Seattle to San Diego?

An ac transmission system would be like a full wave antenna from Seattle to Boston.
People accross the country would wonder why there is a hum in their audio systems
and the inefficiency of power transmission would be tremendous.

Submarine crews could use it to get navigation fixes.
They already use VLF (very low frequency 3 to 30 kHz) transmitting stations for this.

Jon

All good stuff - apart from perpetuating the misunderstanding about radiation. 50/60 Hz transmission lines do leak, but this leakage does not grow disproportionately with line length. You do get inductive and electrostatic coupling, but the two lines are in antiphase; because they are only a small distance apart (~ λ/1000000) true radiation is negligible.

I suspect you are likening the long transmission line to the sort of closed-loop dipole built of 600-Ohm transmission line that is often used at VHF. That has its ends short-circuited, and broadside radiative coupling is observed because self-resonance causes that the phase to be quasi-uniform throughout the antenna.

The following indicates why longer lengths of transmission line do not normally radiate disproportionately relative to shorter ones; it is a considerable oversimplification, but broadly correct: for power transmission the phase varies along the line, so if you stand at a distance to one side you will receive the delayed average of the phases, which will cause the signals to cancel. If the line velocity is exactly c you will see coherent addition in front of the lines - but that is the position of a true zero in the radiation pattern.

BTW even at 3-kHz those VLF transmitters use loops with diameters of 100s of metres - and then rely on resonance to get half-way decent efficiency.

P.S. Of course, even if the line separation were sufficient that synchronous addition from different sections of line became significant, all you would need to do to cancel this would be to provide the occasional fractional twist - this should be quite straightforward.
So, although radiation is not a significant issue for long distance power transmission, you could very easily bring it back down to short-range levels if it were.

Phys,

So you do agree that detailed and costly methods are used to prevent ac losses, including electromagnetic radiation, not necessary with HVDC.

Your explanations are excellent.

Jon

No to electromagnetic radiation*, but yes to resistive losses and arcing. Nevertheless, for distributed populations the advantages of transformer access still favour AC until you reach the point where phasing issues mean that you can't use transformers for reciprocal load balancing.

*Even were it necessary, moving the wires round on pylons by a single position every mile or so would be quite inexpensive