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Author Topic: How many mA in an Ampere?  (Read 22999 times)

Mike Sokol

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Re: How many mA in an Ampere?
« Reply #80 on: April 09, 2016, 01:05:37 PM »

I had no idea that there was a DC power distribution system.
While DC has obvious advantages, I always thought everything was AC.

Believe it or not, a few blocks of New York City was still powered by DC until November 2007. See below. I first found out about this when one of the guys I worked with in the 70's said his apartment in NYC was DC power and he had to get an inverter to power his hi-fi system. Of course incandescent light bulbs don't care, but anything with a transformer would be in trouble. I've got a few AC-DC radio sets laying around with all the tube filaments in series, but they typically had one side of the power line tied directly to the chassis. So without a polarized plug they could be deadly since the chassis would be at 120-volts DC (or AC) above ground potential.

Off Goes the Power Current Started by Thomas Edison

By Jennifer 8. Lee November 14, 2007 12:53 pm November 14, 2007 12:53 pm   
Consolidated EdisonCon Edison’s original power plant on Pearl Street. (Illustration: Consolidated Edison)

Today, Con Edison will end 125 years of direct current electricity service that began when Thomas Edison opened his Pearl Street power station on Sept. 4, 1882. Con Ed will now only provide alternating current, in a final, vestigial triumph by Nikola Tesla and George Westinghouse, Mr. Edison’s rivals who were the main proponents of alternating current in the AC/DC debates of the turn of the 20th century.

The last snip of Con Ed’s direct current system will take place at 10 East 40th Street, near the Mid-Manhattan Library. That building, like the thousands of other direct current users that have been transitioned over the last several years, now has a converter installed on the premises that can take alternating electricity from the Con Ed power grid and adapt it on premises. Until now, Con Edison had been converting alternating to direct current for the customers who needed it — old buildings on the Upper East Side and Upper West Side that used direct current for their elevators for example. The subway, which has its own converters, also provides direct current through its third rail, in large part because direct current electricity was the dominant system in New York City when the subway first developed out of the early trolley cars.

Despite the clear advantage of alternating current — it can be transmitted long distances far more economically than direct current — direct current has taken decades to phase out of Manhattan because the early backbone of New York’s electricity grid was built by Mr. Edison’s company, which had a running head start in the first decade before Mr. Tesla and Mr. Westinghouse demonstrated the potential of alternating current with the Niagara Falls power project. (Among the customers of Thomas Edison’s Pearl Street power plant on that first day was The New York Times, which observed that to turn on its lights in the building, “no matches were needed.”)

But direct current clearly became uneconomical, as the short distances that it could be transmitted would have required a power station every mile or less, according to Joe Cunningham, an engineering historian. Thus alternating current in New York began in the outskirts — Queens, Bronx, Upper Manhattan and the suburbs.
lightbulb

The direct current conversion in Lower Manhattan started in 1928, and an engineer then predicted that it would take 45 years, according to Mr. Cunningham. “An optimistic prediction since we still have it now,” he said.

The man who is cutting the link today at 10 East 40th Street is Fred Simms, a 52-year veteran of the company. Why him?

“He’s our closest link to Thomas Edison,” joked Bob McGee, a Con Ed spokesman.
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Keith Broughton

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Re: How many mA in an Ampere?
« Reply #81 on: April 09, 2016, 02:02:40 PM »

You're talking about the dumbell-looking weights on the lines outboard of the insulators?

If so, those are vibration dampeners. They're weights on flex couplings designed to eliminate (or at least reduce) line vibrations caused by wind from adversely affecting the insulator's outer attachment points.   https://en.wikipedia.org/wiki/Stockbridge_damper

EDIT: Added wiki link.
Finally! After all these years of wondering. Thanks  :)
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Jonathan Johnson

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Re: How many mA in an Ampere?
« Reply #82 on: April 09, 2016, 05:45:00 PM »

The great thing about this forum is I learn something pretty much every day. :)
I had no idea that there was a DC power distribution system.
While DC has obvious advanteges, I alway thought everything was AC.

For long-distance transmission, DC is preferable as there is less inductive loss* and no skin effect to worry about. The problem was that in the beginning, there was no efficient means to boost DC voltages. That's where the AC transformer proved key to long distance transmission: it could boost the voltages high enough that currents could be kept low enough to use a manageable (in terms of cost, weight, and handling) wire size.

For shorter distance, the benefits of DC don't outweigh the costs of conversion.

The megavolt DC transmission lines are made possible by first boosting an AC supply to a high voltage with a transformer, then rectifying it to DC, then converting back to AC again at the far end and stepping the voltage down as needed with ordinary transformers.

If we produced DC at the generator, in order to boost it we'd probably have to convert to AC, transformer boost it, then rectify it back to DC. More efficient to just generate as AC before the boost & conversion.

* I could be wrong about the inductive loss. I am not an engineer.
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David Buckley

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Re: How many mA in an Ampere?
« Reply #83 on: April 09, 2016, 06:01:33 PM »

For long-distance transmission, DC is preferable as there is less inductive loss*

Close.  Capacitive.

You can demonstrate capacitive losses on HVAC lines; go at night with a fluorescent tube and hold it up beneath the power lines, it lights up.  Because you are - literally - in the dielectric of the capacitor between the lines above you and the ground below.
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David Buckley

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Re: How many mA in an Ampere?
« Reply #84 on: April 09, 2016, 06:34:24 PM »

The Sylmar converter station is, in an oversimplified sense, a massive audio amplifier. It's powered by DC, and takes a low-amplitude 60Hz signal as an input, outputting a high-amplitude 60Hz signal. I don't know what its bandwidth is.Seems I've heard that the power flow can be reversed

I'd suspect that all HVDC lines can operate both ways, it's a very simple switch function, so why wouldn't you.  Except if one end is a generating station with no other outputs, no point in backfeeding that, it would be a hell of an expensive way to get blackstart capability.

Most HVDC links are bipole links, so one wire is positive from ground, and the other line is negative to ground.  The lines can operate independently, and there is a ground return path that takes the out-of-balance current.  The ground electrode stations are impressive in their own right; it's not a ground rod, the thing has to be able to pass maybe 2500A to ground, so it requires a lot of connection and a very low impedance.

Fun fact: once you have a connection "to ground", the resistance of the connection doesn't change by distance, whether that distance is one mile, or half way round the planet.

Because the two poles operate independently, you can have one sending power one way, and the other the other.  One can test the system at full power whilst using almost no power in that mode.  Thus a typical converter station with a bipole line is actually two independent converters in one place.

Converters operate in one of two modes, either as a rectifier, in which case they take the AC input and convert it to DC, or as an inverter, in which case they have a DC input and output AC. 

A station uses two transformers (or multiples in parallel) one is delta/delta, the other delta/star, and use twelve thyristors to switch the various secondaries in various combination between the AC transformers and the the DC bus.  The reason for the different type of transformers is that the delta/star gives a phase shift, and thus fills in the gaps of the waveform of the delta/delta

At the receiving end, the thyristors are switched on sequentially to match the AC waveform of the receiving grid.  As folks may know, once a thyristor is switched on, you cant switch it off; it stops conducting thus switching itself off when there is no current flowing through it.  So the converter station has to operate into a strong AC grid with lots of mechanical inertia, so that the grid can drive the current down to zero through the thyristor at the right point in the waveform.  This is called commutation, and is an essential part of the inversion, it cant work without.

A big problem with this kind of rough and ready inverter generating a stepped sinewave is harmonics.  A large amount of the real estate of a converter station is harmonic filters to remove the undesired harmonics, to stop the whole electrical grid becoming a huge VLF transmitter.  Ditto with ground, the currents flowing through the ground can be massive, and can destroy datacomms signals, particularly in places that had phone systems using a ground return.

Bit of a fan of HVDC systems, I drive under one every day, sometimes twice in one journey!  The New Zealand inter-island link, when it was commissioned in the 60s was the longest and most powerful HVDC in the world, and at the time of the last upgrade it was the last line still using mercury arc tubes.  But now its all modern thyristor technology, should be good for another half century.  Transpower, operators of the link, published a book, White Diamonds North: 25 Years' Operation of the Cook Strait Cable 1965–1990, which is almost unobtanium, but is a fascinating bit of history preserved.
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Jonathan Johnson

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Re: How many mA in an Ampere?
« Reply #85 on: April 10, 2016, 12:57:02 AM »

The Sylmar converter station is, in an oversimplified sense, a massive audio amplifier. It's powered by DC, and takes a low-amplitude 60Hz signal as an input, outputting a high-amplitude 60Hz signal. I don't know what its bandwidth is.

At the receiving end, the thyristors are switched on sequentially to match the AC waveform of the receiving grid.  As folks may know, once a thyristor is switched on, you cant switch it off; it stops conducting thus switching itself off when there is no current flowing through it.  So the converter station has to operate into a strong AC grid with lots of mechanical inertia, so that the grid can drive the current down to zero through the thyristor at the right point in the waveform.  This is called commutation, and is an essential part of the inversion, it cant work without.

The surest way to learn the truth is to say something incorrect with authority.  ;D
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John Roberts {JR}

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Re: How many mA in an Ampere?
« Reply #86 on: April 10, 2016, 10:31:15 AM »

The surest way to learn the truth is to say something incorrect with authority.  ;D
while misleading the lurkers.  :'(

JR
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Frank DeWitt

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Re: How many mA in an Ampere?
« Reply #87 on: April 11, 2016, 06:32:27 PM »

Believe it or not, a few blocks of New York City was still powered by DC until November 2007. See below. I first found out about this when one of the guys I worked with in
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Mt Wilson Observatory still runs on 120 VDC.  It was built that way so that it could run on batteries at night.  The batteries were recharged during the day so that they didn't need to deal with exhaust or vibration at night when the work was being done. Fun side note.  When the staff would hear the generator started they would grab there towel and head for the shower next to the generator building.  The cooling water from the generator could be routed to the showers and was the only hot water available.

They now have "city power" and a big inverter.  The generator has been restored and is shown off from time to time. There are a number of pictures and videos on line of the original switch gear (still in use) videos of motor start relays drawing nice long arcs, and of the generator running.  I have had the privilege of seeing all the above in action. 

Some may remember that I posted some pictures on this forum but I had to take them down.
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Frank DeWitt

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Re: How many mA in an Ampere?
« Reply #88 on: April 11, 2016, 06:35:53 PM »

Hi,
This audience might appreciate this story. I was reminded by the plumbing analogy. My dad worked for Laurens Hammond, inventor of the Hammond Organ. While in college, Mr Hammond walked into the wrong final exam and took a test for a class which should have been a year or two later in his electrical engineering studies. He was called in and accused of cheating as he couldn't have known how to answer the questions. He explained that he converted the problems to hydraulics, solved them and re-converted the results. Apparently the professors realized they had someone special in him.
Best,

A friend of mine has a degree in Optics.  He makes his living doing FM antenna design and placement for Christian radio stations.  When I asked him about how he learned to do it he said Physics is Physics.  Just a different frequency.
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Re: How many mA in an Ampere?
« Reply #88 on: April 11, 2016, 06:35:53 PM »


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