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• ##### double the power for given cable budget?(1+ / 0-)
Recommended by:
PrahaPartizan

Three-phase ac power is a thing of beauty, perhaps it made Tesla go crazy.

Three wires carrying ac voltage, spaced 1/3 apart in the cycle.  If each carries the same magnitude (yet sinusoidal) current, the total power delivered to the load is dead flat. No variation in power at all, no more sinusoid, unlike a household outlet which delivers fluctuating sinusoidal power. (you have Excel? good, do the math ;)

That's why old school machine tools use three-phase motors, removing 60 Hz variations in torque causing distortions in the cutting.  The only thing cleaner (because three phase currents maybe not perfectly equal) is to use DC power.  The old WWII and later Monarch lathes had massive vacuum tube rectifiers/amplifiers to run the motors on DC, which is completely flat, not to mention a joy to speed-control.

Back to the point, DC uses two wires.  For power line transport it has no continuing losses from those energy-eating reactive things like capacitance and inductance, which only dine when voltages are changing.

That is one advantage.  The other is the possibility of using Mother Earth herself as the return line (sometimes called earth or ground), so only one "hot" wire is deployed, so for the same money it can be thicker than two smaller ones. That was talked about years ago, not sure where it stands now.

• ##### Thomas A. Edison lost this battle to George(1+ / 0-)
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eyesoars

Westinghouse.  Line losses make long distance transmission of DC current far less efficient than AC.

From Smithsonian, an article of their rivalry:

http://blogs.smithsonianmag.com/...

Republicans are like alligators. All mouth and no ears.

[ Parent ]

• ##### DC is not less efficient.(3+ / 0-)
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jam, Ohiodem1, justajoe

At a given voltage, DC has lower losses than AC.  Also takes fewer wires.  The problem is that it's harder to make DC at very high voltages than AC.  It used to be a lot harder.  The problem is, to switch AC, you just need a transformer; the voltage conversion is the ratio of the number of turns between the two coils.  But that doesn't work with DC because transformers work by induction (changing magnetic fields inducing a current in a different conductor), and since DC is constant, its magnetic field is constant, and thus, no induction.  So they used to have to make motor-generators where a DC motor spins a DC generator.  A super-high-voltage, high power DC generator.  Not a cheap or easy task, inefficient, high maintenance, etc.

Nowadays though modern switching electronics makes high voltage DC power transmission realistic and a better than AC in certain circumstances.  That is, instead of a motor generator, power is stored (capacitors, magnetic fields, whatnot), raising the potential of a discharge.  The problem is that due to the fact that power is voltage times current, and you're discharging at a higher voltage, the discharge inherently can only run for a fraction as long as you're charging it.  So you must charge and discharge multiple banks together, in succession, with something carefully keeping them in phase.  And to do this well and affordably takes super-high-power (both voltage and current) electronics like FETs which didn't become widely available and affordable until relatively recently.

Basically, HVDC requires relatively expensive switching stations instead of simple transformers but saves on wire cost for a given amount of current transfer - thus it excels at long run transmission but is not economically viable for short runs - on land at least (in saltwater it's pretty much a "must", as AC induction losses in saltwater are huge (remember, changing magnetic field + conductor (aka seawater) = inductive currents).  HVDC is also nice in that it lets you easily exchange power with out-of-phase grids and even grids operating at different frequencies.

Anyway, the diarist and many of the commenters have got a lot of this backwards.

1) The innovation is not the concept of having the arms be nonconductive.  This is patently obvious to anyone designing a tower that the distance between a wire and the first thing its capable of discharging to is going to limit your voltage, and so your design has to balance tradeoffs of cost, strength, and discharge voltage.  The innovation here is a ready system to uprate existing lines by effectively insulating their existing arms.

2) The problem being addressed concerning wind is not "power line losses".  There's actually rather little power lost in the grid - the US average is nearly 93% efficiency, power plant to point of consumption.  The issue at hand is maximum power transmitted.  You can only pump so much current through a line before the wires start to overheat and sag too much.  But since power is current times voltage, if you increase the voltage you increase the power even if the current remains fixed.  That is, you can transmit more power without building whole new lines if you do this relatively minor retrofit to existing lines.  These lines are not "poorly designed" in that they couldn't handle the higher voltages; the issue is when they were built, there was no need for higher voltages, for more power to be transmitted.  But when you suddenly put all of this new generation capacity in the middle of nowhere, suddenly there is a need that these existing lines can't meet on their own.

Which makes me think of something.  One would expect that the windier (and especially, the rainier) the day, the faster heat would be lost from the conductors, and thus the more current they should technically be able to handle.  Yet the lines are designed for a specific max current given the rate of cooling in calm air on a hot day.  Windy days are also when the most wind power generation occurs.  I wonder if they could actually simply uprate many lines by... doing nothing.  Simply measure or estimate line temperature under the current conditions and change how much current you limit yourself to based on it.  Or is that being done already somewhere?

Another thing the conversation makes me think of: I wonder if eventually, now that DC switching is much more easily accomplished, if  people will move over to DC power for home distribution (DC sockets instead of AC).  Takes 5 times as much DC current to feel a spark as AC and is far less likely to kill you for a given current (despite the lack of skin effect) and less wiring is needed.

[ Parent ]

• ##### I've thought of it(0+ / 0-)

I call this an "IBM problem". There's an old saying about new technology - "nobody ever got fired for buying IBM." In other words, go with what works.

I tried to convince a team of engineers that we could go with a smaller, cheaper transformer at a low/medium wind site because of
1) line losses
2) efficiency losses
3) wind conditions - i.e. the wind farm would only be producing full power something like 200 hours per year, or about 2.2% of the time.

Taking all of that into account, you can run a transformer hot (105-120% of rated capacity) - utilities do it all the time during peak loads.

I was completely ignored and belittled for the idea, so we spent an extra \$200K.

Javelin, Jockey details, all posts, discontinue

[ Parent ]

• ##### Ah, the old oversimplistic view of safety factors.(2+ / 0-)
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Ohiodem1, jam

"The book / the official documentation / whatever says that this is the safe X for device Y, so we can always run at X, and we can never exceed X."

The real world is, of course, not that simple, and depends on the circumstances of how you're operating.  Sometimes your circumstances are such that your safety factor for device Y becomes compromised if some other system fails, and so it's not really a safety factor at all.  Sometimes you gain additional safety from a whole host of other factors related to other systems and so the safe X for device Y is actually way higher and you're wasting money pointlessly.  Systems must always be looked at as a whole, not as a sum of individual components.  Here's all of the possible use cases, here's all of the possible failures - what happens when we combine them and what are the odds of that situation?  Yes, that's a more challenging analysis, but that's the dang reason why engineers take home a salary, to do the analysis right.

I see this sort of attitude a lot in discussions about wiring.  For example, I've heard a lot of electrical engineers argue that fast charging of electric cars is impossible without dangerously high voltages.  Why?  Because "the book" shows you the maximum rated current for a cord of given thickness with various insulators, and oh look, the cord would have to be too thick to bend or lift!  Great... except "the book" is talking about constant current flow in simple, fixed applications.  That's not taking into account that it takes time for a cord to heat up enough to where you're burning the insulation or risking irreversible deformation, and even if you get to that point, all you have to do to counter it is cool the cord.

[ Parent ]

• ##### And some transformer standards are built around(1+ / 0-)
Recommended by:
jam

a concept that if you use the transformer within its ratings, 100 per cent of the time, you can expect some figure, X hours, frequently I have seen the number 20,000 hours as the expected life.  So, if you run it above its ratings, then that number 20,000 hours may be expected to drop to maybe 19,000 hours if the temperature excursion is defined and predictable.

Design activity may be undertaken to account for temperature excursions as well.  If for instance the insulation system rating is 180 Deg C, and the excursions would dictate a rating of 220 Deg C, then the act of purchasing the 220 Deg C insulation system may be a low cost alternative to a bigger transformer.  You are sized for 95 per cent of usage, with capability to handle a larger load 5 per cent of the time, without degradation of the expected life of the transformer.

On the example of cool the cord, in my opinion, I believe a better, more reliable solution is to purchase a cord or cable with a higher temperature rating, for instance, replacing a 90 deg C wire with 105 or 130 Deg C wire which has higher ampacity in the first place and can take the higher operating temperature, which of course is a fire risk if the wire is used in excess of its capacity.  I have long experience in the electrical safety business, and if you submitted that kind of arrangement to a certification lab, it would be rejected out of hand.

Alternately, if short term current excursions were expected, you could put a duty cycle rating on it  to cover for the excursion, but duty cycles are cumbersome and some consumers would ignore them, leading to a fire or electrical shock risk.

Republicans are like alligators. All mouth and no ears.

[ Parent ]

• ##### that was the other thing(0+ / 0-)

temperature, I forgot. The highest winds (at this location) were only during the winter with ambient temps usually around 0-10 C. And the transformer we put in is ONAN. If temperature became a problem, they could always slap a couple of pumps on it to become OFAF, right?

Ultimately, I'm not a transformer guy, I'm a little wires guy.

Javelin, Jockey details, all posts, discontinue

[ Parent ]

• ##### Had to look up ONAN and ONAF(0+ / 0-)

A succinct explanation is here:

http://www.pacificcresttrans.com/...

They agree that adding forced air cooling to make ONAN an OFAF is an acceptable procedure.

About 20 years ago, I was more knowledgable about transformers than I am now.

Republicans are like alligators. All mouth and no ears.

[ Parent ]

• ##### The cord cooling approach is actually being used..(0+ / 0-)

because realistically you need an order of magnitude or so better cooling than passive air cooling alone can provide, not something that a mere uprating of wire type or insulation can provide.  Rapid charging can be hundreds of amps for 5-15 minutes; the tables for passive air cooled steady-state wires state something like 0.5-1" diameter, give or take depending on the details - way too heavy and cumbersome.

I don't know what sort of coolant they use, but I would expect that it is relatively nonconductive and non-flammable.

[ Parent ]

• ##### and your #1 point(0+ / 0-)

is spot on. I think I tried to say something similar somewhere here in the comments.

Javelin, Jockey details, all posts, discontinue

[ Parent ]

• ##### When I say DC line losses, I am referring to (0+ / 0-)

I^2R (read I Squared X R) losses.  The conductor has a known  resistance per 1000 meters or per mile, and when current flows in the conductor, voltage is dropped per ohm's law.  And voltage is dropped in the return line because the same current must flow in all parts of the circuit.  Given that I am not fully schooled in high voltage transmission, DC or AC, and likely never will be, I will leave it up to the IEEE to work through these issues and come up with "the real answer".  I'm not confident that this will be found in my lifetime since even with Grounding, that topic has been being fought out for over 120 years, is not fully settled.  A facinating disucssion on this topic can be found in the "Soare's book on grounding", available, if I remember correctly, from the NFPA (National Fire Protection Association).

Republicans are like alligators. All mouth and no ears.

[ Parent ]

• ##### I thought DC was slightly more efficient(1+ / 0-)
Recommended by:
Ohiodem1

because there is no skin effect like you have in AC. Same I^2R losses.

But, again, I'm not a high voltage guy.

Javelin, Jockey details, all posts, discontinue

[ Parent ]

• ##### It is. (1+ / 0-)
Recommended by:
Ohiodem1

And there's a number of other factors that make DC more efficient, too.  There's only one case where AC wins out on (ionization of the air around the wires), but it's a relatively small effect overall.

[ Parent ]

• ##### I read it on Wikipedia, it must be true(1+ / 0-)
Recommended by:
Ohiodem1
Even though HVDC conversion equipment at the terminal stations is costly, overall savings in capital cost may arise because of significantly reduced transmission line costs over long distance routes. HVDC needs fewer conductors than an AC line, as there is no need to support three phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect. These factors can lead to large reductions in transmission line cost for a long distance HVDC scheme.

Javelin, Jockey details, all posts, discontinue

[ Parent ]

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