AC versus DC transmission and distribution 1

[Mbrooke]

Starting a new thread not to hijack another one on a different subject. AC vs DC. Which is better? Which do people believe will be used in 50-100 years time?


My thoughts are this: AC is a waste of copper, aluminum, iron, steal, space, resources just to name a few. AC is complex to control, and more unstable than DC in operating large systems. AC is a more difficult to engineer, especially as systems grow. AC power is holding back the burial of transmission and distribution lines. Its holding back renewables. Its holding back energy storage. Its holding back long distance power delivery and exchange. The conversion of AC to DC wastes energy. AC systems can not tolerate large scale none linear loads. AC systems emit fields which can be hazardous to human health. In short AC is impractical and it has always been a bad idea when all is said and done.

 

[Skogsgurra]

DC grids are a reality also in very limited areas like villages, server halls and offices. I have done quite a lot of work on such installations and also been active in evaluation and to some extent in the development of RCD:s, which is a whole lot different in DC that in AC - transformers cannot be used.

A few early examples are the Gnesta, Karlstad, Glava and Tibro installations. The AC/DC ability is already there on many houshold appliances and lamps (LED lamps, incandescent were born DC) and mains adapters. Either explicitly marked so or just "because they can".

DC is not only the future. It is already here.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Mbrooke]

I beg to differ, transformers can be used and are essential to a large DC system. Think of the charge adapter for your phone or brick on a laptop. I ask, why would someone go through the trouble over a 50/60Hz core trafo, a bridge of diodes, zener and caps?

 

[Skogsgurra]

There are no 50 or 60 Hz transformers in them today. They are all switchers. Next try, pls.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Utility-scale generators produce AC. A DC machine of 500MW output would be quite impressive to say the least, and that isn't even a big machine anymore.

 

[Skogsgurra]

Already getting out of hands. Generators: 3-phase AC. Transmission: Both AC and DC. Distribution: Still mostly AC with DC increasing. Installations: AC and DC, with budding DC. Appliances: AC and DC, with surprisingly hich number of DC-ready units.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Mbrooke]

@Skogsgurra: Yes, there are no 50/60Hz trafos, thats what I said. But they still have trafos:

https://en.wikipedia.org/wiki/Switched-mode_power_supply#/media/File:SmSntWoIc.jpg

@ScottyUK: Errr, for now. Solar, wind, fuel cell, ultra cap... all DC. BTW, a variable turbine or at least a generator that can vary speed in a steam plant might have some efficiency gain.

 

[BrianPetersen]

Switched-mode power supplies are only capable of reducing an input voltage (e.g. from mains distribution to the voltage that your laptop computer wants). They cannot increase voltage, and the nature of their operation creates electrical noise. For the few watts that a laptop computer uses, that's not an issue. On a megawatt scale ... it's probably an issue. The Wiki article above suggests an efficiency of 60% - 70%. For powering a laptop or a phone charger, hardly an issue. For megawatts, it's a big issue. Big transformers have very low losses.

Mains distribution is going to stay AC for the foreseeable future.

 

[waross]

Hi Scotty.
This is a question, not a challenge.
I am visualizing a generator built as an inverted DC generator.
A rotating magnetic field and a stator that is wound in a similar fashion to a generator armature.
The leads normally connected to the commutator may then be commutated electronically.
What would the current and voltage limitations be based on the semi-conductors used in the convertor stations for DC transmission?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MatthewDB]

They cannot increase voltage,

As someone who's designed 160kV X-Ray power supplies operating at 25kHz I'd beg to differ.

The Wiki article above suggests an efficiency of 60% - 70%.

That's very dated information. That was true in the 1970s but no more. I haven't seen anything under low to mid 90's since then. 95% plus is pretty typical. They are driven by efficiency standards and customer requirements for better efficiency.

 

[MatthewDB]

I am visualizing a generator built as an inverted DC generator.
A rotating magnetic field and a stator that is wound in a similar fashion to a generator armature.
The leads normally connected to the commutator may then be commutated electronically.
What would the current and voltage limitations be based on the semi-conductors used in the convertor stations for DC transmission?

You don't need anything that complex. The generator can be completely conventional, other than it is no longer constrained by grid frequency for pole count and rotational speed. The mechanical engineers are free to pick whatever operating speed they want on the shaft, and the electrical side can pick their pole count (most likely 4 pole unless the speed is rather low).

The output of the generator is then stepped up via 3 phase transformer and rectified by diodes. Nothing more complex is needed as inversion back isn't needed and output power can be regulated by excitation.

 

[MatthewDB]

Starting a new thread not to hijack another one on a different subject. AC vs DC. Which is better? Which do people believe will be used in 50-100 years time?

I think that it will be DC "someday" but not anytime soon. Technology has a ways to go for limits of technology and the costs involved.

My thoughts are this: AC is a waste of copper, aluminum, iron, steal, space, resources just to name a few. AC is complex to control, and more unstable than DC in operating large systems. AC is a more difficult to engineer, especially as systems grow.

It is true that AC is more complex. You have to worry about voltage control, stability and so on. But the claim AC is more difficult to engineer as systems grow runs into a massive obstacle: HV DC circuit breaker technology has a ways to go. Right now they're frightfully expensive and limited. When your system is always point to point, not a network, that is rather limiting.

AC power is holding back the burial of transmission and distribution lines

Not that I see. Certainly not in the United States. 500kV AC can be buried now. Multiple HVDC island power supplies have been decommissioned and replaced with AC. New distribution in the US is almost always underground. The tradeoffs between overhead and underground is comparable for both. As an example, the HVDC systems in the US are all aboveground. They're that way because it's less cost, more reliable, and longer lived when we're talking about transmission voltages.

Its holding back renewables. Its holding back energy storage.

How so?
Just because solar panels are DC doesn't mean you can hang them on a DC power line. They have a 2:1 voltage range and you need maximum power point optimization that varies with insolation level and temperature.

Just because battery technology is DC doesn't mean you can just float it on the system. LiIon batteries voltage over the charge / discharge cycle varies around 40% you can't let it vary that much.

If you have DC distribution with either, you have to have a DC-DC converter. If you go with AC, you can have a inverter do the same job (you design the inverter for a large modulation index). Typical large inverters today are 97%-98% efficient. The inverter's advantage is you can hang a transformer on it and have any voltage you want out. With DC, you either are forced to match the battery and line voltage or have a more complex isolated DC-DC converter.

Its holding back long distance power delivery and exchange.

I'm not so sure of that. UHV AC was initially developed by the US, Russia and Japan. Other than some 765kV in the US, it hasn't been used much here. Russia has backed down and Japan never implemented anything over 400kV. Since then, India and China have become the leaders in UHV AC. India has achieved a single grid and now operates the largest grid in the world. They still use less power than the US, but the US has 3 major grids, not one. China is headed to a single grid someday. The Eastern and Western interconnections could be synchronized at 765kV by extending AEP's system West just as well as it could be connected more than at present with DC.

The conversion of AC to DC wastes energy.

1st: That conversion has to be done for many sources and loads. Anything powered mechanically is an AC generator with conversion. Anything generating mechanical motion is an AC motor with conversion. (Even a "DC" motor is an AC motor with a mechanical inverter.) So you have to add inverters that aren't needed in an AC system.

2nd: Every change of voltage requires converting DC to AC and back again after a transformation. Non-isolated converters like buck and boost don't send 100% of the power through AC, some goes through at DC; this is analogous to an auto-transformer where some power is transformed and some is coupled galvanically. The advantage is that the frequency can be optimized for the transformer, but when you get to big power, it's only going to be 100's of Hz at the most.

With an AC system, you have at most one electronic conversion with losses. With DC, you've got many more. Remember, you can't have one voltage. At a minimum, you need 4 different voltages. Bulk transmission (100s of kV to 1MV), local transmission (50-200kV), distribution (10-30kV) and utilization (mostly <1kV). That's a lot more AC-DC conversions than you have in a AC system.

AC systems can not tolerate large scale none linear loads.

And? Most SMPS / DC loads are required to be harmonic free. Computers, servers, LED lamps, electric vehicle chargers, solar inverters, battery storage, etc are all low harmonics. Active harmonic filters are going mainstream for those loads that are not.

AC systems emit fields which can be hazardous to human health.

That's not conclusive, at least from what I've ever heard.

In short AC is impractical and it has always been a bad idea when all is said and done.

How is the dominant system that supplies 99% of the world's power "impractical"?

 

[MatthewDB]

DC grids are a reality also in very limited areas like villages, server halls and offices. I have done quite a lot of work on such installations and also been active in evaluation and to some extent in the development of RCD:s, which is a whole lot different in DC that in AC - transformers cannot be used.

I was sold on DC being the wave of the immediate future for server farms myself. It makes a lot of sense.

But then a number of creative trends came along and took out most of the advantages of DC so it is no longer growing.

Server power supplies have sufficient DC bulk capacitance to ride through 2 cycles of interruption. Having a low harmonic front end helped in this regard, because no matter what the input voltage is, they boost to 385V where it's cheaper to store energy than at a lower voltage. Having 2 cycles of ride through means that you don't need to keep the UPS online. It just sits there ready to go at a fraction of a cycle, which is plenty. Out goes the double conversion efficiency of older UPS systems.

The newest server power supplies are rather incredible in their input quality. <1% THD and spot on unity power factor. Older units had EMC filter capacitors at the front end, and they caused leading power factor, particularly at light load that caused trouble. They've moved the filter to after the diode bridge and put the current sensing on the front end. The control loop regulates the input current and can hold it at unit from light load to full load.

Many server farms have gone to processing redundancy and done away with the UPS and even diesel generators.

One more stupid factor in the US was the use of 480V UPS and 120V servers with many transformers to get the 208Y120. There are still server farms like that, but anyone with a brain has gone to 400Y230 distribution. They use US / UL gear designed for 480V, 240V US outlets, etc. and it meets code. All servers work at 230V too so it's a non issue for them. With that, you get the efficiency of the higher AC voltage without having transformers. The other trend is to take 35kV class voltage right to the floor and distribute MV and convert to LV close the the loads as reasonably possible.

Do all that, and AC beats DC.

A few early examples are the Gnesta, Karlstad, Glava and Tibro installations. The AC/DC ability is already there on many houshold appliances and lamps (LED lamps, incandescent were born DC) and mains adapters. Either explicitly marked so or just "because they can".

I don't see any challenges to the DC distribution in the house. The real challenge is how you get from MV to LV DC. If the MV is 10's of kV of DC, you can't use a transformer.

 

[MatthewDB]

Copying from the other thread on this topic.

Considering Amps-turns and Volts-per-Hertz; a 5 KVA transformer rated at 120:1.2 Volts, 50 Hz will transform 500 KVA at 12000:120 Volts, 5000 Hz. Given a 100:1 advantage in material usage, there will be a lot of very smart people working on improving efficiency and insulation and finding various ways to mitigate the various disadvantages of high frequency switching as compared to a low frequency sine wave source.

Sorry, it simple. As the frequency heads upwards, the core becomes loss limited, not saturation limited. As the frequency goes up, more and more exotic core materials are needed to address the higher loss in the core. You simply don't get anywhere near the numbers like 100:1.

Further, 500kVA 12kV 5kHz will run into other obstacles. The biggest challenge to wide band-gap materials today is that the rest of the converter hasn't caught up. They have to figure out how to make capacitors, inductors, transformers, etc. that can handle the voltages, currents and frequencies that SiC and GaN devices can generate. That's why I'll say it is "someday" but it's a while off yet.

I understand that more and more motor driven appliances are using invertor driven three phase motors.
A DC supply will offset some of the extra losses by eliminating the rectifier section.

There is still a rectifier somewhere... that drive is supplied by a higher voltage, arriving from a transformer and rectified. It could be a 50/60Hz transformer or a high frequency one, but you've got to go back to DC with a rectifier. Put it at the drive or put it in the pad mount / pole pig transformer and it's still a rectifier.

Going DC means you go 100% inverter based. Many drives have a mechanical contactor bypass. When there are many parallel motors, only one will be run variable and the others are fixed. Consider applications like chillers, water pumps, etc... Only one has to vary and there an efficiency gain in running some of the motors DOL. No AC to go DOL and you can't gain that efficiency.

Yup, and try 25,000Hz. 500,000Hz. Try air core transformers... The limits are endless. AC is a waste of copper, aluminum, iron, steal, space, resources just to name a few.

I've never seen an air core transformer in an application. I've seen a few inductors that are air core, but never an air core transformer. The point of a ferromagnetic material in the core is to reduce the primary VA burden. Using an air core means LOTS of power electronics devoted to just establishing the magnetic field.

 

[Skogsgurra]

That's massive!

As usual, there are comments ranging from Clueless "Private Label Thinking" to well-informed latest technology based insights.

I am somewhere in-between. Living around 20 miles South of ABB HVDC HQ in Ludvika and done work for them, I think that I have a rather good insight in HVDC systems. That is the "renaissance" of DC after half a century of AC dominance in transmission. I was involved in one of the first DC links (Pas de Calais) and actually met Mr Uno Lamm.

The switchers are here to stay. And they don't mind if they are fed AC or DC. They are lighter, less costly and more efficient than any 50/60 Hz transformer based PSU.

EV:s and HEV:s are coming everywhere. Been there, too. Or, rather: am there. Lots of problems and not yet sure if the batteries will win. But DC will dominate the cars. No matter how it will be produced or stored.

For those that want to get a quick and, as it seems, quite deep insight in SiC and GaN and other high bandgap material systems, there is a Three-day Conference in Stockholm, starting June 10th. I will be there. Yes, educational workshop is on Sunday.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MatthewDB]

Here is an interesting ABB article on their solution to one of the big problems with HV DC going beyond a point to point system: the HV DC circuit breaker.
The Hybrid HVDC Breaker

 

[MatthewDB]

I am somewhere in-between. Living around 20 miles South of ABB HVDC HQ in Ludvika and done work for them, I think that I have a rather good insight in HVDC systems. That is the "renaissance" of DC after half a century of AC dominance in transmission. I was involved in one of the first DC links (Pas de Calais) and actually met Mr Uno Lamm.

I have deep respect for ABB. They have the moxie that so many companies lack. They'll try new, innovative things that no one else is trying.

I've heard they've had financial troubles, and if they die too they'll be a HUGE loss to the industry. I certainly hope that doesn't come to pass in spite of the fact that they are my employer's biggest rival.

For those that want to get a quick and, as it seems, quite deep insight in SiC and GaN and other high bandgap material systems, there is a Three-day Conference in Stockholm, starting June 10th. I will be there. Yes, educational workshop is on Sunday.

The widge band gap technology is there - there are commercial modules on the market today. Expensive, but available. Over time, they will only get cheaper. The real challenge now is how to use their capabilities. It's the passive side of things that isn't ready. The levels of inductance on the DC link have to come down and the capacitances on the AC side have to come down. The current module paradigm won't work to take advantage of the inherent capabilities of the wide band gap materials.

Taking advantage of GaN and the high voltage devices becomes even harder because you have to not only do the above, but do the above at ever higher voltages.

 

[ScottyUK]

Bill - interesting idea! Seems quite complex to achieve the end result, and probably quite lossy. I wonder if a more practical solution will be to use a conventional generator with phase-shifting transformers and a high pulse count rectifier to produce a relatively low ripple DC?

 

[edison123]

As I told in the other thread, my dad did a paper on DC vs AC EHV transmission in early 60's. For it what is worth, it is attached. If I remember right, we had our first DC transmission in 90's.



Muthu
www.edison.co.in

 

[Mbrooke]

@BrainPetersen: Its Wikipedia- certainly you can go above 60-70%.