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.

 

[Mbrooke]

Here is a bank thats even older. I can think of plenty across the US, especially California. But the thing is all these old relics will be replaced eventually.

 

[LionelHutz]

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

So, radiated 60Hz electric fields are bad, but radiated high frequency switching fields are OK?

Of course they want AC, but why is more and more of it coming from VFD? A motor being run high frequency, high slip alone is more efficient than 50/60 directly across the line. Further variable speeds such as compressors and fans are coming as more efficient.

Many VFD's are installed because of stupid energy programs pushing VFD's as instant energy savers no matter the application. If it's not due to a rebate, VFD's get installed because the end users are told they save energy. These programs give money for any VFD installed, no matter if the application is saving money or not. Being in the industry and seeing many of the applications, I can tell you first hand that most are NOT saving energy but rather using MORE energy. I've seen many of them setup to just run at full speed, which means the addition of the VFD has reduced the efficiency of the system by about 5%.

Other times VFD's are installed by lazy engineers who just throw together the system parts and then stick a VFD onto it to get it to work as required, instead of properly designing the system for across the line operation from the get go. Instead of properly designing the pump and piping for the flow and pressure needed, just grab an over-sized pump and use a VFD to run it at the speed needed to get the flow and pressure. This can give a double hit to the efficiency, due to extra losses of the VFD and extra losses due to not operating at the best efficiency point of the pump.

Running a motor at a higher frequency generally means you're running it at a higher speed. Most of the motor load is running fans and pumps. Impellers and casings of fans and pumps wear out quicker when running them at higher speeds. Using gearboxes between the motor and load would kill any efficiency gained when moving the motor to a higher speed. Bearings are harder to get to last at higher frequencies too. I see lots of municipal pumping stations and the old equipment is all generally large low speed pumps. Maybe not as efficient as a higher speed pump, but they last FOREVER and require little maintenance to keep operating. Any pump or fan engineer will tell you that if you want it to last longer you've got to built it to run slower.

 

[marks1080]

Ahhh.... is there any evidence that AC 60hz fields are actually harmful to human health? Considering most people walk around with microwaves held to their heads I don't think engineers should propagate that (mis?)information? Personally, I think the idea that a 60Hz electric or magnetic wave being harmful to health is hogwash.

 

[Mbrooke]

So, radiated 60Hz electric fields are bad, but radiated high frequency switching fields are OK?

Nope, both are bad, the latter worse IMO. Just because its none-ionizing doesn't mean risk should automatically dismissed.

Ahhh.... is there any evidence that AC 60hz fields are actually harmful to human health? Considering most people walk around with microwaves held to their heads I don't think engineers should propagate that (mis?)information? Personally, I think the idea that a 60Hz electric or magnetic wave being harmful to health is hogwash.

Childhood Leukemia is one aspect that has evidence backing it up.

 

[HamburgerHelper]

I can believe it. I don't know what causes issues the magnet or electric field but the magnetic field intensities is in range with a cellphone.

"Even if the seal is intact, microwaves as well as other appliances expose you to EMFs. EMFs are measured in milliGauss (mG). The Environmental Protection Agency recommends that you limit your exposure to 0.5 mG to 2.5 mG. When you are three feet away from a microwave, you are exposed to up to 25 mG."

https://www.safespaceprotection.com/emf-health-risks/emf-health-effects/emfs-in-the-home/

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If you can't explain it to a six year old, you don't understand it yourself.

 

[wroggent]

That website and its products are quackery if I've ever seen it.

 

[Mbrooke]

What can not be denied is that wiring errors, transformers, some appliances and most transmission lines have a significant, measurable EMF presence. There will always be those capitalizing off fears be it real or not via bogus products as with anything like medicine, finance, energy efficiency, self help, ect.

 

[HamburgerHelper]

The charts are from the EPA.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[cranky108]

The EPA sounds like a shoddy group if I've ever seen one. What proof makes them think any of this is bad?

 

[marks1080]

From a health perspective I would treat 60Hz the same as DC. Intensity is only part of it; REAL health impacts come at the higher frequencies.... not 60Hz!

 

[MatthewDB]

The EPA sounds like a shoddy group if I've ever seen one. What proof makes them think any of this is bad?

The EPA is the Environmental Protection Agency, a branch of the United States government. The current director of the agency would agree with you, much to the disgust of most of the employees working there.

 

[MatthewDB]

I'm going to go slightly out of order for your post waross - hopefully that's OK with you.

I suspect that they will retire the old transformers and go with 277 Volt pole pigs for a 480/277 Volt supply.

I'm presuming that is what will happen here because this is a 440V load. I'm not sure what the impact of 10% higher voltage would be for the very old motors in the grain elevator. The transformer for 120/240 has been replaced and I'm sure it's got taps for the higher voltage.

I hope that you can give us an update after the conversion.

Definitely. I went out and took pictures in the area because there is some unique stuff out there. They have many 12kV:12kV isolation transformers to convert delta to wye for underground feeds. They also have some redneck versions with two distribution transformers back to back to do the same.

Grounded with circulating currents?
Floating with higher voltage switching transients?
Fused? One partial solution was to install a fused cutout in the wye point connection. When local maintenance was done, the fuse would be installed in the wye point cutout to avoid switching transients. When all three phases were energized, the neutral fuse would be removed to avoid circulating currents and the wye point would be left floating.
I saw such a bank some years ago. I went back a few years later to take a picture but it was gone.

240 delta service is definitely a challenge to deal with. In the days of 5kV primary, there were no issues. At 15kV there are some, but in the 25kV or 35kV class there are severe problems without good solutions.

I've never seen a bank like that. I have 3 utilities around me, one is 12.5kV and dipping their toes into 34.5kV. One is 12.5kV and 24.9kV. The third is 20.8kV. I know the two with the present higher voltage have done both floating wye point and grounded wye point depending on when the bank was put in. When they ground the bank, you see cutouts dropping out all the time. This is a bank only months old. The fuse is still sitting open two months later. Coincidentally, the open fuse is on the "lighting" phase so that transformer is acting as the ballancer on an open delta bank.

 

[MatthewDB]

So, radiated 60Hz electric fields are bad, but radiated high frequency switching fields are OK?

The higher frequency fields would only be found inside the conversion equipment changing one voltage to another. The power lines going in and out would be predominantly DC.

 

[MatthewDB]

Schneider Electric (APC) doesn't think DC distribution makes any sense inside a data center.

AC vs. DC Power Distribution for Data Centers
A Quantitative Comparison of High Efficiency AC vs. DC Power Distribution for Data Centers
Review of Four Stud ies Comparing Efficiency of AC and DC Distribution for Data Centers

They've made the bet that there isn't an immediate future in DC distribution and are not putting development effort in. That's a pretty big gamble by one of the biggest players in data center power systems.

 

[Mbrooke]

I've never seen a bank like that. I have 3 utilities around me, one is 12.5kV and dipping their toes into 34.5kV. One is 12.5kV and 24.9kV. The third is 20.8kV. I know the two with the present higher voltage have done both floating wye point and grounded wye point depending on when the bank was put in. When they ground the bank, you see cutouts dropping out all the time. This is a bank only months old. The fuse is still sitting open two months later. Coincidentally, the open fuse is on the "lighting" phase so that transformer is acting as the ballancer on an open delta bank.

Thats not a fuse, but a solid copper link. I can tell because of the color, shape and holes in it. Its closed when the bank is being energized one fuse at a time to prevent ferroresonance and opened thereafter to prevent future fuse blowing during faults out on the line.

Unless the customer has a large amount of 3 phase motors the best option for a legacy delta customer is a grounded wye open delta bank. ie, 2 167kva (or over) pigs for that bank.

 

[prc]

Sorry to enter the fray a bit late. In 1880's Thomas Edison vociferously fought for DC transmission not evaluating technical superiority,but from the huge investment he already made in it. He was a shrewd business man than a scientist. So I don't think any point in today saying that Edison was right etc. Those days he purchased an elephant from an Indian Circus company (they were conducting circus shows in New York as on today!)and electrocuted it to death in public to demonstrate the dangers of AC. He even predicted that all great US Prairies will dry up due to AC ground currents from Westinghouse and Tesla planned power plants in US.In 1880s also some high voltage DC(by putting generators in series) transmission was tried in US &Europe but failed soon due to commercial reasons. Anyway DC transmission moved from first Edison project of 110V 100kW to today's 1100 kV 12GW lines put up in China and soon in India. Two bundles of conductors carrying 12000 MW! (2200 kVx6000A)

When the distributed generation comes in a big way will these UHV DC or AC lines will really be required? I don't know. Things are changing so fast in the world we cannot predict anything for sure. A few years back, in India, we put up a short 1200 kV AC experimental line with 1000 MVA 1200/400 kV transformer banks (self involved in it) But the massive solar and wind generation makes us doubt whether such lines will really be required. UHV DC is used for long distance high volume power and UHV AC is planned for short length high power inter connections. Who thought, twenty years back, solar power of current volume will be generated? In 1970s I had some access to engineering magazines from UK, all carrying lengthy papers on experiments on satellite communication. I thought why people waste money and time for such impractical ideas! My dream is man will succeed in creating small suns ( what better model for energy generation !) through fusion reactors and power stations of 20-30GW may be the standard 50 years in to future, requiring 1500 HV DC to evacuate it. But AC is not going to die out and to bring this massive power to the last mile it will be required. Probably distribution transformers may be replaced by electronic units ( it still costs 8-10 times conventional transformers).So both DC & AC will coexist in their own areas.

 

[MatthewDB]

Thats not a fuse, but a solid copper link. I can tell because of the color, shape and holes in it. Its closed when the bank is being energized one fuse at a time to prevent ferroresonance and opened thereafter to prevent future fuse blowing during faults out on the line.

Well I'll be - you're right!

I ran through several exposure settings because my camera was struggling with the overcast sky background. With a different exposure setting, you can clearly see the wiring and that there are three fused cut-outs.

Unless the customer has a large amount of 3 phase motors the best option for a legacy delta customer is a grounded wye open delta bank. ie, 2 167kva (or over) pigs for that bank.

This bank was added when they added air conditioning to the town's city hall. The existing service remains, so all this powers is an AC system, presumably almost all motor loads. No explanation on why it would be delta instead of ground wye - ground wye for brand new service.

 

[MatthewDB]

prc: You're in Southern India, correct? If someone were to find themselves in Bangalore, are there any 765kV or 1.2MV lines in the area?

 

[Mbrooke]

We don't know what Edison was thinking in truth. In all fairness I can see where he was coming from with AC being more dangerous. Back then we did not have the level of isolation and insulation of power systems. Being exposed to a circuit was inevitable. DC for the same voltage is less lethal then AC. However today that point isn't very debatable as you could probably use 347 volts in residential and not worry about it.


Regarding electronic transformers- they are becoming cheaper than conventional units- think halogen lights.

 

[Mbrooke]

Brand new service? That would surprise me. I thought that is what replaced the previous bank you showed which would make sense. Its possible the old system was ungrounded or high leg all for the city hall and this is just feeding the old. That or someone screamed it has to be delta which would be worthless in a new installation. Its a guess. I know of a thread where an EE demanded a 120 volt only drop because he thought it would save money on that extra wire...