5mo ago

DC (direct current) power network

As we all know, AC won the "War of the Currents". The reasoning behind this is that AC voltage is easy to convert up/down with just a ring of iron and two coils. And high voltage allows us to transport current over longer distances, with less loss.

Now, the War of the Currents happened in 1900 (approximately), and our technology has improved a lot since then. We have useful diodes and transistors now, we have microcontrollers and Buck/Boost converters. We can transform DC voltage well today.

Additionally, photovoltaics produces DC naturally. Whereas the traditional generator has an easier time producing AC, photovoltaic plants would have to transform the power into AC, which, if I understand correctly, has a massive loss.

And then there's the issue of stabilizing the frequency. When you have one big producer (one big hydro-electric dam or coal power plant), then stabilizing the frequency is trivial, because you only have to talk to yourself. When you have 100000 small producers (assume everyone in a bigger area has photovoltaics on their roof), then suddenly stabilizing the frequency becomes more challenging, because everybody has to work in exactly the same rhythm.

I wonder, would it make sense to change our power grid from AC to DC today? I know it would obviously be a lot of work, since every consuming device would have to change what power it accepts from the grid. But in the long run, could it be worth it? Also, what about insular networks. Would it make sense there? Thanks for taking the time for reading this, and also, I'm willing to go into the maths, if that's relevant to the discussion.

68 comments

I heard it said many years ago that if DC won the battle, we'd have power stations every 10 miles and power lines as thick as your wrist.
Converting local power is fairly easy, with AC inverters added for universal compatibility.
But, take note of how many DC voltages you use in your house. Devices in mine range from 3v to 25v and some weird one like 19v for a laptop. You'd still have adapters all over the place.
- But, take note of how many DC voltages you use in your house. Devices in mine range from 3v to 25v and some weird one like 19v for a laptop. You'd still have adapters all over the place.
  This is probably true, but every single one could lose the rectifier part, and instead of having to convert from pulsating DC (the output of mains rectification), you get clean DC from the wall instead, which should allow for using smaller capacitors in many places.
- Okay, these are short term problems. "power lines as thick as your wrist" depend on the voltage. If voltage conversion works well enough, that issue disappears.
  But, take note of how many DC voltages you use in your house. Devices in mine range from 3v to 25v and some weird one like 19v for a laptop.
  Yeah, that's why we need some kind of standard for these things.
  
  Ha! Yes! Even today USB 5 volts is pretty sweet for low power stuff. USB PD re-complicates things, but it's not user dependent so that's a plus.
  And you need a loooot of copper to prevent voltage drop especially when a grid of 100 houses 1/2 mile long draw 20-80 amps each. The math starts adding up real quick.

DC is used for long-range transmission in high-voltage DC (HVDC) transmission lines today.
https://en.wikipedia.org/wiki/High-voltage_direct_current
A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages between 100 kV and 800 kV.
HVDC lines are commonly used for long-distance power transmission, since they require fewer conductors and incur less power loss than equivalent AC lines. HVDC also allows power transmission between AC transmission systems that are not synchronized. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows the transfer of power between grid systems running at different frequencies, such as 50 and 60 Hz. This improves the stability and economy of each grid, by allowing the exchange of power between previously incompatible networks.
However, since grids are AC, it's just to send power to a grid or pull from one.
We also do have some increasingly beefy DC in individual households in some forms:
You mention solar PV systems, but more generally, 12V systems used in vehicles (and the related 24V and 48V systems that are sometimes used to push more power) are more common, with lithium batteries that can do many more charge cycles than lead-acid being available.
USB PD can negotiate pushing up to 240W now at 48V, which is a fair bit.
- USB PD can negotiate pushing up to 240W now at 48V, which is a fair bit.
  So if I wanted to wire my home to take advantage of this, supposing I had a house battery on solar, would I have some kind of DC-DC converter from battery to 48V, then cable to outlets with some kind of USB PD adaptor? How much advantage do I get from this, vs using existing 240V outlets + wall wart?

Well, most all DC generators these days are actually AC alternators with the output rectified, because alternators can be run a lot more efficiently. So you're already losing on efficiency there.
You need to consider the consumer side as well. Dinky residential loads like your computer would be fine on DC. But most of the world, especially heavy industry, runs on synchronous or induction AC motors, big ones. Big huge tens-of-megawatts motors that often run upwards of 97% line efficiency, which is insane for any industrial process.
The best you could replace those with would be modern brushless DC motors, which require really expensive inverter controls that die frequently due to the magnetic transients and still top out at an efficiency of only 90% if you're lucky. And that would incur huge costs that just aren't worth it.
- The best you could replace those with would be modern brushless DC motors, which require really expensive inverter controls that die frequently due to the magnetic transients
  Wow, is this why my new brushless cordless tools have had more issues than my 25 year old cordless tools?
  
  I mean, yes and no. A lot of that is modern tools are going to be more carefully engineered to operate as close to failure as possible, as to advertise more power with a cheaper device. Thry have small wires and encoder sensors that can be prone to failure.
  Yes, the driving electronics are also sensitive, however magnetic transients are less of a deal on the scale of a cordless drill. When dealing with huge motors, those can be significant multi-kilovolt spikes that make solid state components Very Very Mad.
  But the brushless motors in a drill also do not have brushes that wear down rapidly in a very dirty/dusty contaminated environment like older power tools would. So it's a bit of a 50/50.

Just to add, 400VAC is rough, but 400VDC scares me
- Same, that alone is reason enough to stick to AC IMO. It's so much safer for the end user (or their kids who stick a fork into the outlet).
- Why though? I thought impedance of the human body is lower at 50/60 Hz than at DC.
  
  AC lets you go if you shock, DC keeps you pulled in. I'm sure if you google this there'd be a video or 2, but it's going to be ugly

PV inverters often have around 1-2% losses. This is not very significant. You also need to convert the voltage anyway because PV output voltage varies with light level.
Buck/boost converters work by converting the DC current to (messy) AC, then back to DC. If you want an isolating converter (necessary for most applications for safety reasons) that converter needs to handle the full power. If it's non isolating, then it's proportional to the voltage step.
Frequency provides a somewhat convenient method for all parties to know whether the grid is over- or under- supplied on a sub-second basis. Operating solely on voltage is more prone to oscillation and requires compensation for voltage drop, plus the information is typically lost at buck/boost sites. A DC grid would likely require much more robust and faster real-time comms.
The AC grid relies on significant (>10x overcurrent) short-term (<5s) overload capability. Inrush and motor starting requires small/short overloads (though still significant). Faults are detected and cleared primarily through the excess current drawn. Fuses/breakers in series will all see the same current from the same fault, but we want only the device closest to the fault to operate to minimise disruption. That's achieved (called discrimination, coordination, or selectivity) by having each device take progressively more time to trip on a fault of a given size, and progressively higher fault current so that the devices upstream still rapidly detect a fault.
RCDs/GFCIs don't coordinate well because there isn't enough room between the smallest fault required to be detected and the maximum disconnection time to fit increasingly less sensitive devices.
Generators are perfectly able to provide this extra fault current through short term temperature rise and inertia. Inverters cannot provide 5-fold overcurrent without being significantly oversized. We even install synchronous condensers (a generator without any actual energy source) in areas far from actual generators to provide local inertia.
AC arcs inherently self-extinguish in most cases. DC arcs do not.
This means that breakers and expulsion type fuses have to be significantly, significantly larger and more expensive. It also means more protection is needed against arcs caused by poor connection, cable clashes, and insulation damage.
Solid state breakers alleviate this somewhat, but it's going to take 20+ years to improve cost, size, and power loss to acceptable levels.
I expect that any 'next generation' system is likely to demand a step increase in safety, not merely matching the existing performance. I suspect that's going to require a 100% coverage fibre comms network parallel to the power conductors, and in accessible areas possibly fully screened cable and isolated supply.
EVs and PV arrays get away with DC networks because they're willing to shut down the whole system in the event of a fault. You don't want a whole neighborhood to go dark because your neighbour's cat gnawed on a laptop charger.
- Oh wow, thanks for the detailed writeup. It's a little above my pay grade (condensers used as localized generators? Wow, what an idea. They must be huge).
  Guess it's time to find an Intro to Powergrids from The Teaching Company
  
  I'll give you a short introduction to the power grid (btw. it's called "stromnetz" (electricity network) in german). The power grid has many "levels", where each level represents a network of cables that transport current at a given, specific voltage. For example, you might have one 220kV level, and then a 5kV level, and a 230V end-consumer level.
  
  Between these levels, there have to be translations. These are "transformers" today, transforming high-level AC into lower-level AC or the other way around. For AC networks, they are basically a ring of iron and a few coils. However, for DC networks, other transformers exists, such as Buck/Boost converter.
  My question basically is: is there anyone who can give me experimental data on how well DC networks would work in practice? Personal experience is enough, it doesn't have to be super-detailed reports.
- Thank you for this well-thought and balanced viewpoint. It took me 19 days to process all the information.
  So basically, I was wrong when I assumed that inverters had an efficiency of around 50%. That misunderstanding comes from the phrase that "filters in the inverter eliminate high-frequency components in the PWM's output". I thought they discard that power. But that's apparently not the case. So the efficiency is more like >95%. So that's good.
  
  Even 95% is on the low side. Most residential-grade PV grid-tie inverters are listed as something like 97.5%. Higher voltage versions tend to do better.
  Yeah, filters essentially store power during one part of the cycle and release it during another. Net power lost is fairly minimal, though not zero. DC needs filtering too: all those switchmode power supplies are very choppy.

buy dc to dc power converter
look inside
ac
- do you have a source for that?
  I know about the buck/boost DC-to-DC converters, but they don't really use AC internally.
  
  Is a square wave not AC? Current is flowing in and out of an inductor 100k times a second.
  Could that 100khz square wave excite a transformer and produce usable current on the secondary? Absolutely it could, and that's how a bigger SMPS works.
  If you're looking for a "pure DC to pure DC" converter, that's called a linear regulator and it's wildly inefficient. They work by varying the conductance of a transistor but are useful for low currents. The extra voltage is converted to heat.

The grid does not work in 230V.
It works from 10kV up to hundreds of kV. Most of your arguments do not count there.
DC is good inside the house, and maybe to the next house. If I would build a new house today, I would build extra wires everywhere for AC and for DC 24V and 5V.
- build extra wires everywhere for AC and for DC 24V and 5V.
  Are there actually any appliances that take DC over a standard plug? Or would you just put in usb receptacles instead or something?
  
  Good question. I have not decided such things, since I am not actually planning to do it.
  I would start some research then about plugs, how others do it.
  Regarding appliances, I would also build many things myself, lights etc.
  
  https://www.sparkydirect.com.au/power-points/usb-powerpoints
- DC can also be used with superconductors for long-distance energy delivery.
- High voltage DC is used for transmission at 10s to 100s of kV already.
  
  Dc transmission lines at 10kv? Dc transmission lines make sense when you need to pass a really long distance or you can only use underground cables because those have a much higher capacitaance, i.e. high reaktive currents.
  If you need to supply your rural area or town you will go from some 20kv lines down to lots of 3kv and then 400v lines with multiple transformers everywhere. Dc transmission doesnt make sense here, but supplying a grid from really far away with a single line? Yea.
- I wonder what it would be like converting DC to DC at those voltages and power.
- The fellow who built my house in the early 1990s was thinking ahead. Dual circuits, one for lighting on 24VDC and one for power on 240VAC.
  If you're referring to 5VDC circuit for USB devices, you can get GPO plates with USB power sockets: https://www.sparkydirect.com.au/power-points/usb-powerpoints

When you have one big producer (one big hydro-electric dam or coal power plant), then stabilizing the frequency is trivial, because you only have to talk to yourself.
Your frequency is still influenced by a million and more consumers. And that's before blind currents come into play.
When you have 100000 small producers (assume everyone in a bigger area has photovoltaics on their roof), then suddenly stabilizing the frequency becomes more challenging, because everybody has to work in exactly the same rhythm.
...and? Everyone is getting the current frequency via the network, everyone knows what it should be, everyone can do their own small part is speeding it up or slowing it down.
The actual issue is that huge big turbines have lots of inertia which, through their inertia, naturally stabilise the frequency. On the flip side inverters (like with solar panels) can regulate the frequency actively, what's iffy is smaller AC generators like wind mills. But then there's also battery and capacitor banks.
It's a thing network engineers have to worry about, but it's not some insurmountable problem. We're already doing it. Insular networks have been doing it for ages, e.g. in Germany Berlin's network wasn't part of the eastern one and they always used stuff like capacitor banks to stabilise it.
All that aside yes in the future there's probably going to be a high voltage DC network in Europe. Less so for private consumers, at least not in the foreseeable future, but to connect up large DC consumers, that is, industry, with DC power sources. If you're smelting aluminium with solar power going via AC is just pure conversion loss.
- what's iffy is smaller AC generators like wind mills
  Not so iffy for bigger wind turbines, these also have significant inertia due to the mass of the rotor spinning (with large mass moment) and grid codes demand active grid stabilisation in most countries.
- All that aside yes in the future there’s probably going to be a high voltage DC network in Europe. Less so for private consumers, at least not in the foreseeable future, but to connect up large DC consumers, that is, industry, with DC power sources. If you’re smelting aluminium with solar power going via AC is just pure conversion loss.
  Thank you, that was exactly what I was looking for. I know about aluminum production processes, and that it requires large amounts of DC power.

How does the efficiency and cost of buck converters compare to AC transformers? Seems like the cost and efficiency of the voltage converter should be the prime determinant, rather than specific applications of generation/consumption.
What would a 400A 10kV utility scale DC converter look like?
- A lot of power blocks rectify the AC and then shift the DC voltage anyway, because transformers are bulky and heavy. This is why power bricks are so much smaller now than they used to be. See the modes section of the Wikipedia page about AC adapters for a longer explanation.
- well, a large part why I asked the question is because I hope that somebody knows more about what buck/boost-converters can do today. I know they work well enough on small scales, but I have no experimental data for them on larger scales.
  I assume they would work well, but I'd like that somebody links me to the right datasheet or something.
  Edit: you have a very important point there. " Seems like the [...] voltage converter should be the prime determinant, rather than specific applications of generation/consumption." YES. So, let me rephrase my question: does anybody have experience with high-power DC voltage converters?

High-voltage direct current (HVDC) is already used for transmission.
- But as OP guessed, would be too much of a hassle (and hazard) for distribution

I lived un a house that had live 120v DC service.
There was an electric fan that ran on it. The outlets were only in the basement and identical to each other.

When you have 100000 small producers (assume everyone in a bigger area has photovoltaics on their roof), then suddenly stabilizing the frequency becomes more challenging, because everybody has to work in exactly the same rhythm.
That's why you have standards and codes, that ensure everybody's equipment is capable of syncing to the grid properly before they are allowed to connect. It's not that hard for an inverter to do. Then you have the constant background supply to stabilize it like battery farms and other energy storage technologies. And a bunch of capacitor banks to correct power factor issues.
I don't think we are getting away from centralized production anytime soon. Even with the move to wind and solar, although I think nuclear should be included in that mix.

Leave the grid out of it. Your PV's charge your battery with DC, that gets used as DC by the USB-PD outlets in your wall. And you have an AC-DC converter for when you need to consume grid power.
- agree

I learned in school, that these days, DC would in theory indeed be more efficient even if you generate a sinus at home for the then legacy devices.

Power grids would mean long distance power transmission, so AC has an advantage. If the point of consumption is near the point of PV generation, DC can and is already being used.
I know factories with solar panels on their rooftops to cut down power bills and instead of converting to high voltage AC, a custom-built DC power system is used.
- How does AC have an advantage in long-distance transmission? Arguably they can be more efficient.
  
  Sorry I wasn't being clear. AC is used for connecting within areas of densely populated cities, e.g. British National Grid. If we are talking about really long distances (> hundreds of kilometers), HVDC is indeed preferred.
  I was talking about a trend of some factories replacing AC from power grids (possibility generated in nearby cities) with DC from solar panels on their rooftops. So it's a long distance compared to that.

FWIW https://insideevs.com/news/731455/solar-dc-to-dc-ev-charger-enteligent-pre-order/

I'm thinking there would be a transition period, where some devices would accept AC, and the others DC.
There will be two different types of power socket outlets, and two converters to convert between AC/DC or DC/AC. (Rectifier and Inverter)

Basically everything runs off 3.7-12v DC at the end of the day
It does make sense to eventually phase out AC in most home and commercial applications
- All the small stuff is low voltage DC, but just about every appliance requires AC (ovens, dishwashers, kettles, toasters, washing machines, aircon). Running an oven on 12v DC would be insane.
  
  Most of those don't actually want AC. They just want higher voltage.
  Ovens are just running resistive heating. They don't need AC.
  Dishwashers might, if the motor is AC.
  Kettles don't care. Same resistive heating thing as ovens.
  Toasters don't care. Same again.
  Washing machines and air conditioners might have motors that run on AC.
  All that being said, I don't know how problematic it'd be to use DC motors.
  Most fans in your house have fixed or several speeds, because they just run the motor off AC directly. I got a DC floor fan recently, and it was kind of nice to have fine-grained control over the speed, something that you can't do with an AC fan.
  kagis
  https://www.tenmenelectrical.com/ac-vs-dc-ceiling-fans/
  Differences between AC and DC ceiling fans
  Now let’s dive into the differences between AC and DC ceiling fans.
  Energy Efficiency
  One of the most significant differences between AC and DC ceiling fans is their energy efficiency. DC motors are more energy-efficient than AC motors and consume up to 70% less power. DC motors also convert more of the energy they consume into rotational energy, while AC motors waste energy in the form of heat. This means that DC motors will save you money on your electricity bill in the long run.
  Speed Settings
  Both AC and DC ceiling fans can have multiple speed settings, but DC fans offer greater control over the fan’s speed. DC motors are usually controlled using a remote control, allowing you to adjust more speed settings of the fan, usually up to 6 speeds. AC fans, on the other hand, typically use a wall controller but can be fitted with a remote control. However, the settings are set to low, medium and high only, which limits the number of speed settings and the ability to adjust the speed precisely.
  Noise
  DC fans are generally quieter than AC fans due to how the motors operate. DC motors operate without the humming sound that AC motors create, making them ideal for bedrooms or other quiet spaces.
  Lifespan
  DC motors are also more durable than AC motors, as they have fewer moving parts and operate at lower temperatures. AC motors, on the other hand, use capacitors to control fan speeds, which have a limited lifespan. When an AC fan loses the ability to change speeds, it is usually due to one of the capacitors failing.
  Price
  DC ceiling fans are more expensive than AC ceiling fans due to their higher energy efficiency, more advanced technology, and quieter operation. However, DC fans’ energy efficiency means that they will save you money on your electricity bill in the long run.
  That being said, they sell fans, so I imagine that they'd rather sell a fancier fan.
- Does it? (Genuine question).
  That seems rather simplistic - I'm sure there are plenty of use-cases where AC is a better choice than DC and vice-versa (though I'm no EE).
  Very rarely is any tech that binary.
  
  Yeah. Basically, the biggest reasons for AC have to do with voltage stepping up and down, and for instant grid load knowledge. Well, and of course, existing infrastructure.
  Both have solutions, but aren't as cheap as they are for AC. But, aside from that, DC has a lot of benefits, particularly in end usage efficiency and transmission over distance.
  Back in the day, the capability to easily bump up or down the voltage of electricity just wasn't there for DC, so AC was the distance winner (high voltage is needed for distance, low voltage typically needed for usage).

68 comments