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Was Thomas Edison Right about DC Power?
Declan McCullagh writes "Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla. But now DC power is being seriously considered for data centers. DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure. The University of Florida has even bought 200 DC servers."
Was Thomas Edison Right about DC Power?
:-)
Oh, well, nothing sensationalist about that headline. (*rolls eyes*)
DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure.
In this case they're right. With that much hardware that close together, it's easier to treat the entire room as a single device. As the article suggests, this cuts down on waste heat produced by inefficiencies in AC->DC conversion. In fact, it significantly cuts down on the amount of equipment needed in the entire room. The concept can be taken as far as to cutting down to a single power supply per rack.
The amusing part about this is that the resulting racks might look a lot like Big Iron servers with pluggable motherboards.
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I heard of this new power system. Seems like a mix of AC and DC, to create the ultimate power form. AC *lightningbolt* DC was the name, and with a lightning bolt in the name, it has to strike you like thunder.
In Washington State, Verizon (Was once GTE) runs almost all DC powered servers and Telco equipment in their Data Centers. Many of the IBM server my company buys support DC power.
Tesla and Edison were both right...and wrong. Like many Slashdotters do when debating which operating system is best for any given job, Tesla and Edison wanted to apply one power system to every job. Its like having a toolbox with only a screwdriver in it. Ever try to drive a nail with a screwdriver?
For moving power over long distances, AC is king. But for short distances with most modern electronics, DC would win. The first thing a desktop system or server does with AC is converts it to DC. So if you have a number of machines all in the same room, why not do the conversion in one spot, and eliminate the redundancy in every machine.
Would it benefit the average user with one or two machines? Not at all. But for a major center with many machines in the same room, I can see quite a bit of benefit with going with DC.
I want a new quote. One that won't spill. One that don't cost too much. Or come in a pill.
Tesla developed AC, and sold the patents to Westinghouse.
How come there is no real difference? Because both modern AC and modern DC supplies start out by converting the power to high frequency AC (on the order of several kHz), and operate on that. That's what you actually want as input, if anything.
The article states:
In other words, the DC supplies they use are more efficient than standard AC supplies, which are the cheap crap and notoriously inefficient.
Er...
"Anisotropic Magnetic Field" has to be the worst offense in terms of technobabble i have seen recently.
Newsflash: there are no magnetic monopoles, so EVERY magnetic field is anisotropic...
HI O WISE PRINCE. WHT TOOK U SO DAM LONG?
Just think, if he'd settled with Tesla back then, today they could be sending people to be killed on the Edison Chair.
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Brilliant!
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First, that would be anisotropic.
It doesn't seem too surprising that AC power would produce an anisotropic field since the current keeps switching, so the magnetic field should be switching direction also. I suppose this would make the magnetic field from a DC current isotropic (invariant with direction, or I suppose in this context constant orientation), but I don't really see why either would be an issue (since you referred to Maxtor, I assume the issue was something that had to do with hard drives). Although if you have a weak, constantly switching magnetic field it might demagnetize (randomize) low-coercivity magnetic grains (domains, whatever - I work with sediment, dammit!), but unless it is a pretty strong field it shouldn't bother the relatively hard (magnetically) magnetic media in use.
I'm too lazy to actually look up what you are referring to, though, so whatever.
Nope, sorry. Please play again. http://en.wikipedia.org/wiki/War_of_Currents Tesla and Westinghouse patented all of the AC equipment. Edison wanted to sell his stuff. He even went as far as designing electric chairs with AC to prove it was "more deadly."
for crap's sake, dc powered servers are nothing new, many have config option of "-48VDC standard telco" supply.
I've seen houses wired with 12V DC from mini hydro and solar - but in those cases it was a long way to the nearest transmission wire and would cost a fortune to get mains power onto the site.
Perhaps we'll see the AC group hitting back with demonstrations of how dangerous these DC powersupplies are to the hamsters and other wildlife native to big server rooms.
Incidentally, that's how the electric chair came about:
[Edison]AC is dangerous! Just watch what happens to these various animals when I close this circuit!
Edison electrocutes some horses
[US_Gov]Ooooo... I'll bet that works on people too!
US_Gov introduces new grisly method of executions, while disregarding the main point of Edison's demonstrations.
The story has a good post script too... some reporters came to Edison to get his take the new, modern form of executions. When asked what name he would give to the method, Edison, in an attempt to forever link his competitor's name with electricity's most grusome application, offered "to Westinghouse someone."
Procrastination Man strikes again!
That said, if space and cooling are an issue it might well make engineering sense to get the transformers, capacitors, and rectifiers out of the computer boxes. Big 5v/12v power busses wouldn't even need to be insulated. So while the reporter badly mangled the story, the engineering sounds reasonable to me.
Actually, that's the norm across the phone industry. Everything, and I mean everything, runs on -48V DC. Okay, not the fluorescent lights....
This goes back to the telephone talk battery, which is -48 V DC. That powered the phones via old cord switchboards, and was the voltage of electromechanical (stepper, and later crossbar) switches, which basically used relays. Electronic gear was then designed to run on the same power plant. A telephone building has a big bank of batteries, powered by multiple "rectifiers" (DC supplies) which, btw, are normally engineered to not run over 40% of load. (That way they can still run the systems and recharge the batteries when one of them is kaput.)
If you then put anything else into one of their buildings, the Network Equipment Building Standards (NEBS), which are Telcordia documents that practically carry the force of law, dictate that equipment be DC powered. Among other things -- NEBS gear has to meet the brick schytthaus test. (Sun Netras and many Cisco routers meet NEBS. Your basic rack server doesn't. And aluminum racks are STRICTLY forbidden; it has to be steel.)
So because of the talk voltage on analog phones, lots of computing equipment is engineered for -48 V DC power. Sort of like the legend (I know, that one is not really true) about the railroad track gauge being based on Roman chariots. But in this case it's surprisingly effective.
The reason you want to use DC is that a computer's power supply converts AC into DC. The power supply of most computers isn't that efficient at it. This basically converts some of your electricity into heat. (Heat in a 1U server in a big rack of 1 U is really bad.) In theory the data center's big AC to DC converter is more efficient and better cooled. Thus you save money in power bills, air conditioning, and rack space (less heat, and power draw means more servers per rack). Plus in theory your servers should last longer as the power supply is one of the more likely points of failure.
IANALBIPOOGL (I am not a Lawyer, but I play one on GrokLaw.)
Wrap the casket in copper, replace the headstone with a magnet, and expose corpse to this article. As Tesla turns in grave, free power.
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[tap tap]
Hey, who turned off the microphone?
In other news, astrophysicists have announced that they now know what all that dark matter is: it's stupidity.
What Rackable is really pushing is a system where AC to 48VDC conversion takes place in a unit at the top of the rack, and 48VDC is local to the rack. That, at least, simplifies the cable management.
One big advantage of 120/240VAC power distribution using US standards is that the connectors are standardized and reasonably idiot-proof. That is, if you can plug it in, you won't overload the power cord or the connector, and if you overload the branch circuit, a breaker will trip. Outlet strips have circuit breakers, so you can't overload the cord to the outlet strip without a breaker trip. There are NEMA standard power plugs for 15A, 20A, and 30A circuits, 120/240VAC, and single and three phase configurations. All this is standardized nationally and enforced by the National Electrical Code.
In contrast, there are no simple standards for 48VDC. Most 48VDC gear has big screw terminals. There are no standard plugs and sockets. Somebody, preferably a licensed electrician, has to check all the data plates, add up the current loads, calculate voltage drops, size the wire and breakers, and torque the big screw terminals to the correct torque, using the correct lockwashers. Every time you add or change a load, somebody has to recheck the math. Errors can cause a fire. None of this is all that hard if you have basic power technician skills, but you can't just go casually plugging stuff in.
Although, since the development of the low-cost clamp-around DC ammeter, things have become easier in the DC world.
I found out that Consolidated Edison still sells DC power.
Yep. My dad was the building superintendent of a church on 96th St. in NYC in the early 70s. The church building has a DC mains supply - mostly to run elevator and fan motors, but some of the outlets in the building were DC, and were identical enough to normal AC outlets that you could plug a regular plug into them. Well, before he knew better, my dad plugged an old TV into a DC outlet. Transformers don't take DC input very well - fireworks ensued. -b.
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Contrast this with a properly designed DC system a la old-school telco: The same front-end of the UPS is used, with a 10% loss converting AC to battery voltage. Then you run that into DC supplies that, with modern electronics, are going to be doing a lot better than an AC supply, so let's say 5% loss. That puts you at better than 85% efficiency.
The critics cited in the article are actually probably not far off in calling the Rackable solution over-hyped, if you only take into account the isolated-rack design. Rackable puts 2-3U of beefy redundant supplies at the top of the rack and does DC to the servers. Efficiency-wise this is only fractionally better than a bazillion AC supplies, and quite possibly dead even because of the DC->DC losses in each server on top of the AC->DC->AC->DC setup implicit with AC-based UPS systems. However, AFAICT from a glance at their site, Rackable's systems are designed to drop right into existing DC datacenters, which eliminates the AC supplies at the top and the DC->AC->DC stages.
The issue is what kind of infrastructure is needed to feed the selected DC voltage (which is going to be -48VDC) into the racks with the lowest bus losses, but this is someone I would expect is either a) already solved by the decades-old telco industry, or b) going to be solved in at the appropriate 384-cores-and-100TB-per-7ft-rack scale RSN, by "the market".
I know that if I were in the position of designing a big datacenter right now, I would be looking very hard at DC systems.
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The origin of the 48 volt number is that it was convenient, and now it just sneaks under the 50-volt "low voltage" cutoff in the NEC, which I think was written with telcos in mind. The glorious thing about this is that you don't need licensed electricians to do power wiring in a central office.
And the reason it's negative with respect to ground goes all the way back to the telegraph system: Western Union initially ran bipolar lines and noticed that the positive ones corroded much faster. Sodium ions (from dissolved salt) are negative, and thus repelled from lines that're also negative. The whole phone system was built with positive ground because of this, and it's saved incalculable maintenance costs. It does tend to mess with people's heads the first time, if they're used to negative ground systems, but you get over it quickly. (A number of traditions use blue for "hot" and black for ground/return, to help escape your "red equals positive" association.)
DC power as used by telcos is also always redundant. There's an A-side and a B-side for everything, and the cables are sized so that the entire load can run from just one side. This leads to some very fat copper, which is cheap compared to downtime. You don't achieve five-nines reliability with a system that contains single points of failure!
Now, about rack-mounting: This was also invented by the telcos, originally in a very wide (40-inch?) format, for the panelboards and Strowger switches. Some of the old crossbar equipment is still in those huge racks, but the 23-inch width is infinitely more common now. All telco equipment is mid-mounted, with the ears approximately in the center of gravity on the shelf, so the force on the screws is shear. There's no torsion on the mounting flange unless you step on the front or back of the shelf. Cooling is always convective bottom-to-top, or occasionally front-to-back with fans. This leads to a "cool" front aisle and a "warm" back aisle between alternating rows of equipment.
Now, the pro audio industry borrowed the rackmount idea fairly early on, but they were mostly mounting control panels and mixers, which are very shallow, so flush-mounting made sense. They also changed the every-inch Western Electric mounting holes to an alternating-spaces "EIA" standard, and narrowed the rack from 23 to 19 inches.
Somewhere along the line, an absolute idiot decided that computers should be rackmounted, but they should be 19 inches wide, flush-mounted, and use EIA hole patterns. I'm sure this has something to do with mainframe legacy getting perverted by peecee people. The current mishmosh of mounting standards (19" vs 23", two-post versus four-post, flush versus mid, inch versus RU, front-cable versus rear-cable) is what every datacenter tech deals with on a daily basis. Throw overhead racks versus raised-floor cabling into the mix, and you've got a recipe for frustration!
If you're familiar with the concept of "blade servers", where common components are separate from processor resources in the shelf, congratulations. Telco hardware has been built like this since the invention of the circuit board. Actually, the concept of replacable plug-in units goes back before that, but it got vastly easier with printed wiring boards and card-edge connectors in the sixties. Most of the "good ideas" in serious computing circles are actually century-old ideas in the telco industry. Spend a week shadowing a central office tech before you design a datacenter, please!
Also consider: If your datacenter is already built for DC, throw some solar photovoltaic panels on the roof. Inverters are a large part of most PV systems' expense, and you can skip that part. Why not start offsetting your grid demand now?
Also also: Edison was flat-out wrong about DC. The modern switching power supplies that make DC transmission lines practical didn't exist in his day. Besides, long-distance power transmission is an entirely other discussion.
Tesla believed that electricity should be free, so he created a tower that transmitted electricity over a distance. http://en.wikipedia.org/wiki/Wardenclyffe_Tower
The article conflates several things.
First off: Digital electronics generally requires several voltages. And they're all low, requiring high currents, massive conductors, and local filtering and regulation. So even if you're providing DC power from outside the room, you'll have a switching power supply (or several) in each piece of equipment to convert whatever the rough DC power is to whatever you need, smooth it, and regulate it.
But while some electronic devices use a common switcher to generate all the voltages with one conversion step, others use a "roughing" supply and a bunch of local supplies. Part of that is to get better regulation - part is because the roughing supply must run from 60 (or 50 or whatever) Hz and thus requires big caps to tide you over the low part of the cycles - caps you don't want taking up space near the components.
If you're going to do it in two stages anyhow, you can put your roughing supply OUTSIDE the room and only have the final supplies inside. The roughing supply has a lot of heat dissipation so you save a bunch on your cooling.
Second: There are two standards for power distribution in electronics rooms:
- Your local power line stuff. (120/240/480/208-3-phase in the US)
- The telco standard: x2-redunant 48V DC.
A lot of equipment - especially networking equipment - is manufactured for sale to tellcos and other operations that use the standard. They might have initially used it because some of their equipment was co-located in tellco sites, where only 2x48VDC is available - and they got a quantity discount for buying a bunch of the same stuff and went to 48V for their own sites. Or they might use it because it's MUCH simpler to do backup power with floating batteries and century-old technology than with a building-sized UPS. (Note that a UPS CAUSES at least one outage when first installed and on the averate at least one more within the first year of operation from some malfunction. And a UPS dissipates more power than a roughing power supply or a battery charger.)
But the standard for 48VDC is REDUNDANT 48VDC supplies, with the equipment only requiring one (and typically doing "cutover" with diodes B-) ). With the equipment already set up for redundant supplies it's not a lot of cost or work to wire both sides and put in two 48V feeds to the equipment room. (Four diodes are a LOT cheaper than a pair of 120V roughing power supplies at each box, too.) So of course the users of such equipment normally give it dual supplies. (Even if it's a single rack and so they just put two roughing supplies in the rack fed from two different 120V feeds.)
The result is that all the equipment has redundant power supply, and keeps operating glitch-free through a number of kinds of partial outages - AND power supply repair and replacement. This is what's responsible for much of the claimed increase in reliability.
The whole Edison/Tesla DC/AC war had to do with the economics of CROSS-COUNTRY power transmission. AC beat DC there because a century or more ago it was virtually impossible to jack DC voltages up to levels suitable for long-distance transmission and back down to levels safe for distribution within houses, while AC could do that easily and efficiently. So Westinghouse/Tesla could ship cheap power from Niagra Falls to New York City while Edison had to build fuel-burning power plants IN the city. It has essentially nothing to do with shipping the power around within a single building.
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Tesla originally worked for Edison, but they had a bit of a falling out, which is possibly why the AC/DC competition was so heated. Edison embarked on a pretty ruthless and gruesome campaign to discredit AC power, at least by modern standards. He electrocuted stray dogs and cats with AC current in public demonstrations intended to show how dangerous AC power was.
In one instance, he even electrocuted an elephant...
During the construction of Luna Park on Coney island, an elephant used as a beast of burden went out of control and killed a couple of people. Topsy, as she was called, was condemned to death. However, there was a wee bit of a problem. Elephants aren't the easiest critters to kill. What happens if you walk up and fire a shotgun at it's head, only just to piss if off? They do have rather thick hides, and we are talking about a homicidal elephant the size of a couple SUV's here. There weren't any cliffs handy to stampede poor Topsy off of, and I doubt dynamite was ever seriously considered. Edison, being the generous person he was, gladly volunteered to execute the elephant with AC current and filmed the whole thing. He showed the resulting film, "Electrocuting an Elephant" (1903) publically on many occasions. It is quite probable that many a cat and dog escaped a crispy fate thanks to this film. If you decide to track down a copy of "Electrocuting an Elephant" today, please be warned that it's a rather gruesome little piece of history, and is not for the faint of heart, or SPCA members.
Edison and Tesla were both right. Remember, the DC vs. AC wars were fought back when the load was mostly made up of lights, motors, very utilitarian things. AC is fantastic for transmission over long distances (and for running three phase motors, but that's another story). DC happens to be better at running precision equipment like computers -- heck, they all run on DC already. All we're really talking about here is taking advantage of an economy of scale by doing one big power supply (or a few, for redundancy) instead of one for each machine.
Ever seen a telco rack? Everything runs on -48VDC. Everything. A telco rack always includes a couple of DC power supplies, and all the equipment just ties in to a common DC bus. The best part of all: the UPS simply consists of four "car batteries" (not exactly, but you get the idea) wired in series and tied directly into the bus! No pesky inverters to deal with.
The telecom industry has been doing it this way for decades. It's about time the computer industry got on board.
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Is this a new Slashdot cliche in the works? Will it be added to soviet russia, elderly koreans and the like?
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Bzzzt. Because 48 Volts is the standard used for all the DC equipment telephone companies have been using for years. Cisco, for example, makes a large portion of their product line with 48 V DC power supplies as well.
Mind you, the 48 V DC systems are not simple or easy to wire. You're talking very significant amperages, which means very beefy conductors, and with batteries in the picture a risk of nasty stuff if you drop your screw driver in the wrong place.
I hate the worship of Edison. He simply hired hordes of scientists and engineers, had them do the work, then took all the credit. I don't know if the story you tell is true, but I certainly can belive it.
No, I don't trust in god. He'll have to pay up front, like everybody else.
Is that quite so? Wouldn't there be taps on the transformer for approximately 12V and approximately 5V, and then the potentials finely adjusted using DC-DC regulators? Wouldn't that have less loss?
Taking this a bit further, why not have an entire rack power supply that can deliver a rail of 3.3V, 5V, and 12V to each server, thus eliminating the need for a high-current DC-DC converter on the target board? I am excluding things like the exotic voltages for CPU and RAM, but still it is the 12V and 5V rails that would have to be able to source significantly more current.
With both AC and DC distribution, there are losses due to the resistance of the wire (I-squared-R losses). The way to minimize these losses is to increase the voltage (V) and decrease the current (I) while transmitting the same power, but there is a limit to how high the voltage can be increased. Air breaks down at about 3x10^6 V/m. To avoid this dialectic break-down, you continue to raise the height of the power line as you increase the voltage.
With AC distribution lines, there are also losses related to the capacitance between the power line and the ground. Increasing the height of the power also minimizes the capacitive losses.
With both AC and DC there are reflections between the source and load which cause further trips from one end to the other. Each reflection is smaller than the previous one, but remember how many people are using electricity and the fact that everyone is constantly adding and removing load from the system. So, even in a DC system, the line voltage will be constantly changing.
Then, we have the conversions. Conversions from one AC voltage to another AC voltage is accomplished with a step-up or step-down transformer. This converstion isn't free, and it doesn't work for DC. It is very efficient and economical however, to convert from a higher DC voltage down to a lower one -- even for moderately high currents. it is very painful however to step a lower DC voltage up to a higher one. There are circuits to do it, but typically (or at least through 1990), it has been easier to convert to AC, go through a step-up transformer, and and then convert to DC. Also, the circuits for up-converting DC to DC are usually fixed at multiples of 2x, 3x, 4x, etc. using diodes.
So, let's put it all together. I can believe there are long-distance DC transmission lines where the savings in capacitive losses are worth the significant capital investment required at both ends of the line for the conversions, conditioning, and to match the source to the line and the line to the load, but in general, in a DC distribution scheme, the DC voltage drops continuously along the line and must be periodically stepped-up by some hard-to-determine amount because it depends on the age of the wire, the distance from the last step-up, and the demands of the load at that moment in time, but the circuits for doing it are inflexible (can only do multiples).
With AC, you get the flexibility that each sub-station is monitoring its own load and it can control the variable-step-down transformers to achieve the desired neighborhood voltages. Ready to increase the height of the lines? Step-up. Ready to drop the height of the lines? Step-down.
In a data center, as several people have said, everything is in one place, so the problem is different. You want to pick a high enough DC voltage so that it is always higher (even at maximum load across the entire room) than any voltage you might need. Then, you use the cheap and economical DC-to-DC conversions _at the point of use_ to take that down to the +5V and +12V that your equipment needs. You may pay marginally extra for larger cabling to handle higher currents, but you save money by not needing step-down transformers in each power supply. Let weight, more compact, and more efficient.
The Edison/Tesla one was about long distance transmission of power, and AC is still the winner there.
TTL logic has to run on DC, so you have to convert the supplied AC to DC. This is just recognizing that instead of converting it individually in each of dozens or hundreds (or more) machines, that it is more efficient and reliable to have one (and perhaps a redundant standby) converter providing DC to the same machines.
Remember the death of Archimedes!
Anyway, the respondant claimed:
Check your history!
Edison died nearly penniless too.
The account which I read described how he ran across some iron-rich sand on a beach, and it gave him the idea to try a new mining technique where the ore would be extracted from non-ore material by dropping the sand past magnets. The idea was a good one (and is still used) but the site he chose to build his iron mine turned out to be almost completely lacking in iron ore. The iron ore in the sand on the beach had apparently washed up from some other source.
Maybe he wouldn't have been desperate enough to try such a risky thing if he had been ALLOWED to sell AC power. I'm sure he could see the advantages of AC for power transmission but Edison didn't have the patents for that, and you can bet that Westinghouse wasn't going to license the technology at a reasonable price to their chief competitor.
So Tesla got ripped off by Westinghouse because he wasn't business savvy and they got ownership of the patents. Then Edison, even though he was somewhat business savvy, got shut out by Westinghouse because they owned the patents. In both cases, patent law helped business-people who didn't invent anything get rich while the real inventors lost out. Shouldn't we remember that the patent system was set up in order to encourage invention?
Edison went further: he lobbied his NY State politico buddies (like the Rockefellers) to use rival AC for the electrocutions. Tesla gave still-famous public exhibitions of AC, voltage/frequency tuned to run only along the surface of human skin, holding an "Edison bulb" in one hand, then grabbing an AC electrode in the other. The bulb glowed violently, Tesla stayed calm and cool. Tesla got the electrocution contract, the power transmission contract, and wide acceptance as "safe power".
Tesla 1, Edison 0.
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make install -not war
I believe the article makes an oversimplification by stating that AC is better for long-distance power transmission. Rather, it's easier to generate AC power (no rectifiers are needed), easier to switch (because the arc when the switch opens is much easier to extinguish - current flow actually stops for a short period of time, and the arc goes out), easier to run a motor from AC (no commutator), and easier to do voltage conversions (you only need a transformer). For really high-power long-distance transmission lines (like between states) they use very high-voltage DC because it is in fact more efficient. But I'm not sure how they do the conversion from DC back to AC in that case (would guess it's just a rotary converter - a motor running a generator). The losses from doing the conversion on both ends are acceptable only when they are less than the losses that would occur in such a long transmission line.
Losses are especially bad in AC transmission lines when the power factor is not correct, because while currents which are out-of-phase with the generated voltage waveform are expressed using imaginary numbers, in fact they are very real currents, and they cause increased heating losses in the transmission line. So the power companies switch large capacitors in and out of the circuit to try to keep the current and voltage in phase. (And they would appreciate if every device on the grid was power-factor-corrected, but this doesn't happen, mostly because motors are inherently inductive, and motors are the largest consumer of electricity. Sometimes they at least manage to persuade large industrial customers to manage their own capacitor bank, to correct for the inductance of their own motors, and give them a discount in exchange.)