Hydropower - AC or DC

Do the generators in a hydropower plant generate AC or DC. If the latter, when does it get converted to AC?

It’s AC, so according to one set of definitions, tecnically they are alternators, not generators.

DC generators are seldom used in large machines, and even smaller ones that supply DC are typically AC machines followed by rectifiers (AKA diodes)
The large AC machines typically have a small DC “exciter” that supplies field current to the main alternator. On older machines these may be true DC generators.

Is this because they ultimately want to have AC to put in the power grid or because AC alternators are cheaper/more efficient than DC generators? The reason I am asking is because I am wondering about what advantages their might be to locate certain businesses near large generating plants.

AC generators are more efficient than DC. Also, AC current travels down electrical wires than DC does, so you need fewer generating stations with AC than you do DC. Back when the US was starting to electrify, Thomas Edison was pushing DC current, while Westinghouse (using technology developed by Tesla) pushed AC. Edison, holding the patents on DC current, stood to benefit greatly if the US went DC. The battle between the two camps got nasty (googling “War of the Currents” will probably yield some detailed info), with Edison using AC current to electricute a dog in an attempt to show how dangerous AC was. They screwed up, though, and zapped the dog with too low voltage and it didn’t die right away. They ended up shocking the poor thing like 13 times before it died, and Westinghouse reportedly said, “It would have been better if they used an ax.”

As a WAG, I’d say that the reason to locate your business close to a power plant is that you’ve less a chance of being without power for a long period of time during adverse conditions (what with the power plant and it’s repair crews being close by).

If you’ve ever played with a toy or demonstration generator, e.g. a simple loop of wire rotating between the poles of a horseshoe magnet, you’ll soon realise that AC is the natural consequence of generating power in this way. As the loop rotates, the current induced in it travels first one way, then the other as it turns over within the field.

Whether the output is AC or DC depends on the nature of the sliding connection between the rotating loop and the outside world. If slip rings are used, then it’s AC. If a commutator is used, then it’s DC. Effectively, all a commutator does is quickly criss-cross the conection every half-turn, so although the current changes direction within the loop, the output current still travels in the same direction. So even a DC generator is AC inside.

This criss-crossing momentarily interrupts the current, and the current has a sinusoidal ripple away, so a simple DC generator doesn’t give clean DC like a battery. Commutators are also prone to wear and arcing, since they are effectively switches that open and close several times a second. For these reasons, almost all generators are of an AC design. E.g. A modern car alternator is AC, and uses a bridge rectifier to convert the output to DC. Older cars such as the VW beetle use a DC generator, because semiconductors weren’t available when they were designed and you have to have DC to recharge the battery.

A second, very good reason why you’d want to generate AC is because you can easily and efficiently change the voltage of AC with a transformer. For long-distance power transmission, you waste much less power in your cables if you tranform the voltage up, transmit at high voltage and transform the voltage down at the other end. This was part of the battle between Edison and Westinghouse - Edison’s vision involved small-scale, local generation and short range DC power lines. Westinghouse’s technology allowed for large scale power generation and long-range transmission.

Reasons to put your business next to a hydro plant would depend on what your certain business is.
If you are selling newspapers and smokes to the plant workers, then being plonked right outside the front gate would be ideal, however I suspect hydroelectric powerplants dont employ that many people in the operations phase.

As a general concept, having a large industrial complex (either single business or many smaler ones) near your power generation facilities is always a good plan to minmse transmission losses and long distance high tension transmission grids are pretty darn expensive.

Powerplants would have ther own step up transformers to 440kV for transmission onto the grid so you would need a step down transformer to go from 440kV to a useable supply. The grid distributes power through a sequence of substations that gradually step down the voltages to useable levels and distributed to an ever enlarging web. Businesses, depending on their power requirements, would have their own step down transformers and would find certain input voltages optimal, so it would be advantageous to be located next to an appropriate sized substation for two reasons.

  1. Minimise supply interuptions as there wil be few points of failure between you and the sustation. Or just go ahead and buy a back up diesel generator, they come in sizes from a few kVA to several thousand kVA (you can bank on shelling out a cool milion for the bigger ones though).

  2. If you are using wads of power and require a dedicated line from the substation to your facility you will have to pay for this, either upfront costs or through increased tarifs, but you certainly won’t get it for free, so closer is cheaper

Almost finally, being up in the hills with a hydro plant would bring additional problems with logistics chains and finding suitable areas of flat land.

And finally, if you are into aluminum smelting then hell yes you want to be sat next to the hydroplant if not actually own your own. If you take a look here you will see they require oodles of DC current.

Having your own generation system that was optimised to producing massive amounts of DC current from a dedicated generator would (and I must admit to going out on a SWAG here) probably be more efficient than taking 440kv AC from the grid and stepping it down to a large DC current.


Very true… from an *overall * efficiency standpoint, it is much better and far cheaper to transfer electrical energy at high voltage and low current vs. low voltage and high current.

But here’s something else to keep in mind: if you were to simply focus on the transmission lines themselves, DC is slightly more efficient than AC. But in either case (DC or AC), the voltage must be boosted/reduced, and it is much easier and more efficient to boost/reduce AC. Hence we use AC.

It makes little difference where your business is loacated with respect to the generating station.
Major distributers in metro areas have a good record for reliability.
Do not locate in rurual areas where distribution is not as reliable.

HVDC High Voltage Direct Current long distance distribution systems are the coming method to carry large amounts of power over long distances.
The DC system originated by Edison in NYC is to be or has recently been shut down due to high maintenance/replacement costs.

I think we’ll see more and more HVDC systems.

Here’s the deal:

Regardless of whether or not you transfer electrical power via AC or DC, the voltage must be high. So which is better: high voltage AC or high voltage DC?

To evaluate this problem, you have to look at the various components in the system, and their efficiency when operating at AC vs. DC. There are two main components in the system:

(a) The conductors (i.e. the high tension lines)

(b) Voltage booster/reducer devices (you need to boost the voltage at the power plant, and reduce the voltage at the customer’s location)

For (a), DC is more efficient than AC. For (b), AC booster/reducer devices have traditionally been a lot more efficient than DC booster/reducer devices.

So who’s the winner? AC or DC?

Yes, DC is more efficient than AC when you’re just looking at the conductors. But the difference is pretty modest. By contrast, AC booster/reducer devices (a.k.a. transformers) have traditionally been a lot more efficient than DC booster/reducer devices. Thus AC wins.

But this could change, as DC booster/reducer devices become more efficient. Once they become as efficient and reliable as transformers, we will see more and more DC systems.

Something else to consider: Let’s say a power company generates electrical power at 60 Hz. One of their customers is located 2000 miles away. The customer has insisted that the power be 50 Hz. In order to do this, the power company must convert the 60 Hz AC to DC, and then convert the DC to 50 Hz AC. The most efficient method is to convert the 60 Hz AC to high voltage DC at the power company, transfer the high voltage DC on the conductors to the customer’s location, and then convert the high voltage DC to 50 Hz AC.

For DC, you have to design the conductors for the voltage across the lines. For AC, you have to design the conductors for the maximum voltage of the sine waves. Plus, you don’t have capacitive losses with DC. You’re going to be able to pump about 40 percent more electricity through the same lines using DC, which I think is more than a modest difference.

DC transmission lines become practical when the savings from the transmission lines becomes greater than the cost of the transformers and converters required. I don’t think DC transformers are ever going to get as cheap as AC transformers. Two coils of wire and a chunk of iron (which is about all there is in most AC transformers) is just hard to beat in terms of simplicity and reliability. I agree with Crafter_Man. We’re going to see more and more HVDC transmission lines as the costs of the transformers and such goes down. While HVDC may be used more and more on transmission lines from one place to another, I don’t think DC is ever going to replace AC for distribution around neighborhoods and such. I don’t think the transformers for DC will ever get cheap enough to replace AC.

I also don’t see us moving to DC generators any time in the near future.

When I was in college, for one of my classes we all had to design (on paper only) a system to power a small cabin out in the woods. There was a stream available which we could dam up, and we were told we had to use a small DC hydroelectric generator and use it to feed batteries in the cabin. We had to search through catalogs and such and design the entire thing out. When we were done, the professor looked it over and decided whether our system would work or not, and we were graded on how well he thought it would work and what the cost was for the system. I don’t know how many practical systems were ever built like this, but I do know it was possible using all off the shelf equipment 20 years ago. Most of the teams in the class came up with a system that cost about $5k to $7k.

Let me give the full context behind my question. It looks like we have a bunch of smart people on this thread. I’m a SW guy and my knowledge of EE does not go much beyond replacing a light bulb.

The company I used to work for built computer servers. One of the big issues was power consumption and heat dissipation. A bunch of servers could be placed in a small area, but data centers were not equipped to supply the required level of power per sq ft and did not have enough AC. So basically, high density servers were a bust.

One of the issues with power and heat was the inefficiencies of converting AC to low voltage DC for use by the computer. The problem was compounded by the fact that the servers required multiple different DC voltages: sometimes as many as 8! This meant that there were a bunch of “converters” (not sure what the right word is), each of which was creating heat and wasting power.

My question is, what happens if you place a server farm near a power generating facility? Could you do something that would eliminate all this waste by running the right kind of power durectly to the facility.

What prompts this question now, is the fact that Google is building a huge computing facility near The Dalles Oregon, which is near the Bonneville Dam. I am assuming that access to cooling water from the Columbia river, cheap land, and cheap power rates are some of the reasons behind this. The rights of way for the power lines may also provide a way to run data lines. I was wondering if there was also another reason to locate in The Dalles, such as the ability to run DC to the facility.

Is the difference really that huge? I thought it was on the order of 5% to 10%. If you’re correct, I agree… it’s certainly not a modest difference.

AC power lines have to be designed to handle the peak voltage. DC always runs at the peak voltage, so it’s average is always sqrt(2) times the average of what you are going to get for AC. Sqrt(2) = 1.41 or a gain of 41 percent. The maximum average current through the line is going to be the same for AC and DC since it’s the average that determines how much you can heat up the wires. Therefore, the total power through the line is also 41 percent higher for DC.

It’s actually a bit more since there aren’t any capacitive losses through the line on DC, but I don’t remember how much capacitance there is for a typical AC line off the top of my head.

No. The low voltages needed by the servers can’t reasonably be transmitted more than a few feet. The very low voltage used by the processor itself, is for example “generated” (down converted actually) just an inch or two from the chip itself.

The power needs to be transmitted at relitivly high voltage, and down converted near the point of consumption. As others have correctly pointed out, that is best done with AC. There is little (if any) to be gained by providing DC input to your power supplies vs AC, and indeed some of the rack fans etc might require the AC.

The root of your problem is that computer power supplies are considered a commodity item. The only competition is who can make one cheaper. The companies that make efficient, high reliability, industrial power supplies (Kepco, lamda, etc) don’t play in that market.

I worked about 10 years for a company that made turn-key systems based on Wintel platforms. The power supplies were far and away the highest failure rate items, and nobody made an “industrial strength” version.

Tucker fan said: “AC generators are more efficient than DC.”

This is not true if you only need a little DC power. A brushed commutator is lower loss than diodes. The problem comes when you try to scale up the machine: The brushes don’t like high current, and if you go to high voltage then the commutator segments start arcing. The commutator is also a high maintainance widget.
One solution might be to cool your server racks directly, rather than cooling the room, and using the air in the room to cool the racks. That would mean sealing them up, and putting fan coil units inside the racks. You could run either refrigerant, or chilled water to the fan-coils. Chilled water would be more maintainance friendly, but refrigerant would be more efficient…if your building used a chiller for the HVAC though, then that would be the obvious way to go.

Uh… a dog?

It wasn’t really spelled out in that article, but Edison actually made sure that the press used the term “westinghoused” to describe people who had just been electrocuted by the electric chair.

And, don’t forget the elephant, because sometimes dogs and horses (and the occasional human) just aren’t quite enough:

By the way, getting back to DanBlather’s original problem, the main heat generator in computers is the processor, not the power supply. Intel seems to be recognizing this, and is moving towards cooler processors for future production. There are also companies like Transmeta who make ultra cool processors so that you can make high density server farms without melting the cieling tiles.

As for the benefit of placing your data farm next to a generator, it isn’t necessary. The advantage of AC power distribution is that the power company can get a heck of a lot of power to you fairly easily with very little loss. The main limitation is not usually getting the power to you, it’s getting the power inside and around the building. At my company’s old location they actually had to come and install a bigger transformer just to handle the load from our computers. The size of the transformer and the size of the wires running around your building are usually the biggest limitation.

Actually, the company I work for makes industrial computers (computers that have to run in manufacturing plants and control things) and there are quite a few manufacturers of high reliability “industrial strength” power supplies. In some of our computers we even have hot swappable redundant power supplies, so if one goes poof the other one takes over and the power supply starts beeping. You just walk over, pop out the broken one, and put in a replacement, all without even powering off the computer.

There are plenty of great PSU’s out there, but good luck convincing a customer that they need a $90 PSU while a $299 dell system commercial is running through their heads is futile.

Had never really considered that a DC system can be at peak voltage 100% of the time. All else being equal, this translates into less current. I did some calculations in Excel, and you’re right… the improvement in efficiency is huge. Looks like I learned something here. :slight_smile:

What’s inside a DC transformer (I didn’t know there was such a thing) and how do they work?

An old fashioned DC transformer is just a DC motor driving a DC generator. Not the most efficient thing in the world, but it works.

Modern DC transformers use semiconductors. For example, you can turn your DC output on and off really fast, then filter the output through a capacitor. How much you vary the “on” time and the “off” time will determine the output voltage, since the capacitor will filter off the ons and offs and you’ll end up with an “average” of the output. Google “pulse width modulation” if you want more details of this sort of thing.

You can also basically convert your DC to AC, use a regular transformer, then take the output of the transformer and convert it to DC again.

There are a lot of other circuits you can use as well. This site should give you some ideas:
(warning - assumes a bit of electrical knowledge)

Basically, no matter what you do, it’s going to be a heck of a lot more complicated than an AC transformer.