Electromagnets

[John Connor]

How are electromagnets in power station generators powered? Is it from batteries or from the same source? I can't imagine it to be from the same source because how would you have started to produce power?

The reason I ask is because I'm watching a show on H2 called Life After People and in the beginning of the show they talked about how power would be the first to go because 70% of the nations power is produced from fosil fuels. And without humans to feed the burners to produce power the system comes crashing down. This I can see, but I said 'what about Hoover Dam?' As soon as I said that that is the exact thing they talked about. The show had a guy on who works at Hoover dam and he said that if he and all the employes were to leave the plant would still produce power weeks, months and years latter. But I question how the electromagnets in the generator shaft are powered? I did some Googleing and found out that indeed the Hoover dam uses electromagnets, but how are they powered?

I thought it was interesting that when the show talked about nuclear power plants that in 2 days the plants would just shut down.

 

[Matt1970]

N/M

 

[PottedMeat]

At the top of the main generator rotor is a small exciter generator. It generates the current needed to energize the main rotor windings.

...what they use to power the exciter and controls i can't seem to find, though it could be easily be done with a small diesel generator or something similar.

Page 7 Conventional System (though i think they've upgraded to some digital brushed system)
http://www.whitbyhydro.on.ca/pdf/5.2.C_GenBasics.pdf

 

[Micrornd]

Simplified, but -

It all works the same as an engine driven Arc welder.

A small exciter generator on the end of a common shaft with the main generator is driven and produces enough exciting current for the fields of the main generator to begin producing power, and then power from the output of the main generator is used to provide regulated exciting current to main generators fields.

The small exciter generator is permanent magnet based, similar to generators in older autos, so as long as it's shaft turns, it produces current.

Pretty much self-sustaining as long as something turns the shaft at a speed fast enough to produce the current needed for the output required.

 

[Mark R]

Probably powered off the main power plant substation, like all the other equipment at the plant.

The substation, as it is grid connected, would stay energised if the main alternator tripped out, allowing plant controls/pumps/conveyors/etc. to keep running.

In general, most power plants will need a stable grid supply to be able to start up - this is especially the case with things like nuclear and coal, which have complex control systems, heavy pumps, heavy conveyors, etc.

A big coal plant may need 100 MW of power to start up. You can't realistically get that from on-site diesels. It has to come from the grid.

Hydro is different. A modest diesel generator should be able to power the alternator exciters, controls and the hydraulics. The ability of a power plant to be able to self start this way is sometimes termed "black start". This tends to be restricted to hydro and natural gas plants, as they don't need huge amounts of heavy machinery (or a stable grid connection) to start.

 

[SecurityTheatre]

Hydro is different. A modest diesel generator should be able to power the alternator exciters, controls and the hydraulics. The ability of a power plant to be able to self start this way is sometimes termed "black start". This tends to be restricted to hydro and natural gas plants, as they don't need huge amounts of heavy machinery (or a stable grid connection) to start.

I don't see any reason to need a diesel generator to start up a hydro turbine. In a hydro plant, the shaft is spinning due to hydro power and a small generator based on permanent magnets can produce electricity from that, which can then be used to energize the electromagnets of the turbine.

 

[PowerEngineer]

As described above, older generators usually received the DC power for their rotor's electromagnet from a smaller generator attached to the end of their shafts. The control windings on these smaller generators allow the plant operators to change the level of "excitation" on the rotor and thereby the voltage at the generator's output terminals.

With the advent of silicon rectifiers, excitation for newer generators is provided by rectifying three-phase "station service" (a term for the internal plant electrical system used to run ancillary equipment and usually fed from the terminals of the generator or a back-up source). These rectifier-based excitation systems respond faster to system disturbances that the older ones.

In order for the generators to keep running, there has to be a place for their electrical output to go (i.e. to be consumed). It'd be pretty unlikely that the uncontrolled collapse of the electrical grid would leave many generators running with the right amount of load for stable operation.

 

[SecurityTheatre]

As described above, older generators usually received the DC power for their rotor's electromagnet from a smaller generator attached to the end of their shafts. The control windings on these smaller generators allow the plant operators to change the level of "excitation" on the rotor and thereby the voltage at the generator's output terminals.

With the advent of silicon rectifiers, excitation for newer generators is provided by rectifying three-phase "station service" (a term for the internal plant electrical system used to run ancillary equipment and usually fed from the terminals of the generator or a back-up source). These rectifier-based excitation systems respond faster to system disturbances that the older ones.

In order for the generators to keep running, there has to be a place for their electrical output to go (i.e. to be consumed). It'd be pretty unlikely that the uncontrolled collapse of the electrical grid would leave many generators running with the right amount of load for stable operation.

That's a really interesting point. There have been a number of chain-reaction events on grids that cause region-wide outages, such as those in the North East and a recent one in the San Diego area (2011, IIRC).

I recall they are pretty difficult to recover from because the load and generation has to be balanced, requiring bringing up one generator at a time on a small grid and gradually growing it while bringing new generation online.

Things like coal and nuke plants, once they go into an emergency shutdown from a loss of load, takes DAYS to spin back up.

But, are you saying that modern hydro plants would suffer from the same fate, or do they have the internal capability to power up without input from other plants, in the event they are isolated, but still have a load to service?