Why do power plants need electricity to start up?

[John Connor]

As the title says. Why is that? The only thing I can think of is resistance in the line?

 

[sandorski]

All the machinery required to control it. I suspect anyway.

 

[John Connor]

Hmmm, that's a good guess, but can't they just run an industrial diesel generator prior to start up?

When I was reading about the threat of an EMP I read that it would take weeks for power plants to go back online.

 

[Tsavo]

Hmmm, that's a good guess, but can't they just run an industrial diesel generator prior to start up?

Wouldn't that be classified under something that provides electricity?

 

[John Connor]

Well, yeah... It's not a city capable generator though.

 

[Paul98]

I would guess because they already have the power lines coming into the power plant.

 

[SecurityTheatre]

As the title says. Why is that? The only thing I can think of is resistance in the line?

Most power plants use turbines that rely entirely on electromagnets.

Your small electric motor or DC generator uses a permanent magnet (sometimes a ferrite, or if necessary, expensive NiB "rare earth" types).

In order to make them reasonably cost efficient and sufficiently strong, however, industrial turbines can't use a permanent magnet.

This means that they must spin up the current in the wires before the turbine will generate any electricity.

Some older turbines had smaller permanent magnetic "startup generators" for this purpose (especially old hydro plants), but they slightly decrease the overall efficiency of the system and are more prone to maintenance issues, so they aren't that common anymore.

In addition, power plants (other than geo/hydro/solar) also require a fuel to be delivered, coolant to be pumped, water to be pumped, etc.

Even a LNG (natural gas) plant has to pump in compressed gas, supply water for the turbines, engage the control and monitoring systems, etc.

Some plants DO have backup generators to facilitate a cold start, but some don't. I think it's just a cost and complexity issue.

 

[SecurityTheatre]

Hmmm, that's a good guess, but can't they just run an industrial diesel generator prior to start up?

When I was reading about the threat of an EMP I read that it would take weeks for power plants to go back online.

Some types of plants need to build up slowly. Nukes, for example, can't (safely) just be switched on in an hour. It takes days of control checks and gradually ramping up the power.

The region-wide outages experienced over the last few years (the most recent i know if was in Southern California and Baja in 2011) demonstrated that balancing the power draw against the supply is also important and very difficult to do.

In San Diego, they tried to spin up a few plants too quickly without correctly balanced loads and ended up tripping safetys that required them to power back down. The essential element here is that the plants must be carefully regulated to match output with draw, or the frequency of the system drops. This is a non trivial (frankly, very difficult) thing to manage. If the whole region is TRYING to draw power, but you are completely offline, you have to break the grid up into small bits and power them on one at a time, carefully matching input to output. Making a mistake will cause a collapse and may lead to a chain reaction that downs the whole system all over again.

Power generation is quite a bit different than just hitting the "on" switch.

 

[Mark R]

Hmmm, that's a good guess, but can't they just run an industrial diesel generator prior to start up?

When I was reading about the threat of an EMP I read that it would take weeks for power plants to go back online.

They could, but the electricity demands can be substantial.

A large coal power plant (say 2000 MW) can have an internal electrical load of about 100 MW. That's way beyond diesel generator size, and is a respectable sized power station in itself.

Remember, you've got conveyors moving many tonnes of coal a minute lifting it up large heights. Coal crushers which grind thousands of pounds of coal a second to a fine dust. 10,000 horsepower water pumps to supply water to the boilers, heaters to get the boiler up to temperature so that the coal can ignite, 10,000 hp compressors for pressurising the combustion chamber, etc.

A nuke plant has several diesel generators, but these only power the bare minimum safety systems, yet they typically use 10,000 hp gensets, which are just about enough to run the reactor emergency cooling pumps, rather than be able to power all operational systems.

It then comes down to cost. Is it necessary to have so much diesel generation on site, just to be able to "black start" a power station? In general, it's not. If you've got a grid supply, it's much easier to use this, and only maintain "black start" capability at a few select plants (often those where the startup demands are minimal - hydro and nat gas), and then use a sequenced start.

A battery is used to start a diesel genset, which is able to operate the hydraulics on a hydro plant. The water turns the turbine and starts the main generator, Once up to speed, the generator is connected to the grid. A nearby nat gas plant connects to the grid and using grid power it starts its turbines, and synchronises the generator with the grid. Once connected, a coal power station can start its conveyors, crushers and compressors and start warming its boiler. Once up to temperature it starts its turbines and synchronises with the grid. Now the grid is strong enough to start spooling up lots more power plants, and start reconnecting loads.

 

[Gillbot]

There are some plants that can do a "black start" but to generate power they need steam into the turbine and that's not available at the flick of a switch.

Generating plants using steam turbines require station service power of up to 10% of their capacity for boiler feedwater pumps, boiler forced-draft combustion air blowers, and for fuel preparation. It is uneconomical to provide such a large standby capacity at each station, so black-start power must be provided over designated tie lines from another station. Often hydroelectric power plants are designated as the black-start sources to restore network interconnections. A hydroelectric station needs very little initial power to start (just enough to open the intake gates and the excitation of the generator field coils), and can put a large block of power on line very quickly to allow start-up of fossil-fueled or nuclear stations. Certain types of combustion turbine can be configured for black start, providing another option in places without suitable hydroelectric plants.

 

[John Connor]

Isn't excitation of the generator field coils provided by an excitator its self? You would think it would be dangreous to rely on external power to provide excitation of the generator field coils.

I've seen old power plant photos and they do have a separate excitator. I just wonder how Edison got his plant on line for the first time.

 

[PowerEngineer]

I'll add that electric utilities usually have restoration (i.e. black start) plans to follow in the event of a system collapse. These plans will identify one or more "black start" units that can restart themselves without outside power. These units are used to reenergize the transmission lines and back feed power to other generating plants (that can't restart without outside power). As others have pointed out, the power required to run the necessary equipment in a steam generating plant is generally too much for any reasonably sized on-site emergency generators. Hydro and simple-cycle gas turbines are most often equipped for "black start" capability because of their much more modest start-up power requirements.

Most all generators of any size use what amount to electromagnets to provide the magnetic fields for their rotors. Not only is it more economical (than permanent magnets), it provides the ability to control the strength of the magnetic field and thereby the voltage produced at the generator terminals. The strength of the field needed to maintain the desired voltage increases as the power output of the generator rises.

The older generators use exciters which are really themselves just small generators (connected on the same shaft as the generator itself) where their output is rectified to become the DC voltage applied to the generator rotor. The output of the exciter is in turn determined by the voltage applied to its rotor (you can think of the exciter as an amplifier). The exciter field voltage might itself be the output of an amplidyne (another amplifier). The output of the amplidyne was controlled by a low voltage (analog in the old days) voltage regulator.

The advent of silicon-based power electronics have made it much cheaper and reliable to use the rectified output of the generator itself (or back fed from the electrical system) to provide the DC voltage for the generator's rotor. It's not really "dangerous".

P.S. -- Edison's first plants were DC rather than AC, and therefore significantly different.

 

[dkozloski]

The local power utility has the largest electric storage battery in the world for back-up when the grid goes down.
http://www.google.com/url?sa=t&rct=...iX7aqbEKup56j-g&bvm=bv.58187178,d.cGU&cad=rja

 

[WHAMPOM]

As the title says. Why is that? The only thing I can think of is resistance in the line?

To energize/excite the magnetic field in the generator field windings.

 

[John Connor]

To energize/excite the magnetic field in the generator field windings.

A diesel generator could do that. But as already mentioud it has more to do with the equipment to go online like conveyors, etc.

 

[hextet]

Well in power plants they have machines that run on electricity, if they didn't have an initial source of electricity, how would they be able to start!

 

[Franz316]

I've wondered about wind turbines. If we want to eliminate the use of rare earth magnets in them, we should use electromagnets instead. Once the wind turbine began spinning would it possible to siphon off a small portion of the electricity to power the electromagnets?(seems unlikely since no electricity would be generated). Or would it have to come from an outside source, such as a battery. That would just be for the initial start up phase, much like an alternator. It feels like the amount of control yielded by electromagnets would really work well with wind turbines given the variability of the wind.

Anyone know if any non permanent magnet turbines exist?

Also, it appears that photovoltaic is by far the most straight forward form of electricity generation requiring no outside assistance other than the sun.

 

[John Connor]

I think this is a patent for one. http://www.google.com/patents/US7425771

 

[imagoon]

I've wondered about wind turbines. If we want to eliminate the use of rare earth magnets in them, we should use electromagnets instead. Once the wind turbine began spinning would it possible to siphon off a small portion of the electricity to power the electromagnets?(seems unlikely since no electricity would be generated). Or would it have to come from an outside source, such as a battery. That would just be for the initial start up phase, much like an alternator. It feels like the amount of control yielded by electromagnets would really work well with wind turbines given the variability of the wind.

Anyone know if any non permanent magnet turbines exist?

Also, it appears that photovoltaic is by far the most straight forward form of electricity generation requiring no outside assistance other than the sun.

You can use a standard exciter on a wind turbine. A small permanent magnet generates enough DC to excite the electromagnets to start the generator. From that point you can siphon the power from the output. I thought the main issue with wind was the variable speeds and bursty nature makes it less useful because a certain speed has to be attained to generate enough power to excite the coils. This is a common "issue" with rotary exciters anyway. Static exciters use an external source to start up. The issue with photovoltaic should be pretty obvious though. You typically need to aim them to generate enough power to be useful. They also don't work at night and might not generate enough power start the exciter when cloudy etc.