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Surplus electric power in grids

  1. Feb 16, 2012 #1
    I start by apologizing if my question is a stupid one. Nowadays there are many concerns with the surplus electric power that can go on the grid from wind farms in their peak productions often not coinciding to a correspondent demand, and how to manage them, shutting down power stations or having to use methods of storage as upstream pumping of water in hidroelectric stations.
    My question is what can this extra input in the grid cause? What happens to momentary excess power in the grid, when supply exceeds demand, that did not correspond to the expected?
    Thank you
     
  2. jcsd
  3. Feb 17, 2012 #2

    gerbi

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    Frequency and voltage in the system goes up
     
  4. Feb 17, 2012 #3
    And that can`t be managed?
     
  5. Feb 17, 2012 #4

    gerbi

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    It is managed all the time. All the time some loads are disconnected and other are being connected to the grid so You have some rapid changes in grid load. It is handled by central automatic regulation system.
    All big generating units are equipped with regulators (to regulate real /quanity of steam sent on turbine/ and reactive power /changing excitation current/) to mantain frequency and voltage in the system. It will change however, but those changes are small.

    The problem is with the small sources, and especially when there are large number of them (like large number of wind turbines). Then the regulation system have a tough job. The power produced by wind farm can change rapidly - then other sources must be aviable to maintain system main parameters (frequency and voltage).
     
  6. Feb 17, 2012 #5

    vk6kro

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    Yes, it can be managed. It happens automatically.

    Power stations face varying demand all the time and yet have to maintain a constant voltage and frequency.

    It is done by use of an automatic throttle control (like a "gas" pedal in a car) that just reduces the fuel being used by the engine that drives the large generators.
    So, the power station uses less fuel and normal voltage and frequency are supplied.

    Note that the generators in a power station are huge rotating machines and it may take many seconds to react to sudden changes in load. There are circuit breakers which can cut the power if a sudden load is placed on the generators and which they may not be able to react to in time.
     
  7. Feb 17, 2012 #6

    gerbi

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    There is some energy stored in rotating masses of all turbines and generators in the system so quick and not enormous changes of load won't lead to system collapse. Then the automatic regulation system comes to work (few seconds) to maintain parameters of energy.

    Circuit breaker usually comes to work in critical situations. There is almost no risk regulator wil not react in time - sometimes it simply cannot react properly at all.
     
    Last edited: Feb 17, 2012
  8. Feb 17, 2012 #7
    Thank you both for your replies!
    What I am thinking is if there is a sudden increase in wind power, and for some reason it's not viable to reduce the power being supplied by other sources (nuclear for example) can`t it just be disconnected from the grid? Why is that a problem, wind is free...
    And if there is an overload in the grid what would be the consequences?
    Regards
     
  9. Feb 17, 2012 #8

    jim hardy

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    It takes hours to start a fossil boiler, days to start a Nuke.

    The short term variability of wind power is causing headaches for the people who need to plan ahead just which plants are to be run when.

    An immediate change of ~10% load won't upset a plant
    but a big wind installation of say a few hundred megawatts needs to have its generation planned for so the other generators in the region can accomodate it.

    There's a budding science of micro-wind meteorology, predicting wind at windmill sites a few hours in advance so that utility grid operators can plan ahead at least that far.

    An overload on the grid results in customers being disconnected, or sometimes large blackouts.

    Peruse AWEA's site...AWEA.org
     
  10. Feb 17, 2012 #9

    vk6kro

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    Excessive power is not a problem because the power used by a system depends on the load. It is only potential power until it has a load on it and current flows.

    What can happen is that the voltage may go a little high until it is corrected by regulating circuits.

    Solar panel inverters have to have circuitry to stay exactly in phase with the mains input and to avoid driving the mains voltage too high locally.

    They also have to switch off if the mains switches off.
    While this seems odd, it is to avoid your solar panels trying to supply power to your neighbours during a power failure and possibly doing a bad job of it and giving them reduced voltage.
    This may operate their lighting to some extent but it could damage the motor in their refrigerator.
     
  11. Feb 17, 2012 #10

    AlephZero

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    There are situations where wind farms have to be disconnected form the grid because they are generating too much power in high winds. This has been a problem in Scotland, which has lots of potential for wind and wave power generation but not much demand. Building of wind farms has outrun the capacity of the grid to transport the peak power output to England where there is a demand for it.

    It is a lot more practical (and quicker) to disconnect a 3 MW wind turbine and "feather" the blades so it isn't acting as a windmill, than to mess about with the output power from a 3000 MW "base load" nuclear power station. Obviously the wind farm operators don't like having to do this, but in the end it's an economic decision - it may not be worth the cost of upgrading the grid to handle the peak wind power that would only be generated for a few days of each year.
     
  12. Feb 17, 2012 #11

    nsaspook

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    Wind-power shutdowns happened in Oregon last year due to excess power generation with no place to send it. During the spring melt period there is low demand in the Northwest and in Southern California via the pacific intertie. We can't just spill the water and turnoff the base load generators because 1. The grid would become unstable. 2. It would harm the protected fish in the river. 3. Wind-power cost more.

    http://ecotrope.opb.org/2011/05/bpa-shuts-off-wind-power-to-make-way-for-hydro/

    Oregon was giving away free power but nobody wanted it. It's a big mess up here.
    http://www.oregonlive.com/business/index.ssf/2011/07/the_bonneville_power_administr.html

     
  13. Feb 18, 2012 #12
    I have a simple question. In 1935, how did producers of electricity generate something with a sure enough frequency to operate electric clocks and sure enough voltage to stay within the absolute +/-10 % limits seen at the end of the line.

    Give you a clue, not with "electronic regulators."

    There's so many generators online that no one of them makes a huge difference. Older machines used speed governers when coming online and the combination of steam flow and field strength to adjust the output, contribute to the system frequency, and power factor. Even in the nineties, I would compare oscillators against the line and see it speeding up and slowing down as the frequency varied.

    The Russians tried running long lines with this method from a remote plant a suffered oscillation and "bumping" due to the impedance between the stations.

    As for extra generation, turn away from the wind, turn the blades, use a brake (an old method used in generators of the 80's), disengage...

    I think it would be cool to see the US use the excess capacity for industries that can utilize varying availability - such as copper refinement.
     
  14. Feb 18, 2012 #13

    nsaspook

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    Generator frequency slipping is the common method used to balance power between stations. With very long lines the amount of reactive power in the transmission lines makes AC power distribution unstable with rapidly changing load but using High Voltage DC line feeds isolates the frequency matching requirements to just a local AC grid and because the DC grid loss is resistive this can make power control easier.

    I've seen some research in using SMES to store power but it's just not practical yet.

    http://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage
     
  15. Feb 18, 2012 #14
    Thank you for all the answers!
    If an increase in grid load is not avoided what can happen, is it dangerous, or the extra power is just grounded?
    How do you manage the frequency, and how does it relate to voltage/load in the grid?
    Regards
     
  16. Feb 18, 2012 #15
    Sure enough, DC lines have been fairly common in Europe. They represent a tremendious capital expenditure, but are best for long distances.
    There has been much research about tapping into the abundant energy resources of Iceland via an underwater cable system. Such system would be comprised of a coaxial line and pipe and likely pressurized with SF6. It would bear the current as DC.
     
  17. Feb 18, 2012 #16

    nsaspook

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    Last edited: Feb 18, 2012
  18. Feb 19, 2012 #17
    I shall read those articles. Thank you!
     
  19. Feb 20, 2012 #18
    If an overload is not avoided, what can really happen?
     
  20. Feb 20, 2012 #19

    vk6kro

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    This implies that because power can be generated, it has to go somewhere. This just isn't true.

    Take a simple example.

    If you have a large car battery capable of delivering 600 amps (that is 7200 watts), is it safe to put a 1000 ohm quarter watt resistor across it?
    Would the battery blow up with all that power inside it? Will the resistor be vapourized in an instant?

    No, the battery will be fine and so will the resistor.

    There will be a current of about 12 mA flowing in the resistor and it will not even get warm with only 144 mW being dissipated in it.

    It is the same with the power grid. It only delivers power if the load takes power from it. There is no power "bottled up" in the generators trying to get out.

    Each component connected to it is arranged with its own protection so that if there was no load or too much load, the equipment would not be damaged.

    For example, a wind generator may tend to spin too fast if it has no load on it. So, it can be turned away from the wind or the blades might be set to present no surface area to the wind.

    The equipment is never sacrificed to supply a demand. If there is too much load, power can just be turned off to consumers, possibly only in a few suburbs, preferably the ones that didn't vote for the current government.

    Large power stations cannot be just switched off, but they continue to generate the standard voltage whether or not much current is taken from that supply.
    As the load increases, it gets harder to turn the generators, so more drive power is used.
     
  21. Feb 20, 2012 #20
    vk6kro, thank you for this answer! The analogy with the battery is great, I wasn`t thinking about that.
    Some clarifications:
    "It is the same with the power grid. It only delivers power if the load takes power from it. There is no power "bottled up" in the generators trying to get out.

    Each component connected to it is arranged with its own protection so that if there was no load or too much load, the equipment would not be damaged."
    But and quoting a previous post your yours - "There are circuit breakers which can cut the power if a sudden load is placed on the generators and which they may not be able to react to in time.", and assuming that such circuit breakers were not present, and also assuming that that reaction time was very long, the extra load would act as a brake in the generator? What else would happen, like damages in the system?
    The question would also apply for your example of wind turbines, if they were not able to be "turned away from the wind or the blades might be set to present no surface area to the wind".
    "The equipment is never sacrificed to supply a demand. If there is too much load, power can just be turned off to consumers", why would the power be turned off to consumers in excess load? It means you would have to switch of all power generators to lower the load? I am confused.
    Also, and since you seem so knowledgeable in this matter, I am going to take some more advantage of that, if you don`t mind. I posted in another threat some questions that nobody is answering in a way that enables me be clear about, so here they are for you, if you have the extra patience:
    Utility poles have a grounded wire. Is this just for lightening? Is not connected the neither one of the 3 phase wires, is it? Is the static electricity wire always present, and is it grounded (at every pole, or every x poles)? What does the multi-grounded neutral wire do, and has it always have to be present? The 3 phase wires are only grounded at destination when electricity is being consumed?
    If domestic consumers receive just one phase current, why there are 3 wires coming from the pole to the house?
    Is ground always the return path for electrons? Is there a ground connection at every house directly or at the nearest utility pole?
    In power plants the wires in the generator are connected to the ground, to close the circuit (assuming the return path for electrons is the ground?

    Thank you very much!
    Regards
     
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