Question to all you EEs: Very large inverters and frequency correction

I would like to learn more about grid frequency regulation logistics; I figured there are EEs on the board that might know enough to educate me:

How is fast response frequency regulation typically achieved when non-traditional means are employed (ie: not pumped hydro or gas turbines with synchronous generators)? Are solid state electronics used, like SCRs in a large motor VFD? If the storage medium is DC, such as lithium titanate batteries, what kinds of inverters are used? How much of this is typically off the shelf components? Does anyone make 1MW+ inverters with variable frequency output? The more specific the better.



  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    Re: Question to all you EEs: Very large inverters and frequency correction

    I not on a computer at the moment... But try google search for DC Transmission Lines.

    From there you can find more about large inverters and frequency control of regional and national power grids on Wikipedia.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,481 ✭✭✭✭
    Re: Question to all you EEs: Very large inverters and frequency correction

    Different power sources have different rates of 'throttle' control.

    A natural gas turbine can be adjusted pretty quickly. A nuclear reactor is the slowest to respond to more or less output power demand. Steam boiler carries on for a while even if heat source is removed. Nuclear boils water so there is double trouble if there is a sudden release of electrical load. That is why nuclear plants have massive cooling facilities to take excess heat away if their steam driven alternators go off line for some reason. It takes a long time to cool down the nuclear reactor core.

    As spec'd on grid-tie inverters, the spec for grid in U.S. is -0.7 Hz to +0.5 Hz around 60 Hz nominal. It is usually much better then this.

    An alternator, whether hydro, steam, or turbine gas driven, is brought on-line by first regulating rpm's to produce close to 60 Hz. Water flow rate, steam pressure, or turbine jet engine is throttled to get rpm's close to required. When phase aligns to grid the switch is thrown to put alternator on-line. At this initial point the alternator is 'neutral' on the grid, neither pushing or pulling any significant power to grid.

    Once on-line, the alternator power source can 'put the pedal to the metal' to push current into grid. No single alternator can make much impact to grid frequency. If a number of alternators push excess power to grid it can raise the grid frequency slightly. When people get home from work, turn on air conditioners and electric stoves, the load increases significantly and can slow down the grid. The power companies are always playing a game to anticipate grid power requirements based on statistical history and 'stoke the coals' before demand causes a drop in grid frequency or power down just as load lightens.

    The power companies keeps track of overages and underages in grid frequency and will intentionally overrun or underrun to bring average back to exactly 60 Hz. This keeps all the clocks that rely on 60 Hz for timebase accurate over the long run.

    There are three main power grids in the continental U.S. These can be further broken down (de-linked) to up to ten sub-regions if necessary. Electric motor to alternators are used to transfer power between non-syncronous power grids. Some of the transfer points are high voltage D.C.

    The U.S. system is generally old and some say a 'house of cards' just on the verge of collapse. It is a major decision for a sub-grid section to release from the larger grid as it is complicated and time consuming to reconnect. But waiting too long to release from main grid can cause major damage to the sub-grid equipment if there is a major failure within the larger system.

    The rate of frequency change is very slow due to load changes on the system, at most a very small fraction of a hertz per minute.

    To your final questions, many D.C. to A.C. sinewave inverters use pulse width modulation at high frequency to develop the 60 Hz sinewave. You can make a mega-watt inverter, just takes some big components or a lot of smaller parallel components. In theory, you could be take about one hundred and seventy Xantrex XW6048's selling into the grid to produce a mega-watt of power.

    There are larger units available commercially. Some emergency backup power generators have inverters to create uninterrupted electric. The inverter is syncronized to grid. When grid power goes out the inverter takes over the load until a diesel generator is started and brought up to stable speed. The inverter then slowly slews the phase to match to diesel generator then lets the generator takeover. When grid comes back on the inverter again is used to transfer the freq/phase from generator to grid. The inverters are fed from batteries or a D.C. generator flywheel.
  • FreeWattsFreeWatts Registered Users Posts: 14
    Re: Question to all you EEs: Very large inverters and frequency correction

    I learned a lot by that post
    thanks RCinFLA
  • MangasMangas Solar Expert Posts: 547 ✭✭✭✭
    Re: Question to all you EEs: Very large inverters and frequency correction

    Learning too RcC.

    Thank you.
    Ranch Off Grid System & Custom Home: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, ICF Custom House Construction, all appliances, Central A/C, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Propane furnaces, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
  • arcturusk1arcturusk1 Solar Expert Posts: 26
    Re: Question to all you EEs: Very large inverters and frequency correction

    Great post RC; thanks for the information. I learned something new today!
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