Grid control or Generator Control!!

Discussion in 'General Electronics Chat' started by sheknowsjoy, Apr 2, 2009.

  1. sheknowsjoy

    Thread Starter New Member

    Oct 17, 2007
    When a generator is synchronised with the grid does the generator or for that matter the entire power plant loses it control over Generation?

    I think it should, coz since electricity cannot be stored generator must supply what the grid demands.

    If yes what does AVR and turbine governor control do?
  2. leftyretro

    Active Member

    Nov 25, 2008
    Once a site's generator is synchronised to the grid, the grid is in effect controlling the frequency of power on the grid. If a specific generator leads or lags past a small amount, large currents will flow and safety protection relays will isolate or trip the generator(s) from the grid.

    Generator controls are use to start-up and shutdown stages and enable the proper synchronization to the grid. Once that is accomplished the generator controls are used to monitor and measure power being exported and of course the protection and isolation tasks already mentioned.
  3. KL7AJ

    Senior Member

    Nov 4, 2008
    Actually, the energy that can be stored in the grid can be tremendous....generally in the inductance and capacitance of long transmission lines. The big power outage in New York in the 1960's was actually caused by transmission line currents "sloshing around" after a large load was inadvertently disconnected. Standing waves represent REAL energy!

  4. recca02

    Senior Member

    Apr 2, 2007
    Hi sheknow,

    I concur with lefty, The grid frequency gives an idea about whether the total generation matches the demand, paralleling a generator would tend to raise the grids frequency, loads can be said to be constant*. My guess is if a new generator is paralleled the load gets distributed evenly (incase AVRs and Governors are not used) and thus a partial load is put on the generator. If it is not designed to handle this load or the power input to the TG set is not adequate the TG set will trip.

    The role of AVRs and Generators comes in that they try to stabilize the TG set over a range of grid conditions. I suppose you know/recall that the mode of governor is kept different(speed,load,pressure) at different stages (for start up its in speed mode IIRC). For an underspeed the governor will increase the steam input. The AVR will increase the voltage if voltage dip is sensed. If the TG set is still unable to keep up with the load or voltage dip then the set will trip.

    Hope that helps.
    Last edited: Apr 3, 2009
  5. sheknowsjoy

    Thread Starter New Member

    Oct 17, 2007
    From the discussions above and my understanding, can I put it this way-

    1. The generation of a power plant is entirely dependant on the grid requirement.
    If the Grid demands more power, the power plant/generator will have to provide the excess or vice versa if the grid requirement diminishes the generation will subside. We do not have control over this.
    2. Avr during these conditions maintains the voltage by increasing or decreasing the generator excitation.Governor, accordingingly increases or decreases the steam flow to maintain frequency by controlling the speed of turbine.
    3. The only control (i think this cannot be called as control) the power plant as a whole has, is to trip the generator when the grid tries to draw more power than the generator can produce.

    The speed, load and pressure control that you refer in effect carry out point number 2 above. Suppposingly we do not have the TRIP control, but have all other components (AVR, Governor, etc.), then even a 210MW generator will try to provide the entire deficit of power in the grid ( 2000-3000MW ) and destroy itself in the process and AVR and the governor will help the generator in destroying itself.
    The reason why then, the generator doesn't trip frequently is the Load Despatch Centre of the region continously matches the generation and demand. How it does this i can't figure it out. If you have some data regarding this kindly forward.

    Also do tell me whether my line ot thinking above is correct.
    Thanks for responding.
  6. eblc1388

    Senior Member

    Nov 28, 2008
    No. The power plant(Station) does not depend on the grid. It can either produce real power, or reactive power or a mix of them and shutdown as the control engineer see fit.

    No. The control engineer of a power generator has full control over this situation. A generator's output can stay constant during such grid loading changes. Only generators that the control engineer(s) allow to react to changing grid loadings will pick up or drop load.

    AVR miantains certain generator voltage such the generator can operate at a certain power factor. Or AVR can control how much reactive power the generator is pumping out or absorbing. Never the real power that the generator produces.

    One can simply say steam flow control how much real power output and AVR determines what reactive power output. The speed of the generator is always in sync with the grid normally.

    As I told you before, a generator's output can be fully controlled even when it has synchronised to the grid.

    The speed of the power turbine(which is the same as the rotor of the generator rotor) is fixed by the grid. In fact, this is what synchronisation is before the generator is sync to the grid.

    After a power generator is sync to the grid, it can produce real power by applying more mechanical torque(more steam) to the power tubrine.

    If control engineer wants to operate the generator in either lagging or leading power factor, he can easily do that using the AVR.
  7. recca02

    Senior Member

    Apr 2, 2007
    As far as I know, generating more power will definitely increase the frequency of grid, rotor speed and thus voltage (if no AVR is present), a rise in these may also increase the load consumption.
    As stated by Chung, a generator can only supply as much power as much mech input is being fed to the TG set (steam input in case of thermal power plants). If demand is more the generators will run at lower RPM, voltage will dip as opposing torque increases. These fluctuations if not within permissible limits can lead to blackouts(grid collapse). This is where Load Dispatch centers help. I guess this is done usually by shedding off of load of a particular area.:mad:
    It might not be entirely correct but loads can be considered as resistors in parallel, more load means more resistances in parallel and thus lesser equivalent resistance connected to a voltage source, which in turn means more current through the generators. This will lead to a more opposing torque acting on TG rotor.
    Though load can not change generation (if Governors do not increase steam input) generation may change the load.( There has been discussion over this, you might want to go through this topic I have had some discussion with the OP on this over PM and wud share our thoughts incase you require)

    Absolutely. Note that AVR only changes excitation to keep voltage constant.
    Isn't that the best control an operation engineer has?:p

    that is why AVRs have limiters and governor can never supply more to the TG sets than what the steam flow is at full opening of control v/v (that is full load). Ofcourse, a TG set without tripping protections will be badly damaged at high loads (and faults).
    Those ppl continously monitor the grid frequency and then shed the load of a region accordingly (I am as clueless as you as to what exactly goes on there). There are islanding systems in private plants requiring huge power and having their captive power plants synchronized to the grid.

    Your line of thinking is always correct.:)
    Last edited: Apr 3, 2009