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Discussion Starter · #1 · (Edited)
Hi Folks

Can some one with good knowledge confirm if what is written below is correct ?

(Any generator, whether a grid-tied inverter or a rotating machine, has to raise it's voltage above the grid until it's full load current flows. In fact, that's how you control load on a generator on the grid, by raising or lowering it's output voltage. Large generators have a voltage regulating control that will keep the output KW constant by lowering or raising the field voltage. Grid-tied inverters must have some way of knowing output current, and likely the software adjusts the output voltage depending on grid, and input power to the inverter.)


Thanks in Advance and Merry Christmas :jester:
 

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Bilge Rat
motors and controls.........
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The statement is correct, but it's a fairly small part of the control scheme.

In addition to voltage, waveform and phase angle need to be addressed as well, and all of these will change slightly on a continuous basis.

Simply paralleling a generator to the grid (or another generator) will not allow the full use of its capacity, though if there are only two generators and they are identical, they'll get along pretty well.

If the generators are of different capacities (or if one source is the grid) and to an even greater degree, if they are of different pitches, they will not do well together.

This is best solved by the use of a VAR controller or some sort of load-sharing device that has control of each generators field.
 

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Discussion Starter · #3 ·
So in short, the amount of power the generator outputs is controlled by adjusting the generator voltage with respect to synchronizing the phase sequence, angle, Hz

Am I wrong here ?
 

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Electron Factory.Worker
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So in short, the amount of power the generator outputs is controlled by adjusting the generator voltage with respect to synchronizing the phase sequence, angle, Hz

Am I wrong here ?
Yes you are wrong. Changing generator voltage does not effect the real power output of a generator. It only changes the reactive power. To increase real power of a generator you must increase the torque from the prime mover (diesel engine, steam turbine, gas turbine etc.

On a synchronous generator that is grid connected once parralled to the system will be in sync with the grid. Its going to spin at synchronous speed 3600 for 2 pole 1800 for 4 pole. In order for the generator to output megawatts the generator has to be set at a speed greater than synchronous speed. Because the speed is fixed the increased torque causes amps to flow in the generator and you get power.

The AVR will adjust field excitation to keep the voltage at its setpoint for a given load. As the load goes up excitation will need to increase in order to keep the voltage at set point. If you increase the generator voltage above the grid voltage the generator is said to be over excited and VARS with flow out of the generator into the grid. Likewise if the generator voltage is less than the grid voltage the generator will be under excited and start absorbing VARS from the grid.
 

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Discussion Starter · #5 ·
On a synchronous generator that is grid connected once parralled to the system will be in sync with the grid. Its going to spin at synchronous speed 3600 for 2 pole 1800 for 4 pole. In order for the generator to output megawatts the generator has to be set at a speed greater than synchronous speed. Because the speed is fixed the increased torque causes amps to flow in the generator and you get power.
How is this done in the case of solid state devices like a grid tie inverter ?
 

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Bilge Rat
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How is this done in the case of solid state devices like a grid tie inverter ?
A VAR controller or a grid-tied inverter will track the utility voltage and make the auxiliary source follow it, but push it at the same time, if that made any sense.

If a basic diesel generator was paralleled to the grid, and the voltage regulator was turned up to the point that the engine throttle was wide open, it certainly would push power into the grid. But because the waveform and phase angle of the grid and the gen are different, it'd be pushing more KVAs than KWs.

Not very efficient, and the different waveforms would cause excessive heating of the gen stator.
 
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