AC generator output voltage to grid question

  • Thread starter girts
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  • #1
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Hi,

In a given area there are different types of power plants , lets assume they are (coal, nuclear, wind and solar) Now as I have learned most of these plants (except solar and some others) use synchronous generators as the last step between mechanical energy conversion into electricity.
But since the plant sizes and output power (thermal) differ from one to another I assume also the generator sizes, rotor pole count and stator winding size and turn ratio differs correct?
If this is so then I also assume that the output voltage of the stator windings is different say from a 1000MW rated 10 generator Hydro plant to a single nuclear reactor two turbine/two generator set plant and maybe even differ from a single generator single turbine coal plant correct?

So by now I know that in AC grids it is important to synchronize before switching online a generator, aka the sinus waveforms need to match in frequency and in angle but how about the voltage? I do realize that every generator out there in a large plant is switched to the grid via a transformer which usually steps up the stator voltage to line voltage which then is fed into the high voltage lines that come to the station.
The question is do all of these transformers located at different plants with different sized generators have to be synchronized in terms of their secondary winding (the one attached to the HVAC grid) so that the voltages are the same?

For example in my country we have three large Hydro plants, they are all connected by 330Kv HVAC lines but their capacity differs so I assume the generator stator output voltage also differs so the transformers need to be made so that the secondary voltages are the same?





Another question, what would happen if for example I had a generator attached to the grid via a transformer but somehow suddenly my generator would output a lower voltage while everything else stayed in sync, (I know synchronous generators don't work this way but this is just a thought question) in other words the frequency and angle stays the same just the amplitude of the sinewave decreases aka voltage decreases, obviously the transformer secondary output would also decreases yet it is still attached to the grid , what happens? is the generator still able to supply energy to the grid even though its output voltage is lower than that of the grid or does energy now flow from he grid into the generator?
 

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  • #2
Baluncore
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Your question is too long.

Each part of the national grid is three phases that operates at the nationally specified frequency.
Where different parts of the grid with different voltages join there are transformers with fixed voltage ratios. The frequency is the same on both sides of the transformer.

The output voltage of a generator = alternator, at synchronous speed, is adjusted by changing the field current. If the field current is low the alternator voltage will be low, so the alternator will be driven like a motor drawing energy from the grid. If the field current is high the alternator voltage will be high and the alternator will be a generator transferring energy to the grid.

So each individual alternator's field current is adjusted to transfer available energy to the grid.
 
  • #3
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But I assume that the field current can only be adjusted within a margin for each generator otherwise the rotor will either be dragged along and the generator will work as motor(too low field current) or if the field current is too high and the prime mover is not capable of turning the rotor it can break out of sync with the stator field correct?



Yes I apologize , my question get too big. In short what I wanted to ask is:, Does a generator also work as a motor if only it's output voltage is slightly lower than that of the grid attached to the generator?
Because from previous learning I understood that if a generator's prime mover somehow loses power, then the mover cannot drag the rotor field against the stator field within the fixed stator field frequency and if the rotor field current is not decreased accordingly then the sator field now drags thew rotor along instead of the mechanical prime mover so the generator starts to work as a motor.

My question was what if the prime mover has enough torque reserve and moves the rotor along in sync with the stator frequency but say somehow the stator coil turns ratio or something else is different so that the stator output voltage is lower than that of the grid transformer winding attached to the stator, does then the generator also becomes a motor or not?




ps. let me rephrase the question in a way that would cause less confusion. Imagine I have a solid state generator, aka a transformer whose primary winding is fed by a perfect lossless switch with a pure sinewave, the secondary winding of this transformer is attached to grid directly at some fixed line voltage.
Now I set up my switch and power source such that the sinewave is in sync and 0 angle with respect to the grid and the secondary output voltage matches that of the power line, I am now supplying the grid with whatever power the grid draws from my transformer at the moment, say I suddenly decrease the amplitude of my sine wave while everything else stays the same, do I still supply power to the grid , just less or do I now only draw power from the grid? (waveform, angle and frequency stay the same and in sync at all times)
 
  • #4
Baluncore
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But I assume that the field current can only be adjusted within a margin for each generator otherwise the rotor will either be dragged along and the generator will work as motor(too low field current) or if the field current is too high and the prime mover is not capable of turning the rotor it can break out of sync with the stator field correct?
You are confusing yourself.
The alternator output voltage is set by the field current.
The output voltage is set by the specification of the grid. Field current is very predictable.
The generated current is not proportional to field current, it is proportional to difference voltage between alternator and grid voltage.
 
  • #5
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I assume also the generator sizes, rotor pole count and stator winding size and turn ratio differs correct?
If this is so then I also assume that the output voltage of the stator windings is different say from a 1000MW rated 10 generator Hydro plant to a single nuclear reactor two turbine/two generator set plant and maybe even differ from a single generator single turbine coal plant correct?
All those assumptions except size are incorrect. The questions based on those wrong assumptions are meaningless.

In two previous threads you have been told that your concepts about the field are wrong. We provided links to the insights articles describing the correct descriptions.

I feel that you are just repeating your past themes. Thread closed.
 

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