answer for the problem
- Thread starter Firas900
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thank you for your help... I have another question I hope you can solve it >> If two power systems connected in parallel and one power system increase its power output to 100MVA, if the second power system not respond simultaneously to the change, the second system will:
a) increase the frequency b) decrease the frequency c) the voltage will rise d) the freq. not change
a) increase the frequency b) decrease the frequency c) the voltage will rise d) the freq. not change
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If they are connected in parallel they can't run at different frequencies.
JFletcher
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- Williamsburg, VA
double post
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JFletcher
Senior Member
- Location
- Williamsburg, VA
I dont believe that quite accurate. Perhaps I was taught incorrectly, but when paralleling with the POCO from the plant genset, the genset has to run slightly higher frequency than the grid to 'pick up' the load, and when transferring back, the generator must drop below the grid frequency to transfer the load back to the grid. ofc all of this has to happen in synchronicity lest you want to turn your switchgear into a very expensive fireworks display. and it happens quite quickly (few cycles?). Am I wrong?
eta: as to answering the OP's 2nd question, without knowing the capacity of the generator or transformers, my guess is that a 100MVA (!) load would cause a severe overload, voltage dip, the generator would struggle for a moment to supply the power it cant, and would trip offline. If the POCO is supplying 99.9% of that load already, you might not see any reaction from a small increase in demand on the primary side. Personally, I think it is a piss-poor worded question.
@JFletcher,
Besoeker is correct. There will be one frequency in a system. If one generator hugs more than the others, and the other generators can't adjust their speeds immediately, the system frequency will tend to rise until the speed controllers take control and bring back the frequency to the specified Hz.
Besoeker is correct. There will be one frequency in a system. If one generator hugs more than the others, and the other generators can't adjust their speeds immediately, the system frequency will tend to rise until the speed controllers take control and bring back the frequency to the specified Hz.
I'm afraid your are. Generators or, more correctly alternators, have be synchronised and in phase with the supply the are to be switched into. In the good old days that was sometimes done manually with incandescent lamps showing the difference and the plant attendant would wait for them to stop rotating and be in the 12 o'clock position before closing the breaker. More modern synchronising systems detect it instrumentaly and and allow the breaker to close only when phase and frequency are matched.
Think about it. Draw out the voltage waveforms for a few cycles with one running at 60Hz and the new one at, say 66Hz, just to make the difference apparent over a few cycles.
It isn't really my field. I've done just a few. But I'm sre there are others here who can offer better explanations.
Bugman1400
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- Charlotte, NC
Perhaps folks are getting confused between frequency and phase angle. The generator and the grid have to run at the same frequency. Otherwise, pole slipping and instability will occur. For a generator to supply MW (Amps) onto the grid, the phase angle has to be ahead of the grid. This angle is part of a simplified Power Flow eqn Pf=VsVrsin(angle)/X.
If you mean alternators, they are generally, perhaps almost always, synchronous machines. The load angle may change but the frequency does not.
I mean a synchronous machine driven as a generator http://xn--drmstrre-64ad.dk/wp-content/wind/miller/windpower web/en/tour/wtrb/syncgen.htm http://www.eecs.ucf.edu/~tomwu/course/eel6208/notes/09 Synchronous Generator and Motor.pdf
the frequency/speed will droop
the governor will compensate
A single autonomous machine may. Not one connected to a system which is what the OP described.
yes it does
see my prior post 17 (interesting article btw)
it describes a large multi-unit grid
fig. 5 shows the 2 units running at different speeds vs load until the controls reach equilibrium (the disturbance in the op's case to upset from equilibrium is a 100 MVA step change)
Did you miss this?
"Governors using speed droop or speed regulation require a sustained change
in system frequency to produce a sustained change in prime mover control"
The machines can't run at different frequencies.
Draw the waverforms and you will see why.
apparently you missed the point
sure they do
during the transient period of load change
exactly as the op described
the 2 machines operate at different freq until the governors restore synch/equilibrium
Speed governors vary prime mover output(torque) automatically for changes
in system speed (frequency). The speed sensing device is usually a flyball assembly
for mechanical-hydraulic governors and a frequency transducer for electro-hydraulic
governors. The output of the speed sensor passes through signal conditioning and
amplification (provided by a combination of mechanical-hydraulic elements,
electronic circuits, and/or software) and operates a control mechanism to adjust the
prime mover output (torque) until the system frequency change is arrested. The
governor action arrests the drop in frequency, but does not return the frequency to
the pre-upset value (approximately 60 Hz) on large interconnected systems.
Returning the frequency to 60 Hz is the job of the AGC (Automatic Generation
Control) system. The rate and magnitude of the governor response to a speed
change can be tuned for the characteristics of the generator that the governor
controls and the power system to which it is connected
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