# Thread: answer for the problem

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## answer for the problem

four alternators of 20 MVA has 16% reactance are connected in parallel. if symmetrical fault occur at the bus bar what will be the short circuit level?

2. sounds like homework
what is the contribution of 1 alternator?

what if it were a transformer with a pu Z?
Last edited by Ingenieur; 03-20-17 at 10:21 AM.

3. 500 MVA

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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
Last edited by Firas900; 03-20-17 at 03:21 PM.

5. Originally Posted by Firas900
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
Is it right?
show me how I determined it, please

6. Originally Posted by Firas900
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
If they are connected in parallel they can't run at different frequencies.

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double post
Last edited by JFletcher; 03-20-17 at 08:58 PM.

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Originally Posted by Besoeker
If they are connected in parallel they can't run at different frequencies.
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.

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@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.

10. Originally Posted by JFletcher
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?
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.

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