Generator minimum voltage

[golubgolub]

What is the minimum voltage that generator can provide us, and on what does it depents on. For example if the generator is 195MVA. Is ti possible to generate voltage less than 1kV?

 

[waross]

Sure. How low do you want? There may be limits to how low you can set the AVR but with a manual field control you can probably dial it down to under 10 Volts.
A 0.48kV set may generate less than 5 Volts from the residual magnetism. That would be you lower limit. If you have any load left at that low voltage the opposing field resulting from the load current may reduce the residual magnetism and drop the voltage even closer to zero.
1kV and 195MVA, That works out to over 100,000 amps on my calculator. The residual magnetism may be a little higher than anticipated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

What is the minimum voltage that generator can provide us, and on what does it depents on. For example if the generator is 195MVA. Is ti possible to generate voltage less than 1kV?

With the limited information you gave, the answer is generally, "If you have enough money - do most anything".

However, generally one does not do science fair experiments with a 195MW machine. That is really a lot of money if it gets screwed up.

Give us some context. What is it you are trying to do?

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[struja]

Hi iceworm... The point is to use generator as a source for, for example, to make a stability test of transformer. Generaly, question is, is it possible to get around 415 V and, while have short circuit in HV side of transformer, to get around 1000A from generator, and keep it like this for some time (for example 15 minutes)...
thanks..

 

[waross]

You never have told us the rated voltage of the generator.
Residual magnetism may produce around 1% of rated voltage.
Does your set have a PMG?
It probably does have a PMG but if it is self excited you may encounter voltage collapse trying to use the AVR to produce and control low voltages.
It is probably best to use a DC power supply or possibly a DC welding machine to power the field.
What is the prime mover?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Yes, this is entirely possible and works well as a source of injection current. The source of power for the field will depend on whether it is a rotating rectfier type or a slipring machine because slipring machines generally need a lot more current. It's equally possible to use an LV or MV diesel set in the same manner with an external laboratory-type power supply feeding the field. If it is a rental set it's normally better not to mention that type of thing to the rental company. ;-)

You need to check whether the prime mover can sustain operation at virtually no load for a prolonged period: steam turbines don't normally like sitting at synch idle, and the HRSG attached to a combined-cycle GT will usually produce a lot of LP steam and little or no HP steam at synch idle.

 

[struja]

Hi..
Rated voltage is 15kV and it is a gas turbine. I am not sure does it have PGM.
And one subquestion....to generate some voltage on generator terminals, normaly we start as FSNL condition. But if we have FSNL condition,, do we have nominal voltage at that time (for example 15kV) or we have full speed but 0V, and then we can slowly increase voltage...I suppose this was not very smart question but I would like to Know how this process is going.

 

[waross]

Connect the generator to the load.
All load circuit breakers closed.
Start the gen-set with NO EXCITATION and bring it up to speed.
Slowly increase the excitation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[struja]

...thx waross...
As it is plan to make stability test of step-up transformer 15/400kV with short ciruit on HV side...please if you can just clarify next:
1. In our case, load will be thre-phase short circuit. Is this ok?
2. While increaseing excitation, means we will increase voltage from 0V up to some voltage until we get satisfing amount of current, suitable to check stability? For example 30A on HV side, means around 600A on 15kV side (generator), voltage on generator terminals in that case should be around 200V.
3. Is there going to be any problem for generator to ''supply'' short circuit? Can some Protection operate in that case, and which protection?

Thanks very much in advance.

 

[ScottyUK]

[ol 1]
[li]Yes, fine[/li]
[li]Makes sense.[/li]
[li]You may cause a voltage-controlled or voltage-restrained O/C relay to operate with high currents and low terminal voltage. You may need to defeat parts of the AVR's monitoring scheme, for example the under-excitation trip.[/li]
[/ol]

 

[waross]

ScottyUK has given the same answers that I would have given.
Thanks Scotty.
This may work well but you are getting close to the lower limit.
The sets I work on typically develop from 0.5% to 2% rated voltage from the residual magnetism. Full load current will bring the voltage down to about 1/2 of the open circuit voltage.
If this was my project I would start the machine up open circuited and check the residual voltage. If the residual is too high for your test, then reverse the excitation current in the field.
There will be some residual magnetism in the brushless exciter and some residual magnetism in the main rotating field.
Reversing the connections to the brushless exciter field should drop the residual voltage enough.
If this doesn't drop the voltage enough, you may have to reverse the connections to the field on the rotating diode plate.
Hopefully the voltage will be low enough that this is not an issue.
If you have to drop the residual voltage, tell us about the excitation scheme.
PMG?, Brushless Exciter?, slip rings?, Solid State exciter?.
We have to know what you have before we can give more detailed instructions.
The generator should be able to produce any current up to rated current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[panter]

Struja and golubgolub *it looks like nicknames from former Yugoslavia countries*

For colleagues should be given more information for source * generator nominal voltage and current * and short circuit voltage of transformator which is marked on the nameplate typically as a percentage.
Then is relatively easy should be calculate voltage and short-circuit currents on 15 kV winding as well as expected current to 400 kv transformer side .
Good luck !

 

[struja]

Hi...
....scottyUK and waross...
many thanks for your help. It is very hepful for me to get such detail comments.
At the moment I don't know exactlly all dana that you waross asked from me because
generator and everything regarding generator is a scope of another company so I have to
search a little bit for that, but for sure I will get back with this data. I would like to
understand that completly.

...panter...
Yes I have some connections with former Yugoslavia, I hope this is not problem
It is no problem to calculate short-circuit current for performing stability test of transformer, but may main conser was
''is ti posible to generate such a small voltage, like 400V from 15kV generator''. And i am very satisfy with
the answers that I get from scottyUK and waross.

 

[waross]

An anecdote to illustrate the sensitivity of the field to small changes in current.
When I was working on a lot of generators, from time to time I would come across a set that had lost its residual magnetism.
In my tool box I had a medium sized flash-light battery cell, a "C" cell in North America.
A couple of leads were soldered to the ends of the dry cell and taped up.
When I had a set that would not build up voltage, I would connect the "C" cell in series with the AVR. The extra 1 1/2 Volts was enough to make the difference and the set would build up voltage.
The sets I used this on were generally 25 KVA to about 250 KVA and 240 Volts to 480 Volts. Very much smaller than your set, but the point is that it will take very little voltage to drop the remove the residual magnetism. Too high a voltage and you will re-establish the residual in the opposite polarity.

Bill
--------------------
"Why not the best?"
Jimmy Carter