xformer vs generator

[electrics]

for the same power rating what can u tell about the comparison of theshort circuit currents of these two ? I read somewhere generators have lower shorts ....

 

[ron]

Generators add short circuit current to the system and transformers reduce.

 

[electrics]

what does it mean? it means something like negatif resistance?

 

[electrics]

so generators short circuits are higher do u mean?

 

[Crionics]

Without getting too detailed, generators are a source of short-circuit currents. Transformers, while often in electrical systems are viewed as a source of short-circuit currents, are really just an electrical device. They will step the voltage up/down with an inverse effect on current, but do not add current to the system.

And the closer you are to the generator, the higher the available short-circuit current will be.

 

[ron]

electrics,
No such thing as negative resistance.
A generator is a source of current, where a transformer is a load (or impedance) to equivalent current.
When a transformer is a step down, it may appear the the current as gone up in relation to the primary-secondary ratio, but it is effectively reduce in relative terms due to internal impedance.

 

[John Paul]

Here's an example of the difference between a generator and a transformer wrt short circuit contributions:

Based on the impedence of the transformer the short circuit contribution is limited or finite. Isca = Xfmr FLA/Xfmr Z. Usually you can find the max contribution from a utility by using this simple formula. We use Z= 5% as a standard inpedence. By the way, what does POCO mean?

A generator is a source capable of delivering current at its rated capacity. So a 1800kW, 480V generator, .9pf, will contribute 2400A.

A 2000kVA, 480V secondary, transformer with 5% impedence could contribute 2408FLA/.05 = 48,000A.

Here's a useful link from the man himself.

http://www.mikeholt.com/documents/calculations/formulas/FaultV6.2.xls

 

[mivey]

...By the way, what does POCO mean?...

Finally, a question I can answer:
POwer COmpany.

 

[mivey]

...A 2000kVA, 480V secondary, transformer with 5% impedence could contribute 2408FLA/.05 = 48,000A...

It may be a minor point, but "transformer could contribute" is probably not the best way to phrase that as it might lead to confusion down the road for some.

 

[jghrist]

Here's an example of the difference between a generator and a transformer wrt short circuit contributions:

Based on the impedence of the transformer the short circuit contribution is limited or finite. Isca = Xfmr FLA/Xfmr Z. Usually you can find the max contribution from a utility by using this simple formula. We use Z= 5% as a standard inpedence. By the way, what does POCO mean?

A generator is a source capable of delivering current at its rated capacity. So a 1800kW, 480V generator, .9pf, will contribute 2400A.

A 2000kVA, 480V secondary, transformer with 5% impedence could contribute 2408FLA/.05 = 48,000A.

Here's a useful link from the man himself.

http://www.mikeholt.com/documents/calculations/formulas/FaultV6.2.xls

You have given full load contribution of the generator. Similar to the transformer, the generator will contribute fault current inversely with its impedance. Typical impedance (subtransient reactance) for an 1800 kW generator would be 0.1 pu so the contribution would be 2400FLA/0.1 = 24,000A.

 

[jim dungar]

It may be a minor point, but "transformer could contribute" is probably not the best way to phrase that as it might lead to confusion down the road for some.

Along this line, I think a better phrase would be 'a transformer lets through' an amount of short circuit current.

 

[John Paul]

Accidently double posted.

Edit to delete but couldn't find a delete option. Must be a moderator only function.

 

[John Paul]

You have given full load contribution of the generator. Similar to the transformer, the generator will contribute fault current inversely with its impedance. Typical impedance (subtransient reactance) for an 1800 kW generator would be 0.1 pu so the contribution would be 2400FLA/0.1 = 24,000A.

Interesting. I don't understand how a generator that produces a max current based on the size of its mag field and copper windings can produce more current without changing speed or the field strength. A transformer has the strength of the POCO high voltage (thanks for the definition by the way) behind it 'let through' high Isca.

I was thinking POCO was maybe Point Of Common cOupling with the utility. But we're not speaking of harmonic currents.

 

[gar]

100113-1218 EST

ron:

Yes there is negative resistance. See the discussion at
http://users.tpg.com.au/users/ldbutler/NegativeResistance.htm
. There are many other such discussions. An arc discharge has a negative resistance region.


Relative to the other various comments.

A generator can be represented as an ideal voltage source with a series internal impedance. Same for a power system. The last transformer probably is the major source of the internal impedance as viewed by the load in the power system.

.

 

[jghrist]

Interesting. I don't understand how a generator that produces a max current based on the size of its mag field and copper windings can produce more current without changing speed or the field strength.

It can't produce it for long without burning up. Just like a transformer can't withstand the fault current for a long time without burning up. As gar said, the generator can be represented by an ideal voltage source ahead of an impedance. In the case of a generator, the impedance also changes with time. The subtransient reactance that I used in my example is only valid during the first cycle. After that, it increases (and the fault current decreases).