Why is the L-G fault higher on on a typical Genset?

[majesus]

Can someone please explain why you get a higher L-G fault over a 3 phase bolt fault on a typical "run of the mill" Genset? For clearity, a typical Genset would be a diesel/generator set say ~100kW upto 2.5MW manufactured from Cat, Cummings, etc.

http://www.cumminspower.com/en/products/generators/diesel/

BTW, please confirm that the same short-circuit theory applies to large hydro utility turbine generators.

Just curious,
Thanks,
Maj

 

[dpc]

Because the zero sequence reactance is often less than the positive sequence reactance.

 

[davidbeach]

Yep, what dpc said. Same thing can happen close to the wye side of a delta-wye transformer.

 

[alehman]

That's one reason they are frequently grounded through an impedance.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[cranky108]

Hey guys can't give a better answer than that (while it is correct).

Like the internal magnetic interaction of the three phases provides a higher impedance for faults that don't include ground, than for faults that do include ground. Which is why a neutral impedance is frequently employed.

Just because we are all electrical guys dosen't mean we all understand systermetrical componets (or can spell it correctly).

 

[ScottyUK]

"symmetrical"


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If we learn from our mistakes I'm getting a great education!

 

[alehman]

See faq238-314 for a very brief overview of symmetrical(?)
components.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[majesus]

Thanks guys,
Yup the zero sequence reactance is typically less than the positive sequence reactance. WHen I do my calculations, I see that. But I was curious on the reason why that is... Is there a theory to explain it? That's what I couldn't find in my book.

 

[dpc]

Zero sequence reactance can be measured by energizing all three phases from a single-phase source. Since everything is in phase, there is less leakage flux and less air gap flux, so reactance is less than normal positive sequence reactance.

 

[prc]

majesus, let me try to explain.When we say zerosequence reactance is same as positive sequence reactance,it means the zerosequence current flowing in a winding has a compensating ampere turns in another winding.(eg YNd transformer -Zo=Z1)Some times such a path will not be available.Then the reactance becomes pretty high.( eg Yny transformer with 5 limbed core.Zero sequence flux in the core will have a ready path through side limbs. ie less zero sequence curent required and hence reactance high.)When the flux has no path (eg Ynyn transformer) through the core, it will jump in to air and go to metal tank to form a closed path.This will have higher magnetic reluctance and hence more current required to maintain the flux and there by resulting less zero sequence reactance.

 

[majesus]

Thank you. As always I appreciate the help and your interest in answering my questions.

 

[umrpwr]

majesus, to try and further clarify your question and provide a more detailed explanation below is a section excerpted from the "Electrical Power Disctribution Handbook by Tom Short" which discusses why the zero sequence impedance for a synchronous generator can be lower and the L-G faults higher than a 3 Phase fault.


The zero-sequence impedance of a synchronous machine can have extremely low impedance. It is enough of a problem that many generators are ungrounded or grounded through an impedance to prevent the flow of zero-sequence current. Many generators are not braced to handle the fault current for a line-to-ground fault at the terminals of the machine. Single phase faults cause more mechanical stress and are higher magnitude. Ground fault currents are 30 to 40% higher than three-phase fault currents (E/Xd" vs. 3E/(2Xd"+X0)=~ 1.3 to 1.4E/Xd"). The zero-sequence impedance is the same whether it is under steady-state, transient, or subtransient conditions.

The reason that the zero-sequence impedance is so low is that magnetic fields from zero-sequence currents in the stator winding tend to cancel each other. If the fields cancel and couple very little to the rotor, the impedance
is very low.

The zero-sequence impedance varies significantly with design. The most prominent difference is due to the pitch of the stator winding. A pole pitch is the number of degrees that the rotor has to move to change from one pole
to the other. In a 2-pole machine, one pole pitch is 180
degrees, and in a 4-pole machine, it is 90 degrees. The pitch factor(or just the pitch) of the stator winding is the portion of the pole pitch that the stator winding spans. A full-pitch stator winding spans the full pitch. A fractional pitch winding spans less than the full pitch. The 2/3-pitch winding reduces the zero-sequence impedance the most. Because the two conductors in each slot have current in opposite directions, the fields cancel almost completely (since a = b = c = -a' = -b'= -c' for zero-sequence current). Other common pitch factors are 5/8 and 3/4.

 

[msquared48]

Because the typical Genset is not designed for a midget.

Mike McCann
MMC Engineering

 

[majesus]

Umrpwr,
Thank you.

I saved the thread as a pdf for future reference. Much appriciated.

Majesus