Why is the L-G fault higher on on a typical Genset?
Why is the L-G fault higher on on a typical Genset?
(OP)
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://w ww.cummins power.com/ en/product s/generato rs/diesel/
BTW, please confirm that the same short-circuit theory applies to large hydro utility turbine generators.
Just curious,
Thanks,
Maj
http://w
BTW, please confirm that the same short-circuit theory applies to large hydro utility turbine generators.
Just curious,
Thanks,
Maj






RE: Why is the L-G fault higher on on a typical Genset?
RE: Why is the L-G fault higher on on a typical Genset?
RE: Why is the L-G fault higher on on a typical Genset?
Alan
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"It's always fun to do the impossible." - Walt Disney
RE: Why is the L-G fault higher on on a typical Genset?
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).
RE: Why is the L-G fault higher on on a typical Genset?
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If we learn from our mistakes I'm getting a great education!
RE: Why is the L-G fault higher on on a typical Genset?
components.
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: Why is the L-G fault higher on on a typical Genset?
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.
RE: Why is the L-G fault higher on on a typical Genset?
RE: Why is the L-G fault higher on on a typical Genset?
RE: Why is the L-G fault higher on on a typical Genset?
RE: Why is the L-G fault higher on on a typical Genset?
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.
RE: Why is the L-G fault higher on on a typical Genset?
Mike McCann
MMC Engineering
RE: Why is the L-G fault higher on on a typical Genset?
Thank you.
I saved the thread as a pdf for future reference. Much appriciated.
Majesus