Fault Calculation

[Danilo917]

Gents,

Just want to get your opinion regarding the fault calculation on 33kV feeder line

if the load are neglected and do not contribute into the fault and are fed by 4 x 33kV feeder cables 5km long and terminated to 33kV generator terminal bus at one end , should the impedance of cable (5km) be neglected as well in fault calculation assuming the fault occurred at one of the feeders near the 33kV bus?

many thanks,

Danilo

 

[davidbeach]

If the fault is truly at the utility bus and the cable impedance is significant, there will be current that goes out on A, B, C, and comes back on D but it may be low enough in magnitude to be a rounding error. As the fault moves down cable D from the utility bus to the generator bus the amount of current on A, B, and C will increase as the amount of current leaving the utility bus on D will decrease. At some point it will really matter. Unless you're trying to do the calcs by hand the cables don't add any particular difficulties.

 

[7anoter4]

In my opinion, it has to be something like this:

 

[Danilo917]

Thanks 7anoter4,
if for instance Zdu is zero when fault occurred at the terminal of end of the cable, what sort of analysis should follow?

 

[7anoter4]

A complete short-circuit diagram it is not possible to draw since we don't know what kind
of short-circuit you intend to analyze: phase-to-phase [3 phases],phase-to-ground ,phase-to-phase-to
ground. For the last two you need to introduce zero impedance [considering the Z2 [negative impedance] it will be still Z2=Z1].The system neutral could be solid grounded, resistance or reactance grounded or ungrounded.

 

[rockman7892]

This is an interesting thread

So are we saying that with the generator off and a fault on "D" close to the utility bus there will be fault current from the utility bus directly to fault on D but there will also be additional fault current going down lines A, B, & C and back up to fault on D? So total fault contribution at D will be fault current directly from utility bus (majority of fault current) and fault current down A,B&C (minority of fault current)

I always assumed that for a fault cacls you looked at the upstream impedance (utility impedance in this case) and used the shortest impedance path for calculating max fault current. But I can see now from this conversation that the "total fault current" will be a function of current divider and although path directly from utility bus to fault on D represents the shortest impedance you still technically need to include the small amount of current flowing down A, B, & C in order to get the "total" amount of fault current at D.

Is this how most commercial software analysis programs calculation fault current for this particular situation?

 

[waross]

The fault current will not be the full fault current.

So are we saying that with the generator off and a fault on "D" close to the utility bus there will be fault current from the utility bus directly to fault on D

This current will be limited by the impedance of the cable "D" from the utility bus to the point of the fault.

but there will also be additional fault current going down lines A, B, & C and back up to fault on D?

This current will be limited by the impedances of cables "A", "B" and "C", and the part of cable "D" back up to the fault.

The sum of both these currents will not exceed to full fault current at the bus.

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

 

[davidbeach]

Current takes all available paths. The current division between paths obeys Ohm and Kirchhoff.