Why Does Generator Fault Current Increase When Connected to the Grid?

[ge.vasiliou]

Hi guys,
For one more time I need your opinion in a kind of strange issue, which escapes my understanding.

Performing a single line to ground short circuit calculations (Low Voltage Side) in a simple Generator power plant using a step up to transformer, we can observe the two following facts:

1. For a stand alone generator (not parallel to the grid) single line to ground fault current (low voltage side) equals to 30kA. This value actually represents that behavior of the generator itself.

2. If generator works in parallel to the existed grid, results of the short circuit fault current calculation equals to 86,4kA. This total fault current splits to 46kA coming from Generator side, and rest 40,5 kA come from grid side.

The question is:
Since generator as a source is capable to provide 30kA on it's own, how is it possible in a parallel to grid configuration to have 46 kA coming from the generator side?
It seems 16 kA have been added to the generator side. Is this fault current actually going thru generator windings?

PS1 : Please notice that the above results in both cases can be reached either by a simulation software or by hand calculations using symmetrical components analysis.

PS 2: Quick Description of equipment:
Generator is 1,9MVA , cosF 0,8, 400Volts, 50Hz, Star connected, solid grounding, Xd'' = 0,13 p.u and Xo=0,02p.u. Step Up Transformer is 2,5 MVA, 6% impedance, Delta connection at LV Side, Star Connection (solid grounding) at 20kV Side. Grid is 20 kV , 250MVAsc, star – solid ground.

 

[jim hardy]

Xd'' = 0,13 p.u

are we perhaps mixing direct, transient and subtransient reactances?

I'm not a relay guy but here's a handy looking article..

http://www.google.com/url?sa=t&rct=...xoWPBw&usg=AFQjCNF2ekJRZNzlorkDaWs8d5z2vJKs_Q

it's PDF , if above doesn't work search on "Power topic #6008 | Technical information from Cummins Power Generation"

sorry i don't have an exact answer .

old jim

 

[ge.vasiliou]

No, we don't mix transient reactances. The currents appear in my sketches are short circuit current (symmetrical rms) during subtransient state of the fault, meaning actually the short circuit current for the first 0.2 - 0.5 seconds (before the whatever protective device operates). For the calculation of short circuit current - subtransient state we need Xd'' of the generator (subtransient reactance). The link you uploaded is nice, i already have this document. Thanks anyway.

I can understand why the fault level is increased when in parallel to grid:
Generator impedance are in parallel to the grid impedance resulting in a total smaller impedance meaning a bigger fault current.

What i don't know is if this extra current of 16kA which seems to come from my Generator side is actually going thru my generator windings.

 

[jim hardy]

i never have done short circuit calculations, so won't pretend to know.

But i look forward to learning from whoever does respond !This is a single phase fault?
What is state of CB1 for your two conditions? Which side of it is the fault on?
Perhaps because of the delta connected transformer the other two phases of generator can contribute current to the faullt through the transformer windings?
In other words, CB1 connects the unfaulted phases to the fault through transformer winding.

that's just a guess.

old jim

 

[ge.vasiliou]

Jim,
Yes it is a single phase fault. CB1 is closed - normal operating conditions - in both sketches. Your guess doesn't apply to the second scenario that current is increased in Generator Side. This increased current is due to the existence of the grid (the only difference between my two schemes).