L-L-G VS L-G Fault Level

[Cerkit]

Hi,

Between a L-L-G fault and a L-G fault which would you expect to give a higher MVA?

I am getting a study with a higher L-G fault level, but I though that the L-L-G fault level should be higher?

Thanks

 

[mparenteau]

Location in the circuit? If it's near a transformer secondary, the L-G fault will be higher.

 

[7anoter4]

See IEC 60909-0/2001:

 

[WHiPCPL]

What type of installation is it (can we get a circuit diagram)?

If your installation is near the transformer on the secondary side then L-G should be the same as L-L-G, this is of course under the pretence that all the impedances are equal.

However! Since both L-G and L-L-G are dependent on the zero sequence they are therefore both dependent on the return path for the fault and the transformer/source earthing.

If we strictly look at the equations assuming E is the phase pre-fault voltage they will look like this:

A note about the L-L-G fault, The left and right hand side of the equation corresponds to a faulted phase, so let's say fault current in phase b corresponds to the left equation and phase c to the right equation. Then from a strictly mathematical perspective if Z0 is greater than Z1 or Z2 if they're equal. Then one phase will have a higher fault current than the other, which can be verified in a simulation tool or by just doing the calculation, although they shouldn't vary greatly to a point where this difference is significant. Then there's an entirely different equation that calculates the fault current in the ground

However if all sequence impedances are equal then the L-G and L-L-G faults are equal.

If Z0 is 0, then L-L-G will exceed the L-G short circuit.

All of this however is speaking strictly from a mathematical perspective and not a practical perspective, as I don't have the amount practical experience the majority in this forum has.

The point I'm trying to make: Look at your circuit, where does the fault happen, what's the return path (does it have a high or low impedance?) and what's the system earthing. All of these factors will affect the fault current in the two scenarios you're examining.

 

[7anoter4]

 

[Kiribanda]

If the transformer is WYE (high side)/ delta (low side)then the a L-L-G fault
current on the WYE side is higher than the L-G fault. Typical example is a WYE/delta
transformer connected to the Utility at small distributed energy resource (DER) stations.

 

[cuky2000]

LLG fault is larger than SLG fault if the following conditions are satisfied:
1) 0.7 ≤ Z2/Z1 ≤ 1.0 and 2) 1.0 ≤ Z2/Z0 ≤ 1.4
The graph below indicates a linear approximation of the region were LLG fault is larger than SLG.