Electronic – What determines what type of short-circuit current is highest

faultshort-circuitthree phasetransmission line

I'm doing a short-circuit analysis of a system, and started wondering about the following: What determines which type of fault current is the highest (and lowest)?

The screenshot below shows the result for 4 different switchboards. As can be seen, for the two first switchboards, an LG-fault results in the highest current, while LLG and 3P-faults result in the highest currents for the two last switchboards.

I know that in an isolated system, the LG-current will be (close to) zero, but other than that, I'm not really sure what makes up the differences. The results below is for a solidly grounded system, where the zero-sequence impedances are 3 times the positive sequence. None of the switchboards are connected (only through a high voltage distribution bus).

enter image description here

Does anyone know why it differs from switchboard to switchboard? I would think that for a solidly grounded system, one of the types would always be the largest. Are there any system characteristics that could tell me (before I run an analysis), which of the types is most likely to be highest?



As Li-aung Yip (thanks) points out, the zero sequence impedance is normally a bit lower than the positive sequence in a transformer. I see the question text above is not completely correct: The zero sequence impedance is 3 times the positive sequence for lines, while for transformers, it looks something like this (this data is used when achieving the results above):

enter image description here

Best Answer

A power transformer will commonly have a lower zero-sequence impedance than positive sequence impedance. See the J&P Transformer Book. Typically a transformer will have \$ 0.8 Z_1 \leq Z_0 \leq 1.0 Z_1 \$.

If the zero-sequence impedance is lower than the positive-sequence impedance, then the line-to-ground fault current will be more than the three-phase fault current.

Generally in my experience the line-to-ground fault current is no more than 1.2 times the three-phase fault current. Depending on the system earthing, the line to ground fault current may be much less than this.

Impedance earthing is commonly used to limit earth fault current and hence touch/step potentials. So it's not unusual to see a switchboard with 25,000 A three-phase prospective fault current, but only 500 A single-line to ground fault level. On certain low-voltage substations with fault levels of 10 kA, I have seen the earth fault level limited to as little as 4 A.