ETAP not adding fault currents correctly

[kaspor]

Hello

I have a 33kV bus modelled in ETAP with multiple power grid sources feeding into it. All of these grid sources are connected to the bus via 3 main CB's (via downstream CB's). I've tried to complete a short circuit analysis (3phase fault) using IEC60909 simulation, but the numbers don't add up and I cannot work out why.

One of the CB's is feeding in 7.106kA from grid source group A, the others are feeding in 655A each from grid groups B & C. Theoretically the total fault current at the bus should be 8.416kA (not 8.15kA as shown).

When I look at the detailed report, it appears as the grid sources are out of phase with one another. This is why they're not adding properly and I cannot work out how to 'sync' them.

Any thoughts?

 

[beyond86]

Can you make a screenshoot from ETAP?
I tried to combine two grids and all was good.

https://i.ibb.co/51C2Qxt/Untitled.png

 

[IrishPower]

Kaspor,

I suggest that you are too quick to suggest that ETAP is “not adding fault currents correctly”. The detailed printout you provided clearly shows that the real and imaginary components add up at the bus and Kirchoff’s current law is satisfied. Yes, the different currents have different X/R ratios, but that would appear to be quite typical for what appears to be a network equivalent model (with X/R =25) and two downstream RMUs which presumably have some motor loads contributing through some higher-resistance (smaller) cables, with a lower X/R = 1.1.

If you find the way that the currents contributed from each source/feeder and the total SC current at the faulted bus is shown to be confusing or non-intuitive, you may find it useful to change the display settings in the SC module in ETAP to show this information in a different way (showing the angle, for example). But I don’t think there is anything wrong with what ETAP is doing.

Hope this helps.

 

[kaspor]

All good, I had different X/R ratios in the grid sources, thanks for the help!