What X/R ratio should I consider for Bus Design using IEEE Std 605? 1

[veegrid]

What X/R ratio should I consider for Bus Design using IEEE Std 605?
for grounding study, I typically enter X/R=20 in CDEGS software. However, for bus calculation, can I consider X/R=10?

 

[dpc]

X/R of what? X/R is highly dependent on voltage level as well as distance from transformer or generator.

 

[veegrid]

X/R for the system/source. It is not available, So looking guidance about what number can be assumed for conservative number. OR how can I derive that.
I have transformer test report on hand.

One of the white paper I was reading, mentioned that when X/R for source is not provided it is safe to assume 12 to 15. However, I did not see any other detail with that statement, so not sure if that is okay to use.

 

[dpc]

Again, it will depend on the voltage level 12 to 15 could be a reasonable assumption for 15 kV systems. But seems low for a 345 kV system. Generally higher X/R the higher the voltage.

 

[veegrid]

ok, thanks. Actually this is giving me good start for thinking further. I was looking for X/R for design consideration for 115kV tubular bus for 115/13.8kv system. so 12-15 is incorrect.

 

[dpc]

Distance to the source is another key factor. At the end of along line, it can be quite a bit lower. At a generating station, it might be 20-25. Using 12-15 is probably not going to be that far off in most situations but could be low for high voltage sources close to your substation.

 

[7anoter4]

Let's use the X/R from ANSI/IEEE C37.10 for power transformer and system short-circuit power according to IEC 600076-5 Table 2 – Short-circuit apparent power of the system- for 100 to 123 kV.
I agree with dpc. If -for instance- there are 5x 440 MVA 22% transformers [in a power station] supplying a total 10000 MVA short-circuit power at 115 kV [with X/R=40], at the end of a OHL of 2 parallel lines of 500 MCM ACSR 1mile length[x/r=3] then total X/R=10.5.For 10 miles length X/R=4 and so on.
So X/R =10 for the supply system of 115 kV it could be close to actual [maximum].