IEEE 80-2000 Choose the curves to approximate split factor Sf

[karls2]

Hello All,
I see that in the IEEE 80-2000 Curves to approximate the split factor Sf we have the choice between Transmission line and feeders for the curves.
I understand that transmission line is when you have only 3 phases (HV MV),
and feeders when you have 3 phases + neutral (LV).
It seems that there is no difference between 3 phases line cables and 3 phases buried cable with shield. I am sure that there is something I havn't catch.
If I have for example a substation with 2 incoming lines in 33kV cables (so only 3 phases) and 10 11Kv outgoing feeders by buried cables (only 3 phases also but buried with shielded cables).
In The substation I have 2 Delta / Wy transformers (not in parallel each transformer supply his 5 feeders or 1 supply 10) The worst case is 1/5 I suppose.
Neutral is limit to 1000A (at the upstream substation) at the primary and limited to 300 Amps by a resistor in the current substation at the secondary.
I have a substation grid resistance of 2 Ohms
I am in 100 % Remote contribution (no generator in the substation)I think I have the choice between figures C13 C14.
Difference between C13 and C14 is the value of Transmission GND 15 ohms and 100 ohms.
As I don’t know these values I could choose the worst case,100 ohms for transmission ground. (how I can obtain these value ?)
So for my example I will have approximatively 35% of Sf.
I s it right ? I don’t see in this example how I make the difference between buried cables (with shield) and line (with only guard cable) .
Thanks by advance for your patience and your explanations.

 

[7anoter4]

The short-circuit phase-to-ground is supplied from the 33 kV source only. The secondary outgoing cables don't have galvanic contact with the high-voltage.
The short-circuit phase-to-ground is returning to the source by Ground and by 33kV cable shield.
No return through 11 kV cable shield. Only if one [or more] of these cables will supply the source auxiliary also-then the shield of this cable will be parallel with the incoming cable shield.

 

[jghrist]

There will be return through the 11 kV cable shields because they are connected to the station ground grid and the return current can flow through the cable shields, into the earth at the ground points of the cable shields and back to the source. The 11 kV shields are not as effective as the 33 kV shields because the distance from the return path to the phase conductor carrying the source current is large, making the reactance of that path large.

The IEEE curves were developed for transmission lines with overhead shield wires and overhead distribution feeders with neutrals. You will have to judge if the stated assumptions are close enough to your situation. In particular, is the distance between shield grounds less than or equal to the assumptions of tower and pole grounds?

 

[7anoter4]

Thank you, jghrist. I agree with you, the shield of 11 kV cables may convey short-circuit currents-if they are grounded both-side- usually there is only a single grounding point. However, since this current it is not a split part of the total short-circuit current before it starts to flow through the grounding grid , but as extension of the grounding grid-in my opinion-may be neglected for splitting factor appreciation.

 

[7anoter4]

In IEEE 80/2000 Annex C [informative] is mentioned:
“The original presentation of this concept was published in Garrett [and others]:
Determination of Maximum Substation Grounding System Fault Current Using Graphical Analysis".
In this article it is noted:
"Using SMECC computer program the system network of Figure 1 was used to perform the parametric analysis."
As one can see no [underground] cable is shown, so these graphical figures are not indicated for underground cables split factor calculation [in my opinion].

 

[karls2]

Ok thanks for your explanations, I will read them carrefully and try to understand that better.
Maybe I will still need your help because the ground grid calculations could be very interrsting topic.