grounding fault in LV network 2

[kookypedia]

Guys,
We usually use HRG for LV networks which limits the grounding resistor current to 5A-25A.
If we take a 3A charging current into account, all ground grid can see in LV network is 25A+3A=28A ?!
This number looks low for designing a ground grid but it is all can flow through ground path right?

 

[7anoter4]

It looks like good. See[for instance]:

http://www.postglover.com/download/neutral-grounding-resistors-technical-information/

 

[basilasq1]

If the transformer is solidly grounded on the high side, you might want to consider that return path as the design criteria

 

[mgtrp]

The grounding of the local transformer on the high-side is less important than the grounding of the upstream transformer supplying the system. Assuming that the upstream system is solidly grounded, the ground grid must be designed for the fault current that would flow from the upstream system in the event of a fault on the HV side of the transformer (assuming that the transformer is located within your facility ground grid).

With regards to the low-voltage system, you should also consider the possibility of a second fault occurring on a different phase, which would result in phase-phase fault currents flowing within your facility ground grid conductors. Unlikely you say? Consider that when the first ground fault occurs, it raises the voltage to ground on all of the other equipment connected to the system, which might just be enough to finish off anything that was just hanging in there before.

Note also that the grounding resistor in the local transformer does not need to be sized with consideration to either of the above.

Cheers,
mgtrp

 

[kookypedia]

Thank you all. Yes, it seems that I should design the ground grid based on HV side of transformer since short circuit current coming from the utility (which is grounded), will flow to the grounding grid.

 

[krisys]

It is better to consider the highest of the two, namely, High voltage earth fault current or secondary side second earth fault current. But, this is as a liberal design.

 

[kookypedia]

krisys,
Because of delta/delta transformer, there is no ground current in the secondary.

 

[GroovyGuy]

LV NGRs are never rated as high as 25A. (MV NGRs are rated a minimum of 25A).
The old rule-of-thumb is 1A for every MVA of transformer capacity. [ie In a typical industrial environment, a 2A resistor should be adequate for a LV 2MVA xfmr).
GG

"Wish I didn't know now what I didn't know then." -- Bob Seger