About low-resistance grounded systems

[GBuigues]

Dear colleagues,

I was searching information about low-resistance grounded power systems, when an apparent contradiction among different important documents came up. It is related with the Ro/Xo ratio in this kind of systems.

On one side, there are some standards/documents in which it is stated that this ratio (Ro/Xo) in low-resistance grounded systems has to meet the criteria of being less than or equal to 2 (Ro/Xo=<2). These documents would be IEEE Std. 142-2007 (IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems - Green Book), ABB Protection Application Handbook, etc.

On the other side, there are other documents in which it is stated exactly the opposite, that low-resistance grounded systems should meet the criteria of being more than or equal to 2 (Ro/Xo=>2). These documents would be IEEE Std. C62.92.1-2000 (IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems - Part I: Introduction), the article "Characteristics of Different Power Systems Neutral Grounding Techniques: Fact & Fiction" (by F.J. Angelini & D.D. Shipp, 1991), the book "Electrical Power Equipment Maintenance and Testing" (by Paul Gill, 2nd edition, CRC Press), etc.

Between these two possibilities, what would be the correct one? What would be the criteria that the ratio Ro/Xo should meet in low-resistance grounded systems? Should it be less than or equal to 2, or more than or equal to 2? Why?

I would be very grateful if you could answer me and clarify this doubt.

Thank you very much in advance

 

[living2learn]

Where the ratio of R0/X0 is equal to or greater than 2, the ratio of X0/X1 may be greater than 10 without danger of severe transient overvoltges, but make sure R0 is lowest engough for relaying purposes.

from what i recall to minimize transient over voltages the ground=fault current must not be less than 25% of 3-phase fault which corresponds to a ratio of X0/X1=10