Earthing Analysis Software

[AusPowEng]

What standalone software packages are available to assess the performance of buried earth grids with respect to touch & step potentials? I am aware of SES C-DEGS, but would be interested to hear from anyone who actually uses this calibre of software on a regular basis.

I am keen to find a software tool that is effective for smaller installations and industrial plant rather than larger switchyards.

I would also appreciate any personal recommendations for good earthing/grounding engineering books (NB: I am already familiar with IEEE 80).

At the risk of opening multiple discussion threads, is there any manufactures performance standard for earth/ground resistance testers, as I have used several that vary greatly with measured frequency, injected current and of most concern, results.

I look forward to all your comments.
Regards,

AusPowEng

 

[jghrist]

We use SES CDEGS software for the design of small to large size substation ground grids. The main reason for using the software is to get an economical design that limits touch potentials in an outdoor substation. Usually this is more difficult in smaller substations. For industrial building grounding systems, you are not usually concerned with touch potentials, so the software is not applicable. There are SES modules available that allow very detailed analysis of electromagnetic interference and surge performance of grounding systems, but I have not used them.

The best things I like about the SES software is the ease of inputting the grid geometry and the method used for determining the split of fault current among line shield wires/neutrals and the ground grid. I normally start off with an Autocad drawing of a grid and then import it into CDEGS, modify it with the included SESCAD program then export it back to Autocad to produce a final grounding plan. The fault current split module uses data available from a fault study; I find this easier to use and understand than the method used by the EPRI SGW/WinIGS programs.

For industrial system grounding, I recommend the IEEE Green Book, Std 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems and the Emerald Book, Std 1100, Recommended Practice for Powering and Grounding Sensitive Electronic Equipment. I don't know of any good books other than IEEE-80 for substation grounding; I've mostly depended on gathering a lot of technical papers. The IEEE-80 bibliography is a good place to start.

 

[AusPowEng]

Hello jghrist,

Thank you for your considered reply to my question, I will have a good read of the IEEE standards you mentioned.

Here in Australia and I imagine many other countries, there is an overall requirement to ensure that in any workplace and public area, during earth faults, the voltages that occur on the ground surface and on exposed metal parts (ie. touch & step potentials), are disconnected in a timely manner (5 seconds) and, that the voltage magnitude is maintained within established safe limits limits. Crushed-rock, grading rings and earth grids provide a means to control the exposed voltage within safe limits. Sensitive relaying schemes provide timely detection and clearing of faults. So far so good...

What I struggle with is that in a combined earth system, the HV & MV bonded parts of transformers and switchgear are taken to a common earth bar (same for MV & LV combined earths). Therefore, during earth faults the ultimate potential on the earth bar (max value of Vp/sqrt(3)) is mitigated throughout a facility by protective conductors on feeders and sub-mains, which in are turn connected to earth bars in areas remote from the switchyard. Metal clad buildings, panel doors and loads can therefore be energised in these areas and, in the absence of grading rings, is there not a touch potential hazard to anyone standing on 'true-earth' and in contact with the exposed metal parts.

Is it the case that the standards focus on substations because of the high voltages of transmission systems (ie>66kV) are much more difficult to keep within safe touch and step potential limits? If find it difficult enough for distribution voltages (3.3-33kV) and there are several major facilties in my region that are islanded from transmission grids, with installed loads of >100MW (i.e. signficantly large).

I look forward to any replies.

Thanks you,

AusPowEng

 

[DanDel]

I believe ETAP by OTI has a Ground Grid module.

http://www.etap.com

 

[jbartos]

Suggestion: The IEEE 80 Standard may be used to create a little spreadsheet in Excel, Lotus, etc., e.g. the Annex D approach.
It works fine. Everything may fit one page.
The IEEE 80 references many papers, books, and publications in Annex A, including EPRI and German publications.

 

[jbartos]

Suggestion: Visit

http://www.avointl.com/cae/products/Index.asp

etc. for a ground resistance tester

 

[jghrist]

AusPowEng wrote:
[red]Therefore, during earth faults the ultimate potential on the earth bar (max value of Vp/sqrt(3)) is mitigated throughout a facility by protective conductors on feeders and sub-mains, which in are turn connected to earth bars in areas remote from the switchyard. Metal clad buildings, panel doors and loads can therefore be energised in these areas and, in the absence of grading rings, is there not a touch potential hazard to anyone standing on 'true-earth' and in contact with the exposed metal parts.[/red]

The ultimate potential (Ground Potential Rise or GPR) is the fault current flowing from the grid into the earth times the grid resistance. Keep in mind that faults whose source is within the plant area, such as on the low side of delta-grd wye transformers do not produce any currents flowing from the grid into the earth because there is a metallic path back to the source. This is true in the normal case where the resistance of the metallic path (equipment grounding conductors and the ground grid) is much less than the resistance of the earth path. The worst case fault is usually on the high side of the transformer where the source is remote from the station. Faults on secondary feeders that serve areas remote from the station may also produce earth currents into the grid, and potential rise, where the fault current is returning to the transformer source in the station.

The GPR may be transferred outside the station by conductors (such as communication wires) or pipes. See the section on transferred potentials in IEEE-80. The grounding conductors on feeders that go to remote areas might cause a touch hazard where equipment is connected to these conductors. This would certainly be the case if the equipment were insulated from the earth where someone could stand and touch the equipment. If the equipment is insulated from the earth, then there will be no current in the grounding conductor and the potential will be the same at the remote end as at the source station. This is mitigated by connecting the equipment to a local ground electrode. The touch potential at the remote area is then a function of the current flowing through the remote ground electrode.

The current from a primary side fault to the station grid returns (partially through earth and partially through primary shield or neutral wires) to the source. Little of the current will flow through the secondary feeder grounding conductors to the remote ground electrode and then back to the primary source. This path is a higher impedance path for the return current. With little current flowing in the remote ground electrode, there will be little touch potential.

The situation can be analyzed with grounding design software such as SES CDEGS. The current flowing in the secondary feeder ground conductor is determined by the fault split module. The remote ground system can be analyzed to see what touch potentials will be produced by this current.

 

[pacha]

SES, has engineering modules that can address most of your concerns. The AutoGrid Pro software (price starts at$999) is designed to handle simple grounding systems. The other packages can handle urban, suburban and GIS ground systems.
Visit their site (

http://www.sestech.com)

for more information.