Common practice for 13.8 kV grounding

[oldfieldguy]

I am fighting a proposal by our 'management' to require the utility company to provide us with an ungrounded 13.8 kV supply for a couple of large motors and station auxiliaries.

I work on the US Gulf Coast. I have NEVER seen one of my former clients use an ungrounded 13.8 system. The common practice in industrial facilities appears to be a neutral grounding resistor to limit ground current to a few hundred amps, while most utility transformers for distribution use a hard ground.

Management is concerned that if we ground our system and tie our ground to the utility, then the large amount of copper in the earth will have adverse effect on cathodic protection for our pipeline. I contend that refineries I've worked have huge amounts of underground piping AND grounded electrical systems and ALL the ground grid is copper.

I'm citing safety concerns as well as common practice, but I need more help to back up my position.

Help is appreciated.

old field guy

 

[davidbeach]

Arcing ground faults on ungrounded systems can create havoc. The problem is that there is no such thing as an ungrounded system, it becomes a capacitively grounded system and they are nasty. Ungrounded is pretty old school, so 1950's. You can gain all of the perceived benefits of an ungrounded system through use of a high impedance (resistance) grounded system as long as you can supply more current into a ground fault than the system charging currents. I can't cite chapter and verse at this point, but would never personally consider a so called ungrounded system.

 

[dpc]

I agree with David. Ungrounded systems are not recommended for industrial systems. Refer to the IEEE Green Book. This is especially true when you have motors on the system. The motor winding insulation is the weak link in the system and will fail before cable insulation during overvoltage conditions.

Also, using an ungrounded system does not remove the need for grounding conductors.

The recommended practice for industrial medium voltage system is generally a low resistance grounded system.

You might also refer to "Industrial Power Systems Grounding Design Handbook" by Dunki-Jabobs, et al.

 

[odlanor]

FACTS
1-Traditionally All US industry is against ungrounded system.

2- There are ungrounded system in the world:
- French-demand ungrounded medium voltage system for hospital installation.
- Spanish uses ungrounded MT services auxilares of thermoelectric.
- Peruvian uses 10kV source with delta distribution sistem for using fuse-protected sensor for detection of ground fault to earth.
- Sweden and japanese uses ungrounded medium voltage system.

3- Management is concerned that if we ground our system and tie our ground to the utility, then the large amount of copper in the earth will have adverse effect on cathodic protection for our pipeline.

Conclusion :
you should look for references to non-grounded system in countries that use this practice.

 

[7anoter4]

So you could use lead or tin covered copper instead of bare copper grounding conductor and provide a Cathodic Protection. A minimum of 10 A through
a grounding resistor will avoid Ground Potential Rise to the faulted phase potential and will reduce the risk. In Eastern Europe the Medium Voltage distribution [overhead lines] used to be ungrounded. In 3 years I saw many "havocs" occurred as people dead and building in fire-nevertheless shields was provided in some important places [crossing roads , telecommunication lines].

 

[acog]

I agree with 7anoter4, why not just use stainless or galvanised steel, tinned copper etc. as the conductors in your grounding grid, then use a neutral earthing resistor to limit the fault current. Everybody will be satisfied and the system will be balanced and safe.

 

[dpc]

If corrosion is a concern, qualified corrosion engineers should be consulted. There are a lot of variables when dealing with pipeline corrosion.

 

[Kiribanda]

oldfieldguy,
Making an "ungrounded" 13.8 kV netwrok with motors etc creates other problems.As acog mentioned,if there is an existing CP system for your U/G pipes,then the solution is to design a galvanized steel ground grid with safe GPR,Touch & Step voltages.Then the steel pipeline and the galvanized ground grid donot have large potential difference in the electrochemical series thereby your existing CP system will not be loaded.If you put a copper ground then the CP system will be loaded and also finally the pipe line will get corroded.Eventhough you put a galvanized ground grid,practically not possible to make 100% galvanic isolation between Utility ground and your steel ground.But if steel is used then the impact from possible corrosion on the CP protected line is minimized.

 

[odlanor]

the question is what material is adequate for grounding grid.
I only know bare copper wire and copperweld

 

[Kiribanda]

odlanor,
As I mentioned in my post,it should be galvanized steel wire of adequate size (to be calculated) with galvanized steel rods.No copper at all.They are available in the market.If no budget constraints then stainless steel is the best.

 

[odlanor]

kiribanda,
is galvanized steel wire a malleable material easy to be manipulated during installation of the grounding grid?

 

[oldfieldguy]

RE: use of non-copper grounding conductors for the grid. The transformers in question are owned by the utility company. Their station ground grid is copper, and lots of it. and naturally, the grounding for the incoming transmission lines is also copper, so we have a LOT of copper already in the ground.

I am aware of, and we've actually used zinc-coated grounding conductors for the ground grid in other stations. I have a problem with this, seeing as how zinc is much higher on the activity scale than the iron of the pipeline. Am I wrong in thinking that my ground grid will dissolve like the zinc anodes on my boat's propeller shaft?

Still, first concern is the ungrounded 15 kV system. I contend that we cannot have an ungrounded system at these voltages.

old field guy

 

[dpc]

Well, you can have ungrounded systems, but if you have motors on this system, it's a really bad idea.

Yes the zinc will be sacrificial. I'm not sure it's allowed by the NEC. You can get copper-clad steel ground conductors.

But I still don't see how going with an ungrounded system helps with the corrosion issue. Everything still has to be bonded to ground.

 

[Kiribanda]

odlanor,
is galvanized steel wire a malleable material easy to be manipulated during installation of the grounding grid?
I believe the uploaded photo will give some answer.

 

[stevenal]

The 13.8 is AC, cathodic protection is DC. You just need some handy device that makes this distinction to keep the two separate under normal non-fault conditions and tie them together during faults.

http://www.dairyland.com/decoupling-from-utility-grounding-systems.html

http://www.dairyland.com/pcr-polarization-cell-replacement.html

 

[oldfieldguy]

stevenal-

I am passingly familiar with the Dairyland devices. I need to get with my corrosion guru and see if that's the answer to this dilemma.

old field guy