Ground Potential Rise in an existing Generation Facility

[mbk2k3]

I have a question about ground potential rise:

We are working on a cogeneration plant, where we are replacing one 13.8kV steam turbine with three 4.16kV gas turbines.

The main step-up transformer is being downsized from 25MVA to 5MVA, 25kV-4.16kV wye-delta configuration (wye on the utility side).

All of this is located in a pre-existing facility, enclosed in a building that has a pre-existing grounding grid.

Now the utility wants us to perform and submit a ground potential rise study. I've modeled the existing grounding grid to the best of my ability using the plant's older drawings.

Every time I run the study, it fails when it comes to tolerable limits for step+touch potential. I did some digging around in IEEE standards and noticed that the main contributing factor why this failed is due to the ground fault current coming from the utility side (contributes 4.5kA).

So my question is this: if I have no way of limiting the ground fault current coming from the utility side, and I have no way of adding new grounding conductors (since its an existing facility with an existing grid in pre-cast concrete floor), what are my options here?

additional note: when I set up ETAP so that the ground grid encompasses the LV switchgear only, and not the main utility side 25kV switchgear, all my tests pass and there are no issues. This is how I determined that the issue lies with the utility side ground fault contribution at the 25kV switchgear level.

I've attached some ETAP screenshots in PDF format to give you a better idea.....

 

[7anoter4]

If the 5 MVA transformer is located on the premise site-that means on the grounding grid- then only the current from utility matters-it is the factor determines GPR, indeed.
Usually it does not change if you'll change the local contribution in short-circuit case.
The remote potential is always zero and the current depends on series reactance.
So, in my opinion nothing it will change if you'll increase or decrease the generating power.
By the way, it is hard to check your calculation due to lack of equipment and line data.
At a first glance it seems to me inconsistent.

 

[mbk2k3]

@7anoter4

thanks for the reply.

i can provide more info from ETAP that might help? what info would you like to see?

 

[cuky2000]

Every time I run the study, it fails[/highlight] when it comes to tolerable limits for step+touch potential
b)... why this failed is due to the ground fault current coming from the utility side(contributes [highlight #FCE94F]4.5kA[/highlight])]

COMMENTS:
1) For modeling purposes, rather than use the full SC current of 4.5 kA, consider to use the fraction of the current injected into the ground. This could be estimated using the split factors as a function of the neutral & shield grounded conductors following the IEEE standard.

2) Use the appropriate clearing time of the backup protective device. For MV usually this is greater than 30 cycles (0.5 seconds).

3) Take advantage of the steel rebar embedded in the concrete floor. This is a natural supplementary grid that will significantly help to create an equipotential surface minimizing the impact of exceeding the allowable step and touch potential generated by ground faults.

4) Rather than calculate the step and touch potential using inaccurate and uncertain parameters, it is more practical and reliable to measure directly the step and touch potential in the area using an appropriate instrument and method.

 

[mbk2k3]

@cuky2000

thanks for commenting.

1. which IEEE standard is this?
2. isn't a higher clearing time worse?
3. how would i go about modelling the steel rebar? just as 2/0 grounding conductors spread all around? We don't have good drawings for existing facility (its a 30 year old building).
4. this is a good suggestion, I will ask our testing subcontractor about this.

 

[magoo2]

Look at IEEE 80.

 

[7anoter4]

I agree with cooky2000 and magoo2.I’ll try to extend their answers.
1) See IEEE 80/2000 ch. 15.9 Computation of current division and Annex C.
2) You need two short-circuit currents and two protection clearing times:
a) In order to state GPR level you need only the current from the utility reduced with division factor and the protection
clear time 0.1 usually is fair enough.
b) In order to state the grounding conductor size you need all the current flowing through grounding grid and locally
even low voltage current. The protection clearing time standard is 1 sec- usually.
3) You cannot modeling the foundation grounding and in my opinion if it is not prepared when was built it is not possible
to use it.
4) This is a good suggestion as you said.

 

[basilasq1]

You might want to complete a Fall of Potential test if possible to see what the ground resistance of grounding system is like. If bonded proprely, the building rebar would be very beneficial in dissipating ground current.
If possible, you may also consider installing a ground wells and remote ground grids.

Ground wells are a common solution for GIS substations installed in urban areas with desert-type sandy soil.

 

[mbk2k3]

i have a follow up question for cuky2000 and magoo (and anyone else really):

i have a testing subcontractor that can measure GPR + step/touch potentials, however is claiming that the best place to do this is outside the plant building in the back field.

i don't understand that. if there are any GPR/step/touch potential hazards to be concerned about, wouldn't they be inside the building where the equipment is and the operators may be walking around? what would measuring GPR/step/touch outside the plant building accomplish?

the grounding studies i've seen in the past are done using modelling the grid, soil resistivity, etc and running simulations that produce nice graphical views of the GPR/step/touch potentials INSIDE the plant....

 

[cuky2000]

Grounding measurement requires inserted probes into the earth in the target measured area. For indoor testing on the power plant, holes need to be made in the concrete slab and some contractor may be reluctant to hit something below grade that gets into liability issues.

I was involved a few years ago in an unusual grounding analysis for a new indoor substation to support a superconducting project to be built in for an old coal power plant building built in 1920. The building was housing a modern combustion turbine in operation with the original grounding conductor running above the basement floor but connected to each steel columns and foundations. The plant manager does not allow cutting the slab and installing a new ground grid.
To determine if there was any issue with the step and touch potentials, testing was performed using a Hood-Patterson & Dewar Smart Ground Meter, Model SGM-2001 with Win SGMD 5.1b software. We convinced that plant manager to allow drilling in the concrete slab of the basement 1 inch diameter set of holes to insert test probes 3/8 inch in diameter to a depth of 18 inches. These holes were then filled with a soil mixture in order to guarantee a good connection between the probe and the surrounding earth/concrete. Underground utilities as well as rebar were present during the time step and touch voltage measurements were taken and analyzed in case impact the results obtained.

To the amount of underground metallic conductors including steel rebar in the concrete slab the end result was satisfactorily without the need to cut the concrete and insert a new ground grid. To maintain the safety integrity on the basement in the event of large fault from the new substation and the superconductors the new substation grid and the old plant grounding system were interconnected to few deep ground electrodes built across the street and interconnected with the steel encased of pipe-type cable and the sheath of new HV solid dielectric cables.
I hope this help.

http://www.ap-concepts.com/win_sgm_func.htm

 

[cuky2000]

Below is an except of the Step and touch potential measurement inside a building.