Grounding Grid Design Using Plant Grid???

[ThePunisher]

HI all,

I have been designing several industrial substations in the past and I always design the substation grounding mesh grid per IEEE 80 and as a "stand-alone" grounding grid. This means, I do not consider any bonding to the overall plant grid and design to be inherently safe and not rely to plant grid bonding.

I was given a grounding report (35 kV level and 3 kA L-G fault, solidly grounded system)and the engineer was not able to meet 3000 V GPR and touch and step potentials with expanded substation grid. So he used and modeled the plant grid along in ETAP and of course got his Vstep and Vtouch within limits. However, I am not comfortable relying on plant grid for the safety of the substation. Is this acceptable to some extent or exception???

I was thinking using deep ground rod (40-60 meters) instead of relying on plant grid.

Appreciate all comments and suggestions.

 

[Kiribanda]

1) If the allowable GPR is 3000V and the L-G current is 3000A (3kA), apparently
the designed ground grid should have 1 OHM resistance. Is it not possible to achieve one Ohm
ground grid resistance at that site?
2) Does this GF current really flow through the soil of the 35kV substation so that one is exposed to Touch & Step?
Could you please upload the grounding layout plan?
3) Plant ground grid is mainly to connect all exposed (motor body etc) & extraneous (overhead pipe racks etc)
conducting paths to the ground. Therefore, it is in fact a "SAFETY GROUND" whereas the substation ground grid
is the "SYSTEM GROUND". In my opinion we should keep them separate.

 

[jghrist]

You will not be able to keep the plant grid separate from the substation grid because the low voltage service will have a ground wire that connects to both grids. It is acceptable under IEEE 80 to consider the split of fault current between the earth and both the 35 kV neutral and the low voltage ground conductor to the plant. Part of the return current from a 35 kV ground fault will return through the service ground and through the plant ground grid and earth.

 

[7anoter4]

1) If we consider the power plant as a part of substation then according to IEEE 80/2013 chptr.17.8 Separate grounds "The practice of having separate grounds within a substation area is rarely used for the following reasons
a) Higher resistances for separate safety and system grounds are produced than would be the case for a
single uniform ground system.
b) In the event of insulation failures in the substation, high currents could still flow in the safety ground.
c) Because of a high degree of coupling between separate electrodes in the same area, the safety
objective of keeping the GPR of the safety grounds low for line faults would not be accomplished.
d) Often dangerous potentials would be possible between nearby grounded points because decoupling of
the separate grounds is possible, at least to some extent.
e) Separate grounds can result in large transient potential differences between components of electrical
equipment during lightning or other surge events, causing equipment misoperation or damage.
2)The old IEEE 665/1995 [withdrawn] chapter 5.2.4.1 Generating station to substation interconnection
Generally, one or more high-voltage substations are located on the generating station site. When the substation
is located in close proximity to the generating station (250 m), the substation ground grid should be tied
into the generating station grid by multiple conductors. These connections should include conductors
installed directly under the tie lines to the station step-up transformers. This should benefit both the generating
station and substation grounding system by increasing the available area.
3) We can calculate the common grounding resistance following
Design of Switchyard Grounding Systems Using Multiple Grids

 

[7anoter4]

I have an old xls file for multiple grounds resistance calculation according to Kercel.[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1669016120/tips/Multiple_grounds_Kercel_y8b1ab.xlsx[/url]

 

[DBL-E]

The IEEE Std 80 really provides criteria for touch, step, and hand to hand voltage thresholds. The connection to the plant grid as others mentioned there are benefits in providing a direct metallic bond between the grids (reduce the impedance -> reduce GPR -> reduce touch/step voltages). IEEE 80 generally provides guidance and concepts but the actual design is dependent on the engineer's application. With that, where is the 3 kV GPR limit based upon? I recall there is a Canadian standard that name 5 kV, but gives an exception for studied systems.

Another area you may look first is if your fault current clearing times are accurate, conservative, or overly conservative. The compliance criteria is going to be driven by the soil resistivity, surface covering, and fault clearing time.

IEEE Std 80 section 14.4 understates that there are significant limitations to the calculations (and simple software tools). Sparse grounding, long linear paths (like ground wells), and soil structure all play into how well the simple software/hand calculations aligns with the real world environment. XGSLab and CDEGS are the only tools I'm aware that have options for the complex soil/metallic systems. WinIGS does consider voltage drop, but only has a 3-4 soil layer modeling option.