neutral grounded conductor transferred GPR

[cmelguet]

Hi,

If I have a utility transformer, lets say 13.8 kv/0.6V DY1n, 2MVA, with the neutral grounded at the MV/LV utility substation ground grid, and then I am going to distribute the energy with a 4 wire cable (3ph + neutral)and connect the neutral to ground at every main service point with a grounding electrode.

Correct me if I am wrong but it seems to me that the substation ground grid and the electrodes at each service point are then all connected through the neutral wire.

What happens in case there is a fault at the MV size of the substation? Then all the ground grid at the substation and the electrode system at the service point will be rise to GPR that can be more than the 600 V of a LV system. Does this mean that the electrode systems at every service point need to comply with the IEEE 80 requirements for step and touch voltage?

Why then the NEC recommend this practice (250.24)? Shouldn't we isolate the neutral from the substation ground grid and connected only to the service point electrodes?

Thanks.

 

[jghrist]

Then what happens if there is a Ø-grd fault on the LV side of the transformer at the substation? There will be no return path to the transformer neutral except for a long path through earth to the service point neutral-earth connection.

 

[cmelguet]

jghrist

I understand that if there is a ground fault at the LV side of the transformer the only return path for the fault current will be the earth. This will produce a GPR at the Substation and also at the electrode earthing point at the service. But I think the GPR produced by a ground fault of a LV system is less than the GPR produced by a fault at the MV side. no?

 

[jghrist]

My concern is the ability to sense and trip the fault. Do you have sensitive earth fault protection?

 

[eamonkerrigan]

"What happens in case there is a fault at the MV size of the substation?"

If you have a SLG fault of the MV side of the substation, I assume that this is a grounded source (let's say substation transformer is 115kV Delta - 13.8kV grounded Wye). The ground fault current will circulate within the substation ground grid back to the nuetral point on the transformer. There will be no GPR, just circulating current.

The current will return to the source, not down the distribution line. The multi-grounded distribution nuetral is not really part of the circuit for a substation bus fault. It would be for a HV fault, but that's another story...

(before I get too much backlash, there will be some small GPR, but because both the fault and the source are located within the same ground grid, there will be minimal leakage current into the soil, therefore minimal GPR)

 

[jghrist]

I think the substaion referenced by the OP is the 13.8 kv/0.6V DY1n, 2MVA transformer, not the HV/MV substation.

 

[cmelguet]

eamonkerrigan

If I understand correctly you talk about the following system>

115kV / 13.8 kV Dyn1---line---13.8kV / 0.6 V Dy1n

If you have a fault at the MV size of the 13.8/0.6V substation you will have a GPR at this substation that will rise the neutral and everything connected to it by an electrical conductor. Thats why I have a concern about connecting the neutral to the earthing grid at the this substation.

jghrist

I will have an RCD at the main service.

 

[pwrtran]

Utilities normally bring the neutral along the distribution lines to feed the customer transformers, in your example a typical 13.8kV/0.6kV 2MVA DY1 transformer.

If there is a L-G fault at the 13.8 side of the 2MVA transformer in the substation, say a down wire. Earth is not the only return path, it will paralleled with the system neutral all the way back to the upstream station. Utility tie the neutral to ground every 3 - 5 poles, so the fault current will split between the earth path and the neutral path.

To control the step and the touch voltage, you may vary the size of the mesh and put 6" thick clear gravel in the substation.

 

[jghrist]

I will have an RCD at the main service.

Is there an RCD at the MV/LV substation? Is this what you are referring to as the main service or are there only RCDs at the individual service points? If not, what is the protection at the MV/LV substation?

 

[odlanor]

If we assume 13.8kV remote source is grounded, a ground fault between source and 13,8kV-MV substation, will not circulate ground current at substation.
If grount fault is at 13,8kV busbar substation, will circulate ground current at substation.
Circulating current at neutral 0,6kV could be ocurred only by mutual indutance. GPR will not ocurr at main service point for these faults.

 

[jghrist]

If there is GPR at the station for a MV fault and there is a metallic connection from the station grid to the services (a neutral grounded at the station), then there will be GPR transferred to the service. The GPR at the station will be limited because most fault current will flow back to the HV/MV source through the MV neutral, not through the earth. Some of the fault current will also flow through the LV neutral, further reducing the current flowing through the earth and reducing GPR.

 

[odlanor]

My answer refer to the Cmelguet question:
"... and the electrode system at the service point will be rise to GPR that can be more than the 600 V of a LV system."
I guess faults on 13,8kV system, in theory,will not affect electrode system.

 

[cmelguet]

In my original question the MV transformer is delta connected on the primary side. There is no neutral being carried by the utility.

Odlanor, I dont understand why you said the electrode system at the service point is not affected by faults at the MV side? Can you explain. Thanks.

 

[odlanor]

cmelguet,
1) Some definitions extracted from IEEE Std 80-2000
3.14 ground potential rise (GPR):
The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance.
NOTE—Under normal conditions, the grounded electrical equipment operates at near zero ground potential. That is, the potential of a grounded neutral conductor is nearly identical to the potential of remote earth. During a ground fault the portion of fault current that is conducted by a substation grounding grid into the earth causes the rise of the grid potential with respect to remote earth.

3.15 ground return circuit:
A circuit in which the earth or an equivalent conducting body is utilized to complete the circuit and allow current circulation from or to its current source.

2) Your question 1.
I believe that your 13,8kV system has the neutral grounded at remote substation. This is enough to create a ground return circuit(3.15 above).
If is really not grounded , there will not be short circuit fault.


3) Your question 2.
You will not have GPR at the service point ,because there is neutral current return at 0,6kV system for a fault at 13,8kV system. This is because current is not is conducted by at the service point electrodes into the earth to cause the rise of the grid potential with respect to remote earth.

There is a metallic connection from the station grid to the services (a neutral grounded at the station), then there will be GPR from MV voltage transferred to the service point.
It can be mesh voltage or touch voltage. These voltages are tolerable potentials as your MV grounding grid was sized very well.
See Figure 12- basic shock situations in IEEE Std 80-2000: assume Etrrd=Emm or Etrrd=Emm.

 

[odlanor]

cmelguet,
I'm sorry. There are corrections.

3) Your question 2. ERRATA
............................................
............................................

There is a metallic connection from the station grid to the services (a neutral grounded at the station), then there will be GPR from MV voltage transferred to the service point.
It can be touch voltages. These voltages are tolerable potentials as your MV grounding grid is connected to service points grounding electrode.

 

[cmelguet]

Odlanor,

I cant understand why the voltages at the service points will be tolerable?
If the electrode and the MV ground grid are connected, then both need to be dimensioned for the MV fault current, step and touch voltage, hence the electrode at the service point need to be a ground grid instead of just an electrode. This is like having two connected earth grids. If I dont have a grid how i will be able to control the step and touch voltages at the service point?

 

[odlanor]

cmelguet,
I am only refering about transfered potential from MV to service points.
groundind system service points should be dimensioned for ground fault at service points. They wil define the cables and necessary tolerable potentials.
Tolerable potentials are same at MV substation or at service points.

 

[cmelguet]

I attached a drawing of the question. Hopefully you will understand. Thanks.

 

[pwrtran]

cmelguet

With regard to your original "transferred GPR" question, I have made a sketch to try to explain that a multiple ground system will address your concern.

Please see attached PDF sketch.

Regards

 

[cmelguet]

Pwrtran

Thanks for your sketch. Still have one doubt, how can you prove that Ig2 will be smaller than Ig1? This will depend on the equivalent resistance of the grounding grid and the electrode resistance at the V2 Bus bar. Thanks and regards.