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]

Referring to pwrtran's sketch.

If the 0.6 kV neutral impedance is low compared to the ground rod resistances, then the voltages V1, V2, etc will be nearly identical (no significant voltage drop in neutral). The currents will split in inverse relation to the grid and ground rod resistances. Higher service ground rod resistance gives lower ground current, but the same voltage.

The single point ground system could result in very high GPR at the services during a ground fault at the service because all of the fault current has to flow back through the earth.

There is no sketch of the OP's suggestion of isolating the low voltage neutral from ground at the MV/LV transformer and grounding it only at the services. In this case, there is no GPR from a 13.8 kV ground fault because the neutral is not connected to the ground grid. If there is RCD on the LV at the MV/LV transformer, then the concern for tripping ground faults at the MV/LV station may be resolved (if it is sensitive enough), but there may be not selectivity with RCD at the services. A ground fault at a service may result in tripping the RCD at the station and all services may be interrupted for a ground fault at one service.

 

[pwrtran]

how can you prove that Ig2 will be smaller than Ig1?

Because Ig1 is out from the MV/LV station ground grid that is normally a mesh of ground electrodes, but Ig2 is out from a pole ground normally it is only one ground rod. So, Rg2 is normally > than Rg1, plus the neutral conductor also has some impedance thus some voltage drop.

 

[cmelguet]

Pwrtran

Ok I agree with you that Ig1 will be higher than Ig2 in most of the cases.

Should I dimmension the ground rods at the service point as a ground grid instead? I ask becouse even if Ig1 is higher than Ig2 you still need to warranty the step and touch voltage at the service points. Strange but i have only see single electrodes at the service points, i dont understand why?

 

[pwrtran]

cmelguet

I would say come to this point you should go by the local electrical safety code to control the step and touch.

 

[odlanor]

cmelguet
I opened your attachment but only found a page with no drawing. I did not understand.

pwrtran
the attachment shows my point of view. I confess that it is necessary a more detailed analysis of the object. Perhaps transformer: phasor diagram, polarity (additive / subtractive), type (core / shell), load.

 

[jghrist]

odlanor,
Your sketch is the same as pwrtran's Multiple Ground System except that it does not show the return current through the service ground rods. There will be some because these are paths in parallel to the main return directly through earth from the substation grid.

How much will flow depends on the relative impedance of the parallel paths. Let's make a simplified example. It is simplified because it does not take mutual impedances into account. Assume a 1 ohm grid resistance and 25 ohm ground rod resistances at each of two services, the first 500m from the station and the second 1000m away. Assume 5000A 13.8 kV ground fault current.

2 MVA at 600V is 1924A, so let's assume a neutral of 2-500 mm² conductors. Impedance will be about 0.05 ohms/km. Total parallel impedance of the three grounds will be approximately 0.926 ohm. Total GPR will be 5000·0.926 = 4630V. Current through earth directly from the station grid will be 4630/1 = 4630A. Current through each of the service grounds will be about 4630/25 = 185A. Voltage drop through the neutral to the first service will be 185·2·0.05·0.5 = 9.25V, leaving a GPR of 4630-9 = 4621V. Voltage drop through the neutral to the second service will be 185·0.05·0.5 = 4.6V, leaving a GPR of 4621-5 = 4616V.

I've made a bunch of simplifying assumptions like adding impedances to resistances arithmetically, but the point is that there is not much voltage drop in the neutral if the impedance is low compared to the ground rod resistance. The GPR at the services is about the same as at the station. Is it too high? It depends. The current flowing through the service ground is low, so there won't be much voltage gradient around the ground rod. Touch potentials are low if the voltage gradient is low. There could be a high voltage between equipment connected to the power ground and telephone lines that are grounded remotely.

This is all very difficult to calculate accurately. The situation is quite common and not considered in most low voltage service safety codes. Have you ever heard of anyone being electrocuted by contacting grounded equipment at a service remote from a substation during a station MV fault? I haven't.

 

[cmelguet]

jghrist

Great example and calculation. It actually explain my doubts. The problem is that you not allways have a high resistance at the service points, compare to the main grid at the MV substation. Imagine is not a house service point, instead is a compresor house with a 10m x 10m earthing grid area. I might have depending on the ground resistance a smaller equivalent earth grid resistance than the resistance at the Substation grid. How I will dimension in this case the ground grid at the compresor house?. Should I follow the IEEE 80 std , which I understand is for substations and not for LV service points?.

 

[odlanor]

I do not agree with the flow of ground currents of Pwrtran anexx because:
1 - 13.8 kV system does not transfer ground current to 600V system because of 13,8kV-delta of winding SE-Cmelguet . All ground current ,return from SE-Cmelguet to SE- Source
Hence, the flow has no sense of IG2, IG3, Ign to SE-Source.
2-On the other hand, if really GPR at SE-Cmelguet injects ground current at 600V neutral system, then the return current should be came back to the remote-earth of SE-Cmelguet or disappears at the service points remote-earth.

I have no familiarity with low-voltage design. You are calculating the currents of a 4-wire system instead of 3- wire system shown in the Pwrtran Annex.
I think the Cmelguet system representation is more representative in my Annex, extracted from Std. IEEE-80 -2000.

 

[pwrtran]

odlanor

Then what is the 448A current flow in IEEE 80 Fig 31 and where does it flow to?

On the same page of the standard, Section 15.6 gives you the answer!

 

[odlanor]

pwrtran,
There are 3 point or bus to consider in grounding system:
neutral bus(system), ground bus(enclosure), earth bus(grid).
IEEE 80 Fig 31 , 448A current flow from earth bus to ground bus ,not from earth bus to neutral bus.
In your Fig. it goes from earth bus to neutral of 600V star winding.

Section 15.6 doesnt apply in my interpretation.

 

[trosszilla]

The neutral connection will reduce the voltage rise at the substation especially if it is a dense MEN as in a city. You can have literally thousands of MEN connections so the transfer hazard is negligible. If fact disconnecting the MEN may cause hazards in the vicinity of the substation as the voltage rise of the earth mat may increase considerably.

Disconnecting the neutral would only be considered if the MEN is sparse such as in a rural environment and the transfer voltage causes problems, or if you are supplying a mine which has much more stringent safety criteria.

The only way to determine is there is a problem is to test and measure the transfer directly.

 

[pwrtran]

odlanor

Unfortunately, current will find its way back no matter you like it or not, or how you describe it as a neutral bus or a ground bus, as long as there is a path.

In AC system, current will flow if any of the below two conditions is true:
A) potential difference
B) torque angle difference

You have to respect the ohm's Law. In your example, the Wye winding of the MV/LV station transformer is grounded, which means it is tied to the station grid. Also the neutral conductor is multiple grounded along the distribution line. When a MV line-ground fault occurs in the MV/LV station, the GPR will bring the neutral conductor to the same potential at the point where it tie to the grid. As the neutral conductor leaving the station, there will be potential difference between the neutral and the pole ground where the neutral conductor also bonded to ( which is a so called multiple ground system), thus there will be current flow. That is why IEEE 80 shows how to calculate the current splitting factor.

When designing the ground grid, one fact you need to take into account is the sky wires, neutral conductor, and even other metallic objects as the auxiliary ground electrodes, which will dissipate the Ig. Please refer the Section 9.5 of the IEEE 80.

I don't think your statement "Section 15.6 doesn't apply in my interpretation. " holds stand.

 

[jghrist]

One thing that is different in the IEEE 80 figure from the OP system is the existence of a metallic ground wire from the substation grid to the remote source. In the OP system, there is no metallic MV ground wire, so all fault current has to flow through the earth, either directly from the station grid or through the LV neutrals to the service grounds. A MV ground wire would greatly reduce GPR by providing a low impedance path for return current other than through the earth.

 

[odlanor]

pwrtran ,
You Fig represents a 0.6kV 3-wire system instead of 4-wire Cmelget. So,"...the pole ground where the neutral conductor also bonded to (which is a so called multiple ground system.. ", is not true.
Neutral of 0.6kV at Cmelguet-SE is bonded to ground grid and thats all!
2-On the other hand,in your Fig., if really GPR at SE-Cmelguet injects ground current at 600V neutral system, then the return current should be came back to the remote-earth of SE-Cmelguet or disappears at the service points remote-earth, instead return to Source-SE.
I have been analysing your Fig. without any existence of metallic ground wire, counterweight grounding cable and so on.

 

[pwrtran]

Then I would say you looked the wrong link! Please check my post again

 

[odlanor]

pwrtran ,
it will be the second wrong link!First one was from Cmelguet.
I´m sorry. Lets stop here.