Multiple earth network

[pisimatza]

Hi .
For a multiple earthed network(the neutral wire is earthed at the powerhouse, at the point of supply to each consumer and at other points along the line between powerhouse and consumer), I have to use 3 p ACB, that's clear.

But designer wants Ground fault. And CT on neutral.
I know ground fault works like this: is adding all currents in the 4 wires, and vectorial sum different than 0 initiates trip.
Or, in this case... All the time vectorial sum is not 0, since we got earth links everywhere.
Final question: ground fault not applicable to multiple earthed system?

 

[pisimatza]

I add a little sketch.

 

[jghrist]

You have to have either:
1. CT around all phases and neutral, or
2. Current from all CTs summed.

What if you only had one ground at the PEN or a ground only at the PEN and at the load? A ground fault at the load would also bypass the neutral CT.

 

[waross]

You are showing 4 CTs. These should sum to zero under normal conditions. The two grounds that you show on the right side of the drawing are confusing. Are these fault grounds? If so the circuit will work. If these grounds represent multiple earth grounding, the neutral CT must be past the last multiple earth ground. Normally, the last earth ground will be in the service equipment at the location of the customers breaker. The neutral must be free of intentional grounds between the neutral CT and the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pisimatza]

Waross, you said: "The neutral must be free of intentional grounds between the neutral CT and the load. "
No. Those groundings are all intentional.
Those grounds are the problem, and the normal current(not fault current) will have the shown path in my drawing, i believe! So, the earth fault protection will trip all day long.
Correct?
PS: i talked to an experienced guy; he said that i will have some small, csmall urrent flowing on my shown path(on normal, not fault conditions), but most of current will go through PEN wire. And UEF has smallest setting 40% of ACB current(in this case 0,4*2500=1000 A).

 

[waross]

If you are using ground fault protection inside a facility, You must either remove the grounds past the CT or move the CT. It may be necessary to install ground fault protection at more than one location. If you are using ground fault protection on the distribution lines, it must be set considerably higher to allow for the ground path sharing the neutral current. We have some regulars with transmission line and distribution experience. They are beter able than myself to comment on distribution line ground fault protection on a system with Multiple Protective Earthing.
Can you describe the physical layout and the distances involved, please?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

Waross is quite right. You cannot use sensitive earth fault protection if you have Ø-neutral connected loads with the neutral grounded beyond the CTs, whether multiple grounded or grounded only at the load. Sensitive earth fault protection is normally only used on MV distribution lines if there is no neutral and all loads are connected between phases.

Ground fault protection on multiple grounded neutral systems is set above the maximum zero-sequence load current. Normally, it is either from a summation of the phase currents or from a relay element in the CT residual lead. A separate neutral CT is not normally used for feeders. The ground fault protection for the station transformer would use a CT in the neutral of the transformer as shown by 7anoter4.

 

[7anoter4]

I am very sorry! You are both right waross and jghrist! As the neutral is actually a grounding wire it is impossible to detect-on supply side-the small grounding leakage but only shortcircuit current enough above the load current. Usually in these low voltage net the only protective device is the fuse. The multiground purpose is to avoid neutral disconnection and so to avoid overvoltage due to neutral potential shifting.
Best Regards

 

[7anoter4]

Revising my cancelled post I still think that the sketch is correct. Only a few sentences have to be revised, indeed.
At Transformer side the Ground Fault Relay is connected between transformer star point and the Grounding.
From experience the current imbalance flowing through the neutral conductor will not be more than 50% of the maximum phase current.
For example: S=1000 KVA RatedVoltage=400 V Iphase=1.44 kA let's say maximum per phase current would be 1.2*1.44= 1.73 kA.
The breaker grounding trip current has to be set let's say 0.5*Iphasemaximum.
Then if the setting of ground fault will be Ig=1.73*.5=0.865 kA time=0 sec that means the ground fault will be cleared fast and at still low ground fault current.
At consumer site the GFCI [Ground Fault Circuit Interrupter] will act as usual.
Thanks, waross for your help.

Best Regards