Fault Current in Substation Grounding Design

Earthing has to been done based on the worst case earth fault current based on IEEE 80 consdering touch and step potential since its 138kV system.

IS THERE A FREE REFERENCE FOR THE IEEE 80?

Please contact me privately if interested.This can be done by professional designer for a cost.

http://www.omicron.at/fileadmin/user_upload/files/pdf/en/2006-Earthgrid-Impedance.pdf

http://www.iaei.org/magazine/?p=3165

Your question is not clear. Do you want to design earthing system for the switchyard. You cannot choose fault current for the grounding system. It is as per your question and to my understanding 21KA on the LV side. Please clarify then we can discuss further.

BB Raina

Hi

Thanks for your reply. I want to design the earthing system for 138/25 kV substation. I am given the ultimate fault current as an input for my grounding design. The ultimate fault current is given below.

138 KV Single line to groung fault(SLG)-22KA

25 KV Single line to groung fault(SLG)-22KA

25 KV feeder 2 line to groung fault(LLG)-14 KA

The transformer is Delta/star. LV secondary is grounded through NGR.

My question is which side fault current would be worst case and I could consider for the grounding design. My understading 138kV side would be the worse case because if any fault inside the substation in 138kV side, the 22KA current flows through the ground grid to source. If any fault in 25 KV feeder side, the feeder reactor will reduce the current. If any bus fault in 25 KV side, max current flows through NGR. Portion of current flows through ground grid. Please correct me If I am worong.

Thanks

Bishwajit

My question

As I understand, your 138 KV side is the line coming into your substation, and 25 KV is your distribution side; I do not think you can ground 138 kV, for it is probably grounded somewhere else. What you should "put on the ground" is all metal non-current carrying parts of transformers and gears, fences, guards, etc., and neutral of your output of transformer. You shall bond them up and ground them. The main idea of grounding is to make sure that there is no potential between the earth and your metal parts even if the ground fault occurs; that is why you drive roads into the earth and connect them to your equipment with GEC (Grounding Electrode Conductor). As the earth is not good conductor, it "saturates" with current very quickly, so you do not need consider very large GEC: the maximum size is 3/0 copper or 250 kcmill aluminum (you can put bigger wire, but it will be waste). So, to connect the earth (actually, your grounding rod system) with your metal parts you need that big wire. After you put it to work, no meter what happens inside your transformer or gears, what kind of fault is there inside or from outside, how much sparks or blasts occur -- in any condition, when someone touches any metal non current-carrying parts, he/she will be safe, because current will not come through the body; GEC and human body are in parallel in this condition, and human body is not good conductor, so the major current will travel through the GEC to the earth, leaving for body just very small amount that person will never feel. The size of GEC is based on the size of your biggest current carrying conductor:

over 1100 kcmill copper or 1750 kcmill aluminum you need 3/0 copper or 250 kcmill aluminum wire.

over 600 through 1100 copper or over 900 through 1750 aluminum you need 2/0 copper or 4/0 aluminum.

over 350 through 600 copper or over 500 through 900 aluminum put 1/0 copper or 3/0 aluminum.

Put your electrodes as far as possible one from another (around the substation -- distance meters), drive them deep, and bond them up; bond every section of your fence together (they should be electrically connected between each-other and all with grounding system).

Put on your safety gloves and glasses while working! Be safe and make others safe on your substation! Good luck!

Hi

Thanks for your reply. Which side fault currrent should I consider for the grounding design? It will be 138Kv side fault or 25 kv side fault? The 138 KV side is delta connetion but if any line to ground fault in this side inside the substation, then all 22KA current will pass through the ground grid to source. If fault in 25 kv side, even the fault current is 22 kA also, the all current will not pass through ground grid. Part of cuurent will be pass through NGR. In this case I sould consider 22 kA 138 kV side for my design. Please correct me if I am wrong.

I have one more question. One of my switching substation's calculated ground resistace is 2.16 ohm. The design fault current was 3 KA. After ground resistance test , the measured value is 0.86 ohm. I found the relay panel box which is grounded ith main ground grid, one 120V Phase come to this relay panel box with neutral from outside single phase transformer. The transformer installed on pole connect to 25 kV line. The LV of transformer is 120V which comes to substation to orepare disconnect switch. The neutral conductor comes from 25 kv line neural and the size of conductor is #3 Ascr. If any fault occurs, the neutral conductor size is ok to carry the current?

Don't make a mistake: grounding system is not designed to open circuit in case of fault (there are other fault-current-protecting devices for that purpose); as I said, the purpose of grounding is to eliminate dangerous electrical potential between metal non-current-carrying parts and ground. So, you connect the earth (with grounding roads and Grounding Electrode Conductor) to all metal parts and bond those parts to eliminate potential between them too (you can ground those parts and bond their rods together). GEC is sized based on the size of your largest current-carrying conductor in your system, not on ampere rating of the system.

What may happen is that any current-carrying conductor (inside or outside) of any voltage can accidentally touch metal non-current-carrying parts like transformer shell, fence, etc. (It is not necessary that you will see sparks or hear the sound when it happens). In this case, if system is not grounded properly, whoever touches those metal parts will get shot, for current will travel through the person's body to the earth (or to another metal part if they are not bonded together and the person touches both of them).

To eliminate this, you connect all metal non-current-carrying parts together and connect them all to the earth. In low-voltage systems it is enough to have 20 Ohms resistance or drive two grounding roads 6' apart. In high-voltage system, however, it is not enough because there is another danger: Step Voltage. This is the voltage between your feet when you walk (it may kill). That is why you need several rods driven into the earth as far from each-other as you can afford (may be you can afford 5 miles distance; you don't have to do so -- somewhat around the substation, near the fence is OK).

Step-voltage occurs when 138 kV wire falls on the ground or to metal parts that touch the earth; the current intends to come to its source (back to 138 kV transformer that is grounded), but first it spreads around the place where the accident happens; the closer to the place -- the higher the voltage on the earth and vice verse (because earth is not a good conductor). That is why you may feel the electricity coming through your legs if one foot is closer to and another is farther from the place of the accident. Nothing like that will ever happen if you have several rods in the earth bonded together (or simply said, grid).

So, you can drive your rods near all your transformers, gears, fences, and all metal parts that potentially can be energized (always consider the worst case that can happen, like "what if my 138 kV touches the fence and people can be there," or something like that, and design your grounding system accordingly). Bond all your rods together so that you have one single rod system; connect your rods to your metal parts with GECs, and that is it.

Do not put aluminum into the earth! Rather use copper (aluminum only inside).

If your GEC comes through the metal conduit, make sure that it is choked (electrically connected) to the conduit on both sides of it. Otherwise, in case of fault, the strong magnetic field may destroy your GEC, and you will do without grounding on some part of your system.

Your zero point of transformer secondary and/or neutral are, probably already bonded to its metal parts.

Your second question. If you have less resistance, it is even better.

"If any fault occurs, the neutral conductor size is ok to carry the current?" The wires coming from 25kV/120V transformer are called Service; they are not usually protected up to the service box, but the neutral of that transformer should be grounded (again, the size of GEC is based on the size of the largest conductor of that transformer). The neutral of the service is good to withstand current faults; there are over-current-protective devices in service box and after it that will open faulty circuits, so your neutral should be OK. Every transformer, including that for substation service should be grounded and connected to the main ground grid; that's OK. The neutral from your LV transformer is also called "Grounded Conductor". It should be grounded in the transformer, then it comes to the service box where it is bonded to the box and Equipment Grounding Conductor (EGC) with bonding jumper. EGC is different from GEC; it is larger because its purpose is to carry ground-fault currents, and its size is based on the ampere rating of the circuit. Fault current will travel back to the source, not to the earth. Grounding Electrode Conductors are smaller because they carry only as much as needed to eliminate potential between earth and metal parts (this current will not open the circuit breaker or activate your relay to open the circuit, but it will make the potential on the earth equal to the potential on metal parts, and you will be safe, if you survive the blast <just kidding>).

I hope it helps, good luck!