Earthing resistance and calculations 5

[AusLee]

Hi,

I'm a bit new at substations grounding and I have given myself an exercise to do it for a 1 MVA 11/0.4kV kiosk substation but I need assistance please:

1. Is there any Free software that can help me with this?

2. A friend of mine at work uses a decent software in his department (ETA Power Station). I was surprised when he showed me the screen for the soil resistivity: you specify only the cover, upper layer (and its depth) and lower layer. Shouldn't there be a procedure to go on site and use a "Shepperd" stick (vertical rod with battery and two electrodes) and take measurements across several points on the site? And if that is done, what do I do with all this "valuable" earth resistance site survey if a software as powerful as ETAPS does not need it?

3. I understand that IEEE 80 and the rest of the stuff is more towards "zone substations"/"outdoor switch yards" with sizeable area. What about a 1 MVA kiosk substation? Historically, i think all that is required was two deep driven electrodes, one at the HV and one at the LV sides. As i'm trying to understand this better, the fault level of this substation at the LV side is around 25kA. From the equation giving the voltage at a distance r away from the electrode or the grid:

Vr = Icc * rho / (2 * pi * r)

if the short circuit current Icc alone is 25kA, the Vr will be in the order of 3-4 kV, which is way above the acceptable level of a step voltage (should be not more than 100V if the protection trips in 0.1s to my understanding, maybe IEEE 80 allows a bit more).

So if this calculation is correct, has it been "always wrong" to use only a couple of rods, and indeed a grid that covers and extends beyond the area where people might be standing near the substation?

I hope someone has a short and quick step by step guide they would like to share.

Looking forward to some instructions

 

[EddyWirbelstrom]

On row 34 of the Ground Mat Results tab of file Eng-Tips_Thread238-279051-02.xlsx, replace 100A with 222A.

 

[ScottyUK]

Eddy - Why not ask the moderator to remove the last few posts and then re-post the corrected spreadsheet? The 'Red Flag' button is a useful way to contact the site owner.


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If we learn from our mistakes I'm getting a great education!

 

[AusLee]

Eddy,

In: Eng-Tips_Thread238-279051-01.xlsx

Can you please detail a bit the equation on row 77?

Thanks.

 

[EddyWirbelstrom]

The data on rows 54 to 91 of the spreadsheet Eng-Tips_Thread238-279051-01.xlsx is incorrect and should be removed.
Rows 54 to 91 had incorrect data on the prospective single phase to earth fault current on the 415V side of the 11 / 0.415kV, 1.0 MVA transformer. Only the 11kV system should be considered when determining the touch and step voltages.
(For interest rows 54 to 77 of tab 1_Input Data of attached spreadsheet Eng-Tips_Thread238-279051-03.xlsx shows calculated bolted earthfault on 415V terminals of 26425A

I have used SKM Ground Mat software to calculate the touch and step voltages for two earthing configurations of the transformer kiosk assuming :
1. Kiosk occupies an area of 5m x 3m,
2. 11kV system prospective short-circuit current is 250 MVA,
3. System z1 = z2 = zo,
4. Standard IEEE 80, person weight 70kg barefeet, fault duration 0.2 sec, No Decrement factor used, soil resistivity 100 Ohm-m, No surface blue-metal, Earth Electrode L = 4.5m, DIA = 8 mm, equal current flows in earth and OHEW, Depth to top of Electrode /Grid = 0.5m.
5. Calculated total current 3Io for an earth fault of the 11kV system at the transformer Kiosk = 443A.
6. Current flowing into the ground = 222A

The results of both the following configurations are shown on the attached spreadsheet.

Configuration 1.
Electrodes at diagonally opposite corners (distance between electrodes 5.83m)
See tabs prefixed with 1_ on attached spreadsheet Eng-Tips_Thread238-279051-03.xlsx

Configuration 2.
Horizontal meshed conductors spaced 1.0m form a grid with earth electrodes at each corner.
See tabs prefixed with 2_ on attached spreadsheet Eng-Tips_Thread238-279051-03.xlsx

The tab 2_More Electrodes shows the effect of adding electrodes to each intersection of the horizontal conductors at the grid periphery.

 

[jghrist]

5. Calculated total current 3Io for an earth fault of the 11kV system at the transformer Kiosk = 443A.
6. Current flowing into the ground = 222A

You assumed that all of the current flowed through the earth when calculating the 443A.
3 x Ibase / ( 3 Rg + R1 + R2 + Ro )
You can't then assume that only half flows through the earth. The half that flows through the ground wire will not be restricted by Rg.

How did you determine that the current split is 50%?

 

[AusLee]

Thanks for that.

1. I second jghrist's question regarding 50% split(?)

2. As per Eddy's calculations (and sorry for the caps)

TWO ELECTRODES ONLY ARE NOT ENOUGH FOR A SIMPLE KIOSK SUBSTATION FED FROM A NORMAL SOURCE (250MVA) AT 11kV

I am not questioning Eddy's calculations, it's just that no matter how you twist the calculations, you will always get that result. I have seen really countless kiosk substations approved with only 2 electrodes.

Can it be that the concrete plinth >>is<< the earth mat that appeared in Configuration 2?

3. This one is very confusing: in ETA Power Station software, draw a very simple diagram:
Utility (250MVA, 11kV0, connect to Bus 1, connect to TX 1MVA, connect to Bus 2, connect to any load.

Run load flow then run short circuit (select both buses for fault evaluation).

Place an earth grid on the diagram, right click and select Update Fault kA: it will take the largest fault which is around 25kA for the secondary side of the TX. Even if I exclude Bus 2 from fault analysis, it will evaluate 13kA for the primary and carry that value into the earthing grid module. Once this value is in the earthing grid module, there is nothing you can do to get the kind of touch and step voltages you're getting here. If anyone uses ETA Power Station can you please advise what can be wrong?

and even when you select the IEEE 80 method, it will still carry on the highest fault value (which is on a secondary LV side) even though IEEE 80 (as discussed above) considers only the HV side.

Eddy can you please upload the original calculation file in PTW Ground Mat format?

 

[EddyWirbelstrom]

You are correct jghrist, I have corrected my spreadsheet. If there are other return paths for earthfault current - such as overhead earth wires (OHEW)- then 3Io will be calculated using grid resistance (Rg) in parallel with the OHEW resistance. I used a 50/50 split as suggested in Auslee's posting of 16 Aug 10 18:30.

I am new to touch and step voltage calculations. Can anyone suggest a method they use to calculate the split of earthfault current (3Io) between the ground and the overhead earthwire of an overhead line ? My 50/50 current split is a guess only.

The 13kA at 11kV you mention AusLee is for a bolted earthfault with no earth grid resistance. If two electrodes are used the earth grid resistance (Rg) = 13.85 Ohm. Assuming there was no other parallel path ( such as OHEW) then the total earthfault current would be 443A.

My configuration 2 consists of a mesh of 8mm dia conductors at 1.0m spacing with a 4.5m, 8mm DIA electrode at each corner. It does not model a concrete plinth - I will investigate how to simulate a concrete plinth.
For the grid in Configuarion 2 the Rg = 6.6 Ohm which would result in a total earthfault current of 897A. If there is an OHEW the total earthfault current would be larger and some of this current would return to the source via the OHEW.

Tab 4_Ig=1193A shows a third configuration where 4.5m earth electrodes are installed at the intersection of each earth grid mesh. The earthgrid resistance is 4.84 Ohm resulting in a total earthfault current of 1193A flowing only in the earthgrid. Touch and step voltages exceed the tolerable levels.

If the overhead line includes an earthwire ( OHEW ) connected to earth electrode(s) at each pole, then detailed information on the overhead line, earth wire, span, earth electrode details, earth resistivity would be required to determine the current split. Is star point of the source transformer solidly earthed ?
Current split between ground and earthwire is required to calculate touch and step voltages.

I have used SKM’s Ground Mat software for the above calculations.