Isolated neutral - Ungounded system - Earth fault calculation

[amadeng]

I am doing a protection coordination study for an 20KV isolated neutral industrial grid.
I need to calculate the earth fault current when an single phase to earth fault occurs. I am not sure about the correct formula. I can neglect the capacitive current from the transformer and I wand to focus on the capacitive current from the cables.
I have found a fomrula Ic=sqrt(3) * 2 * pi * f * C *Vpolar
Is this correct?
Any advice?

 

[aussiejohn2]

Hi Amadeng,
It can be useful to draw out the circuit and understand where the current flows (through the fault to earth, then back through core-screen (or core-ground) capacitance of the other two phases (paralleled), with the circuit completed through the driving source (tx windings). Once you have that circuit straight calculating the current is straightforward. You then need to work out how the capacitance data provided by the cable manufacturer maps onto that circuit - be careful here and check that the (unfaulted) charging current is as expected.
The trouble with relying on formulae is that the way the capacitance is expressed can vary (zero sequence, per-phase, or net capacitance (one core in series with the other two paralleled).
John.

 

[7anoter4]

For capacitive current see:

https://www.postglover.com/download/high-resistance-grounding-application-guide/

However, the phase-to-ground-to-phase presents a high short circuit current you have to take into consideration.

 

[Fuse4]

Dear all.
I am reviewing this topic since time ago. Here the comments:
1. Obtain the Equivalent Circuit of the Ungrounded System: Is a series VLine, Zfault, C circuit
VLine/V3 = Ifault *(Zfault + Xco/3) (Complex quantities)
2. For Bolted Fault (Zf = 0) The total capacitive charging current is:
If= 3(E/Xco)= V3.VLine/Xco = V3*2*pi*f*Co*VLine (similar to Mr. amadeng says)
3. I agree with the comments that Co [uf/km] is key in the calculations (or B0 [Micromhos/km]), and is necessary consider the lengths of the circuit of the all isolated neutral system (very big in distribution systems). Some software help with the model of cables if detailed model is necessary too.

References
- Novak, T., “Analysis of Very-High-Resistance Grounding in High-Voltage Longwall Power Systems”, IEEE Transactions on Industry Applications, Jan/Feb 2001)
- Earth fault Calculation: (Reference: Matti Lehtonen, Tapio Hakola, "Neutral Earthing and Power System Protection", ABB Transmit Oy, Vaasa Finland, 1996)
- Paul, D. Venugopalan, S.I., “Low-Resistance Grounding Method For Medium Voltage Power Systems”, Industry Applications Society Annual Meeting, IEEE 1991.

 

[PUNGLU]

One other aspect that will come into focus is how you connect your current transformers to
measure the fault current. A protection single line diagram would help understand the the co-ordination requirements