Unrestricted Fault Current / Isolation Transformer

[acog]

I have a problem with a new substation which is fed from an unrestricted 11kV feed. The substation upstream has an 11kV fault level of approx 7kA and is located 200m away.

I want to reduce the fault current so that the touch voltages within the new substation comply with IEC/AS standards.

Currently the plan is to install an 11kV/11kV isolation transformer (Dyn) with an earthing resistor on the neutral.

However, a fault (for example) from the incoming side (delta) of the isolation transformer to the tank will bypass the earthing resistor, and put a full 7kA fault to the ground grid again.

To solve this I have suggested moving the isolation transformer to a remote location and restricting access. This is problematic due to space contraints and high EPR.

Is there a better way we can pull the fault current down?
Are my assumptions correct on the earthing transformer?

 

[dpc]

You might want to verify the available fault current for both three-phase and line-to-ground at your site. As the distance from the source transformer increases, the line-to-ground fault current will generally fall off more quickly than the three-phase.

I don't know anything about the IEC touch potential requirements, but 7000 A of fault current in a substation is not really that high, at least around here. For outdoor substations, we deal with this using ground grid, crushed rock surface, and extensive grounding of all equipment, using IEEE 80 as a guide.

 

[ScottyUK]

How is the source end earthed? Is 7kA the line-earth or line-line fault current?



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

 

[acog]

3 line to ground fault current at upstream bus is 7.6kA (all + seq current)

Single line to ground fault is 7kA (0 seq, + seq and - seq currents are all 2.3kA)

Phase-phase fault current unknown.

Source end is solidly earthed with a grid resistance of approx 1 ohm. Unfortunately we can not modify the source substation, it is not ours.

 

[jghrist]

The touch voltages depend on the amount of Ø-grd fault current flowing through the earth back to the source substation. If the substation is only 200 m away, most of the return current will flow in the 11 kV line neutral if there is one, not through the earth. If there is no 11 kV line neutral, you should consider adding one or otherwise metallically connecting your ground grid to the source substation ground. The isolation transformer will do nothing to improve touch voltages for a fault on the primary of the transformer or on the cable leading to it, no matter how far away it is.

 

[acog]

Thanks for your input jghrist,

No neutral/over head earth wire exists between the two substations and we are not allowed to install one. We are also not allowed to bond the substation earth grids together.

The isolation transformer is intended to be placed a significant distance away from our substation. no access will be permitted within a calculated radius of the isolation transformer (fenced off with non-conductive fence) I agree with your comment, we can not restrict the incoming fault current.

Our substation will be fed from the Dyn isolation transformer secondary windings (neutral current limited to a few amps via resistor on the neutral). This should provide a restricted earth fault current, with no touch potential issues in the new substation.

Is there a better way to do this than what I have just described?

 

[stevenal]

Do what dpc said. This is not done at the source, it is done at your facility.

 

[acog]

stevenal / dpc:

How can I verify the reduction in fault current at my location when there is no earth wire (if by verify you mean a site test)?

I have the grid resistance to remote earth at both substations,and know the conductor type, so I could estimate by simple calculation the earth fault current at my location. This would only be reduced by a small amount.

 

[davidbeach]

Fault current is what it is. Line reactors would lower the fault current, but then you'd have line reactors to deal with forever and always. Just design your grounding system to deal with the available fault current, which is already really low anyway.

 

[jghrist]

Your solution might work if there is no metallic connection between the isolation tranformer ground and your substation. Cable sheaths grounded at both ends would transfer the potential to your substation. It would be better to provide adequate grounding at your substation as dpc and davidbeach recommend.