Unrestricted Fault Current / Isolation Transformer
Unrestricted Fault Current / Isolation Transformer
(OP)
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?
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?






RE: Unrestricted Fault Current / Isolation Transformer
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.
RE: Unrestricted Fault Current / Isolation Transformer
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RE: Unrestricted Fault Current / Isolation Transformer
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.
RE: Unrestricted Fault Current / Isolation Transformer
RE: Unrestricted Fault Current / Isolation Transformer
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?
RE: Unrestricted Fault Current / Isolation Transformer
RE: Unrestricted Fault Current / Isolation Transformer
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.
RE: Unrestricted Fault Current / Isolation Transformer
RE: Unrestricted Fault Current / Isolation Transformer