Ground fault return path

[abc_poster]

Hello fellow engineers,

We have a system with the Grid feeding into a YNd TX and earthed on the secondary side by a grounding TX (zig-zag TX). The system (battery inverter) is connected to the above system through a Dy TX.

There are 2 possible sources that contribute to the ground fault current: Grid & Solar/Battery Inverter

Ground Fault Locations (please see image):

A -

Grid Contribution: I'm almost certain earth fault will flow through the path in green.

Inverter Contribution: Since we have a Dy TX and the ground fault is on the Delta side, there is no return path and therefore is there no fault contribution from the inverter? (if the inverter neutral is grounded does this still hold true?)

Another question is if we did not have the grounding transformer, is there any earth fault contribution from the grid? (since the fault is on the delta side so I think not?)

B -

Grid Contribution: If a fault occurs at B, is there a ground fault current return path? (since the Wye side is ungrounded I think not?)

Inverter Contribution: If the neutral of the inverter is grounded, there will be fault contribution from the inverter.

 

[7anoter4]

If the transformer neutral point is insulated and isolated from earth then
there are two possibilities for ground fault:
1) Inverter neutral grounded
2)phase-to-ground-to phase fault.
If the inverter neutral is connected with transformer neutral point the transformer will contribute to the grounding fault.

 

[7anoter4]

I think this is an explanation:

 

[wcaseyharman]

My opinion:
Both the grid and the inverter (assuming the inverter can source negative sequence current) will contribute to the ground fault through the zig zag transformer. One way to think of it - both sources provide the power to zigzag transformer to provide ground fault current.
In symmetrical component language the grid and inverter provide a positive sequence source and a negative sequence path, the zig zag provides the zero sequence path allow fault current to flow.

B
Assuming that the inverter is ungrounded, which appears to be the most common topology, no significant fault current will flow since the transformer is ungrounded. On the battery I worked on we put a Bender ground detection relay in that location to detect ground faults on that bus - that relay continuously measured insulation resistance to ground.

 

[waross]

Ground fault current will pass the delta as phase to phase current.
All three phases will contribute.
For unbalanced current on A phase, a phase will supply 1/2 of the unbalanced current and B and C phases will between them supply 1/2 of the unbalanced current.
Of B and C phase, the current on one phase will be leading and on the other phase it will be lagging, both at 50%.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!