Neutral CT flashing in 3-phase transformer bank 3

[any23]

On charging and loading a spare 765kV, 500MVA 1-ph transformer unit with other two transformer units in ICT-bank operation in a sub-station, sparking was observed at terminal connection of CT provided after star formation at neutral of individual units.
What may be the reason for the following occurrence and probable root cause analysis.

R-phase - Spare transformer - impedance 12.87 - Current on Load - 435A
Y-phase - existing transformer - impedance 14.12 - Current on Load - 380A
B-phase Existing transformer - impedance 14.12 - Current on Load - 340A

 

[waross]

Probably an open secondary circuit. Possibly a poor connection.
Depends.
Is the arcing between terminals or at a single terminal?

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

 

[prc]

Normally the impedance variation between single-phase units in a bank is limited to 3%. In this case, it is almost 9 %. Probably this is contributing to the unbalance in load and naturally, there will be a neutral current due to this variation in load between phases. The sparking may be due to the looseness in connection as there will be a current through the neutral lead to ground.

 

[any23]

@waross (Electrical), @prc (Electrical)

I have received a report from the site regarding the issue. @@waross (Electrical), I have to get an update regarding whether - the arcing between terminals or at a single terminal?

what may be the resolution to this issue?