Synchronizing Problem

Maybe you need to check your PT at B and make sure it's secondary output is equal to the secondary output of the PT at A so your sync relay can read equivalent values between A & B.. It seems the PTs used at A & C has equal ratios so your sync relay was able to synchronize because they read the same voltage level unlike B..

I would recommend to stick with the configuration of A & C..

Good luck

Thanks for the comments.

Would the phase shift on the transformer be the problem?

Yes.

Yes phase shift can be the main reason actually unless the connection of the PTs or your relay took care of the phase shift.. And you are welcome

If you sync in MV, my recommend your VT both utility and set on MV side A and C, but if you sync in LV you put the VT on B and before ACB set.

Is there a difference between synchronizing on MV side or LV side?

Yes, if the transformer vector groups differ.

No, I mean which one to choose if we can synchronize it in a different voltage level?

I think it would make most sense to synchronize at the voltage that will actually be connected in parallel.