Many posts have discussed about transformers in parallel, however in one of them many people commented: "There is not circulating current if both primary transformers are connected at the same bus".

That answer is wrong and is a myth to think that.

Let me share you the following:

Two three-phase 10 MVA 66,000Δ-12,470Y volt transformers were in parallel operation in a substation. The primaries of the two transformers are connected to a 66 kV transmission line through a single air break switch.

This switch is designed to interrupt magnetizing current only, which is less than 1 A. The transformers were being removed from service and the secondary loads had been removed. A switchman then started to open the air break switch, expecting to see a small arc as the magnetizing current was interrupted.

Instead, there was a loud "bang" and there was a ball of flame where the air break switch contacts had vaporized. Something was obviously wrong.

What happened?

The answer is: Circulating current.

Closer inspection, revealed that the two transformers had been set on widely different taps: The first transformer was on the 62,700 V primary tap and the second transformer was on the 69,300 V primary tap. Both transformers had a 7% impedance. Because the turns ratios were unequal, a 6 ampere per phase circulating current was set up even without any secondary load.