2-Phase 415V Load Using 3-Phase Transformer (Dyn11)

Why do you think current will flow to earth if your load is connected between two phases of a 3 phase system?

This will not have any deleterious effects on anything down stream of that transformer, because nothing else is connected. If something else were connected to the 3rd unused phase, there might be a voltage drop because of the heavy load on the other two, and that would have created a severe imbalance. If you had another 3 phase load connected to that transformer, it would definitely have a voltage imbalance and that may adversely affect that load.

Then on the line side of the transformer, there may be an imbalance issue depending on the total connected load on the primary, but without knowing anything else about the primary side and how much of a burden this transformer represents to it, there is no way to tell.

When your welding device is connected between 2 phase connections: you refer to U and V, than your transformer delivers power out of 1 phase and not 2 as you mention.

(in a delta configuration)

You give no explanation about Y or delta connection on the secondary. If you connect in delta, and your whole transformer is rated twice the power of your welding equipment, then this phase could be overloaded, resulting in warming up, heating up and eventually melting the copper windings. It all depends on how "continuous" you will charge the transformer. As far as your specs, the transformer will probably do the job.

When your welding device is connected between 2 phase connections: you refer to U and V, than your transformer delivers power out of 1 phase and not 2 as you mention.

Just FYI, elsewhere in the world, they refer to two-out-three phases as "2-phase" power. Here in North America we don't do that because we do actually have true 2 phase power still in some areas, it would be too confusing, as evidenced by what happens when we mix in forums like this.

The neutral must be grounded otherwise a phase to earth fault will go undetected until you get a second fault on another phase. Then things will go with a big bang!

Why is the third phase absent?

I want to know if the transformer is your own or government own electricity supply company?

Such unbalancing might cause negative sequence currents to flow in the transformer and might cause permanent core saturation. One solution could be to go for a three-phase/single phase, 415V/415V intermediate transformer, so that the welding machine can be connected to the single phase 415V secondary of this IT, whose 415V, 3-Phase Primary may be connected to the LV 415V side of your Dyn11 Trf. Thus, balancing on the three phases of the Dyn11 secondary could be achieved.

A LeBlanc transformer is the ideal solution.

How much money have you got?

Current flow to earth/ground constitutes a safety-related fault, which must be disconnected by the circuit protective device(s). The remedy is to isolate and remove the fault before re-energising.