As well as demonstrating that it can be done, the research paper (which was presented at the Cigre Study Committee B5 Colloquium) shows the formulas necessary to transfer the fault current from one side to the other side of the special transformer based on the actual phase shift angle.
The calculations were done for angles between 60 and -60 degrees, as few special power transformers go beyond such angles, but within those parameters it proves possible to create a set of formulae enabling to predict how a fault current will be transferred to the other side of the transformer.
So, for example, in the case of a 20kA single phase to ground fault, on the load side of a 600MVA, 400kV/400kV, phase shifting transformer with an angle shift of 40 degrees we can calculate the source-side currents will be 85, 45, and 30%, resulting in currents of 17, 9, and 6kA respectively.
The same process can be applied for any type of fault and even for a converter transformer, permitting the calculation of High Voltage side currents from the faults on any low-voltage winding.
It has been shown that individual phase currents on the source side can be bigger (as a percentage) than the load-side currents, in the case of a phase-to-phase or phase-to-phase-to-ground fault on the load side. If this isn’t allowed for in the relay settings it could cause problems with coordinating backup protection, which should be a concern for anyone having commissioning variable-phase special transformers.