"A three-phase 230 V, 27 kVA, 0.9 PF (lagging) load is supplied by three 10 kVA, 1330/230 V, 60 Hz transformers connected in Y-Δ by means of a common three-phase feeder whose impedance is 0.003+j0.015 Ω per phase. The transformers are supplied from a three-phase source through a three-phase feeder whose impedance is 0.8+j5 Ω per phase. The equivalent impedance of one transformer referred to the low-voltage side is 0.12+ j0.25 Ω. Determine the required supply voltage if the load voltage is 230 V."
The way I did this was to first define the load as a delta load, so that each load takes 27000/sqrt(3) VA. Then, through KVL, the line currents can be found. From the line currents, it's just a matter of working backwards by multiplying the line current by two times the line impedance and one times the transformer impedance. Then, this voltage would be transformed to the primary side (along with the current) in order to find the input voltage (line-to-neutral); this would then be multiplied by sqrt(3) to get the line-to-line input voltage.
However, my answer is wrong, and I'm not sure why (I believe it has to do with the line currents)...
If somebody could help me work this out, it'd be mighty appreciated.