A $3-$phase, Y$-,3\mathrm{.}81$ kV: $220$ V transformer is formed by connecting three single$-$phase $2\mathrm{.}2$ kV: $220$ V$,50$ kVA transformers. The equivalent circuit of each single$-$phase transformer referred to the low$-$voltage side gives a total leakage reactance of $0\mathrm{.}05$ and a total resistance of $0\mathrm{.}03.$ Core losses and magnetizing currents are negligible. The low$-$voltage side of the $3-$phase transformer is connected to a balanced $3-$phase load, rated $220$ V$,100$ kVA, $0\mathrm{.}9$ lagging power factor. Determine: $($a$)($b$)$ The rms line currents and the transformer winding currents on the low voltage and high voltage sides. $[6$ marks$]$ The per$-$phase equivalent circuit referred to the high voltage side, indicating the values of all components. $6$ marks$]($c$)($d$)$ The primary line voltage required to supply the load at rated voltage. $[5$ marks$]$ The percentage voltage regulation of the transformer.