Three $25-$kVA$24000780-V$ single phase transformers are connected in $-.$ The open$-$circuit test was performed on the low$-$voltage side of three phase transformer and the following data were recorded: $V_{OC}=480V,$loc$=3\mathrm{.}5A,$Poc$=840W.$ The short$-$circuit test was performed on the high$-$voltage side of three phase transformer, and the following data were recorded: $V_{SC}=1200V,I_{SC}=2\mathrm{.}00A,P_{SC}=1050W$

a$)$ Find the equivalent circuit of this transformer referred to the high voltage side.

b$)$ Find the voltage regulation of this transformer at the rated load and $0\mathrm{.}85$ PF lagging.

c$)$ What is the transformer's efficiency under these conditions?