A deltawye connected threephase transformer is rated at 38 M

A delta-wye connected three-phase transformer is rated at 38 MVA with primary and secondary line voltage ratings of 345 and 125 kV, respectively. The transformer p.u. reactance is 10%, the p.u. no-load current is 0.1, and the iron p.u. power loss is 0.06 per transformer rating. The transformer carries a load of 21 MW at a power factor of 0.8 lagging, Draw the equivalent circuit, Determine the transformer equivalent circuit parameters at the secondary side, Calculate the magnetizing current, If the transformer is supplied by the rated voltage, compute the load current and voltage.

Solution

the transformer is delta-star connected so on primary side line voltage is equal to phase voltage

V_pp = V_pl = 345kv

secondary line voltage,

V_sl = 125kv

secondary phase voltage,

V_sp = 125/3^(1/2) = 91.5kv

Transformation ratio (k) = V_sp / V_pl

k = 91.5 / 345 = 0.26

magnetising current,

I_m = 0.2A