Three transformers are provided with the following rating values:

1) 2400/240 V, 55kVA, Req1 = (0.1 + 0.0Y) Ohm and Xeq1 = (0.15 + 0.XX) Ohm.

2) 240/120 V, 25kVA, Req1 = (0.15 + 0.0Y) Ohm and Xeq1 = (0.30 + 0.XX) Ohm.

3) 240/120 V, 20kVA, Req1 = (0.20 + 0.0Y) Ohm and Xeq1 = (0.40 + 0.XX) Ohm.

All impedances are referred to the 240 V side. Transformer no. 1 can be operated as an autotransformer (Notice; the equivalent impedance as an autotransformer is calculated by dividing the original impedance over the turn's ratio of the autotransformer).

A 240 V, 60 kW, 0.9 power factor lagging load is to be supplied by the above transformers. The load will be connected for one full week, and the energy cost is 0.10 SR / kWh. As it can be observed, the load has to be supplied by a combination of the transformers. Assume that both 2640 V and 120 V supplies are available.

Determine the best design (best combination) to supply the load so that the operating cost is minimum. Show all the steps in your calculation.

Note:

 "XX" is the 7th and 8th digit of student ID, like (200891XX0).

"Y" is your section number (e.g., 01, 02).