Assuming that we have a 66/11-kV primary substation with three66/11-kV, 20MVA (rating of each transformer), power transformers,then the installed capacity is 60MVA and the firm capacity is48MVA. I'd like to know what is the difference between installedand firm capacity. Furthermore, how was the firm capacitycalculated from the installed capacity? What I know is that thefirm capacity is calculated as follow: Firm Capacity = 20 + 20 + 4+ 4 = 48MVA, and that the equation (n - 1)x120% is used, where n isthe no. of transformers.
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