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1. Why per-unit method is important in analyzing power systems? In the given picture, would there be any difference in calculation of the transformers, if you choose primary or secondary side? Explain in words. (3) 111 kv/13.8KV 13.8KV/5KV H tole 67 - load X=10% x=8% 20 MVA X=4%

Answer 1

were has moun antrent voltage levels and Explanation - Before getting an understanding of pu method it is important to understand why we draw single line diagram! A transmission system consists of huge network of generators, transformers bus bars, isola hars cirovit breakers etc, Now, If we To determine any parameter of the network say impedance, then it would be challengeng find it, also in case of nonsformers it is static device which primary and secondary impedance uslues Here, single line diagram comes into a hoge mansmission network shown in just single line diagram or reactance diagrant Also, now the uslues usloes are defined in per-unit because each compoment in a single line diagram has different unit lice generator is in MUA transformer is so to easily calculate the parameters transmission ber-unit system method is used, à two winding play and IS volts - of

our (alwlation a particulst 'the on both • As different components w) have different values , in perunit, generally usloes are close Ipu, so it makes easy. • Per-unit uslues is defined at reference / base uslues. • In case of transformers, we refrain from finding uslues of resistances primary and secondary sides • In fault analysis, per-unit very helpful. in reducing calculation part. Perunit = Actual Uslue Base uslue in the given picture, we are given one hans former 113.8 'RV let's find the respective actual ohm values transformer at secondary sides, and you des pite having same actual different. is let see primary and will observe that the bu value + will be

Resistance on primary side inpu Rp = Ractural Roase and Roase 16² Base vo voltage Su Base MVA Let's assume S6 = 20m VA, Now Row= Ractualip) Rosse 0.0 = Ractuale) 16²/56 0.10= Ractual (P) (111) 720x10 0.06162 Ractual, Ractual 2 Resistance on secondary side fimilarly, Rpw X Roase = 0.10X (13.8) 20x10t 0.0009522 R So, now observe that the resistance uslues at the primary and secondary sides are different, So, there would not be any difference it the uslues are being calwlated as bo uslue remains same both sides a transformer. inpu.