Tutorial 4

Transformers

February 18, 2015

Problem: 1 The three phase transformers each rated 25 MVA, 38.1/3.81 kV, areconnected Y-4 as shown in Figure below, with the balanced load of three 0.6, Y-connected resistors. Choose a base of 75 MVA, 66 kV for the high-tensionside of the transformer and specify the base for the low tension side. Determinethe per-unit resistance of the load on the base for the low tension side. Thendetermine the load resistance RL in referred to the high tension side and theper-unit value of this resistance on the chosen base.

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Ans: RL = 180, 3.10 p.u.

Problem: 2 In power engineering, a one-line diagram or single-line diagram (SLD)is a simpli�ed notation for representing a three-phase power system. Considera system with the single line diagram shown in the �gure below.

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1

The three phase transformer nameplate ratings are listed. The transformerreactances are given in percent; 10 % = 0.1 p.u. The transmission line and loadimpedances are in actual ohms. The generator terminal volatge (magnitude)is 13.2 kV (line-line). Draw the impedance diagram in p.u. The transformerinduced phase shifts can be ignored.

Ans: ES= 0.96 6 0� p.u., Xnew

l1=0.183 p.u., Zline=5.25 �10

�3 (1+j10)p.u.,Xl2=0.08 p.u., Zload=0.63 p.u.

2