Problems 1. Two synchronous machines are connected through three phase transformer to the transmission line as given in Fig. the ratings and reactance's of the machines and transformers are: Machines 1 and 2: 100 MVA; 20 kV; Xd"=X1=X2=20%; XO = 4%; Xn= 5% Transformers T1 and T2: 100MVA; 20/ 345 kV ; X=8% The line reactance's are X1=X2= 15% and X0=50%. The system is operating at nominal voltage without prefault currents when a bolted single line to ground fault occurs on the phase 'a' at bus 3. Determine the subtransient current to ground at the fault

500 kva, 11,000/2300 volts 60Hz transformer has 5.45% impedance and 1.2% resistance. What is the percent regulation at full load 80% power factor lagging?"     A. 3.26     B. 6.23     C. 4.21     D. 5.28

Q1. The following figure shows the differential protection of power transformers with specifications of 20 MVA, 66/11 KV.Trans is No load.If a two-phase short circuit occurs between phases b and ¢ It is desirable to calculate the currents passing through the CTs and relays R, Y, B. If these relays are three overcurrent relays with inverse characteristics as T=0.63 / log (I / Iset) and Iset =0.5A.Will this work for short circuit relays? If yes, which relay works faster and at what time?