Problem 9 Two identical synchronous machines, cach rated 60 MVA, 15 kV, with a subtransient reactance of 0.1 pu, are connected through a line of reactance 0.1 pu on the base of the machine rating. One machine is acting as a synchronous generator, while the other is working as a motor drawing 40 MW at 0.8 pf leading with a terminal voltage of 14.5 kV, when a symmetrical three-phase fault occurs at the motor terminals. Deter- mine the subtransient currents in the generator, the motor, and the fault by using the internal voltages of the machines. Choose a base of 60 MVA, 15 kV in the generator circuit.

True or False When synchronous machines are operated as motors, peak voltages increase in case of inductive loading.

A generator is connected through a transformer to a synchronous motor as shown in the following figure. (All values in this problem are per unit.) Before the fault V,=0.9,1=1 with power factor 0.8 lagging. jX, P + jX ds jX" dm + E. E The reactances are Xd=0.15 , X,=0.1 , X âm=0.35. A symmetric fault occurs at P. Find the per-unit current magnitudes in the fault, the generator and the motor (a) using subtransient voltages. (b) using a Thevenin equivalent.

60 – MVA,15 – kV Problem 4 (Questions 21 through 23)-Two identical synchronous machines, each rated with a subtransient reactance of 0.1 py, are connected through a line of reactance 0.1 pu on the base of the machines rating. One machine is acting as a synchronous generator, while the other is working as a motor drawing 45-MW at 0.80 PF leading with a terminal voltage 14.5-kV. A symmetrical three-phase fault occurs at the motor bus. Calculate the following: 21. The generator fault current contribution (Ru.) is 22. The motor fault current contribution (p.u.) is 23. The total fault current is