Question 1 A 125 MVA, 20 kV, three-phase, wye-connected, 60 Hz, synchronous alternator is connected to a 20 kV power grid. The alternator has negligible armature resistance and synchronous reactance of 5 2/phase. The mechanical limit of the turbine powering the alternator limits its maximum power output to 100MW. (a) Calculate the value of excitation voltage (line to neutral with angle) needed for 50% maximum power (50MW) at unity pf. |E(LN) 8 (b) Find the alternator reactive power import/export if its real power output is now increased to 75% rated (75 MW) with the excitation unchanged. (1) Draw before and after phasor diagrams on the same axes (performance chart) to illustrate the two operational states. (ii) Calculate the alternator reactive power import/export when its real power output is increased to 75% rated (75 MW) with the excitation unchanged. Q3ph Import or export?

5. A 3-phase, 50 Hz, star-connected, 1000 KVA, 2300V alternator gives a short-circuit current of 400 A for a certain field excitation. With the same excitation, the open-circuit voltage was 1328 V (phase value). The d.c. resistance between two lines is 0.412 2. Find (a) effective armature resistance (b) synchronous reactance (c) the full-load voltage regulation at 0.8 p.f. lagging. (Ans. (a) 0.3092/phase (b) 3.312 (c) 49.6%)

For a turbine that is connected to 50 Hz alternator, rated speed is 600 rpm. For an alternator _poles are required.

A 1200 kW,60 Hz synchronous alternator A identical governor droop of 0.0005 Hz/kW.Machine A takes two third of 1000kW bus load at 60 Hz.lf an additional 800 kW load is connected to the bus determine (a) bus frequency (b) load on each machine. is in parallel with a 400 kW,60 Hz alternator B.Both machines have (a) 58.6 Hz (b) 566.67 kW, 433.33 kW. (a) 59.6 Hz (b) 866.67 kW, 133.33 kW (a) 59.8 Hz (b) 666.67 kW, 333.33 kW (a) 58.9 Hz (b) 766.67 kW, 233.33 kW