Answer Happy

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. (2 marks) |EO(LN) d (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. (i) Draw before and after phasor diagrams on the same axes (performance chart) to illustrate the two operational states. (2 marks) (ii) Calculate the alternator reactive power import/export when its real power output is increased to 75% rated (75 MW) with the excitation unchanged. (4 marks) Q3ph Import or export? (c) what is the maximum reactive power that the alternator can export if it delivering 100% rated power (100 MW) and its excitation voltage is set at 200% rated terminal voltage. (i) Draw a phasor diagram to illustrate this scenario. Show all applicable operational limits. (2 marks) (ii) Calculate the maximum reactive power export of the alternator when it delivering 100% rated power (100 MW) and its excitation voltage is set to 200% rated terminal voltage. (4 marks) Q3ph