A three-phase steam-turbine synchronous generator is connected to an infinite bu

Question

A three-phase steam-turbine synchronous generator is connected to an infinite bus. The generator excitation current is IF=1000 A at the time of synchronization with the bus. The generator has a synchronous reactance of Xs =1.5 per unit. With IF unchanged, the steam valves at the turbine are adjusted until the generator delivers an active power of Pg=0.2 per unit. Assuming the infinite bus is the reference voltage (Vt=1 at angle 0),

a)Find the generator load current

b)With the Pg unchanged, the excitation current is increased to 1600 A. Find the new load current.

Please show steps thanks.

Explanation / Answer

a) We know for synchronous machine,

P = Vr* Vs /Xs, where Vr = Voltage at the receiving end; Vs = Voltage at the sending end; Xs= synchronous reactance

Putting the values;

0.2 = 1 * Vs/ 1.5j => Vs = 0.3j V

Hence, generator load current Io = (Vs - Vr)/Xs = (0.3j - 1)/1.5j = 0.69 ang(-106.69) A

b) Now, excitation is increased to 1600 A

New sending end voltage = (1600/1000) * 0.3j = 0.48j V

New generator load current Io = (Vs - Vr)/Xs = (0.48j - 1)/1.5j = 0.739 ang(-115.64) A

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