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The three-bus power system shown in the figure has one alternator connected to bus 2 which supplies 200 MW and 40 MVAr power. Bus 3 is infinite bus having a voltage of magnitude 
p.u. and angle of 
. A variable current source, 
is connected at bus 1 and controlled such that the magnitude of the bus 1 voltage is maintained at
1.05 p.u. and the phase angle of the source current, 
, where 
is the phase angle of the bus 1 voltage. The three buses can be categorized for load flow analysis as
Bus 1 Slack Bus
Bus 
Bus
Bus 
Bus
Bus 
Bus
Bus 
Bus
Bus 
Bus
Bus 3 Slack Bus
Bus 
Bus
Bus Bus
Bus 3 Slack Bus
Bus 
Bus
Bus 3 Slack Bus
Since voltage magnitude is given at bus 1 & angle of current source 
∴ Phase shift b/w V & I across current source = 
∴ P = 0, Hence Bus – 1 is PV bus
Bus-2 has real & reactive power specified ∴ it can be characterized as PQ bus.
Bus-3 has voltage magnitude & phase angle specified
∴ It can be characterized as slack bus.