Note that angle δ is the angle between excitation emf E f and terminal voltage V t, hence it is load angle for generator.So far the transmission line performance equation was presented in the form of voltage and current relationships between sending-and receiving-ends. This is very useful equation and we draw power angle curve of generator using the above equation. You must be very much familiar with the above power equation of the generator. Since r a = 0, therefore α z = 0 as can be seen from impedance triangle. Since the resistance of armature winding is quite low as compared to its inductance, therefore for simplicity this armature resistance r a may be ignored. Power input to the Generator will be the power at the source i.e. Here, E f, V t and Z s are Excitation Voltage, Armature Terminal Voltage and Synchronous Impedance respectively. For power flow through a cylindrical rotor synchronous generator or alternator, E 1 should be replaced by E f, E 2 by V t and Z by Z s. The above expression gives the equation for power flow at the load end or receiving end. This means, we individually have to find their values and get them subtracted to get the actual value of current I flowing from E 1 to E 2. From (1), it is clear that the line current or armature current (for generator) is difference of two values ( E 1/Z) and ( E 2/Z). So we need to find the value of current component in phase with source voltage E 1. = E 1 (Component of I in phase with E 1) …….(2) Since active power is equal to multiplication of voltage and component of current in phase with voltage, therefore Let us now calculate the active power P 1 at the source (sending end) end E 1 of the impedance. The phasor diagram of the above equivalent circuit will be as shown below.
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