Q. Representation of generator winding?
The notation using subscripts is such that VAB is the potential at point A with respect to point B, IAB is a current with positive flow from point A to point B, and IA,IB, and IC are line currents with positive flow from the source to the load, as shown in Figure. The notation used is rather arbitrary. In some textbooks a different notation for the voltage is adopted such that the order of subscripts indicates the direction in which the voltage rise is taken. The student should be careful not to get confused, but try to be consistent with any conventions chosen. The rms values are usually chosen as magnitudes of the phasors for convenience. It is customary to use the letter symbol E for generated emf and V for terminal voltage. Sometimes the two are equal, but sometimes not. If we should neglect the existence of the generator winding impedance, the generated emf will be equal to the terminal voltage of the generator. Although the three-phase voltages are generated in one three-phase alternator, for analytical purposes this is modeled by three identical, interconnected, single- phase sources.
The one-line equivalent circuit of the balanced wye-connected three-phase source is shown in Figure 4.1.4(c). The line-to-neutral (otherwise known as phase) voltage is used; itmay be taken as a reference with a phase angle of zero for convenience. This procedure yields the equivalent single-phase circuit in which all quantities correspond to those of one phase in the three-phase circuit. Except for the 120° phase displacements in the currents and voltages, the conditions in the other two phases are the same, and there is no need to investigate them individually. Line currents in the three-phase systemare the same as in the single-phase circuit, and total three-phase real power, reactive power, and volt-amperes are three times the corresponding quantities in the single-phase circuit. Line-to-line voltages, in magnitude, can be obtained by multiplying voltages in the single-phase circuit by √3.
When a systemof sources is so large that its voltage and frequency remain constant regardless of the power delivered or absorbed, it is known as an infinite bus. Such a bus has a voltage and a frequency that are unaffected by external disturbances. The infinite bus is treated as an ideal voltage source.