What should be the characteristics of the generator parameters like Xd'',Xd' and Xd ,whether saturated or unsaturated ,which should be considered during generator stability studies during transient conditions.?
During stability studies all generator parameters are to be assumed constant around the operating point.
The intuitive reason to have different reactances is: unlike the transmission line reactance which is more or less constant, the reactance of the generator is a function of both time and instantaneous flux distributions caused by various winding in the generator ..... following the disturbance, as each winding has its own time constant, the corresponding flux will take different time to get settle .... that's why there is sub-transient reactance (first 2 cycles, Xd'') and transient (2-4 cycles, Xd') and then steady state reactance (Xd) ..... Transient stability Analysis with transient model (i.e. with Xd') is acceptable but if you do with subtransient model it would be more accurate.......
Along with electrical parameters, rotor inertia(rotating mass), and fast computerized fault clearance, is of prime importance to be considered in transient stability, world is tending to weak link connection for it, even to other countries , to make system more reigied....
There are several models. Each of them have different accuracy level, and corresponding simulation duration for the system. The simplest and fastest model is the classical model where the machines are represented by its transient reactanes. There are also the transient model (transient reactanes +time constants), and the subtransient model which includes the transient model and the subtransient reactanes & time constants. The saturation effect models can be added to the later models. Again, the choice should be based on the desired accuracy and the simulation duration.
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