In power flow calculations, usually a generator is represented as a PV bus, and a simplified method is used to model its reactive power limit. The PQ modeling approach, which converts the generator from PV bus to PQ bus when the reactive power limit is reached, is widely accepted and used by professional load pow programs; however, in reality, a generator on the limit behaves as a constant field voltage Instead of a constant reactive output. This fact has raised frequent discussions on the literature abet the accuracy or errors introduced by the conventional PQ model on pourer pow computations. Unfortunately. the real differences between the two modeling approaches for power pow studies have not been researched by these previous efforts. The purpose of this paper is to carry out comprehensive investigation, on the impact of the two modeling methods for power flow studies. They are compared from three different perspectives: impact on load flour results, impact on voltage stability margin. and convergence behavior. Based on the results. advantages and dis-advantages of the two modeling approaches are discussed, and conclusions are made on the appropriate approach for representing generator var limit for load flow studies.296565576