Comparative Studies on Methods of Modeling Generator Var Limit for Power-Flow Calculations

Abstract

In power flow calculations, usually a generator is represented as a P V bus, and a simplified method is used to model its reactive power limit. The P Q modeling approach, which converts the generator from P V bus to P Q bus when the reactive power limit is reached, is widely accepted and used by professional load flow 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 about the accuracy or errors introduced by the conventional P Q model on power flow computations. Unfortunately, the real differences between the two modeling approaches for power flow 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 flow 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.