LAUSR

In most of the voltage stability studies, simultaneous modeling of transmission and distribution networks is not done because it makes a system with too many dynamic and algebraic equations. Also, voltage stability assessment is often performed by static modeling of power systems (i.e., without time-domain simulation and based on replacing of differential equations by their equilibrium equations). In this letter, time-domain simulations on a simple system show that different reasons can lead to the occurrence of voltage instability. In some cases, voltage stability can be assessed correctly by separate static modeling of transmission or distribution network. But in other conditions, simultaneous dynamic modeling of transmission and distribution networks is needed.