A New Method for Considering Distribution Systems in Voltage Stability Studies

In methods presented to calculate the voltage collapse point, the transmission system is usually the only part of the power system that is completely modeled. Distribution systems are often replaced by aggregate load models because the use of the detailed model of distribution systems in voltage stability analysis not only increases the computation time, but also decreases the probability of convergence of calculations. But this replacement can cause a considerable error in determining voltage collapse point. In this paper, a method for considering distribution systems in voltage stability studies is presented. In this method, a detailed model of distribution systems is used. The loads are connected to the secondary side of distribution transformers. An iterative method is used to calculate the voltage collapse point. In each iteration, first the value of active and reactive power delivered to transmission buses that supply distribution systems is increased. Then for each given value of the delivered power, the power received at the secondary side of distribution transformers is calculated. With this work, the voltage collapse point is determined using separately solving transmission and distribution system equations. On the other word, instead of solving a large set of equations, some smaller sets of equations are solved. The simulation results show the effectiveness of the proposed method.