Incorporation of Nonlinear Load Models in the Transient Energy Function Method

Considerable progress has been made in first swing power system transient stability assessment using the transient energy function (TEF) method. Recently, the TEF technique has been successfully applied to large scale power systems. The technique has also been tested on a variety of applications. In most of the work reported in the literature, the technique has been applied to the classical power system model. In this paper a technique is proposed to remove some of the modeling restrictions of the TEF method by incorporating the effect of nonlinear load models. The basic approach consists of representing the effect of the nonlinear load models on the stable equilibrium point (SEP) and controlling unstable equilibrium point solution. An alternate network solution procedure is used to reflect the effect of the nonlinear load models, via current injections at the internal generator nodes. The power corresponding to these injections is then included in the mismatch equation for the SEP and UEP solution. A new expression for the TEF is developed to include the terms corresponding to the nonlinear load components. A procedure to evaluate some of the path dependent terms is also described. The proposed technique has been tested on a 4-generator system and a 17-generator system. Results of the testing indicate that the proposed technique provides results which compare fairly accurately with time simulation. The technique accurately accounts for the dynamic behavior of the nonlinear loads, and provides good accounting of the transient energy responsible for system separation.