Incorporation of nonlinear load models and identification of the inter-area mode phenomenon 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. However, in most of the work reported in the literature, this technique has been applied to the classical power system model. Some of the recent developments of the TEF method include application to stressed large-scale power systems, incorporation of the effects of the exciter, etc. In the classical model, the loads are modelled as constant impedances. This dissertation aims at removing some of the modeling restrictions of the TEF method by incorporating the effects of nonlinear load models;The basic approach consists of representing the effect of nonlinear load models on the stable equilibrium point (SEP) and the controlling unstable equilibrium point (UEP) 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 term corresponding to the nonlinear load components;The proposed technique has been tested on a 4-generator system and a 17-generator system. The results of these tests compared well with those obtained by time simulation;The other aspect of this research work deals with the application of the TEF method to stressed large-scale power systems. The theory of modal analysis has been used to identify the inter-area mode phenomenon of stressed systems. The inter-area mode phenomenon can be described as the tendency of a large group of generators (including the small group severely disturbed by the fault initially) to separate from the rest of the system as a result of instability. The post disturbance network of stressed systems are characterized by weak synchronizing forces caused by large transmission impedances, and thus generators away from the fault location may also separate from the system;The proposed technique has been tested on three test systems, the 17-generator Iowa system, the 50-generator Ontario-Hydro system and the 126-generator western USA system. Both stressed and unstressed systems have been considered in testing this technique;The technique gives reasonably correct indication of the inter-area mode phenomenon.