Frequency dependent network equivalents for electromagnetic transient studies

Abstract

In electromagnetic transient studies, only a small part of an AC power system can be modeled in detail. The remaining parts of the system should be reduced to equivalent model. Different methods were developed to generate frequency dependent network equivalents. This paper presents a novel approach for obtaining network equivalents for ac power systems using the QZ algorithm. The proposed method obtains accurate resonant frequencies directly from the system equations to be used in finding the network equivalent parameters. Also, the proposed method is a good tool to predict the system response. The use of the QZ algorithm in the creation of the network equivalent model gives the following advantages: 1. Improved accuracy: as compared to the direct frequency response method. 2. Less complexity: because it obtains the minima and maxima frequencies directly from the system equations. 3. Reliable prediction: it gives the complete information about the behavior of the test system and its peaks and troughs before calculating its frequency response. 4. Good evaluation of network's resistance: the proposed method determines the resistors of the network equivalent model as the real part of the impedance function magnitude at the minima frequency points. In this article, a general algorithm is developed to determine the frequency dependent network equivalent model parameters for any given system. Finally a four-bus test system is introduced to show the accuracy and the validation of the proposed method. For simplicity and convenience a comparison between the direct frequency response method and the proposed method, a two-bus system, is also studied in this work. Good results were obtained for network equivalent models for both the frequency and time domains.