Abstract—The unified power flow controller (UPFC) is one of the FACTS devices vastly applied in power systems. This paper presents the modeling of a new structure of UPFC consists of two shunt voltage-source converters and a series capacitor. Through the presented modeling, the transient and steady states responses of UPFC are well investigated. The results of the investigation are presented for the transient and steady states by simulating proposed system in MATLAB changing each system parameter. Index Terms—FACTS devices, steady state modeling, transient state modeling, UPFC. I. INTRODUCTION Stability maintenance is one of the most necessary requisitions of the power system, which has been discussed under several titles such as transient, dynamic, and voltage stability [1]. Recent progresses in high power semiconductor devices and their control techniques fields leads to vast application of FACTS devices in power systems, specially in asymmetric power distribution regulation. UPFC is one of the most important and the most effective FACTS device installed in ac transmission systems [2], [3]. In [4], a new structure, which uses two shunt voltage-source converters along with a capacitor, installed between these converters, is presented for UPFC as shown in Fig. 1 to overcome such problem. The UPFC parameters can be controlled through the capacitor voltage control [4], [5]. Also, the total harmonics distortion (THD) of injected series voltage is decreased in comparison with that of the conventional structures. In addition, the control and measuring systems design is simplified and the insulation and protection levels of transformers are well reduced. In this paper, it is tried to present a dynamic model for the UPFC structure presented in [4], [5] through modeling and mathematical relations calculation as well as stability analysis of this system by investigating the assumed system’s critical eigenvalues condition. II. MATHEMATICAL MODELING A. Voltage Source Converters Modeling The operation of four input control parameters of the Manuscript received March 17, 2013; revised May 6, 2013. Farzad Mohamamdzadeh Shahir is with Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail: f- mohammadzadeh@iau-ahar.ac.ir, f.m.shahir@gmail.com). Ebrahim Babaei is with Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail: e- babaei@tabrizu.ac.ir). presented UPFC ( ,1 E  , ,2 E  , ,1 E m , and ,2 E m ) are introduced and investigated to evaluate its dynamic model. Fig. 2(a) shows the structure of this system in details. The mathematical relations of this structure must be extracted for modeling. In order to achieve this aim, one of the phases is selected and the analyses are carried out for just this phase of shunt voltage-source converters due to the similarity of the structures and performance of the converters. For further explanations, Fig. 2(b), which shows the UPFC components such as dc link, a phase of exciting transformer in shunt voltage-source converter 1, and its related leg is initially considered. Here, it is assumed that C dc , dc link capacitor, is divided into two equal parts and its middle point is named as n. In addition, installed switch,  a , 1 , E and   a , 1 , E switches are bidirectional ones with r s on resistance, which can be considered as the conduction losses parameter of the switches and can be applied in losses analysis. The switches ,1, E a S and ,1, E a S  which define positive and negative switching function, respectively, have two operation modes for on/off states. According to the PWM technique application [6], for voltage-source converter operation control, ,1, E a S and ,1, E a S  are always complementary and the following is valid: ,1, ,1, 1 E a E a S S     The following is obtained applying Kirchhoffs’ voltage law on the circuit of Fig. 2(b): ,1, ,1, ,1, ,1, ,1, ,1, E a E a E aE a Et a E a di L r i v v dt     Fig. 1. Structure of presented UPFC in [4] In (2), , , Egk L , , , Egk i , , , Egk r , , Et g v , , Eg v are used for leakage inductance of transformer, current in shunt converter, resistance of transformer, voltage of shunt Dynamic Modeling of UPFC by Two Shunt Voltage- Source Converters and a Series Capacitor Farzad Mohammadzadeh Shahir and Ebrahim Babaei t I 1 Excitation Transformer ,1 E I ,1 E L ,2 E I ,2 E L 2 Excitation Transformer   se v se X   dc C dc v 1 Converter 2 Converter 476 International Journal of Computer and Electrical Engineering, Vol. 5, No. 5, October 2013 DOI: 10.7763/IJCEE.2013.V5.757