Comparison of Fundamental Frequency Model and PWM Based Model of UPFC S.A. Al-Qallaf, S.A. Al-Mawsawi, A. Haider Abstract—Among all FACTS devices, the unified power flow controller (UPFC) is considered to be the most versatile device. This is due to its capability to control all the transmission system parameters (impedance, voltage magnitude, and phase angle). With the growing interest in UPFC, the attention to develop a mathematical model has increased. Several models were introduced for UPFC in literature for different type of studies in power systems. In this paper a novel comparison study between two dynamic models of UPFC with their proposed control strategies. Keywords—FACTS, UPFC, Dynamic Modeling, PWM, Fundamental Frequency. I. I NTRODUCTION D UE to the increasing demand on power, and with the economical and environmental constraints on building new generating plants and installing new transmission lines, interconnection of the transmission system appeared as an option in order to cover the need for power. This also meant that the system is to be operated and utilized to its limits. Since the transmission system is governed by two limits namely, electrical stability limit, and thermal limit, and by operating outside the permissible range of stability, the system security is compromised. An innovative solution to such a problem was with the introduction of flexible AC transmission system (FACTS). The idea behind FACTS was to increase controllability and optimize the existing power system capacity through the use of power electronic devices. With such devices the transmission system is to be operated near to its thermal limits without compromising the system security and reliability. Since then, the use of FACTS controllers has been popular to solve different problems faced in power systems such as power flow control, voltage support, and even oscillation damping. FACTS installations increase the system operating range, security, and reliability and also provide more functionality over mechanical devices installed in the system. The unified power flow controller (UPFC) was introduced by Gyugyi in 1991[2]. UPFC is a voltage source converter (VSC) based FACTS device. The UPFC is composed of two voltage source converters connected back to back through a common d.c link as illustrated in Fig. 1. Due to its structure, UPFC is considered to be the most versatile FACTS device as it combines the functions of shunt and series connected FACTS devices. Hence, it can control all three parameters of the transmission system. S.A. Al-Qallaf Department of Electrical and Electronics Engineering, University of Bahrain, Bahrain, e-mail:saa-1985@hotmail.com S.A. Al-Mawsawi and A. Haider are with University of Bahrain. Manuscript received June 8, 2014; revised September 05, 2014. Fig. 1. Unified Power Flow Controller (UPFC) Construction In order to evaluate the performance of a UPFC, mathematical models for steady state and dynamic analysis are to be developed. The steady state model is mainly concerned with the incorporation of the UPFC in load flow studies, while the dynamic model is developed to investigate the behavior of UPFC during transients, the control capability, and controller design. Several references in literature have addressed the topic of the UPFC dynamic modeling. In [5], a dynamic model for UPFC was introduced for transient and small signal stability analysis. The problem with this modeling approach was that it did not consider the DC link dynamics which could lead to implications during transients. In [6], a Newton type current injection model is used for transient stability studies. This model consists of a controllable voltage source added in series with the transmission line, plus two current sources added in shunt to balance power flow through the device. As this model has considered the power balance constraint for the UPFC, it has neglected the dynamics of the DC link also. Thus, this model is not suitable for dynamic analysis. This paper presents two dynamic models of the UPFC that were introduced in literature in [4] and in [3].It discusses the control strategies that were proposed for these two models in literature. It highlights the major advantages and disadvantages of the two models and their respective control strategies. II. UPFC FUNDAMENTAL FREQUENCY MODEL A. Model Derivation In [4], a fundamental frequency model for the UPFC was proposed. From Fig. 1, By replacing the VSCs of the UPFC with a controllable fundamental frequency voltage sources as in Fig. 2. World Academy of Science, Engineering and Technology International Journal of Electrical and Computer Engineering Vol:8, No:10, 2014 1585 International Scholarly and Scientific Research & Innovation 8(10) 2014 scholar.waset.org/1307-6892/9999533 International Science Index, Electrical and Computer Engineering Vol:8, No:10, 2014 waset.org/Publication/9999533