IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 4, DECEMBER 1999 4715 Transient Modeling and Simulation of a SMES Coil and the Power Electronics Interface Aysen Basa Arsoy, Student Member, IEEE, Zhenyuan Wang, Student Member, IEEE, Yilu Liu, Senior Member, IEEE, and Paulo F. Ribeiro, Senior Member, IEEE Abstract— This paper presents the modeling and simulation results of a superconducting magnetic energy storage (SMES) system for power transmission applications. This is the largest SMES coil ever built for power utility applications and has the following unique design characteristics: 50 MW (96 MW peak), 100 MJ, 24 kV dc interface. As a consequence of the high-power and high-voltage interface, special care needs to be taken with overvoltages that can stress the insulation of the SMES coil, especially in its cryogenic operating environment. The transient overvoltages impressed on the SMES coil are the focus of this investigation. Suppression methods were also studied to minimize transients. The simulation is based on detailed coil and multiphase gate turn-off (GTO)-based chopper models. The study was performed to assist in the design of the SMES coil insulation, transient protection, and the power electronics specification and interface requirements. Index Terms—Chopper, power electronics, SMES, SMES coil, transient modeling and simulation, transient suppression. I. INTRODUCTION P OWER systems have been experiencing dramatic changes in electric power generation, transmission, distribution, and end-user facilities. Continuing electric load growth and growing power transfer in a largely interconnected network lead to complex and less secure power system operation. Certain factors such as technical, economical, environmental, and governmental regulation constraints put a limitation on power-system planning and operation. Recent developments and advances in both superconducting and power electronics technology have made the application of superconducting magnetic energy storage (SMES) systems a viable choice to solve some of the problems experienced in power systems. SMES is a technology that has the potential to bring essential functional characteristics to the utility transmission and distribution systems. A SMES system consists of a su- perconducting coil, the cryogenic system, and the power conversion or conditioning system with control and protection functions [1]–[3]. Because of its fast response to power Manuscript received May 10, 1999; revised November 17, 1999. This work was supported in part by the National Science Foundation, Department of Energy, under Grant DE-FG36-94GO10011 and the Naval Nuclear Fuel Division, BWX Technologies, Inc. A. B. Arsoy, Z. Wang, and Y. Liu are with the Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060-0111 USA. P. F. Ribeiro is with BWX Technologies, Inc., Lynchburg, VA 24505-0785 USA. Publisher Item Identifier S 1051-8223(99)10427-5. demand and high-efficiency features, it has the capability of providing: 1) frequency support (spinning reserve) during loss of generation [4]–[6]; 2) transient and dynamic stability by damping transmission line oscillations [4], [5], [7], [8]; 3) dynamic voltage support [9]; and 4) automatic gener- ation control [6], [9], thus enhancing security, reliability, power quality, and transmission capacity. SMES systems have received considerable attention by electric utilities and government due to their attractive performance characteristics and potential benefits. The purpose of this study is to investigate the electromag- netic transient interactions between a superconducting coil and the power electronics interface for a flexible ac transmission systems (FACTS) application. Understanding the transient phenomena associated with an SMES system is essential in this investigation. Transient overvoltages can endanger the insulation of a superconducting coil, especially in its cryogenic operating environment, where the insulation characteristics are different from that at normal conditions. The transients may originate from normal or abnormal SMES switching operations and/or faults or lightning and switching surges from the ac and dc systems. They usually take place for a very short time as compared to the steady state, but have the potential to stress the coil insulation. The understanding of the possible transient overvoltages the SMES coil will be subjected to is essential in the design of its insulation and transient suppression schemes. The high-power and high-voltage dc interface of this particular design poses significant design challenges, which need to be well understood and adequate solutions need to be proposed. The transients associated with a SMES coil, which is interfaced with a gate turn-off (GTO) thyristor-based chopper, were simulated using an electromagnetic transient program EMTDC TM (electromagnetic transients for dc) [10]. The fol- lowing transient suppression schemes were investigated to minimize the transient overvoltages: 1) adding filtering/surge capacitor; 2) adding metal oxide varistor (MOV) elements; 3) changing the current sharing inductances. In addition, grounding resistors are used to reduce the terminal to ground voltage stress Section II gives an overview of a general SMES system and transient modeling and simulation concerns. The SMES coil model and power electronics interface circuitry used in the study will be described in Sections III and IV. The transient simulation and suppression scheme study results will be given in Sections V and VI. The last section will summarize the results of this study. 1051–8223/99$10.00  1999 IEEE