IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 30, NO. 3, SEPTEMBER 2015 939 Transient Stability Enhancement of Doubly Fed Induction Machine-Based Wind Generator by Bridge-Type Fault Current Limiter Gilmanur Rashid, Student Member, IEEE, and Mohd. Hasan Ali, Senior Member, IEEE Abstract—Transient stability is a major concern for doubly fed induction machine (DFIM). A DFIM-based wind generator is read- ily affected by faults at the grid side as its stator windings are interfaced to grid. However, the wind generators need to remain connected and continue operation during faults at the grid side ac- cording to the grid code requirements. Therefore, it is important to enhance the transient stability of the DFIM-based wind generators. To achieve enhanced transient stability of the DFIM, a bridge-type fault current limiter (BFCL) is proposed in this study. Symmetrical as well as unsymmetrical faults were applied to the test system to check the efficacy of the BFCL in transient stability enhancement. Simulations were carried out in Matlab/Simulink environment. To demonstrate the effectiveness of the proposed BFCL, its perfor- mance is compared with that of the series dynamic braking resistor (SDBR). Simulation results show that the BFCL is a very effective device to attain better stabilization of the DFIM and outperforms the SDBR in all aspects. Index Terms—Bridge-type fault current limiter (BFCL), doubly fed induction machine (DFIM), grid code, series dynamic braking resistor (SDBR), transient stability, variable speed wind turbine (VSWT), wind generator. I. INTRODUCTION T ECHNOLOGICAL advancement and industrialization has raised the increase in electrical power demand all around the world. Rapid exhaustion and limited reserve of fossil fuels, intensification of environmental concerns have made it urgent to seek for alternative energy sources and to devise improved methods of exploiting renewable energy sources. Among the available renewable energy sources, wind energy is the fastest growing and most prominent option to generate electric power due to its zero fuel cost, no carbon emission, lower maintenance, cleaner, cheaper and renewable nature. It is estimated that about 10% of global electricity demand will be supplied from the wind energy by the year 2020 [1]. Due to flexibility in operation and enhanced features like higher output power, higher efficiency, improved power quality, variable speed operation, lower mechanical stress on turbine hence lower maintenance, decoupled control of the active and reactive power, the variable speed wind generators are becoming Manuscript received June 26, 2014; revised October 21, 2014 and December 6, 2014; accepted February 1, 2015. Date of publication February 26, 2015; date of current version August 18, 2015. Paper no. TEC-00461-2014. The authors are with the Department of Electrical and Computer Engineering, Herff College of Engineering, University of Memphis, Memphis, TN 38152 USA (e-mail: grashid@memphis.edu; mhali@memphis.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEC.2015.2400220 preferred choice for new installations and drawing higher atten- tion than the traditional induction machine-based fixed speed wind generators. Lower cost, durability, simple structure, pos- sibility to cover a wide range of wind speed, partially rated variable frequency ac/dc/ac converter and lower switching loss have made the doubly fed induction machine (DFIM) a superior choice over the other wind generator options [2], [3]. Compared to variable speed wind generators having full rated converter, the DFIM is more vulnerable to grid fault or distur- bances from the stability standpoint, as its stator windings are directly connected to grid while rotor windings are interfaced to grid via the rotor-side converter (RSC) and the grid-side con- verter (GSC) that are connected back-to-back through a dc-link capacitor. At the event of grid fault, terminal voltage of the DFIM goes very low and very high current flows through both stator and rotor winding. This is a threat to stable operation and may eventually burn the machine and the converters. Tradition- ally, to protect from such fault incidents, wind generators were disconnected from the grid. As more and more wind power is in- tegrated into the grid, it has become necessary that wind turbines stay connected to grid during fault. Also, this is a requirement by the grid codes set by different regulatory bodies [4]. For these reasons, the DFIM should have better stabilization and fault ride through capability. The converters of the DFIM topology has the control abil- ity to maintain stability at fault condition. Since the converters have partial rating, their capacity alone is insufficient to ensure stability. Additional auxiliary devices with small capacity are required. Some solutions are proposed to overcome the stabil- ity issue from different aspects in the literature. Some works propose new control methods [5]–[14] that are suitable only for new installations. An auxiliary device like static synchronous compensator is proposed in [15], [16] but it needs additional converter, coupling transformer and harmonic filters. Energy storage systems like flywheel energy storage [17], supercon- ducting magnetic energy storage [18] and superconducting fault current limiter [19] are also proposed, but the high installation cost offsets their good performance [20]. The bridge-type fault current limiter (BFCL) is a new tech- nique with promising applications in power systems [21], [22] and fault ride through capability enhancement of fixed speed wind turbine generators [23]–[25]. However, the BFCL is never applied to enhance the transient stability of DFIM-based wind generators. In this study, performance of the BFCL on enhancing the transient stability of the DFIM in wind energy application is investigated. 0885-8969 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. Copyright IEEE Non Commercial & personal use only Copyright IEEE Non Commercial & personal use only Copyright IEEE Non Commercial & personal use only Copyright IEEE Non Commercial & personal use only