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1 Instantaneous Current-Sharing Control Strategy for Parallel Operation of UPS Modules Using Virtual Impedance Abstract: By regulating the inverter output current every switching cycle, instantaneous currentsharing control strategies are usually employed in paralleled modular uninterruptible power supplies (UPSs). In this paper, virtual impedance, which is usually utilized in the droop method, is firstly introduced to the instantaneous current-sharing control strategy to achieve good load current-sharing performance. The instantaneous circulating current model of the paralleled system is developed and the circulating impedance is derived as well to clearly identify the intrinsic nature of unequal sharing of load current. By inserting the virtual impedance to the circulating impedance with the help of proper control strategy, good current-sharing capability can be obtained. The parallel inductor, e.g., coupled inductor, which is generally added at the output of each inverter to reduce the circulating current among modules, can be, therefore, eliminated, contributing to reduced weight, volume, and cost. In the meantime, the system output-voltage regulation performance, e.g., amplitude and waveform quality, is not affected by the introduction of the virtual impedance. The designation of the virtual impedance is provided and its implementation is simple but rather effective. Experimental results are also provided to verify the feasibility of the proposed method. INTRODUCTION: I N RECENT years, uninterruptible power supplies (UPSs) have been widely employed to provide continuous and reliable electrical power to critical loads. With the ever-increasing power and reliability demand, several UPS units are usually connected in parallel. These units often have similar characteristics and power ratings, contributing to the system features of N+X redundancy operation and modularity [1], [2]. Due to the voltage source nature, proper current sharing is of extreme Manuscript received October 25, 2011; revised January 10, 2012 and March 24, 2012; accepted May 5, 2012. Date of current version September 11, 2012 Recommended for publication by Associate Editor J. M. Guerrero. Y Zhang and Y Kang are with the Huazhong University of Science and Technology, Wuhan 430074, China (e-mail: [email protected]; [email protected]). M. Yu is with the Dongguan Leke Electronic Company, Ltd., Dongguan 523718, China (email: [email protected]). www.frontlinetechnologies.org [email protected] +91 7200247247 2 F. Liu is with the Huazhong University of Science and Technology, Wuhan 430074, China, and also with the Department of Electrical and Computer Engineering, Ryerson University, Toronto, ON M5B 2K3 Canada (e-mail: [email protected]). Architecture Diagram: CONCLUSION: The virtual impedance, which is usually employed in the droop method, is firstly proposed for the instantaneous current-sharing strategy in this paper to achieve good load current- sharing performance. By developing the circulating current model of the paralleled inverter system, the circulating impedance is derived and the intrinsic nature of unequal load current sharing is clearly demonstrated. With the help of the virtual impedance, the stability of circulating current is guaranteed. Another significant feature is that the virtual impedance does not cause additional voltage drop and distortion in the system output, which are common in the traditional virtual impedance- based strategies. Moreover, the circulating current caused by the difference between parameters of each inverter power stage can also be well suppressed, contributing to good load currentsharing performance. A simple and valid designation of the virtual impedance is provided. The feasibility and validity of the proposed method are validated by the experimental results. References: 1. J. F. Chen and C. L. Chu, “Combination voltage-controlled PWM inverters for UPS operation achieving an equal current distribution,” IEEE Trans. PowerElectron., vol. 10, no. 5, pp. 547-558, Sep. 1995. 2. L. Corradini, P. Mattavelli, M. Corradin, and F. Polo, “Analysis of parallel operation of uninterruptible power supplies loaded through long wiring cables,” IEEE Trans. PowerElectron., vol. 25, no. 4, pp. 1046-1054, Apr. 2010. 3. S. J. Chiang, C. Y. Yen, and K. T. Chang, “A multimodule parallelable series- connected PWM voltage regulators,” IEEE Trans. Ind. Electron., vol. 48, no. 3, pp. 506-516, Jun. 2001. www.frontlinetechnologies.org [email protected] +91 7200247247 3 4. Y. Li and Y. W. Li, “Power management of inverter interfaced autonomous microgrid based on virtual frequency-voltage frame,” IEEE Trans. Smart Grid, vol. 2, pp. 30-40, Mar. 2011. 5. W. Yao, M. Chen, J. Matas, J. M. Guerrero, and Z. Qian, “Design and analysis of the droop control method for parallel inverters considering the impact of the complex impedance on the power sharing,” IEEE Trans. Ind. Electron., vol. 58, no. 2, pp. 576588, Feb. 2011. 6. J. Matas, M. Castilla, L. G. D. Vicuna, J. Miret, and J. C. Vasquez, “Virtual impedance loop for droop-controlled single-phase parallel inverters using a second-order general-integrator scheme,” IEEE Trans. Power Electron., vol. 25, no. 12, pp. 29933002, Dec. 2010. 7. U. Borup, F. Blaabjerg, and P. N. Enjeti, “Sharing of nonlinear load in parallel- connected three-phase converters,” IEEE Trans. Ind. Appl., vol. 37, no. 6, pp. 18171823, Nov./Dec. 2001. 8. J. M. Guerrero, L. G. Vicuna, J. Matas, M. Castilla, and J. Miret, “Output impedance design of parallel-connected UPS inverters with wireless loadsharing control,” IEEE Trans. Ind. Electron., vol. 52, no. 4, pp. 1126-1135, Aug. 2005. 9. J. M. Guerrero, J. Matas, L. G. Vicuna, M. Castilla, and J. Miret, “Wireless- control strategy for parallel operation of distributed-generation inverters,” IEEE Trans. Ind. 10. Electron., vol. 53, no. 5, pp. 1461-1470, Oct. 2006. J. Kim, J. M. Guerrero, P. Rodriguez, R. Teodorescu, and K. Nam, “Mode adaptive droop control with virtual output impedances for an inverter- based flexible AC microgrid,” IEEE Trans. Power Electron., vol. 26, no. 3, pp. 689-701, Mar. 2011. www.frontlinetechnologies.org [email protected] +91 7200247247