本文以數位控制實現具備援功能之三相四臂T型式換流器並聯系統。為避免換流器斷線無法正常供電於負載，因此使用兩臺換流器並聯改善單臺換流器故障，負載依然由另臺換流器供電，藉此提升系統可靠度。此並聯系統採用無通訊方式之下垂控制法，作為並聯控制策略，達成無通訊方式並具功率分配之功能，且加入虛擬電阻，以確保各換流器輸出阻抗一致，有效降低輸出功率之誤差。
換流器架構採用三相四臂T型式拓樸架構，此架構特點為輸出電壓為多階波形能有效降低輸出電壓諧波含量，且第四臂可控制不平衡電流。針對不平衡負載，使用正、負、零序分別獨立控制方式，透過抑制負序與零序電壓，使三相輸出電壓維持平衡。透過載波脈波寬度調變，將三相參考訊號注入三次諧波，其最大振幅調變係數與空間向量脈波寬度調變一樣，但無需判斷空間向量區間，易於數位化實現之優點。實驗部分製作兩臺8 kW三相四臂T型式換流器，完成系統並聯測試與驗證。

This thesis proposes the design and implementation of a digitally-controlled three-phase four-leg T-type inverter parallel system with redundancy feature. In order to avoid inverter is disconnected and cannot supply power to the load normally. Thus using two inverters parallel to improve single inverter breakdown. The load is still power by another inverter, thereby improving the reliability of the system. To achieve the power sharing without communication, the system uses the droop control method as the control strategy for parallel operation. Moreover, add virtual resistance to ensure that the output impedance of each inverter is same, effectively reduce the error of output power.
The proposed inverter topology is using three-phase four-leg T-type topology. The characteristic of this topology is that output voltage is multi-level which can effectively reduce the output voltage THD, and unbalanced current can be controlled. Independent control positive, negative and zero sequence for unbalanced loads. Through inhibit the negative and zero sequence voltage to maintain three-phase voltage balance. By Carrier-Based Pulse Width Modulation, the three-phase reference signals are injected into the third order harmonic. The maximum amplitude modulation factor is the same as the Space Vector PWM. The advantage of CBPWM is easy to digitalization and no need to determine the location of space vertor. Finally, the proposed theoretical analysis is verified by experimental result.

[1] M. Schweizer and J. W. Kolar, “Design and Implementation of a Highly Efficient Three-Level T-Type Converter for Low-Voltage Applications,” IEEE Transactions on Power Electronics, vol. 28, No. s2, pp. 899-907, February 2013.
[2] L. Sejpal, Comparison of Two-Level and Three-Level Neutral-Point Clamped Inverters in Automotive Applications, University of Concordia, 2013.
[3] S. M. Shin, J. H. Ahn, and B. K. Lee, “Maximum Efficiency Operation of Three-Level T-type Inverter for Low-Voltage and Low-Power Home Appliances,” JEET, vol. 10, no. 2, pp. 586-594, 2015.
[4] A. Hintz, U. R. Prasanna, and K. Rajashekara, “Comparative Study of the Three-Phase Grid-Connected Inverter Sharing Unbalanced Three-Phase and/or Single-Phase systems,” IEEE Transactions on Industry Applications, vol. 52, no. 6, November/December 2016.
[5] B. Meersman, B. Renders, L. Degroote, T. Vandoorn, J. D. Kooning, and L. Vandevelde, “Overview of three-phase inverter topologies for distributed generation purposes,” in Proc. i-SUP 2010 : Innovation for Sustainable Production, Proceedings, pp. 24-28, April 2010.
[6] R. Aboelsaud, A. Ibrahim, A. G. Garganeev, “Review of three-phase inverters control for unbalanced load compensation,” International Journal of Power Electronics and Drive System, vol. 10, no. 1, March 2019, pp. 242~255, March 2019.
[7] Mohan, Undeland , Robbins，電力電子學，江炫章譯，第三版，全華出版社，2016年，第8-1~8-10頁。
[8] Y.Y. Tzou, H.J. Hsu, and T.S. Kuo, “FPGA-Based SVPWM Control IC for 3-Phase PWM Inverters,” IEEE Transaction on. Power Electronics, vol. 12, no. 6, pp. 953-963.
[9] Y. Jiao, F. C. Lee and S. Lu, “Space Vector Modulation for Three-Level NPC Converter With Neutral Point Voltage Balance and Switching Loss Reduction,” IEEE Trans. Power Electron., vol. 29, no. 10, pp. 5579-5591, October 2013.
[10] M. Hava, R. J. Kerkman, and T. A. Lipo, “Simple Analytical and Graphical Tools for Carrier Based PWM Methods,” IEEE Power Electronics Specialists Conference, vol. 2, pp. 1462-1471, June 1997
[11] J.H. Kim and S.K. Sul, “A Carrier-Based PWM Method for Three-Phase Four-Leg Voltage Source Converters,” IEEE Transactions on Power Electronics, vol. 19, no. 1, pp. 66-75, January 2004.
[12] D. Shanxu, M. Yu, X. Jian, K. Yong, and C. Jian, “Parallel operation control technique of voltage source inverters in UPS,” IEEE 1999 International Conference on Power Electronics and Drive Systems. PEDS'99 (Cat. No.99TH8475), July 1999.
[13] J. M. Guerrero, L. Hang, and J. Uceda, “Control of Distributed Uninterruptible Power Supply Systems,” IEEE Transactions on Industrial Electronics, vol. 55, no. 8, pp. 2845-2859, August 2008.
[14] Z. Li, X. Sun and J. Chai, “Proportional power sharing control for parallel-connected inverters,” 2011 4th International Conference on Electric Utility Deregulation and Restructuring and Power Technologies (DRPT), Weihai, Shandong, pp. 1647-1651, 2011.
[15] Q.C. Zhong, Y. Zeng, “Universal Droop Control of Inverters With Different Types of Output Impedance,” IEEE Access, vol. 4, pp. 702-712, February 2016.
[16] J. M. Guerrero, L. G. d. Vicuña, J. Matas, M. Castilla, and J. Miret, “Output Impedance Design of Parallel-Connected UPS Inverters With Wireless Load-Sharing Control,” IEEE Transactions on Industrial Electronics, vol. 52, no. 4, pp. 1126-1135, August 2005.
[17] K.D. Brabandere, B. Bolsens, J. V. d. Keybus, A. Woyte, J. Driesen and R. Belmans, “A Voltage and Frequency Droop Control Method for Parallel Inverters,” IEEE Transactions on Power Electronics, vol. 22, no. 4, pp. 1107-1115, July 2007.
[18] J. Chen, B. Zhao, X. Chen, M. Liu, and C. Gong, “Droop Control of Three-Phase Four-Wire Parallel Inverters under Unbalanced Load Condition,” Transactions Of China Electrotechnical Society, vol. 33, no. 20, October 2018.
[19] D.G. Holmes and T.A. Lipo, Pulse Width Modulation For Power Converters Principles and Practice. John Wiley, 2003 ,pp. 120-124.
[20] 王嘉紋，具浮點運算數位信號處理器之三相交流-直流-交流功率換流器研製，國立臺灣科技大學電機研究所碩士論文，2010年。
[21] J.D. Glover, M.S. Sarma, T. Overbye, Power System Analysis and Design. Fifth Edition, Cengage Learning, April 2011, pp. 428-431.
[22] 滕大均，一種具補償負相序虛功之低電壓穿越技術，國立中山大學電機工程學系碩士論文，2015年。
[23] Y. Hu, Z.Q. Zhu, M. Odavic, “Instantaneous Power Control for Suppressing the Second-Harmonic DC-Bus Voltage Under Generic Unbalanced Operating Conditions,” IEEE Transactions on Power Electronics, vol. 32, no. 5, pp. 3998-4006, June 2016.
[24] Y. Liu, W. Xu, L. Ke, and F. Blaabjerg, “An Improved Synchronous Reference Frame Phase-Locked Loop for Stand-Alone Variable Speed Constant Frequency Power Generation Systems,” 2017 20th International Conference on Electrical Machines and Systems (ICEMS), August 2017.
[25] IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, IEEE Std. 519-2014, June 2014
[26] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Std. 141-1986, April 1994.