本論文提出以負序電流注入之孤島偵測法及併網過程中相角補償技術來達成微電網中電力轉換器併網及孤島模式之無縫切換。首先介紹兩種模式下的控制策略，併網模式及孤島模式下分別以電流控制模式及自主控制模式來維持系統操作。當孤島事件發生，利用負序電流控制器所注入的負序電流成分(額定電流值的4%)由原本的市電端改流往負載端並且造成責任分界點上的負序電壓上升，此一電壓上升變化可用來做為孤島事件發生的判斷準則。此外利用對稱成分法將電路分成零正負序成分並以負序電路來進行負序電壓變化的定量分析，以支持所提出孤島偵測方法。系統偵測出孤島事件後，將切換至自主控制模式並完成由併網模式到孤島模式的無縫切換。市電端故障清除後，孤島運轉系統必須與市電進行複併，若此時市電端與責任分界點端電壓的相角存有誤差，則市電併上後將產生嚴重的電壓或電流突波進而造成系統及負載損害，因此藉由增加或減少系統操作頻率來進行相角補償與市電同步且達成由孤島模式到併網模式的無縫切換。最後，利用PSCAD模擬軟體及硬體實作實驗來驗證所提出的理論分析。
關鍵詞：無縫切換、電流控制模式、併網模式、自主控制模式、孤島模式、孤島偵測、對稱成分法、市電同步

This thesis presents the seamless transfer between grid-connected and islanded modes for a single voltage-sourced converter (VSC) based microgrid system. Principles of the control strategies at both modes are introduced where current-mode control for grid-connected operations and autonomous-mode control for islanded operations. The smooth transition from grid-connected mode to islanded mode is based on an islanding detection method that actively injects a negative-sequence current (4% of the rated value) as a disturbance signal via a negative-sequence controller which is adopted as the complementary of the conventional VSC controller. The estimated magnitude of the corresponding negative-sequence voltage at the point of common coupling (PCC) is used as the islanding detection signal. A quantitative analysis based on symmetrical component transformations is proposed to support the feasibility of the islanding detection scheme. For the islanded mode to grid-connected mode transition, a phase compensation scheme that increases or decreases the operating frequency of the VSC is applied for grid resynchronization. The detailed process of the seamless transfer between these two modes is illustrated. Finally, simulations in PSCAD software environment and hardware experiments verify the theoretical analysis.

[1] IEEE std. 929-2000, “IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems,” IEEE standards Coordinating Committee 21 on Fuel Cells, Photovoltaics, Dispersed Generation, and Energy Storage, April 2000.
[2] M. E. Ropp, M. Begovic, A. Rohatgi, “Prevention of islanding in grid-connected photovoltaic systems,” Progress in Photovoltaics vol. 7, pp. 39-59, 1999.
[3] M. E. Ropp, M. Begovic, A. Rohatgi, G. A. Kern, R. H. Bonn, Sr., and S. Gonzalez, "Determining the relative effectiveness of islanding detection methods using phase criteria and nondetection zones," IEEE transactions on energy conversion, vol. 15, no. 3, pp.290-296, September 2000.
[4] M. E. Ropp, M. Begovic, A. Rohatgi, "Analysis and performance assessment of the active frequency drift method of islanding prevention," IEEE Transactions on Energy Conversion, Vol. 14, No. 3, pp. 810-816, September 1999.
[5] G.A. Smith, A.Onions, D.G. Infield, "Predicting islanding operation of grid connected PV inverters," IEEE Proceedings Electr. Power Appl., vol. 147, no. 1, pp. 1-6, January 2000.
[6] W. Bower, and M. Ropp, "Evaluation of islanding detection methods for photovoltaic utility interactive power systems," Int. Energy Agency, Tech. Rep. IEAPVPS T5, March 2002.
[7] J. Stevens, R. Bonn, J. Ginn, S. Gonzales, "Development and testing of an approach to anti-islanding in grid interconnected photovoltaic systems," Sandia National Laboratories, Albuquerque, NM, Report SAND2000-1939, August 2000.
[8] G. K. Hung, C. C. Chang, C. L. Chen, "Automatic phase-shift method for islanding detection of grid-connected photovoltaic inverters," IEEE transactions on energy conversion, Vol. 18, No. 1, pp. 169-173, March 2003.
[9] C. F. Wagner and Robert David Evans. “Symmetrical components as applied to the analysis of unbalanced electrical circuits.” McGraw-Hill, New York, London, 1933.
[10] V. Task, "Grid aspects of grid-connected photovoltaic power systems," Tech. Rep. IEA-PVPS T5-01:1998, December 1998.
[11] P. O’Kane, and B. Fox, “Loss of mains detection for embedded generation by system impedance monitoring,” in Proc. Sixth International Conference on Developments in Power System Protection, pp. 95-98, March 1997.
[12] Houshang Karimi, Amirnaser Yazdani, and Reza Iravani, “Negative-sequence current injection for fast islanding detection of a distributed resource unit,” IEEE Trans. Power Electronics., vol. 23, no. 1, pp. 298-307, Jan.2008.
[13] J. Stevens, R. Bonn, J. Ginn, S. Gonzalez, and G. Kern, “Development and Testing of an Approach to Anti-Islanding in Utility-Interconnected Photovoltaic Systems,” Sandia National Laboratories report SAND2000-1939, Albuquerque, NM, August 2000.
[14] Guillermo Hernậndez-Gonzậlez, and Reza Iravani, “Current injection for active islanding detection of electronically-interfaced distributed resources,” IEEE Trans. Power Delivery, vol. 21, no. 3, pp. 1698-1705, July. 2006.
[15] M. E. Ropp, “Design issues for grid-connected photovoltaic systems,” Ph.D. dissertation, Dept. Elect.. Eng., Georgia Tech. Univ., Atlanta, GA, 1998.
[16] F. Gao and R. Iravani, “A control strategy for a distributed generation unit in grid-connected and autonomous modes of operation,” IEEE Trans. Power Del., vol. 23, no. 2, pp. 850–859, Apr. 2008.
[17] R. Tirumala and N. Mohan, “Seamless transfer of a grid-connected PWM inverters between utility-interactive and stand-alone modes,” in Proc. APEC’02 Conf., vol. 2, 2002, pp. 1081–1086.
[18] S. Yuyama, T. Ichinose, K. Kimoto, T. Itami, T. Ambo, C. Okado, K. Nakajima, S. Hojo, H. Shinohara, S. Ioka, M. Kuniyoshi, “A figh speed frequency shift method as a protection for islanding phenomenon of utility interactive PV systems,” Solar energy materials and solar cells, vol. 35, pp. 477-486, 1994.
[19] R. O’Garman and M. A. Redfern, “The difficulties in connecting renewable generation into utility networks,” in Proc. IEEE Power Eng. Soc. Summer Meeting, Jul.13-17 2003, vol. 3, pp. 1466-1471.
[20] Fu-Sheng Pai, and Shuh-Jier Huang, “A detection algorithm for islanding prevention of dispersed consumer-owned storage and generating units,” IEEE Trans. Energy Conversion., vol. 16, no. 4, pp. 346-351, Dec.2001.
[21] R. Jones, T. Sims, and A. Imece, “Investigation of Potential Islanding of Dispersed Photovoltaic Systems,” Sandia National Laboratories report SAND87-7027, Albuquerque, NM, 1988.
[22] R. A. Walling, and N. W. Miller, “Distributed generation islanding- implications on power system dynamic performance,” IEEE Power Engineering Society Summer Meeting, vol.1, pp. 92-96, 2002.
[23] G. Hung, C. Chang, and C. Chen. “Automatic phase shift method for islanding detection of grid connected photovoltaic inverter,” IEEE Trans. Energy Conversion, vol. 18, no. 1, pp. 169-173, Mar. 2003.
[24] L. A. C. Lopes and H. Sun, “Performance assessment of active frequency drifting islanding detection methods,” IEEE Trans. Energy Convers., vol. 21, no. 1, pp. 171–180, Mar. 2006.
[25] A. Yazdani and R. Iravani, “A unified dynamic model and control for the voltage-sourced converter under unbalanced grid conditions,” IEEE Trans. Power Delivery, vol. 21, no. 3, pp. 1620–1629, Jul. 2006.
[26] H. Song and K. Nam, “Dual current control scheme for PWM converter under unbalanced input voltage conditions,” IEEE Trans. Ind. Electron., vol. 46, no. 5, pp. 953–959, Oct. 1999
[27] M. Karimi-Ghartemani and M. R. Iravani, “A method for synchronization of power electronic converters in polluted and variable-frequency environments,” IEEE Trans. Power Syst., vol. 19, no. 3, pp. 1263–1270, Aug. 2004.
[28] M. Karimi-Ghartemani and H. Karimi, “Analysis of symmetrical components in time-domain,” in Proc. 48th IEEE Midwest Symp. Circuits Syst., Cincinnati, OH, Aug. 7–10, 2005, pp. 28–31.
[29] F. Katiraei, M. R. Iravani, and P. W. Lehn, “Micro-grid autonomous operation during and subsequent to islanding process,” IEEE Trans. Power Delivery, vol. 20, no. 1, pp. 248–257, Jan. 2005.
[30] A. Yazdani and R. Iravani, Voltage-Sourced Converters in Power Systems, Wiley IEEE press, 2010.