在電力系統中，濾波器用以消除諧波或是雜訊，被動式濾波器有許多類型，包含L濾波器、LC濾波器、LCL濾波器，其中以LCL濾波器最能有效消除諧波，但是缺點為在控制策略上會較複雜。利用數位訊號處理器晶片可以實現各種控制器，但是，訊號的取樣轉換可能會導致時間的延遲，使得控制器設計更加困難。本文利用LCL濾波器電容器之電流觀察器加到控制迴圈內，不僅能夠減少感測器的數量，而且能夠改善由時間延遲所造成的系統不穩定性。
本文利用Simulink建構出單相轉換器經由LCL濾波器連接電網，並模擬訊號發生時間延遲的情況，再將觀察器加入控制迴圈內，觀察不穩定的波形是否能在啟動觀察器時獲得改善。

In the power system, the filter is often used to eliminate harmonics or noise. There are many types of filters, such as L filters, LC filters, and LCL filters. We use LCL filter to effectively eliminate harmonics, but its disadvantage is that the control would be more complicated. Usually, we use the single-chip like DSP to achieve our control to our system. However, it may result in a time delay because of the signal sampling and conversion, which would deteriorate the power quality. In order to solve the situation, we can use observer into our control loop. It can not only reduce the number of sensors, but also improve the quality of electricity and system instability caused by a time delay.
In this thesis we use Simulink to construct a single-phase converter which is connected to a grid via an LCL filter. We also simulated the situation that the time delay of the signal occurs, and then put the observer into our control loop. We can check that whether the unstable waveform caused by time delay can be improved when the observer is used.

[1] I. J. Gabe, V. F. Montagner, and H. Pinheiro, “Design and implementation of a robust current controller for VSI connected to the grid through an LCL filter,” IEEE Trans. Power Electron., vol. 24, no. 6, pp. 1444–1452, Jun. 2009.
[2] O. Vodyakho and C. C. Mi, “Three-Level Inverter-Based Shunt Active Power Filter in Three-Phase Three-Wire and Four-Wire Systems,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1350–1363, May 2009.
[3] B. Bolsens, K. D. Brabandere, J. V. D. Keybus, J. Driesen, and R. Belmans, “Model-based generation of low distortion currents in grid coupled PWM-inverters using an LCL output filter,” IEEE Trans. Power Electron., vol. 21, no. 4, pp. 1032–1040, Jul. 2006.
[4] R. Pena-Alzola, M. Liserre, F. Blaabjerg, R. Sebastián, J. Dannehl, and F. W. Fuchs, “Analysis of the passive damping losses in LCL-filterbased grid converters,” IEEE Trans. Power Electron, vol. 28, no. 6, pp. 2642–2646, Jun. 2013.
[5] J. Dannehl, F. W. Fuchs, and P. B. Thøgersen, “PI state space current control of grid-connected PWM converters with LCL filters,” IEEE Trans. Power Electron., vol. 25, no. 9, pp. 2320–2330, Sep. 2010.
[6] G. Shen, D. Xu, L. Cao, and X. Zhu, “An improved control strategy for gridconnected voltage source inverters with an LCL filter,” IEEE Trans. Power Electron., vol. 23, no. 4, pp. 1899–1906, Jul. 2008.
[7] R. Pena-Alzola, M. Liserre, F. Blaabjerg, R. Sebastián, J. Dannehl, and F. W. Fuchs, “Systematic design of the lead-lag network method for active damping in LCL-filter based three phase converters,” IEEE Trans. Ind. Informat., vol. 10, no. 1, pp. 43–52, Feb. 2014.
[8] M. Liserre, R. Teodorescu, and F. Blaabjerg, “Stability of photovoltaic and wind turbine grid-connected inverters for a large set of grid impedance values,” IEEE Trans. Ind. Appl., vol. 42, no. 5, pp. 1146–1154, Sep./Oct. 2006.
[9] V. Blasko and V. Kaura, “A novel control to actively damp resonance in input LC filter of a three-phase voltage source converter,” IEEE Trans. Ind. Appl., vol. 33, no. 2, pp. 542–550, Mar./Apr. 1997.
[10] P. A. Dahono, Y. R. Bahar, T. Sato, and T. Kataoka, “Damping of transient oscillations on the output LC filter of PWM inverters by using a virtual resistor,” in Proc. IEEE Power Electron. Drive Syst., 2001, pp. 403–407.
[11] P. A. Dahono, “A control method to damp oscillation in the input LC filter of AC-DC PWM converters,” in Proc. IEEE Power Electron. Spec. Conf., 2002, pp. 1630–1635.
[12] Y. W. Li, “Control and resonance damping of voltage source and current source converters with LC filters,” IEEE Trans. Ind. Electron., vol. 56, pp. 1511–1521, May 2009.
[13] W. Zhao and G. Chen, “Comparison of active and passive damping methods for application in high power active power filter with LCL-filter,” in Conf. Rec. IEEE Int. Conf. Sustainable Power Gener. Supply, Apr. 2010, pp. 1–6.
[14] J. M. Guerrero, L. G. Vicuna, J. Matas, M. Castilla, and J. Miret, “Output impedance design of parallel-connected UPS inverters with wireless load sharing control,” IEEE Trans. Ind. Electron., vol. 52, no. 4, pp. 1126– 1135, Aug. 2005.
[15] J. He and Y. W. Li, “Analysis, design and implementation of virtual impedance for power electronics interfaced distributed generation,” IEEE Trans. Ind. Appl., vol. 41, no. 6, pp. 2525–2538, Nov./Dec. 211.
[16] Y. W. Li and C. N. Kao, “An accurate power control strategy for power-electronics-interfaced distributed generation units operation in a low voltage multibus microgrid,” IEEE Trans. Power Electron., vol. 24, no. 12, pp. 2977–2988, Dec. 2009.
[17] I. J. Balaguer, Q. Lei, S. Yang, U. Supatti, and F. Z. Peng, “Control for grid-connected and intentional islanding operations of distributed power generation,” IEEE Trans. Power Electron., vol. 58, no. 1, pp. 147–157, Jan. 2011.
[18] L. Corradini, P. Mattavelli, M. Corradin, and F. Polo, “Analysis of parallel operation of uninterruptible power supplies loaded through long wiring cables,” IEEE Trans. Power Electron., vol. 25, no. 4, pp. 1046–1054, Apr. 2010.
[19] P. C. Loh and D. G. Holmes, “Analysis of multiloop control strategies for LC/CL/LCL-filtered voltage-source and current-source inverters,” IEEE Trans. Ind. Appl., vol. 41, no. 2, pp. 644–654, Mar./Apr. 2005.
[20] E. Twining and D. G. Holmes, “Grid connected regulation of a three phase voltage source inverter with an LCL input filter,” IEEE Trans. Power Electron., vol. 18, no. 3, pp. 888–895, May 2003.
[21] F. Liu, S. Duan, J. Yin, B. Liu, and F. Liu, “Parameter design of a twocurrent-loop controller used in a grid-connected inverter system with LCL filter,” IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 4483–4491, Nov. 2009.
[22] Y. W. Li, and C. N. Kao, “An accurate power control strategy for power-electronics-interfaced distributed generation units operation in a low voltage multibus microgrid,” IEEE Trans. Power Electron., vol. 24, no. 12, pp. 2977–2988, Dec. 2009.
[23] P. C. Loh, M. J. Newman, D. N. Zmood, and D. G. Holmes, “A comparative analysis of multi-loop voltage regulation strategies for single and threephase UPS system,” IEEE Trans. Power Electron., vol. 18, no. 5, pp. 1176– 1185, Sep. 2003.
[24] M. Vilathgamuwa, A. A. D. Ranjith Perera, and S. S. Choi, “Performance improvement of the dynamic voltage restorer with closed-loop load voltage and current-mode control,” IEEE Trans. Power Electron., vol. 17, no. 5, pp. 824–834, Sep. 2002.
[25] M. Hanif, V. Khadkikar, and W. Xiao, “Two degrees of freedom active damping technique for LCL filter-based grid connected PV systems,” IEEE Trans. on Ind. Appl., vol. 61, no. 6, pp. 2795-2803, Jun. 2014.
[26] M. Liserre, F. Blaabjerg, and R. Teodorescu, “Grid impedance estimation via excitation of LCL-filter resonance,” IEEE Trans. on Ind. Appl., vol. 43, no. 5, pp. 1401-1407, Sep./Oct. 2007.
[27] F. Huerta, D. Pizarro, S. Cóbreces, F. J. Rodríguez, C. Girón, and A. Rodríguez, “LQG servo controller for the current control of LCL grid-connected voltage-source converters,” IEEE Trans. on Ind. Appl., vol. 59, no. 11, pp. 4272-4284, Nov. 2012.
[28] J. Dannehl, M. Liserre, and F. W. Fuchs, “Filter-based active damping of voltage source converters with LCL filter,” IEEE Trans. on Ind. Appl., vol. 58, no. 8, pp. 3623-3633, Aug. 2011.
[29] A. Yazdani, R. Iravani, Voltage- Sourced Converters in Power Systems. John Wiley & Sons, Inc, 2010.
[30] S. Sen, K. Yenduri, and P. Sensarma, “Step-by-step design and control of LCL filter based three phase grid-connected inverter,” IEEE ICIT, pp. 503-508, Feb. 26 - Mar. 1, 2014.
[31] M. Liserre, F. Blaabjerg, and S. Hansen, “Design and control of an LCL-filter-based three-phase active rectifier,” IEEE Trans. on Ind. Appl., vol. 41, no. 5, pp. 1281-1291, Sep./Oct. 2005.
[32] M. Liserre, F. Blaabjerg, and A. Dell’Aquila, Step-by-step design procedure for a grid-connected three-phase PWM voltage source converter. Taylor & Francis, 2004.