應用於傳輸線阻抗不確定性之單相換流器並聯系統雙層迴路分流控制

Two-Layer Current Sharing Control of Parallel System for Single-Phase Inverters with Specified Line Impedance Uncertainties

陳奕誠(Yi-Cheng Chen)

指導教授 ： 鄒應嶼

國立交通大學/電機學院/電機與控制工程系所/碩士(2019年)

摘要

本文主旨是針對無通訊模式下進行交流微電網並聯單相全橋換流器的分流控制。在微電網中，並聯換流器有兩種工作模式，分別為併網模式以及獨立模式，而這兩種工作模式的重點都是在於有無通訊架構，所以採用垂降控制(droop control)來讓各台換流器在獨立模式下，可以毫無通訊的狀況達到同步、負載分流，其中電流的分配條件分別為平均分配以及量力分配，這可以透過垂降控制中的分流比例命令來實現。由並聯換流器組成的獨立模式微電網中垂降控制雖然可以讓各個換流器進行並聯操作並且實現功率分享，但是在換流器連接各個負載之間的傳輸線長度不一致下，使得換流器的輸出阻抗與傳輸線阻抗之間的相互作用，會因為阻抗互相的不匹配而降低垂降控制功率分享的性能，進而衍生出輸出電流誤差的增加。為了降低在並聯模式下由傳輸線阻抗相互不匹配的電流誤差，所以採用虛擬阻抗(virtual impedance)的方式來重新匹配出合理的虛擬輸出阻抗(virtual output impedance)，而該方法在於虛擬額外增加一個阻值在輸出濾波器到負載端之間，利用此方法能有效地降低輸出電流的誤差。另外在微電網系統中，通常是由多台換流器同時並聯操作，而為了實驗驗證以及簡單化，使用雙台換流器來進行模擬以及實驗，並且提出了一種具有在指定傳輸線阻抗不確定下能進行並聯模式操作的雙層迴路控制。首先內層控制器設計在於橋式整流器負載下實現指定輸出電壓THD的所需輸出阻抗，主要是為了滿足單相換流器在進行暫態響應上有著良好的表現; 而外層控制是利用垂降控制與虛擬阻抗迴路所組成的，這層架構實現了分流控制以及解決傳輸線阻抗不一致所衍生出的問題。實驗部份採用德州儀器公司的TMS320F28335數位控制晶片作為實驗平台，並以兩台單相全橋式換流器以功率90W完成系統整合之並聯測試與驗證。

關鍵字

AC交流微電網 ； 並聯操作 ； 電網換流器 ； 強健性分流控制 ； 垂降控制 ； 虛擬阻抗 ； 輸出阻抗 ； 傳輸線阻抗不確定性

並列摘要

The purpose of this research is to investigate the current sharing control of parallel connected single-phase full-bridge inverters for AC microgrid. The inverters in a microgrid system can be operated in two modes, the grid-connected mode and the stand-alone mode. A big difference of these two modes of operations is whether a communication in between or not. In this research, two inverters operated in stand-alone mode are controlled by a droop control method, which achieves synchronization and load power sharing without communication in between. The current distribution can be setup to average distribution or force distribution by the current sharing commands in the droop control. Due to the uncertainties of the lengths of the transmission lines between each inverters and the load, the mismatch of the output impedance and the transmission line impedances reduces the droop control power sharing performance and increase the output current error. In order to reduce the current error in the above condition, a virtual impedance, which is between the output filter and load, is added to rematch the output impedance and transmission line impedance.This method is verified to reduce the current error effectively. In the microgrid system, multiple inverters are operated in parallel at the same time usually,and in order to verify the method and simplify the system, this thesis proposes a two-layer current sharing control scheme for parallel connected inverters with specified uncertainties of line impedances. The inner layer controller is designed to achieve a desired output impedance with specified voltage THD under nonlinear rectified load. The outer layer controller is designed to provide robust current sharing control by using virtual impedance control with specified line impedance uncertainties. The outer layer control is composed of the droop control and the virtual impedance loop. This two layer architecture realizes the sharing control and solving the transmission line impedance uncertainties. The experiment is verified by the test bench with two 90W single-phase full-bridge inverters controlled by Texas Instruments TMS320F28335 DSP.

並列關鍵字

AC microgrid ； parallel operation ； grid inverter ； robust current sharing control ； droop control ； virtual impedance ； output impedance ； line impedance uncertainties

參考文獻

[1] S. Ahmad, S. Sardar, A. U. Asar, and B. Noor, “Impact of distributed generation on the reliability of local distribution system,” INTERNATIONAL JOURNAL OF ADVANCED COMPUTER SCIENCE AND APPLICATIONS, vol. 8, no. 6, pp. 375–382, 2017.

[2] “Renewables 2018 global status report,” REN21, 2018. [Online] Available :http://www.ren21.net/status-of-renewables/global-status-report/

[3] D. I. Narvaez and M. G. Villalva, “Modeling and control strategy of a single-phase uninterruptible power supply (UPS),” 2015 IEEE PES Innovative Smart Grid Technologies Latin America (ISGT LATAM), Montevideo, pp. 355-360, 2015.

[4] A. El Moubarek Bouzid, P. Sicard, A. Yamane and J. N. Paquin, “Simulation of droop control strategy for parallel inverters in autonomous AC microgrids,” 2016 8th International Conference on Modelling, Identification and Control (ICMIC), Algiers, pp. 701-706, 2016.

[5] Q. Lei, F. Z. Peng and S. Yang, “Multiloop control method for high-performance microgrid inverter through load voltage and current decoupling with only output voltage feedback,” IEEE Transactions on Power Electronics, vol. 26, no. 3, pp. 953-960, 2011.