三相市電併聯轉換器共模電壓抑制調變技術之評估

Evaluations of Common-Mode Voltage Reduction Modulation Techniques for Three-Phase Grid-Connected Converters

施智中(Chih-Chung Shih)

指導教授 ： 鄭博泰

國立清華大學/電機資訊學院/電機工程學系/碩士(2010年)

摘要

本論文主要針對與市電併聯的太陽能系統中，適合其轉換器所使用的脈寬調變技術做探討。由於在太陽能市電併聯系統中，效率、重量及體積為重要的考量因素，因此通常使用非隔離型(無變壓器)的電路架構。但由於缺少隔離，大量的漏電流會經由太陽能板對地的雜散電容流入地面，這對安全上是一很大的危害。因此探討使用脈寬調變的技術來抑制共模電流，為本論文討論的方向。

由於共模電壓的振幅對共模電流的量有很大的影響，故本論文以實驗的方式來觀察使用傳統脈寬調變技術與降低共模電壓的調變技術下的差異。在實驗室之測試平台做測試，從測試結果中可發現，由於在切換狀態的選擇上避免使用零向量，降低共模電壓調變技術較傳統的脈寬調變技術有更好的表現。在幾種降低共模電壓的調變技術之間有相異的結果，本論文也在時域及頻域中加以討論。由模擬以及實驗結果可以得知，Near State PWM 在所有調變技術中有最好的表現，不論是在漏電流大小、交流或直流側電流的漣波大小及切換損都有最令人滿意的結果。

最後，針對系統共振的情形，可以由系統參數估測出系統之共振頻率。在操作市電併聯的太陽能系統之轉換器時，此共振頻率可以做為選擇轉換器切換頻率的一個參考，避免操作頻率過於接近共振頻率而觸發共振，使共模電流表現惡化。

關鍵字

脈寬調變技術 ； 三相 ； 共模電壓 ； 共模電流 ； 漏電流 ； 光伏 ； 寄生電容

並列摘要

This thesis presents a survey on reduced common-mode voltage pulse width modulation techniques suitable for grid-connected photovoltaic converters. As the conversion efficiency, weight and size are critical concerns, the non-isolation (i.e., transformerless) photovoltaic system is preferred. However, the common-mode current flowing through stray capacitances is a critical safety issue due to the lack of galvanic isolation. Therefore, reducing common-mode current magnitude by using PWM methods is the target of this survey.

Due to common-mode voltage magnitude has great influence on common-mode current magnitude, conventional PWM methods (Space Vector PWM and Discontinuous PWM1) and reduced common-mode voltage PWM methods (Active Zero State PWM , Near State PWM) are experimentally tested. The performance of all PWM methods has been investigated with laboratory test bench. It can be shown that the common-mode current performance with reduced common-mode voltage PWM methods implemented are much better than that with conventional PWM methods due to the avoidance of using zero state vectors. The differences on common-mode current performance among reduced common-mode voltage PWM methods are analysed either in time domain or frequency domain. In particular, Near State PWM yields the best overall performance including low ground leakage current, low inverter output (phase current) and input (DC link current) ripple, and low switching losses.

When accounting for the system resonance, the resonant frequency in the photovoltaic system can be predicted by system parameters. The predicted resonant frequency can be an index when choosing switching frequency for grid-connected photovoltaic converters avoiding of triggering system resonance, making common-mode current performance worse.

並列關鍵字

pulsewidth modulation (PWM) ； three-phase ； common mode voltage (CMV) ； common mode current (CMC) ； leakage current ； photovoltaic ； parasitic capacitance

參考文獻

[1] “Automatic disconnection device between a generator and the public low voltage grid, paragraph 4.7.1 photovoltaik, dke deutsche kommission elektrotechnik elektronik informationstechnik im din und vde, standard din vde 0126-1-1,” Feburary 2006.

[2] T. Kerekes, M. Liserre, R. Teodorescu, C. Klumpner, and M. Sumner, “Evaluation of three-phase transformerless photovoltaic inverter topologies,” Power Electronics, IEEE Transactions on, vol. 24, no. 9, pp. 2202–2211, 2009.

[3] L. Palma, M. H. Todorovic, and P. N. Enjeti, “Analysis of common-mode voltage in utility-interactive fuel cell power conditioners,” Industrial Electronics, IEEE Transactions on, vol. 56, no. 1, pp. 20–27, 2009.

[4] L. Yen-Shin and S. Fu-San, “Optimal common-mode voltage reduction pwm technique for inverter control with consideration of the dead-time effects-part i: basic development,” Industry Applications, IEEE Transactions on, vol. 40, no. 6, pp. 1605–1612, 2004.

[5] L. Yen-Shin, C. Po-Sheng, L. Hsiang-Kuo, and J. Chou, “Optimal common-mode voltage reduction pwm technique for inverter control with consideration of the deadtime effects-part ii: applications to im drives with diode front end,” Industry Applications, IEEE Transactions on, vol. 40, no. 6, pp. 1613–1620, 2004.