本論文提出一種三階二極體箝位整流器的控制策略，用來實現改善整流器於不平衡電源下之輸入電流，藉此改善當三相三階二極體箝位整流器工作於不平衡之輸入電網電壓狀態時，會造成系統之輸入電流不平衡與輸出直流電壓漣波大等問題。控制方法主要基於分析直流側輸出電容漣波特性，因為在研究中發現輸出直流側電容器上的直流電壓中所包含的交流漣波受到不平衡交流電源條件的影響很大，理想的交流電源平衡狀態下，對於直流側電壓上的漣波只有六次諧波會出現，但在不平衡交流電源下則會多出現二次和三次諧波，上述諧波將明顯影響改變電容漣波波形，進而使得電力品質劣化，藉由利用分析這些漣波特徵於不平衡輸入電網電壓狀態下，且無需使用座標軸轉換、正負相序分離控制、直接功率控制等計算與分析即可針對輸入電壓不平衡情況進行電流控制，來改善系統之輸入電流不平衡度。
整流器之輸出直流電壓由單位功因控制器所控制，並藉由將漣波波形轉為脈波，脈波寬度用於判斷估測交流電源於系統不平衡之條件，最後將其控制方法規劃於系統控制方塊來產生補償電流命令，透過電流控制迴圈控制三相輸入電流之大小，再利用脈波寬度調變產生開關切換訊號，如此以獲得經改善且可調之輸入三相交流電流。
本論文使用MATLAB/SIMULINK進行系統模擬，用來評估系統控制策略的功能，並建立一直流輸出電壓為750伏特、輸出功率2千瓦、功率因數為0.99之三相三階二極體箝位整流器架構，使用數位訊號處理器TMS320F28069作為控制器進行試驗，透過模擬與實測的結果，來驗證所提出之控制方法可行性與實用性。

This thesis proposes a control strategy for a three-level diode clamped ac/dc converter to achieve balanced line current under unbalanced ac source. Consequently, through the control scheme proposed, in this paper, it can not only balance input current but also reduce dc output voltage ripple when three-phase neutral-point clamped rectifier works on input voltage unbalance condition. The proposed strategy is based on the ripple characteristics in the dc filter capacitors deployed in the dc output. The ac ripples contained in the dc voltages of the filter capacitors are highly affected by the balanced condition of the ac source. Under ideally ac source, only sixth harmonics will exixt, while second and third harmonics will emerge under unbalanced ac source. The latter two harmonics will change the ripple waveforms dramatically and then makes power quality deterioration. These multi-harmonic ripple waveforms can be used to compensate the unbalanced line current caused by unbalanced ac source, and no need to use coordinate transformation, positive and negative phase sequence disconnecting method, direct power control that calculations and analysis can control the current imbalance.
The converter dc output voltage is regulated by the coltroller with unity power factor. The ripple waveforms are trsansfered to pulse type, form which the pulse wides are used to estimate the unbalanced condition of the ac sourcecontrol system and then control scheme is planned to control device to generate the compensated comment of line cueernt, at last, through current control loop control three-phase input current, using pulse width modulation generate switch signal, so it can improve and modulate input ac current unbalance factor.
The control scheme of this thesis using Matlab/Simulink simulation system to evaluate the performance of the proposed scheme. Moreover, the a 750VDC/2kW, PF 0.99 three-phase NPC rectifier prototype based on digital signal processor TMS320F28069 also is established to demonstrate the vadility of the proposed system. Both simulation and experimental results show the correctness and practicality of proposed control scheme.

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