轉換器並聯可增加系統額定功率、提高系統可靠度並提升效率，而且可降低成本和減少輸出入電壓或電流漣波。此外，並聯架構非常適合系統模組化設計與規劃，亦可提供較佳的彈性，但是當轉換器直接並聯在一起運作時，不同轉換器開關的操作會形成轉換器之間環流的迴路，此一現象將會造成線路電流波形失真、負載電流分配不均，以及系統整體效能的降低。到目前為止，只有極少數的文獻提到並聯系統零序環流的建模，有關環流的完整建模和較佳的控制方式仍有待進一步的研究。因此，本論文經由系統化的模擬研析與觀察了解相同容量轉換器並聯時的環流產生現象，在國際首先定義出各個轉換器各相之環流，藉此一定義可完整說明環流的物理意義，並清楚的說明並聯三相轉換器環流產生的機制。基於此ㄧ環流定義，本論文亦首度導出正確而完整的三相環流模型，經PSPICE模擬軟體的驗證，此一環流模型與定義之環流幾乎完全吻合，同時從此一狀態平均模型可以看出，環流不僅有零序成分，而且存在非零序成分。從此環流模型可顯而易見的了解環流受到哪些因素的影響，諸如不同的脈波寬度調變切換策略。值得ㄧ提的是，本論文首次提出並聯轉換器本質環流的現象。基於本論文所提之環流模型，吾人更進一步尋找一適當的環流協調控制策略，此策略可消除並聯系統之環流而獲得均流效果。此外，有鑒於逐漸增加的並聯分散式電源系統，作者首先提出環流指標和不平衡率作為量化量測第j台轉換器第k相電源品質的依據，同時更進一步提出廣義的環流定義和廣義並聯三相轉換器環流狀態空間平均模型，適用於並聯轉換器在不同負載電流之操作情形。同時，本論文亦提出一廣義環流協調控制策略，依據額定容量或較佳效率之分佈因素考量可作不同負載之調度，以達成並聯轉換器間環流消除與所需負載電流之分配。最後，本論文作者並實體製作一套並聯三相升壓型整流器之硬體系統，使用數位訊號處理器TMS320F2812 DSP作為控制器，以驗證本論文所提廣義環流協調控制策略，模擬與實作波形同時顯示本論文所提之廣義環流協調控制策略確實可行。

It is well-known that multiphase converters can be paralleled to increase the power rating, reliability, efficiency as well as decrease the cost and current/voltage ripples . In addition, the parallel structure is very suitable for modular system design and system reconfiguration, providing better flexibility. However, when converters are paralleled together, circulating currents will be generated automatically whenever closed loops between different converters are formed by the switching operations. This will result in current distortion, unbalanced load sharing and the overall performance will be degraded. However, up to the present, very few existing papers concerning with circulating current modeling of paralleled systems are available. Further research work about more complete modeling and better control of circulating currents remains to be done. Hence, in this dissertation, definition of circulating currents of multi-phase paralleled converters is first presented and the circulating current generating mechanism is clearly explained. Then, based on this definition an averaged model of the circulating current is proposed. It is seen from this model that the circulating current consists of not only the zero sequence but also the non-zero sequence components. The governing differential equation also shows explicitly the relation between the circulating currents and the affecting factors such as different PWM strategies. With this understanding, a simple coordinate control is then presented to achieve equal load sharing control and reduce the circulating current. The phenomenon of the intrinsic circulating current is also explained.
In addition, in view of the gradually increased parallel systems in distribution power generation systems, the author also proposed a circulating current index and unbalance rate for the kth phase of converter j as a quantitative measure about the serious status of power quality. Furthermore, the proposed circulating current definition and model are also generalized to cover the case of paralleled converters with different loadings. Meanwhile, a generalized coordinate control is also proposed to achieve unequal load sharing control as well as to eliminate the circulating currents according to the desired distribution factors. A prototype system is also constructed and the proposed general coordinate control is implemented by using TMS320F2812 DSP to verify the validity. Both simulation and experimental results indeed show the effectiveness of the proposed general coordinate control.

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