在眾多不斷電系統中，大多都使用變壓器做電氣隔離以降低環流，但變壓器的使用增加系統成本與體積，其運作時產生之熱能會減少功率元件的穩定性與壽命。無變壓器型不斷電系統可經由功因校正電路調節直流鏈與交流電網間的電力潮流，同時由電壓型換流器提供穩定的輸出電壓，配合雙向充放電器與電池達到不斷電之功能，並使用分切合整數位控制法與開關切換方式以克服傳統abc轉dq軸的複雜運算，且納入不同電流下的電感變化可精確的追蹤電流命令，降低電流控制誤差量並能克服環流問題。
本研究提出一無變壓器型不斷電系統，此系統結合功因校正與不斷電之功能，前級功因校正電路採用全橋三相三線式換流器架構，後級電壓型換流器則使用全橋三相四線式架構，數位控制器皆使用微控制器Renesas RX62T 做為數位控制核心，實現本研究之控制法則。
本研究之主要貢獻如下：第一點為有效抑制環流，此系統前級與後級電路間有地線連結，當兩電路在運作時，若開關切換狀態不同，地線則有較大的環流，改變開關切換方式和加入市電與開關同步電路可有效降低環流。第二點為採用分切合整數位控制法，將交流電壓諧波與電感值變化納入控制法則推導，因此換流器可以在交流電壓有諧波成分下依舊輸出低諧波失真的電流，同時可允許寬廣的電感值變化，有效減少電感鐵芯的損失與尺寸。第三點為將分切合整控制法運用在電壓型控制模式下，同時導入重複控制，有效降低輸出電壓總諧波失真。最後經由實測結果驗證本研究所提出之理論應用於無變壓器型不斷電系統之可行性。

In most uninterruptible power supply (UPS) systems, transformers are incorpo-rated for galvanic isolation to reduce circulating current. However, the use of trans-formers increases cost and size of a system. In addition, heat generated during opera-tion lowers the stability and reduces the longevity of power components. A trans-fomerless UPS system regulates the power flow between dc link and utility and stabilizes the output voltage. Rather than the conventional abc to dq frame transformation, the division-summation(D-Ʃ) digital control is adopted instead so that the calculation process can be greatly simplified.
This research presents a transformerless UPS systems which includes a power factor corrector (PFC) and a voltage source inverter(VSI). For the PFC, a three-phase three-wire full-bridge inverter is adopted while a three-phase four-wire full-bridge inverter is adopted for the VSI. Microprocessor RX62T is incorporated in both cir-cuits to implement the proposed control law.
The main contributions of this research are summarized as follows: First, The circulation current is suppressed effectively. Large circulating current flowing through the common ground of the PFC and the VSI due to different switching states is observed. To reduce the circulating current, synchronization of: a) the switching states between the two circuits and b) the input voltage of the PFC and the output voltage of the VSI have been implemented. Secondly, both ac harmonic voltage and inductance variation are taken into account in the control law derivation of the D-Ʃ digital control, allowing a sinusoidal output current even in the presence of harmonic-contaminated grid voltage. Meanwhile, core size can be greatly reduced with the consideration of wide inductance variation. Thirdly, total harmonic distortion of the output voltage can be effectively reduced by incorporating a repetitive control in the D-Ʃ digital control based voltage tracking control.
Finally, the feasibility of the proposed control scheme has been verified by the measurements from the implemented transformerless UPS system.

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[31] Magnetic Powder Cores Ver. 14
[32] HGTG40N60A4