本研究研製一部高切頻併網型換流器，電路架構為LCL三相三線全橋式。為因應切換頻率100 kHz，需要更短的類比/數位轉換時間與多轉換通道，因此以微控制器Renesas RX71M做為控制核心。此併網型換流器主要功能為輸出正實功與特定功率因數下的電流至電網，可依照電網所要求的比率來饋入實、虛功，達到補償電網電壓與頻率的效果。
以往併網型換流器受限開關元件、濾波元件與控制方法的頻寬，換流器切頻一般來說較低，導致系統體積龐大，因此本研究使用LCL濾波器，在與相同濾波效果下比起LC濾波器體積較小，並將系統切換頻率提升至100 kHz，更有效降低系統體積。
在控制方面，本系統採用解耦合分切合整直接數位控制法，將直流鏈電壓、開關切換頻率及電感值隨電流變化衰減考慮進去，並使用零序注入法，精確的計算出下一週期的開關責任比率，且能避免開關責任比率過度調變。再透過分切合整方法使三相三線系統等效成三組單相獨立進行控制，與傳統的abc –dq軸轉換更能有效簡化控制上的複雜程度，大幅減低控制法則推導計算時間，並且可以提昇控制頻寬。此外，解耦合分切合整直接數位控制搭配空間向量調變(SVPWM)，與正弦脈波寬度調變(SPWM)相比，可得到更好的電壓利用率。
在實作上，首先利用模擬來驗證此控制法在三相三線全橋式換流器之可行性，再進行實測換流器之各項功能，最後進行損耗分析。
本論文主要貢獻為:(1)採用分切合整方法推導之解耦合直接數位控制法則，證明三相三線系統能實現單相控制與降低控制法之複雜度。(2)實作一部33 kW高切頻LCL三相三線全橋換流器，可注入實、虛功至電網。(3)設計LCL濾波器，使換流器輸出電流總諧波失真率符合併網規範要求。

This research is aimed at designing and implementing a high switching frequency grid-connected inverter. A three-phase three-wire full-bridge topology with LCL filters is adopted. For the 100 kHz switching frequency, micro-controller Renesas RX71M is chosen as control center of the system because of its having many A/D conversion channels and short conversion time. The main function of the grid-connected inverter is to inject active and reactive power into the grid by following the command of utility company to compensate grid voltage and frequency.
In the past, grid-connected inverters were restricted by switching elements, filter components, and bandwidth of control methods, which only operate at low frequency, resulting in high volume. Compared with an LC filter, the inductor size of an LCL filter is smaller under the same filtering effect. Therefore, the inverter with LCL filters and operating at 100 kHz switching frequency can further reduce the power stage volume effectively.
With regard to the control of the inverter, this research adopts decoupled three-phase direct digital control with the division-summation (D-Σ) process to control the inverter system. Taking DC link voltage, switching frequency and inductance value varying with current into account and combining zero-sequence injection method can achieve fast response and robustness. The duty ratios of next cycle can be accurately determined and duty ratio over modulation can be prevented. The three-phase three-wire inverter can be equivalent to three single-phase inverters and they can be controlled independently. Comparing with traditional abc to dq frame transformation, derivation of the direct digital control can be simplified, reducing the computation time and improving control bandwidth. In addition, the decoupled three-phase direct digital control with SVPWM compared with SPWM has better DC voltage utilization.
First, feasibility of the three-phase three-wire full-bridge inverter with the decoupled three-phase direct digital control and the division-summation (D-Σ) process has been verified by simulation. Then all of the functions are verifing by experimental results, and finally, the loss analysis is conducted.
The major contributions of this thesis are : (1)verifing that the control of a three-phase three-wire system can be equivalent to three single- phase inverters when adopting the decoupled direct digital control, (2) implementing a 33 kW and 100 kHz three-phase three-wire full-bridge inverter, and proving the capability of injecting active and reactive power into the grid, and (3) designing LCL filters for the inverter to achieve the output currents complying with the regulation.

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