| 摘要: | 本文針對獨立運行的交流孤島微電網，側重研究於如何維持多台轉換器之間的功率分配以及微電網電壓與頻率穩定的方法。高度依賴通訊設備傳遞信息的微電網若運行於孤島模式下，發生通訊設備故障、天災等突發狀況使通訊失聯時，集中式或分散式控制的失效，將導致轉換器間功率分配的不均，而第二級控制的失效將導致整體微電網的電壓與頻率劇烈變化。
為了預防微電網發生突發狀況，本文基於垂降控制法提出了一種無通訊功率分配控制。在分散式再生能源轉換器併網下輸出功率時，於初級控制中估算轉換器與責任分界點之間的線路阻抗，並使用負虛擬阻抗加以消除，使微電網進入孤島模式後達成功率自主分配的效果。隨後，透過線路阻抗消除的效應，將第二級控制中的電壓與頻率恢復機制加入本地控制器中，恢復垂降控制於孤島模式運行下所產生的電壓與頻率穩態誤差，以維持孤島微電網的穩定運行。
然而，線路阻抗估算於弱電網下將難以計算，這將影響功率分配的性能，而恢復機制於無通訊下，多台轉換器間的補償量未必相同，長時間運行將造成功率分配的不均。於此本文提出一種分散式的通訊控制策略，利用它台轉換器資訊實現弱電網下的線路阻抗估算，而恢復機制中引入觸發條件的控制策略，結束補償後將多台轉換器之補償量平均化，使各台轉換器之補償量達成一致，以維持功率分配。
為因應再生能源發電佔比的提升，本文基於虛擬阻抗孤島模式的垂降控制下再加入虛擬慣量，於加減載中降低系統頻率變化，提高整體孤島微電網的可靠性，然而當虛擬阻抗與虛擬慣量同時存在時，虛擬慣量產生的功率擾動將引入虛擬阻抗所補償的電壓命令中，而導致轉換器之輸出功率發散，於此本文亦提出解決方案以避免此情況發生。為驗證所提方法之穩定性，本文使用了基於阻抗穩定性準則以及小訊號穩定度分析證明所提方法之穩定性，提供設計參考，最後於模擬與實作中驗證所提研究方法的可行性。;This thesis focuses on the standalone operation of AC islanded microgrids and explores methods to maintain power sharing between multiple converters and stabilize voltage and frequency in the microgrid. Microgrids that heavily rely on communication devices to exchange information may encounter challenges when operating in islanded mode due to communication failures caused by equipment malfunctions or natural disasters. The failure of centralized or decentralized control can lead to uneven power sharing among converters, and the failure of secondary control can cause severe voltage and frequency fluctuations in the entire microgrid.
To prevent such situations in microgrids, this thesis proposes a communication-less power sharing control based on droop control. When decentralized renewable energy converters are connected to the grid, the primary control estimates the line impedance between converters and the point of common coupling. Negative virtual impedance is then used to eliminate the effect, enabling successful autonomous power sharing when the microgrid enters islanded mode. Furthermore, the local controller incorporates the voltage and frequency restoration mechanisms from the secondary control, leveraging the effects of line impedance elimination to maintain stable voltage and frequency during islanded mode operation, ensuring the stable operation of the islanded microgrid.
However, estimating line impedance in weak grids can be challenging, impacting the performance of power sharing. Additionally, without communication, the compensation among multiple converters in the restoration mechanism may not be uniform, resulting in uneven power sharing over time. To address these issues, a decentralized communication control strategy is proposed in this thesis, utilizing information from neighboring converters to estimate line impedance in weak grids. Moreover, a triggering condition control strategy is introduced in the restoration mechanism to average the compensation among multiple converters after the compensation ends, achieving consistency in compensation and ensuring power sharing stability.
Considering the increasing ratio of renewable energy generation, virtual inertia is incorporated into the droop control of islanded microgrids based on virtual impedance. The virtual inertia reduces system frequency variations during load changes, enhancing the overall reliability of the islanded microgrid. However, the simultaneous existence of virtual impedance and virtual inertia can introduce power disturbances from virtual inertia into the voltage commands compensated by virtual impedance, causing converter output power divergence. To address this issue, this thesis presents a solution to avoid such occurrences. To validate the stability of the proposed methods, this thesis employs impedance stability criteria and small-signal stability analysis, providing design references. Finally, the feasibility of the proposed research methods is verified through simulations and implementations. |
