| dc.description.abstract | 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. | en_US |