Design of secondary controllers for microgrids with the consideration of communication issues

• Main Author: Lian, Zhijie
• Other Authors: Wen Changyun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/165570
• Institution: Nanyang Technological University

With the increasing concerns about environmental problems, microgrid (MG) deployments throughout the world have increased. Traditionally, a hierarchical control method is applied to MG systems to guarantee safe and economical operation, which is divided into three layers, i.e., primary, secondary, and tertiary layers. The secondary control is usually realized in a centralized or distributed way, which relies on real-time signal transmissions through the communication channels. Controlled by such communication-assisted cooperative controllers, an MG is regarded as a coupled cyber-physical system (CPS). For such a CPS, communication issues, e.g., time delays and cyber attacks, are of significance as they could pose a dramatic threat to an MG's stable and secure operation. In order to solve the above drawbacks, advanced distributed and decentralized secondary control methods are proposed in MGs in this thesis. To ensure the system stability subject to time delays, a distributed dynamic event-triggered secondary control law is used in the secondary frequency restoration and active power allocation in an MG. Additionally, the communication burdens can be measurably reduced by using dynamic event-triggered mechanisms. The proposed control scheme can restore frequency to its reference while maintaining accurate active power sharing under a sufficient condition established. Based on this condition, an explicit tolerable upper bound of all time delays is obtained, which can be used for the MG system design guideline in the planning phase. Experimental results have verified the effectiveness of the proposed controller. Then to mitigate the negative effects of the communication system, a decentralized secondary frequency control strategy is employed in AC MGs, by which no data exchanging process happens. Thus, MG reliability and robustness can be enhanced. A rigorous theoretical analysis proves that the proposed simple decentralized controller can achieve frequency restoration with a steady tolerant error and provide an accurate power sharing ratio calculated by the tertiary layer as working conditions change. Except for time delay issues, cyber attacks on distributed secondary controllers result in communication faults and thus cause severe stability and cyber-security issues. A distributed resilient optimal current sharing control is proposed for a single-bus DC MG, which not only achieves bus voltage restoration but also realizes the optimal current sharing optimization calculated by the tertiary layer all the time under denial-of-service (DoS) attacks. In addition, a resilient sampling mechanism is designed in the secondary layer to improve communication efficiency against DoS attacks. Based on Lyapunov stability analysis, it reveals that the overall system with the proposed control method ensures the stability of the overall DC MG system under DoS attacks. Besides utilizing the linear matrix inequality (LMI) approach and the theoretical results, a guideline can be given to designers for control parameter selection. Then considering multiple attacks, i.e., false data injection (FDI) and DoS attacks, a distributed resilient secondary control strategy is introduced for DC MGs, which can achieve global voltage restoration and power-sharing objectives with a resilient observer.