Modeling, stability analysis and advanced control of cyber-physical microgrid systems

• Main Author: Yao, Weitao
• Other Authors: Xu Yan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/164902
• Institution: Nanyang Technological University

Microgrids (MGs) are becoming vital parts of the future smart grid with the higher penetration of renewable energy sources. Compared with the traditional power system, MGs have the advantages such as being eco-friendly, flexible and enhancing grid resilience. For the islanded MGs, a typical hierarchical control framework including primary, secondary, and tertiary control is commonly implemented. The droop-based primary control ensures the MG system's rapid stability. The secondary control aims to eliminate the frequency and voltage deviation. The tertiary control is to achieve economic dispatch. Besides, considering a single MG has a limited generation capacity and specific geographical boundaries, the networked-microgrid (NMG) system is also proposed by interconnecting multiple MGs to enhance its resilience against extreme events. MG/NMG is a cyber-physical system with coupled electrical and cyber networks, such a system could involve new stability issues due to the intrinsic characteristics and non-ideal cyber networks, such as low-inertia characteristics, time delays, and cyber-attacks. Hence, the modeling, stability analysis and advanced control methods for cyber-physical MG/NMG systems need to be in-depth investigated. This thesis aims to solve the above-mentioned stability issues, and presents integrated modeling, analysis, and advanced control for MG/NMG systems. The focus is mainly on the following aspects: small-signal modeling of MG and NMG system, advanced control method to enhance the time-delay margin for MG system, cyber-resilient control against cyber-attacks for MG system, layered distributed control framework, stability analysis and time-delay compensation control for NMG system. The detailed descriptions will be respectively provided hereinafter. The time delay in MG control will significantly impact the system's stability. To enhance the time-delay margin for MG system, a weight-average-prediction (WAP) controller is first designed to compensate for the delayed system state. By introducing a time-delayed differential term in the designed control law, the traditional time-delayed small-signal model is transformed into a neutral time-delayed mathematic model. Based on the developed model, the stability analysis is conducted considering both fixed-time and time-varying delays. For the former, a novel graphic analytical method is proposed to evaluate the time delay margin, which eliminates the conservatism compared with existing time-domain methods. For the latter, the stability condition is established by a Lyapunov-Krasovskii function and linear matrix inequalities. In addition, some non-linear WAP control methods are discussed to guide the parameter tuning with a higher resolution. Lastly, the designed method and analytical result are verified in the OPAL-RT real-time test platform. Cyber-attack is another stability issue for cyber-physical MG systems. Considering the different attack intensities for data availability, an attacker can choose denial-of-service (DoS) attacks or latency attacks to impact the stability of MG system. In this thesis, the above two kinds of attack consequences including network jamming and time-varying latency in the communication network are simultaneously studied. Firstly, a metric is defined to quantify the DoS attacks. Then, the time-domain stability study is conducted. Next, a cyber-resilient control strategy is designed with two control modes: (i) An adaptive-gain resilient controller to sustain the fast stabilization of MG systems under the non-uniform time-varying latency attack. (ii) An event-trigger topology reconfiguration controller against excessive latency and damaged cyber connectivity caused by DoS attacks. A switching mechanism for coordinating the above control modes is also designed. The effectiveness of the designed controller under different attack scenarios is verified by OPAL-RT real-time tests. NMG system, as a specific MG system, has a much more complex structure with larger control difficulties and complicated dynamic behaviors. To address this issue, a layered distributed control framework for NMG system is designed in this thesis, which is comprised of two layers: the individual MG control layer and NMG control layer. The NMG control layer generates the voltage/frequency reference for the MG control layer. The MG layer controls each DG to ensure system stability. Besides, a small-signal model for a generalized single-bus NMG system is derived to analyze the impact of coupling among MGs and various parameters on NMG dynamic stability. It reveals that the coupling relationship among MGs will weaken the stability of the whole NMG system while the proper control parameters can enhance the stability. Lastly, the designed control framework and the analytical result are verified by time-domain simulations. The above-designed distributed control framework in NMG systems relies on the information exchange among distributed generators and sub-MGs, which inevitably contain heterogenous time delays that can deteriorate system stability. Hence, the multiple time-delays small-signal stability analysis of an NMG with the above-designed distributed layered control architecture is further studied, and a lead-lag compensation controller is designed. Firstly, a generalized time-delayed DC NMG small-signal model is developed. Then, stability analysis is conducted considering multiple time delays from communication and measurement stages in NMG layer and MG layer. The impact of multiple time delays on NMG stability is quantified. It reveals that both the electrical coupling among MGs and different kinds of time delays in distributed control loops will impact the NMG stability. Subsequently, based on the detailed analytical results, the critical eigenvalue is identified, and a lead-lag compensation controller is designed to enhance the system stability by compensating for the phase lag of the critical eigenvalue. Lastly, the effectiveness of the designed method and the accuracy of the analytical findings are verified by OPAL-RT-based real-time tests. In summary, the overall research problems of this thesis focus on modeling, stability analysis, and advanced control of cyber-physical MG/NMG systems. Various stability issues including time delays, cyber-attacks, and low-inertia characteristics are separately studied. The relevant stability characteristics are analyzed and revealed in this thesis. Based on the findings, the novel control framework and advanced controllers are separately designed to enhance stability, compensate for time-delayed or attacked effects, and achieve multiple control objectives in MG/NMG system.