Model Predictive Control Design for the Secondary Frequency Control of Microgrid Considering Time Delay Attacks

Fast depleting fossil fuels and growing awareness of environmental protection have raised worldwide concerns, aiming to build a sustainable and smart energy ecosystem. Renewable energy generation plays an important role in providing clean power supply. However, the integration of a bulk renewable generation system would also introduce new forms of disturbances and uncertainties to impact the power quality, threatening the secure operation of the distribution network. Microgrid, as an emerging technology, is quite appealing to be interfaced with distribution systems due to its potential economic, environmental, and technical benefits. The microgrid differs from the “smart grid” with different control strategies to accomplish the goal of helping the power grid with load balancing and voltage control and assisting power markets. A hierarchical control structure for the microgrid is commonly designed to address all above issues both in islanded mode and grid-connected mode. On the other hand, concerns about cybersecurity threats in the microgrid are steadily rising, and enormous number of economic losses would occur if defense strategies are not stipulated and carried out. In the modern power system, distributed control system, intelligent measuring devices and Internet of Things (IoT) are highly recommended in microgrid systems, which lead to the vulnerability of communication channels. Cyber threats such as false data injection (FDI) attacks, denial of service (DoS) attacks, and time-delay switch attacks (TDS) can be effortlessly implemented through information and communication centers, compromising the secure operation of power systems. By theoretically analyzing the AC microgrid simulation model, the MPC control strategies, and the modified MPC method based on GCC estimation will be studied in this thesis. In the second chapter, this thesis summarizes the start-art-of microgrid control, introducing a hierarchical control structure: primary control, secondary control, and tertiary control. These control levels differ in their speed of response, the time frame in which they operate, and infrastructure requirements. We focus on the centralized secondary frequency control system, which compensates the frequency deviation caused by primary control—P/f method. Then, in Chapter 3, the isolated AC MG frequency control system including WTG, DEG, PV panel and energy storage system with MPC controller is modeled. Three case studies are designed in MATLAB/Simulink to illustrate the advantages of the MPC method compared with the traditional PI controller. In the next Chapter, since state estimation based on precise status feedback of the system components is essential for the MPC controller to calculate corresponding control signal, the status feedback attack to BESS and FESS is considered. Correspondingly, an online status switching method is proposed to detect the original statuses of BESS and FESS, updating the state estimation function to obtain desirable performance of frequency regulation. Last, considering the time delay attack hacked by the adversary in the sensor, a modified MPC method based on GCC estimation is proposed to detect and track time delay attacks online. The model of proposed method to regulate frequency deviation is built in MATLAB. There are three case studies in this part: a constant time-delay attack with 0.1 pu load increase; a time-varying delay attack with 0.1 pu load increase; and a time-varying delay attack with changing load disturbance. By analyzing results of three cases, the effectiveness of the modified MPC method is proved

Model Predictive Control Design for the Secondary Frequency Control of Microgrid Considering Time Delay Attacks

Fast depleting fossil fuels and growing awareness of environmental protection have raised worldwide concerns, aiming to build a sustainable and smart energy ecosystem. Renewable energy generation plays an important role in providing clean power supply. However, the integration of a bulk renewable generation system would also introduce new forms of disturbances and uncertainties to impact the power quality, threatening the secure operation of the distribution network. Microgrid, as an emerging technology, is quite appealing to be interfaced with distribution systems due to its potential economic, environmental, and technical benefits. The microgrid differs from the “smart grid” with different control strategies to accomplish the goal of helping the power grid with load balancing and voltage control and assisting power markets. A hierarchical control structure for the microgrid is commonly designed to address all above issues both in islanded mode and grid-connected mode. On the other hand, concerns about cybersecurity threats in the microgrid are steadily rising, and enormous number of economic losses would occur if defense strategies are not stipulated and carried out. In the modern power system, distributed control system, intelligent measuring devices and Internet of Things (IoT) are highly recommended in microgrid systems, which lead to the vulnerability of communication channels. Cyber threats such as false data injection (FDI) attacks, denial of service (DoS) attacks, and time-delay switch attacks (TDS) can be effortlessly implemented through information and communication centers, compromising the secure operation of power systems. By theoretically analyzing the AC microgrid simulation model, the MPC control strategies, and the modified MPC method based on GCC estimation will be studied in this thesis. In the second chapter, this thesis summarizes the start-art-of microgrid control, introducing a hierarchical control structure: primary control, secondary control, and tertiary control. These control levels differ in their speed of response, the time frame in which they operate, and infrastructure requirements. We focus on the centralized secondary frequency control system, which compensates the frequency deviation caused by primary control—P/f method. Then, in Chapter 3, the isolated AC MG frequency control system including WTG, DEG, PV panel and energy storage system with MPC controller is modeled. Three case studies are designed in MATLAB/Simulink to illustrate the advantages of the MPC method compared with the traditional PI controller. In the next Chapter, since state estimation based on precise status feedback of the system components is essential for the MPC controller to calculate corresponding control signal, the status feedback attack to BESS and FESS is considered. Correspondingly, an online status switching method is proposed to detect the original statuses of BESS and FESS, updating the state estimation function to obtain desirable performance of frequency regulation. Last, considering the time delay attack hacked by the adversary in the sensor, a modified MPC method based on GCC estimation is proposed to detect and track time delay attacks online. The model of proposed method to regulate frequency deviation is built in MATLAB. There are three case studies in this part: a constant time-delay attack with 0.1 pu load increase; a time-varying delay attack with 0.1 pu load increase; and a time-varying delay attack with changing load disturbance. By analyzing results of three cases, the effectiveness of the modified MPC method is proved

Modeling & Small Signal Analysis of Grid Forming Inverter

There is a rising number of inverter-based resources (IBRs) being integrated with distribution systems are becoming a more common occurrence. With integration of IBRs inverters, power utilities are experiencing an increase of number of operations with regards to voltage and frequency support. To maintain grid stability and reliability, IBRs need to provide some of the services currently (or formerly) provided by synchronous generators. Interconnection standards, like the IEEE 1547. 2018 has include requirements for IBRs to have the capability to provide some of these services—such as frequency and voltage support—and the procurement and deployment of the services can be implemented either as mandatory interconnection requirements or as market products. All the IBRs deployed today are grid-following (GFL), and read the voltage and frequency of the grid and inject current to provide the appropriate amount of active and reactive power. The fundamental GFL IBR design assumption is that there are still enough synchronous generators on the grid to provide a relatively strong and stable voltage and frequency signal, which GFL IBRs can “follow.” But since levels of GFL are increasing, there will be a limit to how far GFL controls can be pushed, and, at some point, new advanced inverter controls (termed grid forming (GFM)) will be needed to maintain system stability. GFM IBRs will also be needed to establish voltage and frequency during operating conditions when there are zero synchronous machines (100 percent IBR penetration). Power systems around the world are at the point of now needing to make this technological leap; however, system operators and planners, equipment owners, and manufacturers today are facing a circular problem regarding the deployment of advanced IBR controls. Which comes first, the requirement for a capability or the capability itself? How do grid operators know what performance or capability is possible from new equipment (and therefore what they could require)? How can they evaluate costs and benefits of having such equipment on the grid? What drives manufacturers to invest in modern technology without its being mandated for interconnection to the grid or otherwise incentivized by the market? The objective of this thesis is to provide a better understanding of ride through fault capabilities of Grid Forming Inverter (GFM) tied into the generation side of the power grid when using control functions. Furthermore, to investigate the robustness of implementing time delay with a PLL system within the control settings for grid forming inverters. To this end, to identify the contributing factors that affects the stability of the time delay to better design and future models of GFMs. As discussed, the microgrid is a potential solution for future distributed generation systems. However, controlling a microgrid is still a complex issue and many proposed solutions, are only based on locally measured signals without any communications; thus, it is difficult to achieve global optimization. Future works on this topic will analyse the role of restoration practices, communication control techniques to better approximate the delay. The specific areas below will be discussed in this thesis

Modeling & Small Signal Analysis of Grid Forming Inverter

There is a rising number of inverter-based resources (IBRs) being integrated with distribution systems are becoming a more common occurrence. With integration of IBRs inverters, power utilities are experiencing an increase of number of operations with regards to voltage and frequency support. To maintain grid stability and reliability, IBRs need to provide some of the services currently (or formerly) provided by synchronous generators. Interconnection standards, like the IEEE 1547. 2018 has include requirements for IBRs to have the capability to provide some of these services—such as frequency and voltage support—and the procurement and deployment of the services can be implemented either as mandatory interconnection requirements or as market products. All the IBRs deployed today are grid-following (GFL), and read the voltage and frequency of the grid and inject current to provide the appropriate amount of active and reactive power. The fundamental GFL IBR design assumption is that there are still enough synchronous generators on the grid to provide a relatively strong and stable voltage and frequency signal, which GFL IBRs can “follow.” But since levels of GFL are increasing, there will be a limit to how far GFL controls can be pushed, and, at some point, new advanced inverter controls (termed grid forming (GFM)) will be needed to maintain system stability. GFM IBRs will also be needed to establish voltage and frequency during operating conditions when there are zero synchronous machines (100 percent IBR penetration). Power systems around the world are at the point of now needing to make this technological leap; however, system operators and planners, equipment owners, and manufacturers today are facing a circular problem regarding the deployment of advanced IBR controls. Which comes first, the requirement for a capability or the capability itself? How do grid operators know what performance or capability is possible from new equipment (and therefore what they could require)? How can they evaluate costs and benefits of having such equipment on the grid? What drives manufacturers to invest in modern technology without its being mandated for interconnection to the grid or otherwise incentivized by the market? The objective of this thesis is to provide a better understanding of ride through fault capabilities of Grid Forming Inverter (GFM) tied into the generation side of the power grid when using control functions. Furthermore, to investigate the robustness of implementing time delay with a PLL system within the control settings for grid forming inverters. To this end, to identify the contributing factors that affects the stability of the time delay to better design and future models of GFMs. As discussed, the microgrid is a potential solution for future distributed generation systems. However, controlling a microgrid is still a complex issue and many proposed solutions, are only based on locally measured signals without any communications; thus, it is difficult to achieve global optimization. Future works on this topic will analyse the role of restoration practices, communication control techniques to better approximate the delay. The specific areas below will be discussed in this thesis

Modulation and Control Techniques for Performance Improvement of Micro Grid Tie Inverters

The concept of microgrids is a new building block of smart grid that acts as a single controllable entity which allows reliable interconnection of distributed energy resources and loads and provides alternative way of their integration into power system. Due to its specifics, microgrids require different control strategies and dynamics of regulation as compared to ones used in conventional utility grids. All types of power converters used in microgrid share commonalities which potentially affect high frequency modes of microgrid in same manner. There are numerous unique design requirements imposed on microgrid tie inverters, which are dictated by the nature of the microgrid system and bring major challenges that are reviewed and further analyzed in this work. This work introduces, performs a detailed study on, and implements nonconventional control and modulation techniques leading to performance improvement of microgrid tie inverters in respect to aforementioned challenges

Frequency and Voltage Control of Islanded Microgrids

Islanded microgrids (MGs), characterized by distributed generators, power consumers, and energy storage systems (ESSs), are designed to signi cantly enhance self-sustainability of future distribution networks and to provide energy for remote communities. In order to have a stable system, both primary and secondary frequency and voltage control of the MG are critical. From a primary control perspective, it is essential to maintain frequency and voltage in acceptable ranges. Conventional controllers are designed to regulate system frequency and voltage solely based on droop control theory, and this is mainly provided by fast-response generation units such as ESSs. Therefore, an intelligent power sharing (IPS) control is necessary to maintain frequency and voltage within acceptable ranges, and to share power not only based on generation units' droop values, but also their operating power capabilities. A mathematical model of small-perturbation stability is presented along with performance analysis. Based on analysis and simulation results, the IPS controller offers advantages such as robust performance under load and renewable energy variations, a dynamic compromise between voltage regulation and accurate reactive power sharing among generators, and enhancement of voltage regulation by an adaptive virtual impedance. From a secondary control perspective, scheduling of generation units based on conventional unit commitment (UC) remains fi xed for the duration between two dispatch intervals; however, demand or renewable generation can continuously change. This stair-pattern scheduling of generation units creates large frequency and voltage excursions at the edge of each dispatch interval. Different from the existing UC mechanisms, a hybrid mid-level the controller is proposed based on communications with a distributed primary controller. It determines optimal power of generation units between two dispatch intervals for the secondary controller while regulating frequency and voltage within desirable ranges. Through several tested scenarios on a CIGRE test system, numerical results show that the mid-level controller can regulate frequency and voltage of the islanded MG. It covers time intervals between those of primary and secondary controllers and avoids the stair-pattern generation scheduling in conventional UCs. Additionally, it reduces both operating cost of MG and degradation of fast-acting generation units' life-cycle. Subsequently, impact of communication delay on islanded MGs is studied. The delay causes local controllers to use outdated power dispatches at the proposed mid-level controller. The outdated reference power deviates frequency and voltage from their nominal values in primary control. Existing primary and secondary controllers use a communication network assuming no time delay or considering a constant time delay. A mathematical model of constant and time-varying delay in islanded MGs is tegrated into the proposed mid-level controller. This formulation addresses the impact of time delay on transient performance of these controllers. A delay-based controller is designed to mitigate frequency oscillation of islanded MGs in the presence of either small or large perturbations. Numerical results are performed on small and large perturbations to evaluate the impact of time delay on realistic 14-bus CIGRE test system