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

Advanced Control Strategies for Voltage Source Converters in Microgrids and Traction Networks

Increasing concerns regarding global warming caused by greenhouse gases, which are mainly generated by conventional energy resources, e.g., fossil fuels, have created significant interest for the research and development in the field of renewable energies. Such interests are also intensified by the finitude availability of conventional energy resources. To take full benefit of renewable energy resources, e.g., wind and solar energy, interfacing power electronics devices are essential, which together with the energy resources form Distributed Generation (DG) units. If properly controlled and coordinated, the optimal and efficient operation of DG units, which are the main building block of rapidly emerging microgrid technologies, can be ensured. In fact, the optimal and efficient operation of any energy conversion systems, e.g., microgrids, traction networks, etc., necessitates some sorts of control strategies. Being structured into two main parts and exploiting two-level Voltage Source Converters (VSCs), this thesis introduces several control strategies in the context of microgrids and electrified traction networks. Although the proposed approaches of this thesis are mainly tailored for two-level VSCs, the methods are equally applicable to other converter technologies. In the first part, adopting an optimization-based loop shaping approach, a vector current control strategy for three-phase grid-tied VSCs is proposed. The proposed control strategy is able to independently regulate the direct and quadrature (dq)-components of the converter currents in a fully decoupled manner and shows very fast dynamic response similar to the existing methods. In order to extend the applicability of the proposed vector control method to single-phase systems, a countermeasure is also proposed. In single-phase systems, to form the orthogonal component of the current needed to create the dq-axes, the converter current is phase-shifted a quarter of a fundamental period. This phase-shift is the reason of strongly coupled dq-axes and oscillatory dynamic response in such systems. To obviate the need for the problematic phase-shifting, adopting a Fictive Axis Emulator (FAE), the orthogonal fictive current is created concurrent to the real one. In such a case, utilizing the proposed decoupled vector control strategy and the FAE, the dq-currents of single-phase converters are also regulated in a fully decoupled manner. Moreover, in this part, using a generalized version of the optimization-based loop shaping approach, three voltage control schemes are proposed for the voltage regulation of islanded microgrids. Since the dedicated loads of islanded microgrids are not fixed, the loop shaping is simultaneously carried out for various operating points of interests, i.e., for various combinations of the load parameters. Two single-stage control strategies and a cascade one are proposed: (i) a single-stage PI-based Multi-Input Multi-Output (MIMO) controller, (ii) a single-stage PI-based MIMO controller in conjunction with resonant terms, which is able to compensate for the adverse impacts of nonlinear loads, and (iii) a cascade PI-based MIMO controller. The cascade control scheme utilizes the proposed decoupled vector control strategy as its inner loop for regulating the converter current. In the second part, this thesis focuses on electrified traction networks and addresses a power quality problem in such networks, i.e., catenary voltage fluctuations. The Active Line-side Converter (ALC) of modern locomotives is utilized as STATic COMpensator (STATCOM) in order to inject reactive power to compensate for the adverse effects of catenary line voltage fluctuations. To determine the proper amount of reactive power, several control strategies belonging to the PI-controllers family are proposed: (i) a P-controller, (ii) a PI-controller, and (iii) a gain-scheduled PI-controller. Among the proposed approaches, the gain-scheduled strategy provides the best performance. The gain-scheduling is performed through identifying the catenary inductance at the connection point of the locomotive to that. The inductance identification is carried out by the injection of harmonic current through the ALC and monitoring its effect on the locomotive voltage. Despite its acceptable performance, the gain-scheduled approach shows several shortcomings. Therefore, utilizing the optimization-based loop shaping technique, a high-order voltage support scheme is also proposed. The proposed high-order scheme does not need any online tuning and/or modification while provides excellent performance for various operating points

Plug-and-play and coordinated control for bus-connected AC islanded microgrids

This paper presents a distributed control architecture for voltage and frequency stabilization in AC islanded microgrids. In the primary control layer, each generation unit is equipped with a local controller acting on the corresponding voltage-source converter. Following the plug-and-play design approach previously proposed by some of the authors, whenever the addition/removal of a distributed generation unit is required, feasibility of the operation is automatically checked by designing local controllers through convex optimization. The update of the voltage-control layer, when units plug -in/-out, is therefore automatized and stability of the microgrid is always preserved. Moreover, local control design is based only on the knowledge of parameters of power lines and it does not require to store a global microgrid model. In this work, we focus on bus-connected microgrid topologies and enhance the primary plug-and-play layer with local virtual impedance loops and secondary coordinated controllers ensuring bus voltage tracking and reactive power sharing. In particular, the secondary control architecture is distributed, hence mirroring the modularity of the primary control layer. We validate primary and secondary controllers by performing experiments with balanced, unbalanced and nonlinear loads, on a setup composed of three bus-connected distributed generation units. Most importantly, the stability of the microgrid after the addition/removal of distributed generation units is assessed. Overall, the experimental results show the feasibility of the proposed modular control design framework, where generation units can be added/removed on the fly, thus enabling the deployment of virtual power plants that can be resized over time

Review on Multi-Objective Control Strategies for Distributed Generation on Inverter-Based Microgrids

[EN] Microgrids have emerged as a solution to address new challenges in power systems with the integration of distributed energy resources (DER). Inverter-based microgrids (IBMG) need to implement proper control systems to avoid stability and reliability issues. Thus, several researchers have introduced multi-objective control strategies for distributed generation on IBMG. This paper presents a review of the different approaches that have been proposed by several authors of multi-objective control. This work describes the main features of the inverter as a key component of microgrids. Details related to accomplishing efficient generation from a control systems' view have been observed. This study addresses the potential of multi-objective control to overcome conflicting objectives with balanced results. 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Stability analysis and robust damping of multiresonances in distributed-generation-based islanded microgrids

International audienc

Maximum current injection method for grid-forming inverters in an islanded microgrid subject to short circuits

In islanded microgrids, when a short circuit or a sudden overload occurs, it provokes an abrupt increment in the currents supplied by the generation nodes, which feed the load collaboratively. This is particularly challenging for inverter-based nodes, due to its reduced power capacity. This work takes advantage of the droop-method basic configuration to propose an additional closed-loop control, which ensures maximum current injection during any kind of short circuit maintaining the underlying droop control. Ensuring that any node injects its maximum rated current during the short circuit, it emulates the most common low-voltage ride-through protocols for grid-feeding sources oriented to support the grid and, in this way, the voltage unbalance is reduced. To develop the control proposal, a model of the faulted system is presented in order to evaluate the stability of the closed-loop system. A general modelling methodology is introduced in order to derive the control for any microgrid configuration. Finally, selected experimental results are reported in order to validate the effectiveness of the proposed control.Peer ReviewedPostprint (author's final draft