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This article investigates distributed event-triggered control (ETC) of power buffers in a direct current (DC) microgrid. In order to facilitate the control design, a linear interconnected system model is derived that captures the physical coupling among power buffers. Then, a distributed ETC law regulates the stored energy and input impedance of each power buffer, and a decentralized dynamic event-triggering mechanism determines when each power buffer communicates with its neighboring buffers. This strategy eliminates the need for both continuous controller updates and continuous communication among the power buffers. The resulting closed-loop system is shown to be exponentially stable under a mild assumption on the communication network. The proposed event-triggering mechanism guarantees not only the exclusion of the Zeno behavior but also the existence of a positive minimum interevent time that can be adjusted by the control design parameters. Simulation studies validate the effectiveness of the proposed theoretical results for a multibuffer DC microgrid. [ABSTRACT FROM AUTHOR]

Emerging distributed control paradigms rely on communication among converters of a microgrid. We investigate the secondary control of dc microgrids with a distributed dynamic event-triggering mechanism. The physical and cyber layers are represented by different graph topologies. To reduce the communication burden, a distributed dynamic event-triggering mechanism is designed. Therein, the Zeno behavior is excluded and in addition, a positive minimum interevent time (MIET) is determined. Asymptotic stability of the closed-loop system dynamics is proven, and the adjustment of the positive MIET is discussed. Compared to the existing static event-triggering mechanisms, the proposed dynamic one guarantees the existence of a positive MIET, whose value can be tuned by the design parameters. Controller/hardware-in-the-loop implementation results validate the efficacy of the proposed method. [ABSTRACT FROM AUTHOR]

This paper develops a fully distributed attack-resilient secondary control framework for DC microgrids, in the presence of unknown unbounded attacks on control input channels. The secondary control of each converter consists of an average consensus-based voltage regulator and a consensus-based current regulator that use relative information from neighboring converters. This distributed control manner relies on localized control and a sparse communication network and, therefore, could be prone to malicious attacks that deteriorate the consensus performance and even destabilize the overall microgrids. In contrast to the existing remedies for bounded disturbances/noises, this paper considers unknown attacks that are intentionally unbounded to maximize their damage on the microgrid. A fully distributed adaptive attack-resilient secondary control framework is established for DC microgrids to mitigate the adverse effect of unbounded attacks. Rigorous proofs, based on Lyapunov techniques, show that the proposed method guarantees the uniformly ultimately bounded convergence for both global voltage regulation and proportional load sharing objectives under unbounded attacks. Moreover, the asymptotic stability of the overall closed-loop microgrid system is achieved in the presence of bounded attacks. Experimental results are illustrated in a hardware-in-the-loop environment to validate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

Emerging converter-dominated dc microgrids employ distributed cooperative control strategies and communication network. Since there is no central entity to monitor and assess the global cyber scenario, microgrids employing distributed control are prone to cyber attacks. This work presents signal temporal logic (STL) detection of two major types of cyber attacks, namely false-data injection attacks and denial-of-service attacks. Such cyber attacks can compromise voltage regulation and load sharing in dc microgrids. STL is a formalism to monitor the output voltages and currents of dc microgrids against the defined specifications, such as operational bounds, over time. Besides detection, the proposed approach also quantifies the attack impact. Moreover, it can be effectively employed for a complex dc microgrid without prior knowledge of its dynamics. This detection technique is successfully demonstrated using a physical microgrid setup or in a hardware-in-the-loop environment, where various attacks are formalized, detected, and quantified. [ABSTRACT FROM AUTHOR]

This paper proposes a fully distributed control methodology for secondary control of ac microgrids. The control framework includes three modules: 1) voltage regulator; 2) reactive power regulator; and 3) active power/frequency regulator. The voltage regulator module maintains the average voltage of the microgrid distribution line at the rated value. The reactive power regulator compares the local normalized reactive power of an inverter with its neighbors’ powers on a communication graph and, accordingly, fine-tunes Q-V droop coefficients to mitigate any reactive power mismatch. Collectively, these two modules account for the effect of the distribution line impedance on the reactive power flow. The third module regulates all inverter frequencies at the nominal value while sharing the active power demand among them. Unlike most conventional methods, this controller does not utilize any explicit frequency measurement. The proposed controller is fully distributed; i.e., each controller requires information exchange with only its neighbors linked directly on the communication graph. Steady-state performance analysis assures the global voltage regulation, frequency synchronization, and proportional active/reactive power sharing. An ac microgrid is prototyped to experimentally validate the proposed control methodology against the load change, plug-and-play operation, and communication constraints such as delay, packet loss, and limited bandwidth. [ABSTRACT FROM AUTHOR]

In this study, a unified distributed static output‐feedback (OPFB) control protocol is presented for multi‐agent systems. The proposed approach is unified in the sense that it is applicable to both homogeneous and heterogeneous multi‐agent systems. Despite its importance, distributed OPFB control design is not considered for heterogeneous systems in the literature. Moreover, as will be shown, existing static OPFB controllers for homogeneous systems are vulnerable to attacks on sensors and actuators. More precisely, it is shown that a compromised agent can make an intact agent turn away from the leader if there is a directed path of any length from the compromised agent to the intact agent. To overcome these shortcomings, a distributed OPFB controller is presented which is (i) applicable to both homogeneous and heterogeneous systems, and (ii) is resilient against attacks on sensors and actuators. The proposed framework prevents attacks on sensors and actuators from propagating across the network and, thus, guarantees synchronisation of intact agents to the leader. To further improve the resiliency and guarantee synchronisation of even compromised agents, a disturbance compensator is designed. A significant advantage of this approach is that no assumption on topological connectivity and the number of agents under attack is required. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed noise resilient control technique for voltage and frequency synchronization in inverter-based ac microgrids. Existing cooperative control techniques assume ideal communication among inverters. The effect of additive noise in communication links among inverters, and between the reference signal and inverters, on the synchronization process, is studied. Distributed least mean-square solutions estimate the reference set points, and a local least mean-square algorithm estimates neighboring inverter frequencies and voltages. The efficacy of the proposed solution, for an islanded microgrid test system under the additive noise in reference communication links and links connecting neighboring inverters, is evaluated for a modified IEEE 34-bus feeder system. An upper bound for the noise-induced deviation in the consensus parameter is analytically derived and verified by the simulated testbed. [ABSTRACT FROM PUBLISHER]

Low generational inertia and lack of damping elements cause stability concerns in dc microgrids. Power buffers have been introduced to damp volatile load demands and improve microgrid's stability. Power buffer is a power electronics converter with large storage components (e.g., capacitors), which can decouple dynamics of a power distribution network and electronic loads by adjusting its input impedance and stored energy. Typically, power buffers are controlled individually to serve local loads. Alternatively, collective operation of power buffers is considered here to extend their damping effect to neighboring loads. Additionally, the group operation allows buffer designs with smaller storage components. A supervisory control to manage energy-impedance profiles of power buffers across a grid would require a complex communication network. Alternatively, distributed control lays a reliable ground to link power buffers with a minimal communication. This work offers a fully distributed feedback control algorithm to collectively manage the power buffers, using a sparse communication network. The controller enables the buffers to collectively respond to any load transient. Hardware-in-the-Loop simulation of a dc microgrid is used to show the controller's efficacy; it successfully groups power buffers in the neighborhood of the affected load and manages their energy reservoirs to shape the power supplied by the grid. [ABSTRACT FROM AUTHOR]

This paper casts the synchronization phenomena in inverter-based ac microgrids as an optimization problem solved using alternating direction method of multipliers (ADMM). Existing cooperative control techniques are based on the standard voting protocols in multiagent systems, and assume ideal communication among inverters. Alternatively, this paper presents a recursive algorithm to restore synchronization in voltage and frequency using ADMM, which results in a more robust secondary control even in the presence of noise. The performance of the control algorithm, for an islanded microgrid test system with additive noise in communication links broadcasting reference signals and communication links connecting neighboring inverters, is evaluated for a modified IEEE 34-bus feeder system. An upper bound for the deviation due to communication noise from the reference set point is analytically derived and verified by the simulated microgrid test system. [ABSTRACT FROM PUBLISHER]

Power electronics-intensive dc microgrids use increasingly complex software-based controllers and communication networks. They are evolving into cyber-physical systems (CPS) with sophisticated interactions between physical and computational processes, making them vulnerable to cyber attacks. This paper presents a framework to detect possible false-data injection attacks (FDIAs) in cyber-physical dc microgrids. The detection problem is formalized as identifying a change in sets of inferred candidate invariants. Invariants are microgrids properties that do not change over time. Both the physical plant and the software controller of CPS can be described as Simulink/Stateflow (SLSF) diagrams. The dynamic analysis infers the candidate invariants over the input/output variables of SLSF components. The reachability analysis generates the sets of reachable states (reach sets) for the CPS modeled as hybrid automata. The candidate invariants that contain the reach sets are called the actual invariants. The candidate invariants are then compared with the actual invariants, and any mismatch indicates the presence of FDIA. To evaluate the proposed methodology, the hybrid automaton of a dc microgrid, with a distributed cooperative control scheme, is presented. The reachability analysis is performed to obtain the reach sets and, hence, the actual invariants. Moreover, a prototype tool, HYbrid iNvariant GEneratoR, is extended to instrument SLSF models, obtain candidate invariants, and identify FDIA. [ABSTRACT FROM PUBLISHER]

Existing cooperative control of DC microgrids rely on a communication network that makes them vulnerable to cyber attacks. A trust-based cooperative controller is proposed that can mitigate adverse effects of attacks on communication links and controller hijacking. It only requires local and neighbor information to detect and mitigate an attack. The proposed solution is evaluated using a hardware-in-the-loop setup under different types of attack. [ABSTRACT FROM AUTHOR]

A distributed control method is proposed to handle power sharing among a cluster of dc microgrids. The hierarchical control structure of microgrids includes primary, secondary, and tertiary levels. While the load sharing among the sources within a dc microgrid is managed through primary and secondary controllers, a tertiary control level is required to provide the higher level load sharing among microgrids within a cluster. Power transfer between microgrids enables maximum utilization of renewable sources and suppresses stress and aging of the components, which improves its reliability and availability, reduces the maintenance costs, and expands the overall lifespan of the network. The proposed control mechanism uses a cooperative approach to adjust voltage set points for individual microgrids and, accordingly, navigate the power flow among them. Loading mismatch among neighbor microgrids is used in an updating policy to adjust voltage set point and mitigate such mismatches. While the voltage adjustment policy handles the load sharing among the microgrids within each cluster, at a lower level, each microgrid carries a communication network that is in contact with the secondary control system. It is this lower level network that propagates voltage set points across all sources within a microgrid. Load sharing and set point propagation are analytically studied for the higher and lower level controllers, respectively. Experimental studies on two cluster setups demonstrate excellent controller performance and validate its resiliency against converter failures and communication losses. [ABSTRACT FROM PUBLISHER]

A cooperative distributed secondary/primary control paradigm for AC microgrids is proposed. This solution replaces the centralized secondary control and the primary-level droop mechanism of each inverter with three separate regulators: voltage, reactive power, and active power regulators. A sparse communication network is spanned across the microgrid to facilitate limited data exchange among inverter controllers. Each controller processes its local and neighbors' information to update its voltage magnitude and frequency (or, equivalently, phase angle) set points. A voltage estimator finds the average voltage across the microgrid, which is then compared to the rated voltage to produce the first-voltage correction term. The reactive power regulator at each inverter compares its normalized reactive power with those of its neighbors, and the difference is fed to a subsequent PI controller that generates the second-voltage correction term. The controller adds the voltage correction terms to the microgrid rated voltage (provided by the tertiary control) to generate the local voltage magnitude set point. The voltage regulators collectively adjust the average voltage of the microgrid at the rated voltage. The voltage regulators allow different set points for different bus voltages and, thus, account for the line impedance effects. Moreover, the reactive power regulators adjust the voltage to achieve proportional reactive load sharing. The third module, the active power regulator, compares the local normalized active power of each inverter with its neighbors' and uses the difference to update the frequency and, accordingly, the phase angle of that inverter. The global dynamic model of the microgrid, including distribution grid, regulator modules, and the communication network, is derived, and controller design guidelines are provided. Steady-state performance analysis shows that the proposed controller can accurately handle the global voltage regulation and proportional load sharing. An AC microgrid prototype is set up, where the controller performance, plug-and-play capability, and resiliency to the failure in the communication links are successfully verified. [ABSTRACT FROM AUTHOR]

A cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids. The conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator. A noise-resilient voltage observer is introduced that uses neighbors' data to estimate the average voltage across the microgrid. The voltage regulator processes this estimation and generates a voltage correction term to adjust the local voltage set point. This adjustment maintains the microgrid voltage level as desired by the tertiary control. The current regulator compares the local per-unit current of each converter with the neighbors' and, accordingly, provides a second voltage correction term to synchronize per-unit currents and, thus, provide proportional load sharing. The proposed controller precisely handles the transmission line impedances. The controller on each converter communicates with only its neighbor converters on a communication graph. The graph is a sparse network of communication links spanned across the microgrid to facilitate data exchange. The global dynamic model of the microgrid is derived, and design guidelines are provided to tune the system's dynamic response. A low-voltage dc microgrid prototype is set up, where the controller performance, noise resiliency, link-failure resiliency, and the plug-and-play capability features are successfully verified. [ABSTRACT FROM AUTHOR]

A distributed-adaptive droop mechanism is proposed for secondary/primary control of dc microgrids. The conventional secondary control that adjusts the voltage set point for the local droop mechanism is replaced by a voltage regulator. A current regulator is also added to fine-tune the droop coefficient for different loading conditions. The voltage regulator uses an observer that processes neighbors’ data to estimate the average voltage across the microgrid. This estimation is further used to generate a voltage correction term to adjust the local voltage set point. The current regulator compares the local per-unit current of each converter with the neighbors’ on a communication graph and, accordingly, provides an impedance correction term. This term is then used to update the droop coefficient and synchronize per-unit currents or, equivalently, provide proportional load sharing. The proposed controller precisely accounts for the transmission/distribution line impedances. The controller on each converter exchanges data with only its neighbor converters on a sparse communication graph spanned across the microgrid. Global dynamic model of the microgrid is derived with the proposed controller engaged. A low-voltage dc microgrid prototype is used to verify the controller performance, link-failure resiliency, and the plug-and-play capability. [ABSTRACT FROM AUTHOR]

Power buffers can help improve the inertia in DC microgrids and supply the additional power demand during short load transients. This article investigates distributed event‐triggered control of power buffers over an unreliable network, subject to packet losses and transmission delays. For each power buffer, a distributed event‐triggered control law regulates the input impedance while altering its stored energy. Moreover, a dynamic event‐triggered communication mechanism determines the time sequence of control updates and data transmissions among power buffers. An appealing feature of this mechanism is a designable positive minimum inter‐event time despite packet losses and transmission delays. The closed‐loop system is shown to be exponentially stable. Controller/hardware‐in‐the‐loop studies validate the theoretical findings. [ABSTRACT FROM AUTHOR]