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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]

In this paper, we propose a topology-cognizant optimal power flow (OPF) paradigm with additional safety constraints for multi-terminal direct current (MTDC) grids. The resulting formulation is concerned with the optimization of controller set-points, i.e., voltage and power levels at each bus, and the switching status of transmission lines that collectively referred to as grid topology. A pair of additional safety constraints are integrated into the problem formulation to prevent voltage violations caused by power fluctuations in between two controller set-point updates. Searching for a grid topology that offers more efficient operation leads to a mixed-integer nonlinear program (MINLP) which is computationally challenging due to: i) Non-convex power flow equations, (ii) Non-convex converter loss equations, and iii) Binary variables accounting for the operational status of transmission lines. Non-convexities of power flows and converter loss equations are tackled by means of a mixed-integer second-order cone programming (MISOCP) relaxation, while the optimal switching status of transmission lines are determined via a branch-and-bound search. Numerical results for the modified IEEE 14, 30, and 57-bus systems are used to verify the merits of the proposed method. Furthermore, this method is experimentally validated using the CIGRE B4 DC grid benchmark in a real-time hardware-in-the-loop platform. [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]

This paper considers a cooperative and adversarial AC microgrid system consisting of cooperative leaders and inverters, as well as adversarial attackers. The attackers aim to destabilize the synchronization dynamics of the AC microgrid by first intercepting the communication channels, penetrating the local state feedback, and pretending to be a cooperative neighbor, and then initiating malicious attacks by launching unbounded injections. A fully distributed resilient control framework is offered for the secondary frequency regulation and voltage containment to ensure system stability and preserve bounded synchronization. In particular, a virtual resilient layer with hidden networks is developed to integrate with the original cyber-physical layer. The proposed resilient control framework is fully distributed without requiring any global information. A modified IEEE 34-bus test feeder benchmark system is emulated in a controller/hardware-in-the-loop environment, where the control objectives are met under different attack scenarios. [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]

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]

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]

A distributed control method is proposed to simultaneously optimize the power sharing among sources of islanded dc microgrids, while regulating the distribution bus voltage. During the optimization process, an economic dispatch problem is solved to minimize the total generation cost by setting the output powers of the dispatchable sources. To this end, the voltage set points of individual dc–dc converters are adjusted using a voltage regulator and an optimizer, which regulates the average voltage of the sources to establish the generation–consumption equality constraint and matches the incremental costs, respectively. Afterward, the proposed optimizer is modified to exclude the sources from the incremental cost consensus protocol upon reaching their generation limits, enforcing inequality constraints. This coregulation and cooptimization paradigm is developed in a fully distributed fashion. The dynamical model of the proposed controller is established. The steady-state analysis verifies the fulfillment of the control objectives, i.e., voltage regulation and cost minimization. Experimental results verify the controller performance and validate its resiliency against cyber and physical faults. [ABSTRACT FROM AUTHOR]

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]

This paper investigates the distributed finite-time restoration of inverter voltage and frequency terms in an islanded ac microgrid. Formulating networked inverters of ac microgrids as a cooperative multiagent system, the voltage and frequency restoration can be cast as synchronization problems, while the active power sharing can be viewed as a consensus problem. One popular distributed control approach is the neighbor-based linear consensus protocol, where the consensus at the frequency and voltage set points is achieved over an infinite-time horizon with an exponential convergence. To achieve accelerated convergence and better disturbance rejection properties, a distributed finite-time control protocol, based on feedback linearization approach, is proposed for voltage restoration, which synchronizes the voltage term at each inverter to the reference value in finite time period. Then, a finite-time control protocol for both frequency restoration and active power sharing problems is proposed to synchronize the microgrid frequency to the nominal value, and share the active power among inverters based on their ratings in a finite time. Rigorous Lyapunov proofs are provided to establish the upper bounds on the convergence times. Numerical simulation studies verify the effectiveness of the proposed control protocols. [ABSTRACT FROM PUBLISHER]

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 networked method for load sharing of parallel converters is proposed. Using consensus-voting protocols, the need for a master converter or a central controller is eliminated. The proposed modular structure does not require a priori knowledge of the number of active converters, which makes it a viable option for a plug-and-play operation. The voltage regulation at the desired set point and the consensus of the per-unit currents are analytically proven for the steady-state conditions. Moreover, in the absence of a centralized controller, measuring output voltages of each converter individually can lead to a measurement mismatch. The effect of this voltage mismatch on the controller performance is analyzed, and a solution is provided. Experimental results verify the proposed distributed control method using a parallel four-converter system and show its efficacy in response to changes in operational conditions and its resiliency against the loss of converters or communication links. [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]

This paper proposes an adaptive and distributed secondary voltage control for microgrids with inverter-based distributed generators (DG). The proposed control is fully adaptive and does not require the information of DG parameters. Neural networks are used to compensate for the uncertainties caused by the unknown dynamics of DGs. The controller structure is fully distributed such that each DG only requires its own information and the information of its neighbors on the communication network. Therefore, this secondary control is associated with a sparse communication network. The effectiveness of the proposed methodology is verified for different loading, outage, and islanding scenarios, as well as variable communication structures in a microgrid setup. [ABSTRACT FROM AUTHOR]

Modular dc-dc converters are popular in dc-power systems due to their advantageous characteristics such as fault tolerance, ease of thermal management, reducing voltage/current stress of the components, and modularity. In this paper, the concept of cooperative control in multiagent systems is introduced for modular dc-dc converters. Each constituent converter is represented by a node in a directed communication graph that models the information flow among converters. The proposed cooperative control scheme enjoys structural modularity, plug-and-play capability, fault tolerance against random failures in the converters and/or communication links, and satisfactory dynamic performance. This paper provides a general analytical framework to study modular dc-dc converters with an arbitrary communication graph. Hence, the designer has the freedom to choose among the various types of graphs based on the available communication resources and the desired level of reliability and fault tolerance. The dynamic model of the cooperative multiconverter system is developed and analyzed. Hardware measurements are presented to verify the plug-and-play capability, fault tolerance in both cyber and physical domains, and dynamic performance of the proposed cooperative control scheme. [ABSTRACT FROM AUTHOR]

In this paper, the problem of optimal voltage and power regulation is formulated for distributed generators (DGs) in DC microgrids. It is shown that the resulting control is optimal but would require the full information of the microgrid. Relaxation of information requirement reduces the optimal control into several controls including the conventional droop control. The general setting of a DC microgrid equipped with local sensing/communication network calls for the design and implementation of a cooperative droop control that uses the available local information and coordinates voltage control in a distributed manner. The proposed cooperative droop control is shown to include other controls as special cases, its performance is superior to the conventional droop control, and it is robust with respect to uncertain changes in both distribution network and sensing/communication network. These features make the proposed control an effective scheme for operating a DC microgrid with intermittent and distributed generation. [ABSTRACT FROM PUBLISHER]

This paper proposes a distributed two-layer control structure for ac microgrids. Inverter-based distributed generators (DGs) can operate either as voltage-controlled voltage source inverters (VCVSI) or current-controlled voltage source inverters (CCVSI). VCVSIs provide the voltage and frequency support, whereas CCVSIs regulate the generated active and reactive powers. The proposed control structure has two main layers. The first layer deals with the voltage and frequency control of VCVSIs. The second layer regulates the active and reactive powers of CCVSIs. These controllers are implemented through two communication networks with one-way communication links and are fully distributed; each DG only requires its own information and the information of its neighbors on the communication network graph. The proposed control framework is verified on a microgrid test system and IEEE 34 test feeder. [ABSTRACT FROM PUBLISHER]

A single-stage multiple-input multiple-output dc–dc converter topology is proposed. The proposed converter can employ arbitrary number of energy sources and can supply arbitrary number of loads. The ability of the converter to regulate the input powers coming from different energy sources, in addition to regulating the output voltages, stands out. A specific switching scheme is proposed, and the dynamic model of the converter is developed based on the designated switching scheme. An appropriate control algorithm is presented, based on which the input powers and output voltages are regulated. Several hardware measurements and numerical simulations are presented to verify dynamic characterizations and to demonstrate the converter's performance. [ABSTRACT FROM PUBLISHER]

A Cúk converter operating in discontinuous capacitor voltage mode (DCVM) is an inherent power factor correction converter. A reduced-order switch-network model of a Cúk converter in DCVM is developed, which is valid for both dc–dc and ac–dc applications. The model can be used for controller analysis and design. Moreover, a fast dynamic response controller is proposed, which is an integration of a switching-frequency control unit and a proportional-integral (PI) controller. The PI controller is in charge of duty ratio adjustment. In the proposed controller, unlike conventional PI controllers, switching frequency is no longer fixed and varies as load changes. The switching-frequency control unit reacts much faster than the PI controller and instantly compensates for the output voltage in case of drastic load change, thus significantly improving the transient response. Furthermore, the proposed control scheme keeps the switch voltage stress constant, which eases switch selection and improves converter reliability. [ABSTRACT FROM PUBLISHER]

This paper proposes a secondary voltage control of microgrids based on the distributed cooperative control of multi-agent systems. The proposed secondary control is fully distributed; each distributed generator only requires its own information and the information of some neighbors. The distributed structure obviates the requirements for a central controller and complex communication network which, in turn, improves the system reliability. Input–output feedback linearization is used to convert the secondary voltage control to a linear second-order tracker synchronization problem. The control parameters can be tuned to obtain a desired response speed. The effectiveness of the proposed control methodology is verified by the simulation of a microgrid test system. [ABSTRACT FROM PUBLISHER]

A multiple-input multiple-output dc–dc converter topology is proposed. This converter is able to accommodate an arbitrary number of sources and loads. The steady-state and dynamic characteristics of the proposed converter are analyzed. A controller scheme is proposed that enables budgeting the input powers coming from different energy sources, in addition to regulating the output voltages. Loss and efficiency modeling and sensitivity analysis to the underlying parameters are performed. Several case studies are presented to verify the analytical models and evaluate the performance of the proposed converter. [ABSTRACT FROM PUBLISHER]

Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed. [ABSTRACT FROM PUBLISHER]