Microgrid Secondary Control Research Articles

Distributed sliding mode control for reactive power sharing in an islanded microgrid

This paper presents a novel microgrid secondary control method based on distributed sliding mode control, addressing the issue of reactive power sharing in islanded microgrids. Acknowledging the complications posed by feeder impedance mismatch, the traditional droop control mechanism fails to facilitate efficient reactive power sharing. Utilizing a distributed framework, this paper initially introduces a reactive power ratio observer, enabling the observation of global average reactive power without the need for precise reactive power calculation as required in a conventional centralized control setup. Subsequently, employing the devised reactive power ratio observer, sliding mode control is integrated to achieve effective reactive power sharing. Furthermore, this paper broadens its scope to encompass both reactive power sharing and voltage restoration, resulting in the design of a comprehensive controller. Notably, the proposed distributed sliding mode controller not only achieves accurate reactive power sharing but also exhibits significant resilience in the face of load disturbances. The simulation results conducted under various scenarios effectively validate the efficacy of the proposed distributed sliding mode control strategy.

Multiagent-Based Nonlinear Generalized Minimum Variance Control for Islanded AC Microgrids

Voltage regulation, frequency restoration, and reactive/active power sharing are the crucial tasks of the microgrid's secondary control, especially in the islanding operating mode. Because sensors and communication links in a microgrid are subject to noise, it is of paramount value to design a noise-resilient secondary voltage and frequency control. This paper proposes a minimum variance control approach for the secondary control of AC microgrids that can effectively perform noise attenuation, voltage/frequency restoration, and reactive/active power sharing. To this end, the nonlinear generalized minimum variance (NGMV) control approach is introduced to the islanded microgrid's secondary control system. The effectiveness of the proposed control approach is verified by simulating two microgrid test systems in MATLAB.

Co-Optimization of Microgrid Secondary Control and Communication QoS: A Cross-Layer Perspective in Cyber–Physical System

Co-Optimization of Microgrid Secondary Control and Communication QoS: A Cross-Layer Perspective in Cyber–Physical System

Optimal Distributed Coordinated Reinforcement Learning for Secondary Voltage Control in Time-Delayed Microgrid

This article proposes an optimal distributed secondary voltage control method based on distributed coordinated reinforcement learning (DCRL) algorithm in the time-delayed microgrid. First, a distributed secondary voltage control protocol through a sparse communication network is designed using consensus-based algorithm and Lyapunov–Krasovskii theory. Then, by combining the consensus control method with reinforcement learning together, a novel DCRL algorithm is utilized to dynamically adjust the voltage controller gains. Since the DCRL method is an online intelligent algorithm, it can adaptively improve the voltage control performance in the presence of various external disturbances encountered in microgrid secondary control. The effectiveness of the proposed method is verified through simulations under different scenarios on a four inverter-based microgrid test system. Simulation results indicate that our method shows superior dynamic voltage regulation response over traditional methods.

Distributed Adaptive Nash Equilibrium Solution for Differential Graphical Games.

This article investigates differential graphical games for linear multiagent systems with a leader on fixed communication graphs. The objective is to make each agent synchronize to the leader and, meanwhile, optimize a performance index, which depends on the control policies of its own and its neighbors. To this end, a distributed adaptive Nash equilibrium solution is proposed for the differential graphical games. This solution, in contrast to the existing ones, is not only Nash but also fully distributed in the sense that each agent only uses local information of its own and its immediate neighbors without using any global information of the communication graph. Moreover, the asymptotic stability and global Nash equilibrium properties are analyzed for the proposed distributed adaptive Nash equilibrium solution. As an illustrative example, the differential graphical game solution is applied to the microgrid secondary control problem to achieve fully distributed voltage synchronization with optimized performance.

Self-triggered [formula omitted] Consensus-based secondary control of AC microgrids with uncertainty of communication

At present, the distributed cooperative control scheme has been widely used in the secondary control of microgrids (MGs). However, instability factors in communication, as well as the low bandwidth and energy of the communication network, affect the safe and efficient operation of the scheme. In this paper, a distributed self-triggered H∞ consensus-based secondary control scheme is proposed to be used in voltage and frequency restoration control of AC MGs system. In the research, the external interference, the uncertainty of the communication link, the time-varying delay, and several common instability factors in topology networks are considered. Different from many existing works, this paper designs a self-triggered communication mechanism based on event-triggered, which reduces the communication burden and eliminates the computational burden of continuous monitoring of trigger conditions in event-triggered schemes. By applying the H∞ theory to the system model, the sufficient conditions for the robust consensus convergence of multi-agents under self-triggered and uncertain time-varying delay communication are obtained using the Lyapunov-Krasovskii theory. Finally, an MGs simulation model was established to evaluate the effectiveness of the proposed control strategy. The results show that the proposed secondary control scheme can restore the voltage and frequency to the reference value, and while greatly reducing the communication burden between agents.

Privacy-Preserving Sliding Mode Control for Voltage Restoration of AC Microgrids Based on Output Mask Approach

Although privacy-preserving is often considered in energy management and energy scheduling of microgrids (MGs), to the best of authors’ knowledge, there are almost no literatures on distributed secondary control of MGs with privacy-preserving. To fill this gap, this article investigates the privacy-preserving problem for voltage restoration of an ac MG. First, the model of the MG is transformed into a linear multiagent system model by feedback linearization. Furthermore, output mask approach, which avoids divulging the initial condition by inserting a dynamic mask on the exchanged state information, is adopted to make the distributed generator agents exactly converge to the reference value rather than the value with a preset certain convergence error by differential private. Meanwhile, the sliding mode control scheme with simple structure is adopted to accelerate the convergence speed of the masked system. However, it is difficult to design the controller directly because of the strong nonlinearity brought by the mask function. To solve this problem, we first implement the controller for the original system, and then extend it to the masked system. After that, we carry out the consensus analysis of the controlled masked system. Finally, the effectiveness of the proposed control scheme is validated in the MATLAB/Simulink environment.

Distributed Secondary Control in Microgrids Using Synchronous Condenser for Voltage and Frequency Support

A high share of distributed energy resources (DERs) in power distribution grids has posed many challenges for system operation and control. Microgrid (MG) application with different distributed control approaches for DERs has been drawn a lot of attention from the research community to provide more flexibility, reliability and resilience for the system. This paper develops a distributed secondary control for DERs in MGs and on top of that using synchronous condenser (SC) participating in the secondary control for voltage support. The proposed distributed secondary control framework of MGs is designed to obtain four objectives as follows: (i) frequency restoration, (ii) average voltage restoration, (iii) arbitrary active power sharing among SGs and BESSs and (iv) arbitrary reactive power sharing among all SGs, BESSs and SCs. The comparison results under different scenarios show that with SC participating in the distributed secondary control in MGs, the system frequency and voltage response are much improved and quickly recovered to the nominal values thanks to the natural inertia response and fast reactive power control of SC sharing with other DERs in the MGs. Additionally, a multi-agent system is implemented to realize the proposed control method in hardware environment.

Distributed fixed-time secondary control of an islanded microgrid via distributed event-triggered mechanism

The fixed-time secondary control problem for an islanded microgrid (MG) through an event-triggered communication mechanism is investigated in this paper. A distributed event-triggered fixed-time control strategy is proposed to achieve frequency and voltage secondary restoration. And a proportional active power-sharing is also achieved through an undirected topology. The given control protocol using frequency, voltage magnitude and active power measurement values of neighboring distributed generations only when the predefined event-triggered condition is satisfied. In comparison with the periodic communication way, the presented method exhibits excellent performance in alleviating communication burden and promoting control efficiency. Moreover, compared with conventional asymptotic control algorithms, the objectives of islanded MG secondary control are obtained within a fixed settling time by using the presented fixed-time control approach. The estimation of settling time is unrelated to any initial states. Several simulation experiments are conducted to demonstrate the effectiveness of the proposed control scheme.

Communication Constraints for Distributed Secondary Control of Heterogeneous Microgrids: A Survey

Heterogeneous microgrids provide a promising solution for accommodating different distributed energy resources (DERs) and enhancing the system performance in terms of reliability, resilience, flexibility, and energy efficiency. With the penetration of microgrids, the cyber-enabled distributed control techniques are playing an increasingly important role in microgrids for coordinating a multitude of heterogeneous and spatially distributed DERs. To identify the influence of communication constraints on the reliability and stability operation of heterogeneous microgrids, this survey presents a comprehensive review and comparison of cyber-enabled distributed control techniques for microgrid's secondary control layer. Local controls of an unavailable state signal are calculated in a distributed and cooperative matter according to merely invoked cyber messages. Then, the recent developments in the communication constraints of distributed controlled microgrids are discussed. Specifically, the communication constraints (e.g., time-varying network topology, communication delay, noise disturbance, limited communication bandwidth, uncertainties of communication links, and cyber attacks) on the operation of the distributed controllers and the possible challenges are presented and compared. Finally, a short discussion section is contained to summarize the existing research and suggest some interesting research directions along with several open issues that are critical to further exploring the promising research area.

Distributed event-triggered multi-timer synchronization scheme for secondary control in islanded microgrids

Consensus-based distributed control has been widely applied in the secondary control for microgrids (MGs) with the assumption that the control actions of each distributed generations (DGs) are triggered at the same time as others in the state-of-art literature. However, in the absence of centralized timer synchronization, the asynchrony resulting from the independent timer localized in each DG would inevitably undermine the system performance or even cause instability. This paper proposes a distributed time synchronization strategy to guarantee the update actions of individual DGs to be triggered at the same discrete instants. First, the asynchrony problem in the distributed secondary control due to the essential distributed characteristic is elaborated and the resultant convergence deviation is deduced. Then, the distributed time synchronization strategy based on pinning consensus is described, with the optimal pinning timer selected to achieve superior propagation performance. Further, the event-triggered mechanism is introduced, which reduces the communication burden significantly compared to the time-driven mode and maintains the global clock synchronization efficiently. The selection of event triggering threshold is formulated for the tradeoff between the trigger frequency and convergence accuracy. The effectiveness as well as flexibility of the proposed control strategy is demonstrated by simulations.

Asynchronous Periodic Distributed Event-Triggered Voltage and Frequency Control of Microgrids

In this paper, we introduce a distributed secondary voltage and frequency control scheme for an islanded ac microgrid under event-triggered communication. An integral type event-triggered mechanism is proposed by which each distributed generator (DG) periodically checks its triggering condition and determines whether to update its control inputs and broadcast its states to neighboring DGs. In contrast to existing event-triggered strategies on secondary control of microgrids, the proposed event-triggered mechanism is able to handle the consensus problem in case of asynchronous communication. Under the proposed sampled-data based event-triggered mechanism, DGs do not need to be synchronized to a common clock and each individual DG checks its triggering condition periodically, relying on its own clock. Furthermore, the proposed method efficiently reduces communication rate. We provide sufficient conditions under which microgrid's frequency and a critical bus voltage asymptotically converge to the nominal frequency and voltage, respectively. Finally, effectiveness of our proposed method is verified by testing different scenarios on an islanded ac microgrid benchmark in the MATLAB/Simulink environment as well as a hardware-in-the-loop (HIL) platform, where the physical system is modeled in the Opal-RT and the cyber system is realized in Raspberry Pis.

Optimization-based primary and secondary control of microgrids

Abstract This article discusses how to use optimization-based methods to efficiently operate microgrids with a large share of renewables. We discuss how to apply a frequency-based method to tune the droop parameters in order to stabilize the grid and improve oscillation damping after disturbances. Moreover, we propose a centralized real-time feasible nonlinear model predictive control (NMPC) scheme to achieve efficient frequency and voltage control while considering economic dispatch results. Centralized NMPC for secondary control is a computationaly challenging task. We demonstrate how to reduce the computational burden using the Advanced Step Real-Time Iteration with nonuniform discretization grids. This reduces the computational burden up to 60 % compared to a standard uniform approach, while having only a minor performance loss. All methods are validated on the example of a 9-bus microgrid, which is modeled with a complex differential algebraic equation.

Multiplex Centrality Measurements Applied to Islanded Microgrids

This paper presents four centrality measurements applied to an alternating current (AC) microgrid (MG) modeled as a multiplex network. The MG secondary control is separated into a frequency and a power-sharing layers, each one with a different adjacency matrix. A physical layer is also considered with an admittance matrix representing the impedances among the inverters. Centrality measures are used to determine the importance of nodes in separate layers, thereafter adjacency and Laplacian matrices are redefined to calculate the role of nodes in the multiplex system. First, a global adjacency matrix is calculated by the matrix sum of each adjacency matrix. Second, the adjacency matrix is calculated by a supra-Laplacian matrix. The first eigenvalue of the perturbed matrix is used to determine the diffusivity in the network using as leaders the sets obtained by the centrality measures. The role of the nodes in the system is verified in a simulated MG model of 37 nodes. Degree centrality and energy Laplacian measures present similar sets of nodes; however, the fastest set of nodes is found using the Eigenvector measurements for uniform and supra Laplacian approach.

Adaptive Resilient Secondary Control for Islanded AC Microgrids With Sensor Faults

In this article, we investigate the adaptive resilient secondary voltage and frequency control problem for islanded ac microgrids (MG) in the presence of sensor faults. Sensor faults or data attacks have a great impact on the stability and quality of MG. Existing methods commonly assume that the sensing information from distributed generations (DGs) is healthy or satisfies some ideal conditions. To achieve resilient secondary voltage restoration, a novel adaptive fault tolerant control scheme is proposed. Since the bounds of the time-varying faults are unknown, sufficient conditions are first derived to guarantee the voltage tracking errors to be uniformly ultimately bounded. It is also theoretically shown that the frequency restoration and power sharing can be ensured stable of the overall ac MG system by using the same control scheme. Compared with the existing distributed control methods for secondary control of MG, the considered problem with unknown boundaries sensor faults is more challenging and still not well explored.

Distributed control of heterogeneous energy storage systems in islanded microgrids: Finite-time approach and cyber-physical implementation

Microgrid (MG) is evolving towards a complex interacted cyber-physical system. In the physical layer, the energy storage systems (ESSs) are installed to mitigate the uncertainty of renewables. In the cyber layer, the distributed algorithms implemented on multi-agent systems are widely proposed to enhance system resilience and flexibility. Therefore, in this paper, a distributed control framework with the finite-time approach and cyber-physical implementation is proposed for ESSs with heterogeneous parameters in islanded MGs. With the proposed control framework, the fair power sharing and state-of-charge balancing among ESSs can be accomplished in finite-time. In the meantime, the MG secondary control objective of frequency and voltage restoration is also achieved. The stability of the proposed controllers is proved via the Lyapunov method. Towards the practical application, the MG entities and primary controllers, as the physical system, are emulated in OPAL-RT, while the distributed finite-time algorithms and multi-agent system, as the cyber system, are realized in multiple Raspberry Pis. The cyber hardware-in-the-loop experiment results demonstrate the effectiveness of the proposed controller design under various scenarios.

Robust distributed stochastic secondary control of microgrids with system and communication noises

This study proposes an advanced robust stochastic cooperative frequency/voltage restoration control considering the effects of both communication/system noises based on Kalman-filtering theory for secondary control of islanded microgrids. Compared with existing ideal and noise-resilient distributed methods, the proposed robust stochastic distributed secondary control method can achieve mean-square synchronisation for voltage and frequency restoration of distributed generations (DGs) along with the real power sharing accuracy through a sparse communication network, even though both the system measurement and communication channels are susceptible to noise interferences and limited band-width constraints. To this end, the graph theory, stochastic theory, and Kalman-filtering theory are employed to estimate the states of each DG based on the information of noisy measurement, and then the estimated signals are sent to the neighbour DGs via a noisy communication environment. The main advantages of the proposed method are its ability to provide the quality of the estimate, low complexity, low computational burden, and working even though both the system and communication channels are susceptible to noise interferences and limited band-width constraints. Finally, the simulation results in MATLAB/SimPowerSystems Toolbox show that the proposed method has better, robust, resilient, acceptable, and desirable performance compared with the recent and effective methods.

Secondary Control of Microgrids via Neural Inverse Optimal Distributed Cooperative Control

In this paper, a new control scheme of secondary voltage and frequency control based on a discrete-time neural inverse optimal distributed cooperative structure is proposed for islanded microgrids. A neural adaptive secondary controller for each distributed generator is developed to achieve the desired goals. The proposed controllers are composed of an on-line neural identification scheme on the basis of a recurrent high-order neural network using the extended Kalman filter and a nonlinear control strategy. Additionally, the proposed control scheme does not require information of all installed distributed generators neither a distributed generator model, which improves reliability. The proposed controllers are validated through simulation for an islanded AC microgrid.

Distributed Resilient Adaptive Control of Islanded Microgrids Under Sensor/Actuator Faults

To compensate for the voltage and frequency deviations caused by the droop control in islanded microgrids (MGs), employing an additional control level, called the secondary control, is necessary. Therefore, distributed control methods based on the sparse communication network, are recently employed in secondary control of MGs, because they are more reliable and flexible. However, traditional methods use the centralized communication network, which is neither economical nor efficient due to its large communication burden. Distributed control methods solve many challenges in the secondary layer of MGs, such as disturbances, uncertainties, unmodelled dynamics, noises, delays, attacks, and actuator faults. This paper proposes the novel resilient adaptive consensus-based voltage and frequency control protocols to compensate for the adverse effects of both sensor and actuator faults in communication network channels of islanded MGs. To this end, local adaptive sensor and actuator compensators eliminate the adverse effects of failures on sensors and actuators. The robust stability analysis based on the Lyapunov analysis is given to validate the results. Finally, the understudy islanded MG system is built in MATLAB/SimPowerSystems to verify the theoretical results. It is shown that the proposed secondary control strategies based on the adaptive protocols can highly increase the reliability and resiliency of the MG system compared with the existing distributed resilient methods.

Distributed Cooperative Secondary Control for Islanded Microgrid With Markov Time-Varying Delays

The network-induced delays have a great impact on microgrid performance and stability. In this paper, we proposed a distributed cooperative secondary control with stochastic Markov time-varying network-induced delays, by which both output voltage/frequency restoration and active power sharing can be obtained. A new small-signal model of the microgrid control system is developed, and a small-signal model-based information Kalman observer is introduced to acquire the estimated state vector. Then, the global closed-loop stochastic system accommodated by the Markov time-varying network-induced delays of the microgrid is derived, and the corresponding global stability theorem is introduced. Finally, the simulation of a microgrid test system is presented to verify the control performance. The results show that our approach has accurate regulations on voltage/frequency restoration and active power sharing in terms of load variations, plug-and-play capability, and network-induced delays. In addition, the control gain parameters and network-induced delays effects on the microgrid secondary control are also verified.