Control Of Islanded Microgrids Research Articles

Bio-inspired distributed load frequency control in Islanded Microgrids: A multi-agent deep reinforcement learning approach

To prevent errors in load frequency control (LFC) decision-making in an islanded microgrid and reduce the waste of frequency regulation resources, a fully distributed “starfish” load frequency control method is proposed. For this method, a novel deep reinforcement learning algorithm called the distributed decomposed multirole multiagent deep deterministic policy gradient (DDMR-MADDPG) algorithm is introduced. This algorithm treats each unit as an agent, enabling centralized training of all the agents. Through the coordination and control of multiple agents, a global optimal strategy can be obtained. During online application, the algorithm employs a distributed implementation strategy, enabling each unit to make decisions autonomously. This decision-making strategy is akin to the distributed neural network of a starfish, and it can fundamentally improve the efficiency and correctness of decision-making. In addition, it employs multiple techniques, including value function decomposition, multirole learning, imitation learning, and curriculum learning approaches. The performance of the proposed method compared with 24 existing methods is demonstrated on a simulated LFC model of the Zhuzhou Island microgrid in China.

Dynamic mater-slave control strategy for transient coordination of inverter based microgrids under asymmetric faults

The ever-expanding inverter-based microgrids (MGs) require the grid-forming distributed renewable energy resources (DERs) to enhance the system safety. However, the different types of grid-forming DERs are prone to have transient conflicts with each other under short circuit faults, leading to unexpected outage or even system collapse. The fast response and poor overcurrent capability of DERs make transient coordination a challenge. This paper proposes a dynamic master–slave architecture for transient coordination control in islanded MGs under asymmetric faults. A dynamic reconfigurable voltage reference unit (DVRU) is designed according to the real faults. In this manner, the other DERs could comply with this DVRU to achieve both system voltage support, inrush current restraining, and power oscillation suppression. It is action signals instead of phase signals need to be transferred by the communication bus with the proposed local fault current differential time delay method (FCDDM). Thus, the low-bandwidth and the enhanced robust communication could be met under transient state. Additionally, the feasibility of the proposed method to combined with relay protection is analyzed. Finally, case study results on islanded MGs consisting of various grid-forming and grid-following DERs demonstrate the effectiveness and robustness of the proposed method under short circuit faults as well as communication failures.

Event-Triggered Fractional PID-Based Load Frequency Control in Islanded Microgrids Under Cloud-Edge Collaborative Framework

To address the issue of frequency stability in microgrids with limited communication resources and bandwidth, this paper proposed an event-triggered control method based on a cloud-edge collaborative architecture. Firstly, to address the limited communication resources, event-triggered detector is set up at the edge to upload state variables and update control instructions only when the frequency deviation exceeds a pre-set threshold. Secondly, a fractional-order PI–PD controller is designed at the cloud control center, which achieves higher control degrees of freedom and stronger robustness than traditional integer-order PID control schemes. Additionally, to obtain better control parameters, an optimized design method based on an improved Big Bang-Big Crunch method (BB-BC3) is proposed for the fractional-order PI–PD controller. At last, a simulation example is given to prove the validity of the proposed method. Simulation results show that compared to the integer-order PI–PD controller, the fractional-order PI–PD controller based on the BB-BC3 algorithm proposed in this paper reduces the peak value of the response curve by 5%, and shortens the setting time by 0.14 s.

A Unified Distributed Digital Control Architecture for Secondary Control in Islanded Microgrids

Distributed control theory has gaining momentum for the global stability of modern Micro-grids (MGs), which can be hence regarded as a cyber-physical energy systems in a networked control systems perspective. Besides the benefits, the control over networks poses several issues and, among them, how to preserve stability despite the unavoidable sampling process is one of the most crucial to be addressed for the implementation in real-world scenarios. The paper addresses this concern by introducing a unified digital control architecture for secondary control of inverter-based MGs working in islanded mode, where both frequency and voltage regulations are ensured via fully-distributed sampled-data control protocols. By choosing a small-enough sampling period h > 0, the proposed strategies can ensure a reduction of the communication burden by avoiding a continuous interactions among electrical entities, thus enhancing the efficiency of the MG. Exponential stability of frequency and voltage closed-loop systems is analytically proven via Lyapunov-Krasovskii theory. The derived stability conditions are in the form of Linear Matrix Inequalities (LMIs) whose solution provides the maximum sampling period preserving the global stability. Simulation results, carried out in MATLAB/SimPower Systems, confirm the theoretical derivation.

Nonfragile power and frequency control in islanded microgrids under abnormal asynchronous stochastic cyber attacks

In this paper, we focus on the real power sharing and frequency regulation of distributed generators in islanded microgrids with abnormal asynchronous stochastic cyber attacks, which is of great significance to the information security and stable operation of microgrids. Firstly, considering the possible cyber attacks in the communication network, a distributed non-fragile controller with coupled memory delay is proposed according to the nonperiodic sampled-data control. Then, the construction of delay-dependent two-sided looped-functional makes the Lyapunov–Krasovskii functional contain more delay and sampling information and relaxes constraints on free matrices. In addition, based on the enhanced integral inequality technique and the linear convex combination method, a sampling-based consensus protocol is presented to solve the issues of real power sharing and frequency regulation in the islanded microgrids. Finally, to verify the effectiveness and feasibility of the designed control strategy, a modified IEEE 34-bus test system is used for the experiment.

Decentralized and Coordinated V-f Control for Islanded Microgrids Considering DER Inadequacy and Demand Control

This paper proposes a decentralized and coordinated voltage and frequency (V-f) control framework for islanded microgrids, with full consideration of the limited capacity of distributed energy resources (DERs) and V-f dependent load. First, the concept of DER inadequacy is illustrated with the challenges it poses. Then, a decentralized and coordinated control framework is proposed to regulate the output of inverter-based generations and reallocate limited DER capacity for V-f control. The control framework is composed of a power regulator and a V-f regulator, which generates the supplementary signals for the primary controller. The power regulator regulates the output of grid-forming inverters according to the real-time capacity constraints of DERs, while the V-f regulator improves the V-f deviation by leveraging the load sensitivity to V-f. Next, the static feasibility and small signal stability of the proposed method are rigorously proven through mathematical formulation and eigenvalue analysis. Finally, a MATLAB-Simulink simulation demonstrates the functionalities of the control framework. A few goals are fulfilled within the decentralized and coordinated framework, such as making the best use of limited DERs' capacity, enhancing the DC side stability of inverter-based generations, and reducing involuntary load shedding.

Modelling the steady-state of secondary control in islanded AC microgrids

The secondary control is applied in islanded Microgrids (MGs), after the primary control, in order to restore the voltages and frequency to specified values. Although existing power flow methods can accurately calculate the power flow of primary and tertiary control of islanded MGs, they present considerable limitations in the modelling of secondary control. The main challenge is that secondary controllers consist of integral parts, which under communication failures, could integrate functions with different integral histories, and thus, can imply non-uniform power production of distributed generators (DGs). This paper proposes a power flow method for calculating, accurately, the steady-state of secondary control in islanded MGs. The method has four distinct features: a) generalized implementation in several communication strategies e.g., centralized, decentralized, consensus, distributed averaging, b) precise simulation of communication links and proportional-integral (PI) controllers, c) low computation time, and d) accurate three-phase network representation. Simulations were executed to validate the proposed method and highlight the importance of modelling, precisely, the secondary control in islanded MGs.

Stability Analysis and Performance Improvement of Power Sharing Control in Islanded Microgrids

Due to the requirement of synchronism and power sharing, droop control and its variations have become one essential component for distributed generator (DG)-powered microgrids. However, the power sharing accuracy and system stability margin may be threatened by the randomness from the load demand and renewable generation. In this paper, a dynamic stability analysis is first performed on a DG-powered microgrid through a produced system frequency response model (SFR). The results point out that (i) the critical system eigenvalues directly vary with the system operating condition; (ii) a fixed-gain power sharing control is prone to be less damped and loses stability easily under some operating conditions. Then, the heuristic adaptive dynamic programming (HDP) strategy is used for power sharing control with the benefit of adapting to real-time disturbances and uncertainties. Through Lyapunov theorem, stability analysis is provided to demonstrate the reliability of the HDP-based power sharing control in islanded microgrids. Finally, simulation tests verify the analysis results and demonstrate the favorable performance of the HDP-based power sharing control under uncertain load disturbances.

Dynamic Performance Evaluation of the Secondary Control in Islanded Microgrids Considering Frequency-Dependent Load Models

Performance analysis is challenging in modern power distribution networks due to the increasing penetration of distributed energy resources (DERs) interfaced by voltage source converters (VSCs). Parameter uncertainty, black-box models of the components, and low inertia are some of the issues that must be addressed. The latter can represent high vulnerabilities to sudden load changes in a microgrid (MG). Additionally, the islanded operation represents a challenge for inverter-based (DERs), where secondary control must regulate the microgrid frequency and voltage to its nominal values. When a secondary control strategy is implemented, it is necessary to validate its performance under several conditions. Most existing research papers focus on the microgrid’s small-signal and transient stability. At the same time, little has been done on the influence of the load model on voltage stability. This type of analysis is required to ensure an adequate transition between the grid-connected and stand-alone modes. This paper provides a voltage stability analysis of a microgrid, considering secondary control based on receding horizon and frequency-dependent load models. Simulation results demonstrate the robust performance of the secondary control and validate the importance of considering or adapting voltage stability indices to ensure adequate microgrid performance.

An optimization model based on the frequency dependent power flow for the secondary control in islanded microgrids

This paper proposes an optimization model for the secondary control of islanded microgrids. It considers the action of the primary control and the references given by the tertiary control. The model is solved in each step, based on a linear approximation of the frequency-dependent power flow equations. The main objective of the model is to carry out frequency and voltages to suitable values, considering the capacity limits of renewable sources and energy-storage devices. Numerical experiments on the CIGRE low-voltage benchmark test system demonstrate the frequency achieve nominal values taking into account the main constraints of the system.

Robust fuzzy model predictive control for voltage regulation in islanded microgrids

An effective robust fuzzy model predictive control (RFMPC) method for secondary voltage control in islanded microgrids (μGs) is presented here. In contrast to the existing techniques, which require a detailed model of μG and an ideal communication network between μG central controller and primary local controllers, RFMPC is synthesized for a non-linear model of the μG with various time delays, uncertainties, and bounded disturbances. The famous Takagi-Sugeno fuzzy approach is adopted to approximate the inherently non-linear model of μG by locally linear dynamics. The Lyapunov–Razumikhin functional method is exploited to deal with time delays. In this regard, sufficient conditions are provided in the form of linear matrix inequalities (LMIs). Then, a sequence of control laws corresponding to a set of terminal constraints is computed offline. Doing so, the online stage is reduced to solving a convex problem with LMI constraints considering the sequence of constraint sets obtained in the offline stage, thereby reducing the computational burden significantly. Robust positive invariance and input-to-state stability property concerning communication network deficiency are then speculated. The effectiveness of the proposed RFMPC is verified via a comprehensive suite of simulations in the MATLAB/SimPowerSystems environment.

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.

Distributed optimal voltage control in islanded microgrids

Distributed optimal voltage control in islanded microgrids

Investigations of Decoupled Trigonometric Saturated and Fuzzy Logic Techniques for the Automatic Frequency Control of Islanded Microgrid

This paper involves the investigation of new techniques for the automatic load frequency control of islanded Microgrids. Microgrids are being established as a part of smart grid environment. In modern power systems, smart grid represents the solution for many of traditional power system problems such as frequency fluctuations. Frequency fluctuations have negative consequences in terms of electrical equipment life, production cost and production losses relative to costumers and electricity producers. So, the frequency of power system must be kept in acceptable range. In order to operate a power system with fixed frequency, it is necessary to always maintain a balance between the generation and the consumption of active power. The frequency droop control methods are widely used to control active power and frequency of the parallel synchronous generators in the traditional power grid. At present, this method has also been applied to the control of parallel inverters to share the load demand in proportion to their ratings. In order to improve the frequency control of traditional droop control technique applied in islanded microgrid, It is proposed to investigate the Decoupled Trigonometric Saturated (DTS) and fuzzy droop control techniques on islanded microgrids which uses meshed parallel inverter systems. To verify the performance of the proposed ALFC based on Decupled trigonometric saturated controller, fuzzy PI controller the MATLAB/SIMULINK environment is used.

Distributed Secondary Control of Islanded Microgrids for Fast Convergence Considering Arbitrary Sampling

This paper proposes a novel distributed secondary control of MGs for fast convergence considering asynchronous sampling. With the employment of the algorithm, optimal power sharing and voltage restoration are implemented simultaneously. First, the hierarchical control objectives concerned with economic operation and voltage quality are introduced. Then, the execution process of the fast convergence algorithm is described for weighted average state estimation, with the illustration of corresponding features and the application in cooperative control. Further, the relevant stability issue is discussed based on large-signal dynamic modeling and a sufficient stability condition is derived based on the Lyapunov–Krasovskii theory. Our approach offers superior reliability, flexibility and robustness because of the loose implementation in terms of its performance concern, which is essential when the distributed consensus protocol is likely to yield toward deviation or even instability under arbitrary sampling and delays. The effectiveness of the proposed methodology is verified via simulations.

Decentralized Virtual Impedance-based Circulating Current Suppression Control for Islanded Microgrids

Parallel connected inverters in islanded mode, are getting momentous attention due to their ability to increase the power distribution and reliability of a power system. When there are different ratings of Distributed Generation (DG) units, they will operate in parallel connection due to different output voltages, impedance mismatch, or different phase that can cause current to flow between DG units. The magnitude of this circulating current sometimes can be very large and damage the DG inverters and also cause power losses that affect power-sharing accuracy, power quality, and the efficiency of the Microgrid (MG) system. Droop control, improved droop control, and virtual impedance control techniques and modifications in the virtual impedance control technique are widely used to suppress the circulating current. However, the addition of the virtual impedance to each inverter to compensate the output impedance is resistive or inductive in nature. The resistive nature of the output impedance always causes a certain voltage drop, whereas the inductive nature of the output impedance causes phase delay for the output voltage. Both problems are addressed by the proposed control mechanism in this paper. Negative resistance, along with virtual impedance, is utilized in the proposed control strategy. The output impedance is to be maintained as inductive in nature to achieve good load sharing in droop control MGs. The simulation results validate the proposed control scheme.

Secondary control of islanded microgrid using robust finite time decentralized method based on adaptive fuzzy sliding mode controller

Secondary voltage control of islanded microgrid is one of the main needs of microgrid hierarchical control from one perspective and the realization of stable smart grids from another aspect. This article aims to achieve several goals simultaneously: First, it provides a decentralized structure for controlling and regulating the voltage of DGs, in which the effects of nonlinear terms on the dynamics of the microgrid system are considered without elimination or linearization. The fuzzy method has been used to approximate the uncertainties, the interactions of agents and nonlinear terms, and the adaptive method has been used to obtain the optimal fuzzy approximation gains. Sliding mode rules are also used to obtain a control signal and using Lyapunov's theory, the finite time and robust stability of the closed-loop control system is guaranteed. The finite time approach ensures the achievement of convergence in a finite and definite time, and the robust approach realizes the dynamic stability of the system against uncertainties and disturbances. The results of the microgrid system simulation show well the proposed controller capability in different operating conditions.

An intelligent ABC-based terminal sliding mode controller for load-frequency control of islanded micro-grids

In today’s electric networks, micro-grids are highly integrated into power system regarding their technical, environmental, and economic advantages. Due to the stochastic behavior of loads and intermittent nature of renewable energy resources, the micro-grids are subjected to frequency oscillations especially in the islanded mode of operation. In this paper, an intelligent Terminal Sliding Mode Control (TSMC) based on Artificial Bee Colony (ABC) optimization algorithm is proposed for load-frequency control in islanded micro-grids composed of several energy resources. In TSMC approach, by designing the nonlinear sliding surface and using fractional power terms in surface, it is ensured that the states of system converge to the origin in the finite time. The finite-time stability not only guarantees the finite-time convergence of the state variables, but also provides a function with high accuracy. The parameters of the proposed terminal sliding mode controller are optimized using the presented evolutionary algorithm. The ABC algorithm introduces an intelligent TSMC showing an excellent performance in micro-grid’s load-frequency control. The simulations are implemented in MATLAB, and the results are compared with PID control approach. The obtained results demonstrate the superiority of the proposed approach in damping of frequency oscillations during severe disturbances and uncertain conditions.

Distributed Secondary Voltage Control in Islanded Microgrids With Consideration of Communication Network and Time Delays

The extensive usage of open communication mechanisms in distributed secondary control of islanded microgrids (MGs) suggests that the effects of the communication network and time delays on system performance cannot be negligible. Based on the relationship between convergence and algebraic connectivity, this paper proposes an optimal design for a directed communication topology and pinning location through the mirror operation and global propagation rates, respectively. Alternatively, a small-signal dynamic model of an MG equipped with time-delayed distributed secondary voltage control is derived, to investigate the delay-dependent stability of MG system in terms of the controller, network and pinning conditions. An analytical formula is then developed to determine the individual delay margins with respect to different sets of controller gains and pinning. Through a series of trials, the qualitative effects of controller parameters and pinning conditions on delay margins can be used to guide the design of both controllers and pinning for the improved system performance. The effectiveness of proposed methodology is verified via the real-time hardware-in-the-loop (HIL) simulations.

Local adaptive pinning synchronisation for distributed secondary control of islanded microgrid

This study introduces a distributed secondary control method-based adaptive pinning control for inverter-based distributed generations (DGs) in an isolated microgrid (MG) structure with frequency and voltage compensation and power sharing. Without MG centre controller or energy management system, local DGs’ controllers have no global information of the whole communication network, which is the second smallest eigenvalue of the Laplacian matrix. In practice, the large coupling weight gains are mostly selected. However, a sufficient large coupling weight may cause instability. It is difficult to choose the coupling weight in the fully distributed complex network. To solve the above problems, the authors investigate a pinning control for synchronisation using the local adaptive pinning algorithm on coupling weight gains. The proposed method has the advantage that the consensus controller design is independent of the Laplacian matrix associated with the communication network. And the collective dynamics of the communication network can be adjusted to steady states without global information. However, the adaptive pinning control scheme is not suitable for the multi-disturbance situation, which will accumulate the coupling weight and even cause integral saturation. Therefore, an improved strategy is proposed to adapt to the load changing conditions for islanded MGs.