AC Microgrid Research Articles

Secondary control with reduced communication requirements for accurate reactive power sharing in AC microgrids

AbstractA secondary control method is proposed for accurate reactive power sharing as well as frequency and voltage restoration in islanded AC microgrids (MGs). The proposed method consists of an MG secondary controller, local secondary controllers for distributed energy resources (DERs), and a low‐bandwidth communication link for broadcasting common data from the MG secondary controller to DERs. The broadcasted data includes the MG point of common coupling voltage magnitude and a common vertical shift for frequency and voltage restoration. Local secondary controllers calculate specific shifts for the Q‐V droop characteristic of each dispatchable DER and the V‐Q reverse droop characteristic of each photovoltaic (PV) system, aligning their operating points with the Q‐VPCC and VPCC‐Q droop characteristics, respectively. By employing VPCC as a common global variable, coordination of reactive power generation of all dispatchable DERs and PV systems is achieved, enabling accurate reactive power sharing. Importantly, in the proposed scheme, the required communication bandwidth and the communication burden are minor and are not increased with the number of DERs. Additionally, the DERs are relieved of the need for data transmission capability. The small signal stability of the proposed method is examined and its effectiveness is validated through Hardware‐in‐the‐Loop (HIL) experimental results.

Development of Control Techniques for AC Microgrids: A Critical Assessment

This article aims to provide a comprehensive review of control strategies for AC microgrids (MG) and presents a confidently designed hierarchical control approach divided into different levels. These levels are specifically designed to perform functions based on the MG’s mode of operation, such as grid-connected or islanded mode. The primary control ensures frequency (f) and voltage (V) stability, whereas the secondary control adjusts their values to their references and the tertiary control efficiently manages the power of distributed generators (DGs) in a cost-effective manner. The article extensively discusses centralized, decentralized, and distributed strategies for each control level, highlighting their differences, advantages, disadvantages, and areas of application. Finally, the usefulness of different control strategies at different levels is demonstrated through practical examples.

A consensus-based control for precise reactive power-sharing in AC microgrids

The objective of this work is to propose a control structure for managing the energy of distributed generators connected through a meshed AC network, which supplies a set of distributed loads. To avoid the need for algorithm switching in the event of faults (such as line losses or generator disconnections), all converters operate in grid forming mode. A control strategy based on nonlinear droop control ensures optimal network operation (equal sharing of active and reactive power) even in the presence of uncertainties. Only local voltage measurements are exchanged between neighboring nodes and used by a consensus algorithm to achieve the control objectives. The design of control parameters and hardware-in-the-loop (HIL) validations are presented in the article. The findings demonstrate that a low communication rate network is sufficient to achieve the desired objectives.

A Detailed Assessment of the Power Quality Improvement of an Islanded AC Microgrid through Upgrading Conventional Grid-Feeding Current-Controlled Converters to Operate as Multifunctional Converters

The decarbonization of the global economy is one of society’s urgent objectives nowadays. Thus, the large-scale adoption of renewable energy sources, like solar and wind energy, seems to be the best pathway to achieving sustainable development. Thus, the microgrid concept has emerged as a solution to address concerns related to the intermittent nature and lack of inertia for such sources. Several studies involving microgrids have been published in the last few years. However, in most of them, power quality (PQ) disturbances and strategies to manage such disturbances are not considered, especially in the islanded operation mode. Hence, in this paper, a detailed assessment of the PQ of an islanded AC microgrid is conducted, considering the coexistence of a nonlinear load, voltage-controlled converters, and a current-controlled converter (CCC). The analyses consider two scenarios, depending on the operation mode of the CCC: (1) the CCC operating as a conventional grid-feeding converter and (2) considering a cost-effective upgrading of the conventional CCC to operate as a multifunctional converter. An experimental test bed is built to validate both scenarios. The presented results provide strong evidence that the AFF significantly improves the microgrid PQ and its suitability for dealing with PQ disturbances in real-world applications.

Distributed feedback optimisation based optimal power flow control in fully inverter based islanded AC microgrids

AbstractA novel distributed feedback optimisation (FO) based control method is proposed to control grid‐forming inverters (GFMIs) in fully inverter‐based islanded AC microgrids (MGs). The proposed controller has two control layers. The upper layer uses FO to calculate the frequency and voltage setpoints of GFMIs, whereas the lower layer makes GFMIs track these setpoints. The proposed control method takes advantage of the flexibility of voltage control to regulate the system frequency, maintain both active power and reactive power sharing accuracies, keep bus voltage within allowable range and meanwhile preserves the optimality of the closed‐loop system in term of optimal power flow. The gradient descent method is used to solve the proposed FO problem based on the real‐time measurements in the MGs, which is implemented in a distributed way, and thus eliminates the need for a central controller. Case studies show the effectiveness of the proposed method.The cover image is based on the Research Article Distributed feedback optimisation based optimal power flow control in fully inverter based islanded AC microgrids by Y. Cheng et al., https://doi.org/10.1049/stg2.12132.

Remote Negative-Sequence Voltage Fair Compensation in Grid-Forming Inverter-Based Islanded AC Microgrids

Remote Negative-Sequence Voltage Fair Compensation in Grid-Forming Inverter-Based Islanded AC Microgrids

Control techniques for operation of roof-top solar photovoltaics based microgrid in islanded mode

This paper presents the control techniques for the operation of roof-top solar photovoltaics based single-phase ac microgrid in islanded mode. The proposed microgrid consists of four roof-top photovoltaics on various buildings, battery energy storage system in each building, and loads. In each building, the roof-top photovoltaic system is connected to the single-phase ac loads and proposed single-phase ac microgrid through the two-stage converter, which consists of dc-dc boost converter and bidirectional single-phase voltage source converter. A bidirectional dc-dc converter is used to connect the battery energy storage system to the photovoltaic system. The control technique of the bidirectional single-phase voltage source converter, using the feed-forward and feed-back control signals, has been suggested into a rotating d-q synchronous reference frame, with the dual controllers, for controlling the power mismatch and rated sinusoidal ac voltage of the microgrid. The control technique of the bidirectional dc-dc converter, used for the battery energy storage system, is implemented with two voltage controllers and one current controller for managing of the power mismatch in the proposed microgrid system. In this work, the power control algorithms have also been developed, which are aimed to manage the power balance in the single roof-top PV based building as well as in the proposed microgrid, under islanded mode, for various operating scenarios including fault condition.

Impact of energy storage devices on microgrid frequency performance: A robust DQN based grade-2 fuzzy cascaded controller

This article intended an advanced deep-Q network (A-DQN) based algorithm for the optimal design of a robust grade-2 fuzzy cascaded controller (G2-FCC) concerning frequency control of an autonomous AC microgrid under several electrical disturbances. The proposed AC microgrid is equipped with a hydraulic electrolyzer fuel cell (HE-FC) that empowers electrically to a variety of local consumers. Apart from the HE-FC, the proposed microgrid model is associated with few fractional megawatts (MW) rated power generating sources such as diesel generators (DEG), micro-turbines (MT), solar power stations (SPS), wind power stations (WPS) and geothermal to maximize the total power of a system. However, to increase system performance and energy quality some energy-storing devices such as ultra-capacitors (UC), flywheel (fw), and battery energy management systems (BEMS) are integrated into the system. Most renewable energy sources such as solar and wind power plants are associated with high uncertainties and exhibit low inertia which severely affects the grid frequency. The proposed fuzzy G2-FCC controller dominates the frequency disturbance gracefully and shows its superiority over a few standard approaches like fuzzy Proportional Integral Derivative (PID) and PID controllers. The energy-storing components also have shown their potential in regard to frequency profile improvement of the microgrid system. Finally, the suggested A-DQN algorithm is proved as the most effective tool to optimal design the parameters of the G2-FCC controller. Finally, it is verified that the suggested G2-FCC controller is found to be the most effective tool to improve the settling time of ΔF by 154.54 and 249.64 % over well-existing grade-1 fuzzy PID and PID approaches, respectively.

Optimal-Consensus-Based Event-Triggered Control Strategy for Island AC Microgrids

This article proposed an optimal-consensus-based even t-triggered control strategy to coordinate multiple distributed generators (DGs) in an alternative current (ac) microgrid. The proposed control strategy integrated the average bus voltage recovery and the proportional reactive power sharing by turning the coordinated control problem into a convex optimization problem, which realized the bus voltage control and proportional reactive power sharing of the DGs in one controller so that the controller structure is significantly simplified. Accordingly, the controller provided an asynchronous event-triggered mechanism considering the finite availability of communication resources, which reduces communication burdens and avoids Zeno behaviors. The sufficient conditions are demonstrated using the Lyapunov function for maintaining the stability of the proposed control strategy. Several simulations are exhibited using the MATLAB/Simulink platform and the results are reviewed to validate the effectiveness of the proposed control strategy for enhancing the ac microgrid robustness and reducing communication burdens.

Distributed Predefined-Time Secondary Frequency and Average Voltage Control for Islanded AC Microgrids

The frequency and voltage restoration along with power sharing is imperative for the safe, reliable and stable operation of the islanded AC microgrids (MGs). In this paper, a novel distributed fractional-order predefined-time sliding mode controller (DFOPTSMC) is proposed as a secondary controller for the islanded AC MGs. The proposed DFOPTSMC can ensure that frequency and estimated average voltage (EAV) converge to reference value along with accurate power sharing within a setting predefined time. The fractional calculus algorithm is implemented to design the DFOPTSMC, which increases the degree of freedom of parameter adjustment and greatly enhances the flexibility of the DFOPTSMC. In addition, a distributed predefined-time observer is presented to achieve the EAV within a setting predefined time. The upper bound convergence time of the islanded AC MGs under the proposed DFOPTSMC is directly equal to a tunable parameter, thus overcoming the disadvantages of finite-time control and fixed-time control. The Lyapunov function is applied to present rigorous proof to analyze the stability of the islanded AC MGs under the proposed DFOPTSMC. An islanded AC MG is taken into simulation on the Matlab/Simulink 2021a platform and the experimental platform to test and verify the effectiveness and superior performance of the proposed DFOPTSMC. The results indicate that the proposed DFOPTSMC can significantly enhance the dynamic performances of the islanded AC MGs.

On design of power sharing for VSC‐based islanded AC microgrids: An adaptive MIMO controller equipped with a predictor

AbstractIn an AC microgrid, the active/reactive power is usually shared among its distributed generators (DGs) based on the frequency‐active power () droop and the voltage‐reactive power () droop. By increasing the resistant/inductance ratio () of feeder lines; however, adverse effects of interactions between these two control loops are intensified. In this paper, an adaptive multi‐input multi‐output (MIMO) current control structure is proposed to tackle this problem in AC microgrids with arbitrary numbers of DGs in the primary control level. A deep analysis based on the relative gain array (RGA) matrix and the diagonal dominance concept is provided to systematically design MIMO controllers. The proposed technique is based on the Lyapunov's stability theory, and the asymptotic stability of the whole microgrid is guaranteed. For each DG, the suggested design procedure is started by defining a model reference in which the desired control objectives, including the settling time and the steady‐state error, are considered. Then, a feedback‐feedforward controller is established where its gains are adaptively tuned by some rules derived from a Lyapunov function. Moreover, a predictor is used to estimate the adverse effects of other DGs which are taken into account as external disturbances during the design process of the adaptive controller. By considering some realistic scenarios through time‐domain simulations in MATLAB/SIMULINK, it is shown that the proposed strategy can be successfully used to solve the power sharing problem in AC microgrids.

Two-level distributed fully-predictive frequency control scheme for inverter-based AC Microgrid considering communication delay

Amid the current development of decentralized renewable energy sources, the research of multi-agent-based control for the next generation of the intelligent power system has become one of the most trending and worth-noticing topics in the smart grid research community. Serving as a contribution to the research orientation, this paper proposes a novel two-layer distributed fully-predictive frequency control scheme for an inverter-based three-phase three-wire AC microgrid. The key feature of the suggested methodology is the incorporation of objective functions and dynamic models as constraints in the predictive formulation of both primary and secondary layers in the microgrid control hierarchy. In our study, the controller is designed based on constrained cost function minimization using Quadratic Programming (QP) and the utilization of local measurements and information from surrounding distributed generators to achieve the desired control action. Along with the integration of virtual impedance, second-order generalized integrator phase-lock loop, and harmonic suppression constraint functions, the proposed control scheme shows great response against all variations of load changes, communication delay, communication network failure, and Plug-n-Play capability test. The performance of the proposed controller is verified and evaluated on the 4-DGs microgrid model using MATLAB/Simulink environment.

Distributed Event-Triggered Secondary Control of Microgrids With Quantization Communication

Taken communication networks with limited resources in practical microgrids into consideration, a communication scheme, combined with an event-triggered communication scheme and quantization technology, is designed in the distributed optimal secondary control for islanded AC microgrids to reduce communication loads. In addition, switching communication networks are considered in microgrids to enhance the flexibility and the extendibility. In comparison with the existing distributed secondary control methods, the proposed distributed control strategy based on the designed communication scheme not only achieves the frequency and voltage restoration and the optimal active power sharing, but also reduces both the communication frequency and the amount of transmitted data in networks. Moreover, the presented communication scheme can exclude the Zeno behavior. Simulation results are given to evaluate the effectiveness of the proposed method.

Iterative Learning-Based Distributed Secondary Control for AC Microgrid

Existing distributed secondary control strategies relying on the microgrid system model have good theoretical performance but are difficult to implement in practice due to the need for real-time observation of numerous system states. This article addresses this issue by proposing a distributed secondary control strategy for ac microgrids based on ILC. During operation, this strategy neither relies on the microgrid system model nor imposes any computational burden. First, this article introduces the microgrid hierarchical control structure and establishes a system model for primary control. Then, the proposed ILC-based secondary control strategy is elaborated, and the corresponding stability criterion is derived using the small-signal analysis method. Finally, the simulation results in MATLAB/Simulink demonstrate the effectiveness of the proposed ILC-based strategy. Specifically, the proposed strategy can limit the transient time within 0.4 s when handling various disturbances, which is comparable to the existing strategies based on MPC and SMC. However, the proposed strategy is easier to implement compared with the MPC-based and SMC-based strategies, which rely on the system model. Furthermore, in plug-and-play and load change scenarios, the proposed control strategy can suppress voltage fluctuation within 1 V, which is better than existing strategies.

Quadratic-Droop-Based Distributed Secondary Control of Microgrid With Detail-Balanced Communication Topology

This article proposes a quadratic-droop-based fixed-time distributed secondary voltage control of an islanded ac microgrid. The microgrid consists of inverter-based distributed generators, connected over a detail-balanced communication topology with bidirectional unequal edge weights. The proposed voltage restoration strategy is derived from the quadratic relation between voltage and reactive power for inductive microgrids against their linear dependence, which has been conventionally used in designing voltage droop controllers. It has been illustrated that the proposed quadratic-droop-based control design strategy shows better performance as compared with the linear counterpart. We provide a rigorous Lyapunov stability analysis for the proposed controller and also obtain an upper bound on the convergence time, independent of the system's initial conditions. In addition, frequency restoration and proportional active power sharing among the generators are realized in the fixed time. Simulations are provided to verify the efficacy of the proposed controllers, including a comparative analysis with conventional droop control and finite-time control through a case study.

An Integrated Human–Cyber–Physical Framework for Control of Microgrids

In this paper, to jointly study the energy dynamic behavior of humans and the corresponding physical dynamics of the microgrid, we bridge two disciplines: systems & control and environmental psychology. Firstly, we develop second order motivation-behavior mathematical models inspired by opinion dynamics models for describing and predicting human activities related to the use of energy, where psychological variables and social interactions are considered. Secondly, based on these models, we develop a human-cyber-physical system framework consisting of three layers: (i) human, (ii) cyber and (iii) physical. The first one describes human behavior influenced by behavioral intervention and motivation, which in turn depend on contextual factors, personal values and social norms. The cyber layer solves an optimization problem and embeds load controllers, which are designed to automatically mimic human behavior. Finally, the physical layer represents an AC microgrid. Thus, we formulate a social-physical welfare optimization problem and solve it by designing a distributed primal-dual control scheme, which generates the optimal behavioral intervention (with respect to a given reference) and the control inputs to the microgrid.

Hierarchical control of island microgrid based on consensus algorithm

When the load inside the microgrid changes, droop control maintains a stable power supply cycle of the microgrid by controlling the voltage and frequency at the parallel network of the distributed generation. Since the traditional microgrid droop control usually uses a fixed droop coefficient to adjust the output active power and reactive power, the frequency and voltage of the inverter parallel network will deviate from the system frequency and voltage rating to varying degrees, so a new layer of control link is needed to restore the target value. Moreover, most of the hierarchical control methods adopt centralized control methods, which have limitations such as high dependence on communication. This paper combines a hierarchical control framework and a consistency algorithm to propose a distributed sag control strategy for islanded microgrids based on a multi-agent system. It achieves equal-to-ratio output when distributed generation responds to power changes while maintaining the stability of the microgrid’s main performance indicators. Since the distributed system with a consistency algorithm does not require a centralized control centre, it increases the system’s robustness and reduces the dependence of the system on the communication network. Finally, an islanded AC microgrid model is built by MATLAB/Simulink, and the effectiveness of the proposed islanded microgrid hierarchical control strategy is verified by simulation.

Nonlinear Feedback Linearized Controller Design for Islanded AC Microgrids

A new way has been proposed in this paper to design a nonlinear feedback linearized controller for a single-input-two-output control problem of a multi-input multi-output system. An islanded AC microgrid linearized by the feedback linearization control scheme. The output voltages and currents are the control objectives to improve the voltage regulation and the power-sharing accuracy among parallel-connected distributed generation systems, respectively. The stability of the proposed feedback linearized control scheme has been investigated using a Lyapunov function for the islanded AC microgrid. The performance of the designed nonlinear controller has been evaluated under different case studies for the islanded AC microgrid.

Distributed cooperative fault tolerant optimal active power control in AC microgrid

In this paper, a new distributed and cooperative fault tolerant control is proposed for the double-function optimal active power control (APC) of distributed generators (DG) in an islanded AC microgrid. Double-function optimal APC consists of the economic dispatch which calculates the optimal active power set-points and coordinates control modules to receive and track the set-points. In the proposed control method, if there is a faulty DG whose capability to satisfy its optimum active power set-point is restricted due to a totally or partially DG failure, the task of reaching the global control goal is re-distributed among the fault-free DGs through designing dynamic feedback set-points. In addition, no central coordinator is considered for the MG, and the new set-points are calculated locally by the fault-free DGs through message exchange over the communication network. Redistribution is performed without requiring any coordination unit and only with the cooperation of DGs on redistributed set-points through local exchange information. The capability of the proposed method is evaluated by considering a microgrid with multiple DGs and analyzing its operational performance under a variety of fault scenarios, as well as providing a qualitative and quantitative comparison with existing literature to demonstrate its efficacy.

An Overview of DC Microgrid Protection Schemes and the Factors Involved

The development of a suitable scheme for protection of DC microgrids is still a major challenge. This is due to the shortage of practical experience in appropriate protection guidelines and standards related to the uncertainty in the network topology, absence of a natural current zero-crossing point in DC systems, and dependence of fault current shape and magnitude on various factors. These factors include network topology, type of converters used in the network, type of fault and its location, fault impedance, and type of system grounding. In this paper, these factors and their impacts on the development of a comprehensive protection plan for DC microgrids have been investigated. Various algorithms that have been proposed in previous studies for fault detection and location are reviewed, and the technical challenges associated with the use of communication links in smart protection systems are described. In addition, the advent of new technologies for development of DC circuit breakers are reported, which are different from the conventional AC circuit breakers due to the differences between the faults in DC and AC microgrids and their respective clearance times. This comprehensive review covers all aspects of the DC microgrid protection including topology, converters, configurations, and types of faults.