Isolated AC Microgrids Research Articles

Intelligent frequency control of AC microgrids with communication delay: An online tuning method subject to stabilizing parameters

Smart control techniques have been implemented to address fluctuating power levels within isolated microgrids, mitigating the risk of unstable frequencies and the potential degradation of power supply quality. However, a challenge lies in the fact that employing these computationally complex methods without stability preservation might not suffice to handle the rapid changes of this highly dynamic environment in real-world scenarios over communication delays. This study introduces a flexible real-time approach for the frequency control problem using an artificial neural network (ANN) constrained to stabilized regions. Our solution integrates stabilizing PID controllers, computed through small-signal analysis and tuned via an automated search for optimal ANN weights and reinforcement learning (RL)-based selected constraints. First, we design stabilizing PID controllers by applying the stability boundary locus method and the Mikhailov criterion, specifically addressing communication delays. Next, we refine the controller parameters online through an automated process that identifies optimal coefficient combinations, leveraging a constrained ANN to manage frequency deviations within a restricted parameter range. Our approach is further enhanced by employing the RL technique, which trains the tuning system using an interpolated stability boundary curve to ensure both stability and performance. This one-of-a-kind combination of ANN, RL, and advanced PID tuning methods is a big step forward in how we handle frequency control problems in isolated AC microgrids. The experiments show that our solution outperforms traditional methods due to its reduced parameter search space. In particular, the proposed method reduces transient and steady-state frequency deviations more than semi- and unconstrained methods. The improved metrics and stability analysis show that the method improves system performance and stability under changing conditions.

A novel weighted exponential sliding mode controller with a modified reaching law for the frequency regulation of a renewable integrated isolated AC microgrid

This paper investigates the development and application of a novel hybrid controller through novel sliding surface and novel reaching law, Weighted Exponential Sliding Mode Control with Modified Reaching Law (WESMC-MRL), for frequency regulation in an isolated Microgrid (MG) with intermittent Renewable Energy Sources (RESs) such as solar and wind. Major contributions include the analysis of frequency regulation of isolated MG controlled by WESMC-MRL under various load perturbations. The performance of WESMC-MRL is compared with conventional Linear Quadratic Regulator (LQR), Sliding Mode Control (SMC), and Integral Sliding Mode Control (ISMC) controllers. Outcomes indicate that the WESMC-MRL effectively regulates the MG frequency with minimal chattering, and outperforms LQR SMC, and ISMC in terms of settling time, overshooting, and undershooting in frequency deviation. With the WESMC-MRL the maximum frequency deviation is found only 0.00724 Hz and undershoot is negligible. The proposed controller is also robust against RES integration variability, disturbances, and nonlinearities making it a promising solution for reliable and stable MG operation.

A distributed coordinated control strategy for isolated AC microgrids based on consensus algorithm considering communication delay

AbstractIn the multi‐paralleled converter microgrid system, the traditional hierarchical control strategy can eliminate the bus voltage amplitude and frequency deviation from the rated value. However, isolated AC microgrids may face extreme scenarios such as communication delays and interruptions in data transmission due to the use of low‐bandwidth communication (LBC) lines. Additionally, the inconsistent line impedance of each distributed generation (DG) unit may result in the inaccurate division of reactive power in multi‐paralleled converter systems, thus affecting system stability. To address these issues, this paper presents a distributed coordinated control strategy for isolated AC microgrids based on the consensus algorithm. The proposed strategy first replaces LBC lines with a filter to alleviate the effects of communication delays. A small‐signal model is established in its state space, and stability of the microgrid system under the proposed control strategy is verified through eigenvalue analysis. Furthermore, based on the above theoretical analysis, a consensus algorithm is introduced, and a distributed control strategy for isolated AC microgrids based on the consensus algorithm is proposed to solve the issue of inaccurate equalization of the system's reactive power. Finally, factors that influence the dynamic convergence performance of the consensus algorithm are analyzed through simulation in PLECS software. Also, the maximum tolerable communication delays of the microgrid system under different communication topologies are also compared, and the system's robustness is evaluated under the condition of sudden communication interruption in a DG unit and sudden weather variation. These analyses confirmed the robustness of the proposed strategy against communication delays, output power fluctuation, and communication interruptions.

Distributed Actor-Critic Neural Networks-Based Structure for Frequency Synchronization in Isolated AC Microgrids

Distributed Actor-Critic Neural Networks-Based Structure for Frequency Synchronization in Isolated AC Microgrids

Improving power sharing and enhancing the stability of an isolated AC microgrid by implementing droop control and virtual impedance

The stability and accuracy of power sharing between DGs in isolated microgrids are key points for favouring renewable energy sources within a wider operating margin to extract the maximum available power. The conventional droop control method CDC and the improved virtual impedance droop control CDC-VI are proposed in this work to enhance the stability, improve the speed, robustness and accuracy of power distribution between two parallel DGs feeding three linear loads of different types connected to the AC bus within an isolated AC microgrid. A Matlab/Simulink simulation is carried out to validate our proposal, robustness, accuracy of power sharing between DGs and stability are well improved by the implementation of the proposed CDC and CDC-VI controllers. The main improvements brought by these controllers include improved power distribution with greater precision, enhanced stability even in the presence of disturbances, and faster response in dynamic scenarios. These improvements are crucial to ensuring efficient, reliable operation of microgrids, particularly in isolated environments where integration of renewable energies and management of critical loads are paramount for uninterrupted service continuity.

Distributed Q-Learning-Based Voltage Restoration Algorithm in Isolated AC Microgrids Subject to Input Saturation

Distributed Q-Learning-Based Voltage Restoration Algorithm in Isolated AC Microgrids Subject to Input Saturation

Droop Control Optimization for Improved Power Sharing in AC Islanded Microgrids Based on Centripetal Force Gravity Search Algorithm

The urgent demand for clean and renewable energy sources has led to the emergence of the microgrid (MG) concept. MGs are small grids connecting various micro-sources, such as diesel, photovoltaic wind, and fuel cells. They operate flexibly, connected to the grid, standalone, and in clusters. In AC MG control, a hierarchical system consists of three levels: primary, secondary, and tertiary. It monitors and ensures MG stability, power quality, and power sharing based on the specifications of governing protocols. Various challenging transient disturbances exist, such as generator tripping, secondary control failure due to communication delay, and drastic load changes. Although several optimal power sharing methods have been invented, they pose complex control requirements and provide limited improvement. Therefore, in this paper, a novel optimized droop control is proposed using a metaheuristic multi-objective evolutionary algorithm called the Centripetal Force-Gravity Search Algorithm (CF-GSA) to improve the droop performance of power sharing, voltage and frequency stability, and power quality. CF-GSA is an improved algorithm designed to address the issue of local solutions commonly encountered in optimization algorithms. The effectiveness and superiority of the proposed method are validated through a series of simulations. The results of these simulations show that the proposed multi-objective optimization droop control method works well to fix problems caused by power sharing errors in isolated AC microgrids that have to deal with high inductive loads and changes in line impedance.

Data-Driven Distributed Q-Learning Droop Control for Frequency Synchronization and Voltage Restoration in Isolated AC Micro-Grids

Data-Driven Distributed Q-Learning Droop Control for Frequency Synchronization and Voltage Restoration in Isolated AC Micro-Grids

Concurrent Cyber Deception Attack Detection of Consensus Control in Isolated AC Microgrids

Concurrent Cyber Deception Attack Detection of Consensus Control in Isolated AC Microgrids

Optimal Distributed ADMM-Based Control for Frequency Synchronization in Isolated AC Microgrids

To restrict transient dynamics of each distributed generator (DG) in isolated AC micro-grids (MGs), an optimal distributed alternating direction method of multiplier (ADMM)-based control scheme, will be explored for achieving frequency synchronization within a given finite horizon. By introducing the consensus variable, we re-phrase the frequency synchronization problem as a linear quadratic tracking problem under the framework of multi-agent systems. The objective function of each individual DG is defined as the accumulation of quadratic frequency errors and quadratic inputs. Since the consensus variable is also part of the decision variable, more flexibility can be gained in developing the optimal distributed ADMM-based algorithm within a finite horizon. Since the direct extension of ADMM for static optimization can not be fully distributed, a fully distributed ADMM-based control is developed by exploring the optimal distributed control theory in each DG. To validate the performance of the proposed method, real-time simulations on OPAL-RT are conducted to validate the effectiveness of the proposed optimal distributed ADMM-based control strategy even under large load variations and plug-and-play operations of DGs.

Optimal Reset-Control-Based Load Frequency Regulation in Isolated Microgrids

In isolated ac microgrids, multiple controllable distributed energy resources (DERs) may simultaneously participate in load frequency control (LFC). To improve system frequency dynamics and reduce the frequency deviation for such a multiple-DER microgrid,this paper presents a novel LFC method based on an optimal reset control (ORC) scheme. The proposed ORC comprises a baseline linear controller and resetting elements, i.e., the integrator outputs, determined by a linear quadratic regulation problem. The overshoot in frequency regulation induced by integral control is reduced, and the settling time is shortened. Besides, the simple structure facilitates its real-time application, making ORC a simple yet effective solution for the LFC of microgrids. By coordinating the reset controllers of all controllable DERs, the ORC further improves the overall frequency control performance. The asymptotic stability of the closed-loop system is theoretically proved. Finally, the effectiveness of the presented LFC method is validated by switch-level simulation studies.

Improvement of Fault Current Calculation and Static Security Risk for Droop Control of the Inverter-Interfaced DG of Grid-Connected and Isolated Microgrids

The contribution current of an inverter-interfaced distributed generator unit during a fault is one of the significant challenges for two modes: grid-connected and isolated AC microgrid. For this challenge, this article is aimed to study two methods of fault current calculation for two modes: grid-connected and isolated microgrids. These methods include a virtual equivalent impedance and a proposed method. The proposed method is a new technique for calculating the fault current contribution depending on the droop control of inverter-interfaced DG. The proposed method can control the contribution short-circuit current of DG within its limit (2 p.u.) where it is dependent on the voltage value of the DG bus to calculate the short circuit current of DG by using the control criterion. Static security risk and load shedding are calculated after fault clearance using an operation scenario in which the distribution system will be divided into small subsystems and is then grid-connected and isolated due to the removal of the faulted bus by protection devices. The proposed technique is applied to a standard IEEE 33-bus distribution network with five DGs. The results show that the contribution current of inverter-interfaced DG during the fault has more effects than the fault current of the nearest faulted bus to the DG bus. The proposed technique improves the calculated fault current value by about 30% for the grid-connected microgrid and by about 50% for the isolated microgrid from its value of the virtual impedance method. The static security risk is improved after load shedding. The static security risk improved by about 0.025%.

Holomorphic Embedding Power Flow Algorithm for Isolated AC Microgrids With Hierarchical Control

Power flow (PF) calculation plays a fundamental role in the steady-state analysis of isolated microgrids (MGs). In this paper, a novel analytical PF algorithm based on holomorphic embedding (HE) is proposed for isolated AC MGs considering hierarchical control (HC). With a deliberate design in embedding technique, the presented algorithm not only inherits the deterministic property of the canonical embedding that enables to derive the upper-branch (operable) solution explicitly, but also is compatible with the HC characteristic of isolated MGs. Furthermore, the embedded PF model features a general structure with a constant recursive matrix, which can accommodate various bus types and network re-configurations (including both radial and meshed networks), as well as non-nonlinearities of voltage-dependent load models. Case studies are carried out on 3 test systems: a 12-bus MG and modified IEEE 33-bus and 123-bus distribution systems. Numerical results reveal the applicability and efficacy of the proposed algorithm.

A novel distributed secondary voltage control method for AC microgrids based on voltage observer

Since the islanded AC Microgrid is affected by the impedance of line, it is difficult to coordinate the voltage regulation and reactive load distribution. A distributed secondary voltage controller based on observer is proposed for isolated AC Microgrids, and it does not need more voltage communications. This method can guarantee the estimated average voltage equals to average of actual output voltages that can converge to the nominal values, and realize accurate proportional load sharing in a microgrid. Then, the convergence of the voltage observer is proved. Finally, the proposed control method is verified by time-domain simulation.

Joint Control Strategy of Microgrid Inverter Based on Optimal Control and Residual Dynamic Compensation

With the continuous development of microgrid, the inverter under traditional PID control is difficult to meet the power quality requirements of microgrid. In order to improve the anti-interference ability and transient response speed of the output voltage of Isolated AC microgrid inverter, a dynamic compensation control structure combining linear quadratic optimal control and disturbance residual generator is proposed in this paper. Firstly, the quadratic optimal controller is established based on the inverter model. Secondly, the residual generator is established to observe the residual value of the disturbed system. Then, the dynamic compensation controller is designed according to the superposition principle, and a control signal opposite to the influence caused by the disturbance is output at the output of the controller. The disturbance signals can cancel each other without knowing the specific disturbance form. Finally, the experiments of load switching, three-phase imbalance and nonlinear load are designed on MATLAB/Simulink simulation software to verify the effectiveness of the control strategy proposed in this paper.

Consensus-Based Single-Phase Control of Three-Phase Inverter for Current Sharing in Three-Phase Four-Wire Isolated AC Microgrids

This work proposes a novel secondary control system based on the Consensus algorithm for balancing the RMS current between the distributed generation (DG) phases in a four-wire isolated Microgrid (MG) containing unbalanced and non-linear loads. The control strategy proposes using a droop control per phase to regulate the voltage magnitude, whereas the harmonic current sharing is performed using the virtual impedance concept. Additionally, the proposed control scheme guarantees voltage regulation, causing the MG to operate at its nominal voltage value. In addition to sharing the RMS value of the current, the consensus algorithm also provides the equal sharing of the RMS value of the harmonic current between each phase of the DGs that comprise the MG and also ensures that the harmonic distortion of the voltage in the PCC of each DG remains within limits defined by the standards and recommendations. Simulation results demonstrate the functioning and effectiveness of the proposed control strategy.

Distributed Predictive Secondary Control for Imbalance Sharing in AC Microgrids

This paper proposes a distributed predictive secondary control strategy to share imbalance in three-phase, three-wire isolated AC Microgrids. The control is based on a novel approach where the imbalance sharing among distributed generators is controlled through the control of single-phase reactive power. The main characteristic of the proposed methodology is the inclusion of an objective function and dynamic models as constraints in the formulation. The controller relies on local measurements and information from neighboring distributed generators, and it performs the desired control action based on a constrained cost function minimization. The proposed distributed model predictive control scheme has several advantages over solutions based on virtual impedance loops or based on the inclusion of extra power converters for managing single-phase reactive power among distributed generators. In fact, with the proposed technique the sharing of imbalance is performed directly in terms of single-phase reactive power and without the need for adding extra power converters into the microgrid. Contrary to almost all reported works in this area, the proposed approach enables the control of various microgrid parameters within predefined bands, providing a more flexible control system. Extensive simulation and Hardware in the Loop studies verify the performance of the proposed control scheme. Moreover, the controller’s scalability and a comparison study, in terms of performance, with the virtual impedance approach were carried out.

Multi-source residual capacity collaborative dynamic compensation strategy based on robust residual generator

In this paper, a dynamic compensation strategy based on a residual generator was proposed to solve the problem of the power quality of the bus in an isolated AC microgrid. Firstly, the disturbance analysis of power quality problems such as voltage fluctuation, three-phase imbalance, harmonic wave is carried out, and the disturbance current is converted into the time domain function in the dq rotating coordinate system. Secondly, the inverter establishes the state space model under the dq rotation coordinate system. Then, the control structure based on residual generator is proposed, the voltage compensator is solved by model matching method, and the current loop secondary compensator is designed to eliminate the influence of the voltage compensator on the current loop. Thirdly, the residual capacity calculation method of the inverter is analysed, and the residual capacity is used to achieve the power quality collaborative governance on the parallel inverter system with the perturbation allocation method. Finally, the control strategy proposed in this paper is verified by the simulation built in PSCAD/EMTDC, and the results can be effectively applied to the low-level control of microgrid, and the power quality control effect of voltage fluctuation, three-phase unbalance and harmonics is good, which improves the robustness of the system to a certain extent.

An enhanced second-order-consensus-based distributed secondary frequency controller of virtual synchronous generators for isolated AC microgrids

Virtual synchronous generator (VSG) control is a promising control method in microgrid (MG) for the inertia support characteristic. However, the VSG inherits the oscillation feature of synchronous generator (SG), thus the active power and frequency oscillation may be observed when VSG control is applied. In MG, the oscillation may cause the unreasonable dynamic active power sharing among VSGs. Subsequently, the VSGs are prone to overload and even unstable, and the existing works based on consensus control cannot suppress the oscillation effectively. In order to tackle this problem, this paper proposes an enhanced second-order-consensus-based distributed secondary frequency controller by introducing the active power second-order consensus control, and reveals the oscillation suppression capacity of the second-order consensus algorithm in depth. Since there is no specific control requirement for the second-order variables, i.e., the change rate of active power and frequency of MG in steady state, the proposed controller can be simplified by replacing the integral control of second-order variables with the proportional control of first-order variables, i.e., active power and frequency, thus the communication data volume will not increase comparing to the traditional first-order-consensus-based controller. This paper also provides a small-signal model of MG with the proposed controller to evaluate the system dynamic performance and design the parameters. Finally, the simulation results verify the effectiveness of the proposed controller.

Fault Tolerant Control of Sensor Faults in Microgrid Inverter Control System

Aiming at sensor drift fault, the design problem of active fault tolerant controller for island-AC microgrid inverter system is studied. The controller has good ability to suppress external finite energy disturbance. Firstly, a closed-loop system state space model is constructed for an isolated AC microgrid inverter system with external finite energy disturbance. Then, when the sensor fault occurs in the control system, the fault information is taken as an auxiliary state vector, and the estimated value of the sensor fault and the system state variable is obtained simultaneously through the proposed extended observer. A fault tolerant controller is designed based on the idea of fault compensation. Finally, the correctness and effectiveness of this method are verified by programming simulation in MATLAB.