Autonomous AC Research Articles

Virtual Synchronous Machine Based Direct Charging Power Control for the Two‐Stage EV Battery Charger

ABSTRACTIn this paper, a direct charging power control strategy (DCPC) based on virtual synchronous machine (VSM) technique is proposed for the two‐stage EV battery charger. In the proposed solution, the two‐stage charger involves a three‐phase full bridge‐based AC‐DC stage and a buck/boost DC‐DC stage. In control of the front‐end AC‐DC stage, the core algorithm of the traditional VSM is adopted. Nevertheless, unlike the traditional VSM‐based battery charging scheme, the DC‐bus closed‐loop in the traditional VSM algorithm is replaced by a positive feedback‐based DC power compensation loop, and meanwhile, the DC‐bus voltage is regulated by the rear stage DC‐DC converter. The proposed method can ensure stable DC‐bus voltage control and direct control of charging power. Due to the existence of power compensation control, the power balance between AC and DC terminals can be indirectly controlled without complex power losses calculation process. In addition, the autonomous AC frequency and voltage regulation abilities of the VSM technique can be inherited in the proposed method, which can appear a grid‐friendly charging pattern to ensure more stable battery charging. To verify the validity of the proposed method, systematic analysis and experiments are performed on a developed test bed.

Small-Signal Modeling and Region-Based Stability Analysis Using Support Vector Machine (SVM) for Autonomous Hybrid AC and DC Microgrids

Small-Signal Modeling and Region-Based Stability Analysis Using Support Vector Machine (SVM) for Autonomous Hybrid AC and DC Microgrids

A Gain Scheduling Control Framework for Mitigation of Time Varying Network Latency in Autonomous AC Microgrids

A Gain Scheduling Control Framework for Mitigation of Time Varying Network Latency in Autonomous AC Microgrids

Design and FPGA-in-loop based validation of predictive hierarchical control for islanded AC microgrid

This paper proposes a decentralized control approach for the flexible operation of an autonomous AC microgrid (MG). AC MG typically consists of two or more voltage source inverters (VSI), capable of simultaneously regulating the voltage at the point of common coupling (PCC) and meeting the local power demand. The classical linear control techniques attain these functions. However, they have many limitations, such as slow transient response, high sensitivity to the parameter variations, and inability to handle the system nonlinearities. This paper aims to address these issues by presenting an improved finite control set model predictive control (FCS-MPC) approach for inverter-based distributed generation (DG). The proposed scheme tracks the voltage trajectory using cost function (CF) over two-step prediction horizons. The proposed control method is employed for the AC MG having two parallel DGs. Droop control is responsible for the power-sharing between the parallel DGs regardless of impedance mismatch at the distribution level of AC MG. The decentralized secondary control is developed to eliminate the deviation in the voltage and frequency caused due to overlooking the primary control. The proposed control scheme has been validated through extensive simulations and real-time controller hardware in the loop tests using an FPGA ZYBO Z7 board. Moreover, the proposed methodology depicts the enhanced transient response, less computational burden than the classic MPC, and shows robustness to parametric uncertainties in terms of THD than hierarchical linear control. The simulations and experimental results visually represent research outcomes, exhibiting the THD of 0.98 % for different dynamic loads, which is within the limit of IEEE and IEC standards. Furthermore, the proposed controller’s mathematical stability is further supported by analysis based on the Lyapunov stability theory.

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.

Control of ILC in an Autonomous AC–DC Hybrid Microgrid With Unbalanced Nonlinear AC Loads

This article presents a multiobjective control scheme for a bidirectional interlinking converter (ILC) of renewable energy based ac–dc hybrid microgrid. The prime focus of the power management algorithm is to feed the ac loads, and dc loads from ac source, and photovoltaic battery energy storage (BES), respectively. The second objective is to utilize the solar power to maximum capacity, by feeding it to dc load, BES, and ac load, in the given order of priority. The ac and dc subgrids operate independently, while supporting each other in case of overloading/underloading, thus reducing the amount of power reserve required in each, and eliminating the need of dump loads. The ANFIS-MN-based power quality control is integrated in order to ensure the total harmonics distortion of ac source current remains below 5% as per IEEE 519 standard, irrespective of the nature of load connected. In addition, the ILC control prevents any negative sequence currents from entering the ac source, even if there is any open circuit fault leading to unbalanced loading on the three phases. The performance of ANFIS-MN-based ILC control is demonstrated to be superior when compared with the conventional and contemporary techniques under different scenarios.

A Comprehensive Solution to Decentralized Coordinative Control of Distributed Generations in Islanded Microgrid Based on Dual-Frequency-Droop

In an autonomous ac microgrid system, a peer-to-peer decentralized solution would be a preferred choice for the coordinative control of distributed generation units. However, due to the dispersed sources and loads, and mismatched line impedances, it is hard to achieve satisfactory power sharing performance and power quality without communication links. Therefore, a comprehensive decentralized solution based on dual-frequency-droop is proposed in this article. With only local information, appropriate virtual impedances could be obtained to compensate the mismatched complex line impedances in the required frequency range, and hence active, reactive, unbalanced, and harmonic powers could be shared accurately among parallel inverters under various load conditions. Then, the frequency deviation caused by the droop control is effectively attenuated without affecting power sharing through a dynamic droop-slope regulator. Moreover, the causes of steady-state ripples on virtual impedance waveforms are carefully analyzed, and the design criterions to eliminate these ripples are provided. Finally, case studies are provided in simulations and experiments to validate the effectiveness of this method.

Coordinated control of parallel three-phase four-wire converters in autonomous AC microgrids

Coordinated control of parallel three-phase four-wire converters in autonomous AC microgrids

An Optimal Controller for Multi-Load-Bus Voltage Control and Reactive Power Sharing in Autonomous Ac Microgrid

An Optimal Controller for Multi-Load-Bus Voltage Control and Reactive Power Sharing in Autonomous Ac Microgrid

A Distributed Hierarchical Control Framework for Economic Dispatch and Frequency Regulation of Autonomous AC Microgrids

Motivated by the single point of failure and other drawbacks of the conventional centralized hierarchical control strategy, in this paper, a fully distributed hierarchical control framework is formulated for autonomous AC microgrids. The proposed control strategy operates with a distinct three-layer structure, where: a conventional droop control is adopted at the primary layer; a distributed leaderless consensus-based control is adopted at the secondary layer for active power and, hence, frequency regulation of distributed generating units (DGUs); and the tertiary layer is also based on the distributed leaderless consensus-based control for the optimal power dispatch. Under the proposed strategy, the three constituent control layers work in a coordinated manner. Not only is the load dispatched economically with a negligible power mismatch, but also the frequencies of all the DGUs are regulated to the reference value. However, the frequency regulation is achieved without requiring any central leader agent that has been reported in the contemporary distributed control articles. As compared to the conventional centralized hierarchical control, the proposed strategy only needs local inter-agent interaction with a sparse communication network; thus, it is fully distributed. The formulated strategy is tested under load perturbations, on an autonomous AC microgrid testbed comprising both low-inertia-type (inverter-interfaced) and high-inertia (rotating)-type DGUs with heterogeneous dynamics, and found to successfully meet its targets. Furthermore, it can offer the plug-and-play operation for the DGUs. Theoretical analysis and substantial simulation results, performed in the MATLAB/Simulink environment, are provided to validate the feasibility of the proposed control framework.

Decentralized Control Based on Hybrid Water Cycle and Moth-Flame Optimization of Fractional-Order Fuzzy PID in a Multiple DGs Faulty Autonomous Microgrid

This study presents the employment of a novel hybrid Metaheuristics optimization technique to improve and stabilize voltage secondary layer of control being applied in Fractional-Order Fuzzy Proportional-Integral-Derivative controllers in the case of a faulted autonomous 3-phase AC Microgrid. For the frequency control part, a Proportional-Integral-Derivative controller is used. The autonomous Microgrid under study comprises multiple distributed generation units connected with impedance lines along with local loads. The Fractional-Order Fuzzy Proportional-Integral-Derivative controller is considered to have superior performance than conventional ones, despite the increase in the number of tunable parameters. Therefore, a hybrid Water Cycle and Moth-Flame optimization algorithm is considered to reach the optimum solution for the applied decentralized controllers. The performance of the controller is compared through varying optimization techniques and changing the controller type. Under the discussed control approach, the AC Islanded Microgrid operation steadiness is augmented.

Sizing optimization and design of an autonomous AC microgrid for commercial loads using Harris Hawks Optimization algorithm

In this study, the sizing optimization and design of an autonomous AC microgrid is performed using the Harris Hawks Optimization (HHO) algorithm. The objective is to demonstrate that more economical and reliable microgrids can be designed using the HHO algorithm. For this purpose, an autonomous AC grid is designed for the Gazi University Technology Park building located in Ankara. A yearlong energy consumption of the building and meteorological data have been measured and recorded. Based on the data, a microgrid structure consisting of photovoltaics (PV), wind turbines (WT), battery energy storage systems (BESS), and diesel generators (DG) is proposed. Finding the optimum capacities of these components is crucial to achieve the low cost and reliability objectives with true autonomous characteristics. On one hand oversizing that may arise from random assessments without any sizing optimization will increase the investment, operation, and maintenance costs. On the other hand, insufficient capacity planning based on prompt decisions can end up with problems in maintaining the energy continuity. Therefore, sizing optimization is a requirement for an effective microgrid design. In this context, after the sizing problem is solved using the HHO algorithm, the results are used in the design of the proposed microgrid. The simulation of the designed microgrid in MATLAB has demonstrated a successful performance providing savings ranging from 1.18% to 18.23% in the total net present cost (TNPC) value compared to other algorithms. The performance of the HHO algorithm is compared with four state-of-the-art metaheuristic algorithms, namely the Particle Swarm Optimization (PSO), the Firefly Algorithm (FA), the Gray Wolf Optimization (GWO), and the Salp Swarm Algorithm (SSA). Consequently, it is shown that the HHO algorithm has a competitive performance in design of cost effective and reliable microgrids.

Performance improvement of an islanded AC microgrid in presence of Plug-in Hybrid Electric vehicles, load and renewable generation uncertainties

In this paper, a performance improvement scheme for an autonomous AC microgrid in the presence of dispatchable and renewable units, and Plug-in Hybrid Electric vehicles has been proposed. In a standalone mode of operation, a droop-control strategy is implemented at the primary control level for a communication-free accurate power-sharing among multiple dispatchable generators. For a network that is neither highly reactive nor highly resistive, active and reactive powers are coupled to both voltage magnitudes and phase angles. A mixed-droop feature is incorporated at the primary control level for autonomous power-sharing among the units for such networks. The mixed-droop feature is considered in this paper. Further, stochastic models of load demands, renewable generation, and plug-in hybrid electric vehicle charging loads are considered. Objectives are to reduce the cost of operation and minimize the emission of greenhouse gases from generation activities. The bi-objective problem is solved in the fuzzy domain. Performance improvement is achieved in two steps, first, by choosing optimal static droop constants using the particle swarm optimization, and then a further performance improvement is achieved by network reconfiguration. The proposed algorithm is tested on a thirty-three bus autonomous microgrid and gives satisfactory results.

Hierarchical Model Predictive Control for Performance Enhancement of Autonomous Microgrids

This study presents a modified Model Predictive Control (MPC) strategy with primary and secondary levels of control, applied to Distributed Generator (DG) units, to account for limiting overcurrent in case of a faulted autonomous AC microgrid operation. Primary layer involves a Finite Control Set-MPC (FCS-MPC) for reference voltage tracing and droop control with Proportional-Integral (PI) control for power sharing between DGs. Unscented Kalman Filter estimator with MPC-based Voltage control and a communication-less event time-dependent protocol for frequency control are proposed for voltage restoration along with frequency supervision as secondary control stage in islanded DGs operation. The performance under faults of proposed controller is compared with that under conventional hierarchical control and MPC unmodified control. Under the mentioned new strategy, the AC islanded microgrid operation stability is enhanced.

Power-Flow-Based Secondary Control for Autonomous Droop-Controlled AC Nanogrids With Peer-to-Peer Energy Trading

Regarding the control of micro- and nanogrids, LC- or LCL-filtered power inverters (acting as interfaces with distributed energy resources such as photovoltaic or wind) commonly perform as grid-forming or grid-supporting units to maintain both the frequency and voltage within pre-set standards. Nevertheless, these power inverters are assumed to be connected at the same point of common coupling directly or via radial feeders; thus, the voltage references are the same for each parallel power inverter, thus requiring a virtual impedance loop. In addition, classic power sharing techniques comply with their individual power rates, and circulating currents among distributed generators are not considered. Under these circumstances, energy trading among prosumers and peer-to-peer contracts is not feasible in autonomous AC micro- and nanogrid operations. This paper proposes a reformulated power flow problem, adapted to autonomous droop-controlled AC microgrids, to be used as a secondary control layer. The entire hierarchical control is implemented and experimentally validated in a laboratory-scale nanogrid with energy storage systems, photovoltaic generators and power converters. The obtained results demonstrate the proper performance of the proposed approach, with successful operation of primary and inner controllers.

Resilient distributed control of BESSs and voltage source converter‐based microgrids considering switching topologies and non‐uniform time‐varying delays

In this study, a distributed resilient control (DRC) scheme is proposed for inverter-interfaced autonomous AC microgrids (MGs). Due to unavoidable telecommunication vulnerabilities, time-delays already exist in communication links, and it seriously impresses the stability and performance of the network. Due to this fact, non-uniform time-varying communication delays are considered on the communication links (here, information exchanges between neighboring local controllers of DGs and BESSs). The authors consider a time-varying topology for distributed energy resources to address the uncertainties of communications links, improve flexibility, and enhance the security of the system against cyber-attacks. Stability analysis is characterised based on the Lyapunov–Krasovskii theory, and sufficient conditions for a precise and comprehensive control algorithm are derived in terms of linear matrix inequalities (LMIs). The provided LMIs yield the upper bound of the time delay, which guarantees the stability of the system. Finally, to evaluate the performance of the control law, offline digital time-domain simulation studies are performed on a test MG system in a MATLAB/Simulink environment, and also the results are compared with previously reported methods.

A Cyber-Secure Distributed Control Architecture for Autonomous AC Microgrid

Security of various cyber-physical systems is a major concern for researchers worldwide. Nowadays, microgrids also form such cyber-physical system to achieve a number of objectives such as enhanced frequency regulation, economic active power sharing, proper reactive power sharing, etc. Communication links are essential in such microgrids to ensure bilateral flow of data to be fed to the secondary controllers of the distributed generators integrated to the power network. This article, therefore, studies the impact of various kinds of cyberattacks viz., false data injection, denial-of-service, and replay attack on the performance of these communication enabled secondary controllers having potential vulnerabilities. Additionally, it presents a unique framework to ensure system stability when communication links are subjected to such attacks. The analysis has been carried out in a microgrid operating in islanded mode where the controller area network (CAN) bus communication network is utilized to form the cyber-physical system. Simulations are performed in MATLAB/Simulink platform and real-time evaluation of the proposed framework has been carried out in a test bed having OPAL-RT (OP5600) simulator and physical CAN devices to form the communication links.

Resilient Synchronization of Voltage/Frequency in AC Microgrids Under Deception Attacks

This article proposes a resilient distributed secondary voltage and frequency control scheme for autonomous ac microgrids considering disturbances and attacks on both sensors and actuators. To achieve the main objectives of the control system, first, a distributed state observer is employed to predict the normal behavior of state variables in the presence of potential deception attacks. Then, a distributed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${H_\infty }$</tex-math></inline-formula> controller is utilized to mitigate the impacts of disturbances and uncertainties. Finally, to overcome the detrimental effects of attack signals, a resilient distributed adaptive algorithm is integrated with the designed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${H_\infty }$</tex-math></inline-formula> controller. The proposed control algorithm has no restriction on the number of agents under attack and guarantees the boundedness of synchronization errors for all units. Also, in addition to distributed generation units, the contribution of distributed energy storage (DES) units in the regulation of voltage, frequency, and active power sharing is considered. Due to the limited energy of DESs, it is needed to balance the state of charge of these units. We evaluate the performance of the proposed algorithm using offline digital time-domain simulation studies carried out on a test microgrid system in MATLAB/Simulink environment, and the results are compared with previously reported methods. Simulation results and comparison with previous works reveal the effectiveness and accuracy of the proposed algorithm in regulating microgrid voltage and frequency.

Resilient H ∞ Consensus-Based Control of Autonomous AC Microgrids With Uncertain Time-Delayed Communications

In this paper, a resilient distributed $H_{\infty }$ consensus-based control scheme is proposed to be used in the secondary layer of the hierarchical control structure of AC heterogeneous autonomous microgrids (MGs). In the presented study, the external disturbance, the uncertainty on the communication links, and fixed time-delays (FTDs) are all considered as the destabilizing components of the time-varying topologies network. So, by applying $H_{\infty }$ theory to the reduced-order model of the system, sufficient conditions for the stability and robustness of the proposed controller are outlined based on Lyapunov-Krasovskii theory (resulting in the feasibility of linear matrix inequalities (LMIs), which yields the upper bound of time-delay that guarantee the stability of system), so that the $H_{\infty }$ performance index for uncertainty and disturbance are satisfied. Finally, to evaluate the performance of the control laws, offline digital time-domain simulation studies are performed on a test MG system in MATLAB/Simulink, and the obtained simulation results reveals the effectiveness, efficiency, authenticity, and accuracy of the proposed method in regulating MG voltage and frequency, providing accurate proportional active power-sharing, and state of charge (SoC) modification (balancing).

Interconnected Autonomous ac Microgrids via Back-to-Back Converters—Part II: Stability Analysis

In this article, the stability of voltage source converter-based autonomous ac microgrids (MGs), which are interconnected through back-to-back converters (BTBCs), is analyzed. The small-signal stability analysis is based on a detailed, comprehensive and generalized small-signal modeling of the ac interconnected MGs (IMGs), which is possible for any number of MGs and interconnections. The large-signal stability of the IMGs is investigated for the case of the initial BTBC dc voltage as a part of paper contribution. A new margin/criterion is determined for the initial dc voltage in different situations of the BTBC operation. According to the proposed criterion, a fundamental difference between very weak MGs and conventional strong grids in the BTBC voltage stability is addressed. Using eigenvalue analysis and participation matrix, the main participating state variables and corresponding parameters in the dominant critical modes are recognized for an equilibrium point. Sensitivity analysis involves changing initial values of the state variables, parameters, and forcing functions to study their different values and find acceptable ranges of the parameters. Particularly, the considerable contribution of the BTBC in the critical modes is found out by analyzing the initial dc voltage, dc voltage controller and PLLs. In order to observe possible unstable situations and verify the transient studies, real-time simulations are provided for two and three MGs interconnected through BTBCs using OPAL-RT digital simulator. The IMGs can be robustly stable only by specifying the stabilizing ranges of the sensitive parameters to the critical modes and selecting their appropriate values.