AC Microgrid System Research Articles

Improvement of an AC microgrid system’s stability by using a fuzzy logic-based PSS controller

This research article presents the low-frequency oscillation damping control of an AC Microgrid. It is integrated with four distributed generation units in which two DGs are renewable energy source based solar PV and wind energy using DFIG and the other two DGs are conventional synchronous generator-based hydro and diesel generators. There is insufficient generation through renewable-based DG then a diesel generator is employed as an emergency unit to fulfil the requirements and maintain and control the deviation of the frequency of the system. The use of renewable-based DG affected the system's dynamic stability and the characteristics of an integrated microgrid differ from conventional sources of generation. This investigation explores the impression of the deployment of multiple forms of the power oscillation damping (POD) controllers to distributed generation sources of grid forming AC Microgrid. The main various types of POD controllers are conventional lead-lag PSSs, MBPSS-4B, and proposed robust fuzzy logic PSS (FLPSS) controllers are implemented on distributed generation units of AC Microgrid for the elimination of low-frequency oscillation. Low oscillation frequencies between 0.1 and 2 Hz are the focus of this work. The system's damping ratios and stability margins because of MG integration are observed using eigenvalues analysis. The results of the simulation test demonstrate the accomplishment of the robust fuzzy logic PSS controller regarding conventional lead-lag PSSs and the MBPSS-4B controller during circumstances of fault.

Design of adaptive protection coordination scheme using SVM for an AC microgrid

Microgrids have gathered a significant amount of attention within the past decade and becoming an essential asset in the energy industry. The ability to integrate sustainable renewable energy generation into the distribution network is one of the main reasons for microgrids' popularity. Besides technical advantages, renewable energy sources integration with the grid poses many technical challenges. Due to randomness in generation and different modes of operation, integrating renewable energy sources into power systems will be challenging. Reliable operation of renewable energy sources integrated microgrids even when faults occur, requires an adaptive protection scheme and relay coordination. In order to tackle those issues, this paper developed an adaptive protection coordination scheme using numerical overcurrent relays and support vector machines with a particle swarm optimization approach. Renewable energy sources based on microgrids use power electronics systems that require relays to operate in milliseconds, and the power system will be safe. particle swarm optimization's program execution time exceeds the primary relays. The machine learning algorithm is faster in computation time. This article proposes a method, particle swarm optimization, to estimate the optimum relay operating time. The machine learning model having a support vector machine is proposed to estimate the relay operating time. The proposed research is developed and tested on an IEEE12BUS and IEEE35BUS system. The support vector machine -based on adaptive protection coordination schemes estimates the operating time of directional overcurrent relays, with a lesser program execution time and a low percentage error rate. The derived result defined that the proposed approach is well-suitable for real-time applications of an AC microgrid system.

Fractional Order Controller For Power Control in AC Islanded PV Microgrid using Electric Vehicles

Background: Microgrids conquer a significant role in the evolution of distributed and modern grids from the traditional electricity system. However, microgrids with renewable energy sources connected to them often incur grid instability issues, due to the intermittent nature of these sources. Objective: This work aims to study Microgrids with Electric vehicles as a backup energy source and maintain the system’s frequency that can overcome this issue. Methods: This paper uses an autonomous control algorithm in an islanded ac microgrid to regulate the active power depending on the irradiation and load scenarios, thereby maintaining the system frequency and stability. The controller also keeps track of the battery's charge level, keeping it from overcharging or over-discharging conditions. The PI (Proportional Integral) and Fractional Order Proportional Integral (FOPI) controllers were compared, with the best controller utilized for system simulations. Results: Simulations are presented with MATLAB/Simulink for an Islanded Photo Voltaic AC microgrid system with the electric vehicle's battery connected to it as a source of backup energy. The system's effect is exhibited under varied irradiations and load levels, and the findings demonstrate the control algorithm's adaptability. Conclusion: This work attempts to discover the capability of the control technique to maintaining the stability of an AC islanded microgrid system under diverse irradiation and load situations, thereby maintaining the system's frequency and the State of Charge (SoC) of the battery of an electric vehicle under specified levels.

Optimized robust control for improving frequency response of delay dependent AC microgrid with uncertainties

Converter-interface-based renewable energy incorporated into modern power systems causes deterioration in power system stability and it is proving to be a main challenge for operators using sluggish traditional generation with a growing share of converter-interface generation, and there is a need for an alternative means to deliver rapid frequency control. This paper proposes two control loops as secondary frequency control, first is an improved optimized delay-dependent frequency control to ensure robust frequency control under daily changes in system parameters that may occur in prospective, renewable-rich future AC microgrids. For a variety of operating conditions, the design process employs a mixed H2/H∞ robust control based on linear matrix inequality (LMI). In second, a speedy-acting improved power stability control loop is utilized as the proposed secondary frequency control, because demand response aggregators can be beneficial for frequency control. Robustness is evaluated against several perturbations and communication delay attacks through dynamic simulation and stability is evaluated by small-signal analysis. Different case studies subject to generation\\load loss, multiple time delay and parameter uncertainties are demonstrated on an interconnected AC microgrid system to verify the performance of the proposed control technique.

Real-time analysis of islanding detection scheme developed for AC microgrid system

Real-time analysis of islanding detection scheme developed for AC microgrid system

Distributed Resilient Secondary Control for AC Microgrid Under FDI Attacks

This paper is concerned with both voltage and frequency regulation problem for AC microgrid under false data injection (FDI) attacks in the secondary control layer. A distributed convergent observer is first designed based on an iterative mean estimation technique, which can accurately estimate the attack signals with theoretical proof of convergence provided. Based on the iterative mean estimation information, a distributed resilient controller is designed to compensate for the effect of cyber attacks in the secondary control layer. It is theoretically shown that the proposed strategy can restore accurate regulation of the voltage and frequency of AC microgrid in the presence of FDI attacks in the secondary control layer. Finally, an AC microgrid system containing four distributed generations and two leader nodes is constructed in MATLAB/Simulink for simulation, and the results validate that the proposed controller is effective for regulating voltage and frequency.

DC Link Voltage Enhancement in DC Microgrid Using PV Based High Gain Converter with Cascaded Fuzzy Logic Controller

Renewable-based sources can be interconnected through power electronic converters and connected with local loads and energy storage devices to form a microgrid. Nowadays, DC microgrids are gaining more popularity due to their higher efficiency and reliability as compared to AC microgrid systems. The DC Microgrid has power electronics converters between the DC loads and renewable-based energy sources. The power converters controlled with an efficient control algorithm for maintaining stable DC bus voltage in DC microgrids under various operating modes is a challenging task for researchers. With an aim to address the above-mentioned issues, this study focuses on the DC link voltage enhancement of a DC Microgrid system consisting of PV, DFIG-based wind energy conversion system (WECS), and battery Energy Storage System (ESS). To elevate PV output voltage and minimize the oscillations in DC link voltage, a high-gain Luo converter with Cascaded Fuzzy Logic Controller (CFLC) is proposed. Droop control with virtual inertia and damping control is proposed for DFIG-based WECS to provide inertia support. Artificial Neural Network (ANN) based droop control is utilised to regulate the ESS’s State of Charge (SOC). The effectiveness of the proposed converter and its control algorithms for maintaining stable DC bus link voltage has been analysed using MATLAB/Simulink and experimentally validated using a prototype model and FPGA Spartan 6E controllers.

Fractional order [formula omitted] controller for microgrid power system using cohort intelligence optimization

Microgrids are the future of energy management systems. Effective deployment and integration of microgrids can provide energy independence to many communities. The effectiveness of a microgrid primarily depends on its stability and reliability. Fractional regulators offer promising solutions to ensure stable microgrid performances. This article proposes a Fractional order Proportional–Integral–Derivative (FOPID) based secondary Load Frequency Control (LFC) for single area and dual-area AC microgrid systems. Cohort Intelligence (CI) based optimization was employed to tune the five major gain values of fractional PID regulators. The performance of the proposed CI tuned FOPID regulator was compared with that of the traditional PID controller tuned by Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) on the same single and dual control area microgrid systems. Four different cases of AC microgrid simulations were explored for LFC performance assessments and effective comparisons among different controllers and tuning algorithms. The first case considered load changes in single area AC microgrid with and without distributed energy system. The second case considered load variations in the two area AC microgrid system. The third and fourth cases included variations in random step load and system parameters respectively. Primary results affirm the superiority of the CI tuned FOPID regulator over its GA and PSO tuned PID counterparts in terms of its better steady-state precision, expressed in terms of the Integral Time Absolute Error (ITAE) selected as the objective function.

Overview of Single-Stage High-Frequency Isolated AC–DC Converters and Modulation Strategies

The single-stage high-frequency (HF) isolated ac–dc converter has the advantages of high-power density, long life, and high efficiency. It has a broad application prospect in distributed power generation, ac microgrid, and energy storage system. In this article, its topologies are classified according to the energy storage components. The structure and operating principle of various topologies are introduced. Furthermore, the development of extended multiport single-stage HF isolated ac–dc converter is introduced. On this basis, the advantages and disadvantages, typical characteristics, cost, and power loss of various topologies are analyzed and summarized. Finally, the key issues that need to be solved are pointed out and the development trends are explored. This article aims to provide a reference for the subsequent research on single-stage HF isolated ac–dc converters and the advancement of their practicability.

Decentralized secondary control for frequency restoration and power allocation in islanded AC microgrids

In this paper, we show that a fully decentralized simple control method can be used to solve the secondary frequency restoration and power allocation problem in AC microgrid (MG) systems. Compared to communication enabled control schemes, this paper makes a breakthrough of realizing frequency restoration under a decentralized secondary control by which no data exchanging process happens and thus, MG reliability and robustness can be enhanced. Different from most of existing decentralized control approaches, rigorous theoretical analysis proves that the proposed simple decentralized controller, which is easier for implementation, can still ensure the stability of the overall AC MG system, achieve frequency restoration and accurate optimal power sharing flexibly. In order to illustrate the proposed scheme, real-time tests are carried out in an islanded AC MG system built in the real-time simulator OPAL-RT. The results do show its effectiveness and also verify the established theoretical findings.

ANFIS-Based Droop Control of an AC Microgrid System: Considering Intake of Water Treatment Plant

Provision of an efficient water supply system (WSS) is one of the top priorities of all municipals to ascertain adequate water supply to the city. Intake is the lifeline of the water supply system and largely effects the overall plant efficiency. The required power supply is generally fed from the main grid, and a diesel generator is commonly used as a power backup source. This results in high pumping cost as well as high operational cost. Moreover, due to operation of motor pumps and other auxiliary loads, frequent maintenance is required. Therefore, to avoid various challenges and to efficiently operate the intake system, microgrid concept has been introduced in this paper. Various distributed generations (DGs) such as solar photovoltaic (PV), interior permanent magnet machine (IPM) wind turbine generator and Battery energy storage system (BESS) are incorporated in the microgrid system. Additionally, a new approach based on adaptive neuro-fuzzy inference system (ANFIS) is proposed, where P-f and Q-V droop is considered while training the ANFIS data; after successful training, the microgrid voltage and frequency are controlled as per system requirement. Simulation of the microgrid system shows good results and comparison with the generalized droop control (GDC) method is done using MATLAB/Simulink software.

Seamless Operation of Master–Slave Organized AC Microgrid With Robust Control, Islanding Detection, and Grid Synchronization

In this article, islanding detection, control, grid synchronization, and power share techniques have been considered for the seamless operation of ac microgrid in grid-connected mode, islanded mode, and during the transition between both modes. A master–slave organization coordinates the power distribution among microgrid sources. The control scheme of each source is designed with a nested loop robust linear quadratic regulator and mixed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H_{2}/H_{\infty }$</tex-math></inline-formula> optimal controller. The islanding detection algorithm is developed with synchro-extraction transform and optimal support vector machine (OSVM). An optimal linear Kalman filter-based synchronous reference frame phase locked loop (OLKF-SRFPLL) method is considered for the reconnecting of the microgrid to the utility grid. For optimal parameter tuning in the OSVM and LKF-SRFPLL methods, a particle swarm optimization technique is used. The seamless operation of the proposed ac microgrid system is verified in MATLAB/SIMULINK, and the practical feasibility is verified with an experimental photo-type setup and dSPACE 1202 control kit. Simulation and experimental results are presented to illustrate the seamless operation of the proposed ac microgrid system.

Distributed Adaptive Fault-Tolerant Control for Heterogeneous Multiagent Systems With Time-Varying Communication Delays

This article considers the distributed adaptive fault-tolerant control problem for heterogeneous linear multiagent systems with actuator faults and nonuniform time-varying communication delays. First, novel distributed switching observers are proposed to estimate the system matrix and the state of the exosystem. The observers allow each agent to share its information with its underlying neighbors only at sampled instants of time, thus making a distinct difference from the existing results that require the exosystem matrix to be accessible to all agents at every continuous instant of time. Second, a sufficient condition is derived such that the observer error systems are exponentially stable under the influence of communication delays among interacting agents. Third, new distributed adaptive fault-tolerant controllers, which promise fewer adaptive parameters, are presented to compensate for the actuator faults. It is shown that the developed controllers are capable to effectively and efficiently solve the cooperative fault-tolerant output regulation problem with reduced computational complexity. Finally, three illustrative examples, including an inverted pendulum system, an electronic double-integrator circuit system, and an AC microgrid system, are presented to show the validity and efficiency of the developed method.

The Single-Phase Voltage and Power Control Algorithm of a 4-Leg Type CVCF Inverter for an Off-Grid Micro-Grid System

In general, severe load imbalances in small AC micro-grid systems can degrade their operational performance and their maintenance. This is because the unbalanced load in the micro-grid affects the energy flow and the voltage regulation functions of each phase. In order to solve the voltage imbalance problem, several algorithms for the 3-phase 4-leg CVCF inverter have been proposed, but the control algorithms are not enough to operate the 4-leg CVCF inverter in a stable manner. Therefore, this paper proposes a single-phase voltage and power control algorithm for the 3-phase 4-leg CVCF inverter based on a dq control in order to improve the voltage imbalance problem caused by a severely unbalanced load, where the single phase voltage control algorithm is composed of an αβ-dq and a dq-αβ transformer, a voltage and a current controller, and an off-set controller and a PWM, and the single-phase power control algorithm is also composed of an αβ-dq and a dq-αβ transformer, an active/reactive power and a current controller, and an off-set controller and a PWM. Additionally, this paper performs modeling of the single-phase voltage and the power controller for a 4-leg CVCF inverter using the Matlab/Simulink S/W. From the simulation results, it is confirmed that the transient stability of the proposed single voltage control algorithm can be improved compared to the conventional control algorithm, and voltage control can also be maintained in a stable manner under extremely unbalanced conditions. Further, it is confirmed that 3-phase currents of the proposed single-phase power control algorithm are controlled in a stable manner under extremely unbalanced conditions.

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.

RES Based Islanded DC Microgrid with Enhanced Electrical Network Islanding Detection

An electrical islanding detection method for DC microgrid (MG) is proposed in this paper. Unlikely conventional AC MG system protection has been challenging for the DC MG system. The goal of the proposed scheme is to detect the islanding intelligently within the agent nodes. The proposed islanding detection technique detects the electrical link failure intelligently, and if any electrical link failure occurs, then the proposed technique maintains the MG operation and load sharing. Islanding detection is carried out using an injection signal and utilizes the existing communication network to check network connectivity. After checking the network connectivity, all the nodes maintain the look-up table of the connected network. This research work illustrates that the proposed electrical islanding detection technique is effective in maintaining the DC MG operation in the case of an electrical islanding/link failure scenario. The proposed scheme’s performance is checked through MATLAB/Simulink for the detection of islanding and maintaining the operation.

Estimation of Non-Synchronous Inertia in AC Microgrids

This paper proposes a comprehensive method to estimate non-synchronous inertia in an AC microgrid system by analyzing measured frequency transients. Non-synchronous inertia includes inertial support from various inverter-interfaced renewable energy sources and energy storage devices in the form of virtual or emulated inertia. In the proposed method, a multi-step approach is used to identify a matrix of parameters that estimates the overall non-synchronous inertial response. Firstly, motivations are drawn from the fundamental system requirements of adequacy in RoCoF and maximum deviation in frequency, for any credible contingency. A systematic classification of various contributors to the non-synchronous inertial response is also carried out. System-identification studies are then performed to identify various component-models that contribute to the non-synchronous inertia. The component models include a faster RoCoF-based response model, a relatively slower frequency deviation based model, and in some cases, a slow, integral-of-frequency based model. Lastly, the concept of releasable kinetic energy and non-linear estimation is used to identify the capability of the virtual inertia sources and improve the accuracy of the overall estimate. The developed methodology is tested in a microgrid network hosting a mix of renewable energy generations and battery energy storage systems. The estimate of non-synchronous inertia will help system operators to accurately estimate the maximum frequency deviation and the time to reach the maximum deviation for any credible contingency. The estimate can also help in deciding the type of virtual inertia that will best maintain frequency stability in the microgrid system.

Modeling and Stability Analysis of a Novel Voltage-Oriented Power Coordination Controlled Constant-Frequency AC Microgrid System

The conventional AC microgrid power regulation method is achieved using dual undulations of the grid-side voltage and frequency, whose complex variables and processes make the system less efficient and stable. For this problem, this paper proposes a novel two-hierarchical small signal model stability analysis for islanded AC microgrid systems with a designed power regulation algorithm under the constant frequency, mainly aimed at the continual switching microgrids, and considers the future expansion. MATLAB/Simulink simulations and experiments are conducted to validate the feasibility. It is found that there is a strong consistency between the stability of primary and overall systems, and that two key parameters affect the stability properties. Discussing the mutual influence of various parameters, k1 is positively correlated with control intensity and k2 determines the proportion of reactive power distribution in each power generation. Contrasting with the previous complex processes, the unique points of this method are the simplicity of calculation, parameter induction, response testability, strong operability, and system extensibility. The conclusion is that this constant frequency power control is simple, feasible, and stable under several specified conditions.

Synthetical Optimal Design for Passive-Damped LCL Filters in Islanded AC Microgrid

In AC microgrid systems, technical issues resulting from the power control effect are the power control method efficiency and the filter resonance since voltage-frequency interference and filtering resonance may cause significant control input reduction and oscillation. A simple and robust solution is to regulate active power flow by adding an optimal passive damping resistor. The optimal LCL filter parameterization process in this paper provides an innovative approach to obtain the optimal quality factor and the optimal damping resistor. The amplitude responses of resonant frequency and high frequency are involved in the cost function. Simulations of various cases are conducted in MATLAB/Simulink. The key advantages of this procedure include flattened admittance peak, weakened high frequency decay, and less damping loss. Through the method in this paper, the optimal passive-damped filter parameters can be calculated in an efficient manner, while the power can be regulated and tracked in a simplified way, and the performance of the control strategy is improved.

Review of distributed generator integratedACmicrogrid protection: issues, strategies, and future trends

In the recent power system scenario, the concept of microgrid is evolving rapidly. The architecture should be robust enough to cater the complexity of integration of distributed generation sources, demand-side management, and storage. The fast growth of embedded generation with other advanced power electronic interfaces and components along with energy storage devices change the operating pattern of the integrated power system scenario. In this context, protection is the vital aspect for qualitative uninterrupted power and making the system resilient to adverse, and faulty conditions. Many associated issues, particularly related to AC microgrid systems like the large difference in the short-circuit level between integrated and isolated mode of operation, bidirectional power flow, unsynchronized reclosing, the blindness of protection, lack of natural zero-crossing current, and false tripping are needed to be focused. In light of the above factors, it motivates us to survey the previous research works on the existing protection strategies to redesign and present the critical analysis of the predicted protection issues of microgrid protection with upcoming advanced technology and methodology. This article presents a critical review of the issues and protection schemes for the DG integrated AC microgrid extensively. Here, the merits, and demerits of each protection scheme, and visible possibilities for any advancement in protection strategies to enhance the reliability, selectivity, and security of AC microgrid is emphasized keeping in mind the different connections and configurations. In addition, this work also focuses on integrating the intelligent approaches/devices that help to elevate the current protection schemes to be smarter and efficient.