Inverter-based Islanded Microgrids Research Articles

Virtual oscillator with delayed feedback for transient mitigation in inverter-based islanded microgrids

Virtual oscillator with delayed feedback for transient mitigation in inverter-based islanded microgrids

Optimized Unsymmetrical Per-Phase Droop for Soft Line Switching of Reconfigurable Unbalanced Inverter-Based Islanded Microgrid

Optimized Unsymmetrical Per-Phase Droop for Soft Line Switching of Reconfigurable Unbalanced Inverter-Based Islanded Microgrid

Power sharing enhancement through a decentralized droop-based control strategy in an islanded microgrid

A decentralized intelligent droop-based control strategy is proposed in this paper for the enhancement of equal active and reactive power sharing in an islanded inverter-based microgrid. Droop control is mostly preferred because it does not need communication facilities for its implementation. However, the presence of different feeder impedances for the different distributed generators (DGs) in a microgrid make reactive power sharing inaccurate. Furthermore, as a result of considerable load changes and different droop characteristics for the DGs, the inverter's output voltage and frequency are altered and these in turn alter the reactive and active power sharing respectively. With these conditions, the generalized droop control (GDC) strategy fails to effectively share load reactive and active power among the DGs. In order to extenuate these challenges, this paper presents a nonlinear autoregressive exogenous neural network droop-based control (NARX-NN DBC) strategy which does not depend on the varying line impedances, droop gains for the different DGs and is less affected by fluctuating loads in the grid. A microgrid, made up of two DGs and a load is modelled in MATLAB/Simulink environment and validation of the proposed control strategy is done through many simulations. Within the simulation runtime, NARX-NN DBC yielded a maximum frequency percentage deviation of 0.46% from the nominal value of 50 Hz whereas GDC yielded 0.62%. Regarding the voltage, NARX-NN DBC gave maximum deviation of 0.026% meanwhile GDC gave 0.079% from the nominal value for 380 V. In addition, during 0.57–0.64 s with load active power demand of 4.6 kW, NARX-NN DBC registered 0.43% power sharing error whereas GDC registered 6.5%. On the other hand, during 0.57–0.64 s, with 5kVAr load power demand, NARX-NN DBC registered 0.2% whereas, GDC registered 2%. These obtained results clearly show that NARX-NN DBC strategy has a better performance compared to GDC strategy with respect to power sharing in an autonomous microgrid.

Cyber–physical anomaly detection for inverter-based microgrid using autoencoder neural network

Contemporary cyber–physical systems have evolved into highly autonomous and distributed entities, enabled by cutting-edge control frameworks and advanced communication networks. This transformation has empowered these systems to efficiently accommodate inverter-based resources, harnessing the grid’s dynamic potential. However, the increased integration of cyber and communication infrastructure has exposed these systems to vulnerabilities stemming from both cyber and physical disturbances. Power system errors in the physical layer of microgrids and malicious attacks on the communication layer pose substantial threats to grid stability and load supply. In response to these challenges, this paper presents an innovative unsupervised learning method for cyber–physical anomaly identification. Leveraging an autoencoder-based neural network, this approach addresses the intricate interplay between the physical and cyber aspects of power system failures and false data injection within the communication network. The method’s efficacy is evaluated on an islanded inverter-based microgrid, representing a realistic and vulnerable testbed. The autoencoder error is introduced as a key performance metric, providing a robust and practical measure of the proposed detection technique’s effectiveness. Simulation results demonstrate the promising utility of this method, positioning it as a valuable tool for enhancing the resilience of contemporary cyber–physical systems against cyber–physical anomalies.

Droop-Free Sliding-Mode Control for Active-Power Sharing and Frequency Regulation in Inverter-Based Islanded Microgrids

This paper introduces a simple decentralized sliding-mode (SM) approach to control active power sharing by regulating the local frequency in inverter-based islanded microgrids (MGs). Its sliding surface arises from the frequency correction term introduced in the droop-free technique; it relates local active power to neighboring MGs’ active power by considering available communications among voltage source inverters. Then, this schema allows one to avoid hierarchical control just as the droop-free method does, and the benefits associated are twofold. First, it reduces the steady-state frequency error while providing accurate active power distribution. Second, the system stays reliable, withstands uncertainties, and provides a fast transient response. A Lyapunov analysis confirms stability, and simulations on a realistic four-inverter MG platform substantiate the control scheme’s effectiveness. Its performance regards frequency regulation while achieving active power sharing, stability, and robustness against clock drifts and load steps.

Centralized Model Predictive Control for Transient Frequency Control in Islanded Inverter-Based Microgrids

Centralized Model Predictive Control for Transient Frequency Control in Islanded Inverter-Based Microgrids

An Experimental Approach for Secondary Consensus Control Tuning for Inverter-Based Islanded Microgrids

A microgrid is a group of interconnected loads and distributed energy resources that can fill the gap between the dependence on a bulk power grid and the transition to renewable energies. The islanded mode presents itself as the most interesting scenario, when local controllers should maintain the power quality standards based on several parameters. A tool specifically focused on the process of parameter tuning of the secondary consensus-based control for inverter-based islanded microgrids was proposed in this paper. One often-quoted drawback in this process is the great number of parameters that must be tuned, even for a very simple microgrid structure. To manage such a large number of parameters, the design of experiments was used in this study. The main motivation for this work was to present an optimized way to define the correct parameters for the secondary consensus control for inverter-based islanded microgrids. The study shows how experimental design methodology can be an efficient tool to tune microgrid parameters, which are typically multi-objective-based experiments. From the results, it is correct to state that the design of experiments is able to reach the optimal setting with a minimal number of experiments, which would be almost impossible to obtain with the trial-and-error method.

Rate of Change of Direct-Axis Current Component Protection Scheme for Inverter-Based Islanded Microgrids

Rate of Change of Direct-Axis Current Component Protection Scheme for Inverter-Based Islanded Microgrids

Harmonic Dual-Setting Directional Overcurrent Protection for Inverter-Based Islanded Microgrids

Limited fault currents in inverter-interfaced islanded microgrids impose immense challenges on conventional overcurrent protection schemes. This paper proposes a sensitive and selective protection scheme for islanded microgrids using a third harmonic voltage generated by inverter-interfaced distributed generators (IIDGs). The generated harmonic voltage results in a harmonic layer formed during short-circuit faults and is decoupled from the fundamental fault current, i.e., limited by IIDGs. Further, the generated harmonic voltage is adaptively adjusted based on fault severity to enhance protection sensitivity and obtain a universal set of relays’ settings. The proposed protection scheme utilizes harmonic directional overcurrent relays (HDOCRs) equipped with a dual-setting time-current-voltage setting that sense the generated harmonic voltages and currents at the relay location to ensure optimal protection coordination (OPC) of islanded microgrids. The OPC with the proposed dual-setting is formulated as a constrained nonlinear program to determine the optimal forward and reverse relays’ settings. The proposed scheme is tested on the Canadian benchmark urban distribution system and compared to the conventional protection scheme, which relies only on a single time-current-voltage trip characteristic. The results ensure the ability of the proposed scheme to protect islanded microgrids without communication and its capability to reduce relays’ operation times.

A signed correlation index-based differential protection scheme for inverter-based islanded microgrids

The different fault characteristics of the inverter-based distributed generation (IBDG) units make the protection of the inverter-based islanded microgrids (IBIMGs) a formidable task. According to IEEE Std 1547.4, the limited fault current levels associated with the IBIMGs may result in misleading decisions of the traditional protective relays during fault occurrence. In this paper, a new differential protection scheme for the IBIMGs is introduced based on the signed correlation technique. The proposed scheme is formulated based on the signs of the instantaneous phase currents at both line ends instead of the current magnitudes. In addition, a signed correlation index is defined to measure the similarity between the signs of the instantaneous phase currents at both line ends employing only a time window of half-cycle. Moreover, a fault detection index is introduced to discriminate the internal faults from other events. In order to validate the proposed scheme, the 9-bus IBIMG and the modified IEEE 33-bus system are simulated on the PSCAD/EMTDC platform, and several scenarios are conducted, including load switching, outage of IBDG units, external bus faults, and internal faults. The obtained results emphasize that the proposed scheme operates correctly considering all types of faults, different fault resistances and inception angles, as well as measurement and synchronization errors.

Primary Voltage and Frequency Regulation in Inverter Based Islanded Microgrids through a Model Predictive Control Approach

A frequency and voltage control strategy based on a decentralized and communication-less approach is proposed in this work and applied to Photovoltaic-Storage-Microturbine islanded Microgrids (MGs). The approach is based on the Model Predictive Control (MPC) technique. Thanks to the use of local measurements, each source can nullify the steady-state voltage and frequency errors by means of a dedicated MPC controller. Consequently, the proposed approach unifies the advantages of classic droop and master/slave controllers due to the absence of communication links among devices and due to the absence of a secondary centralized control loop.

Complex Power Sharing Is Not Complex

This paper presents a distributed complex power sharing approach that solves the problem of active and reactive power sharing in inverter-based islanded microgrids. Previous state-of-the-art strategies based on droop controls with possibly virtual impedance methods and supplemented with hierarchical schemes may provide poor power sharing due to the R/X ratio and mismatched line impedances, even compromising microgrids stability if droop control parameters are not properly set. The novel control approach, that uses a communication network to exchange data among all inverters to fairly inject power, provides a set of appealing properties. First, it achieves accurate active and reactive power sharing for both resistive and inductive power lines with no additional control cost (avoiding unnecessary power injections), and regardless of load changes and connections and disconnections of inverters. Moreover, it ensures an exponential convergence rate, and the tuning of the control parameters can not compromise microgrid stability. The theoretical development relies on a change of variables that linearizes the complex power dynamics, thus easing the control law design task and providing the means for computing the appropriate amplitude and phase for each inverter output voltage. Selected experimental results on a laboratory microgrid certify the applicability and performance of the proposed control scheme.

Mathematical modeling of islanded microgrid with static and dynamic loads

Increasing the penetration level of distributed generation units as well as power electronic devices adds more complexity and variability to the dynamic behavior of the microgrids. For such systems, studying the transient modelling and stability is essential. One of the major disadvantages of most studies on microgrid modelling is their excessive attention to the steady state period and the lack of attention to microgrid performance during the transient period. In most of the research works, the behavior of different microgrid loads has not been studied. One of the mechanisms of power systems stability studies is the application of state space modelling. These studies include the development of state space models of various components of the power system and then linearizing them around an equilibrium point. In this paper, a comprehensive method for modelling of islanded microgrid with dynamic and static loads is presented. The basic step of the proposed method is transformation to a dq0-based model. In order to find a complete and accurate model of islanded inverter-based microgrid, the sub-modules of generation, network and load must be modelled in local dq reference and then transferred to a common reference. The simulation results show the effectiveness of the proposed modelling approach for transient stability studies.

Distributed Sampled-data PID Control for Voltage Regulation in Inverter-Based Islanded Microgrids Using Artificial Delays

Distributed Sampled-data PID Control for Voltage Regulation in Inverter-Based Islanded Microgrids Using Artificial Delays

Synthetic Harmonic Distance Relaying for Inverter-Based Islanded Microgrids

Faults in inverter-based islanded microgrids can be a formidable protection challenge due to (i) low fault current magnitudes, (ii) compromised fault current phase angles, and (iii) bidirectional flow of fault currents. This paper proposes a protection scheme that disregards the fault signals altogether. Instead, it relies on decoupled synthesized signals introduced only during fault conditions. This scheme is achieved by exploiting the existing inverter-based distributed generation (IBDG) controllers to inject synthetic harmonic voltages and currents. These synthetic signals are measured locally by digital relays in the microgrid to develop a novel synthetic harmonic distance relay (SHDR). Apart from the utilization of high-order harmonic signals that enhance SHDR reactance reach, further reach improvement is achieved via the introduction of line reactance magnifiers (LRMs). Transient studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults, contingencies, and different microgrid configurations.

A General Linear Framework for Analysis of Islanded Inverter-Based Microgrids

Simulation and stability analysis of power systems are challenging topics due to nonlinear models. Fortunately, most power components are weakly nonlinear and by taking the advantages of this characteristic and also using the first-order term of the Taylor expansion, a general linear time-variant model for such systems is derived. Specifically, this article analyzes islanded inverter-based microgrids (IBMGs) using the proposed linear model driven directly from a general nonlinear model of IBMGs, and it is just as comprehensive as the reference one. Two methods to linearize the weakly and severely nonlinear state-space models are presented. A new time-step simulation is developed to modify the model dynamically and to calculate time-domain response as well as eigenvalues directly, which are much faster and more efficient than numerical methods and small-signal stability analyses. The capabilities of the proposed models make it possible to easily develop a time-saving eigenvalue calculator as well as a fast- and low computational direct power flow analysis. Different time-domain simulations in MATLAB/Simulink and other tests are investigated to verify the performance of the proposed simulations and their applications.

A Linear Quadratic Regulator With Optimal Reference Tracking for Three-Phase Inverter-Based Islanded Microgrids

This article proposes a power sharing control method based on the linear quadratic regulator with optimal reference tracking (LQR-ORT) for three-phase inverter-based generators using inductor-capacitor-inductor (LCL) filters islanded mode. Compared to single-input single-output (SISO)-based controllers, the LQR-ORT controller increases robustness margins and reduces the quadratic value of the power error and control inputs during transient response. Supplementary loops are used to reduce frequency and voltage deviations in the ac bus without communications. The supplementary loop for voltage regulation is based on the droop controller by reducing direct and quadrature output voltages according to the active and reactive power demand. A model in a synchronous reference frame that integrates power sharing and voltage-current dynamics is also proposed. In addition, a methodology to develop an islanded microgrid model in a synchronous reference frame is proposed. Robustness analysis demonstrates stability of the LQR-ORT controller under variations in the frequency and the LCL filter components. Experimental results demonstrate accuracy of the proposed model and the effectiveness of the LQR-ORT controller on improving transient response and robustness in islanded mode.

Dynamic power flow based simplified transfer function model to study instability of low‐frequency modes in inverter‐based microgrids

This study proposes a new modelling approach for studying low-frequency oscillations (LFOs) in a droop controlled islanded microgrid. Due to the absence of inertia, these sources are more vulnerable to power and frequency oscillations. Their quick response can introduce faster electrical dynamics in the system which should be monitored from time to time. To ease the analysis process, this study proposes a simplified method for finding LFO in the microgrid. Inspired from the small-signal automatic generation control model of the conventional grid, a transfer function based closed-loop small-signal model of inverter-based islanded microgrid is presented in this study to study power-sharing among the various inverters. The proposed model uses the concept of dynamic power flow through the network to find the power output of each source following load perturbations in a system. Time domain simulation results and eigenvalue analysis is provided to verify the effectiveness of the proposed small-signal model. By comparing the results obtained with actual system simulation in MATLAB/Simulink, it is found that the proposed simplified model is able to predict the stability margin and the LFO of the system without using actual detailed state-space modelling procedure.

Distributed LMI‐based control of heterogeneous microgrids considering fixed time‐delays and switching topologies

In this study, a distributed cooperative control protocol for inverter-based islanded microgrids (MGs) is presented. The MG consists of distributed generation units and battery energy storage systems (BESSs). Despite most of the reported works which assumed the ideal conditions and did not considered any delay in information exchange between local controllers, the authors design a controller for the restoration of voltage and frequency fluctuations, achieving accurate active power-sharing and state of charge (SoC) balancing of BESS in the presence of fixed communication time-delay and changing topology of the network. The stability analysis is performed based on the Lyapunov–Krasovskii method, and sufficient conditions are presented based on linear matrix inequalities (LMIs) to guarantee the system stability and to reach consensus under arbitrary switching topologies. The upper bound of communication delay that the system can tolerate is given, too. 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 simulation results reveal the effectiveness, efficiency, authenticity and accuracy of the proposed method in regulating MG voltage and frequency and providing accurate proportional active power-sharing and SoC balancing.

Interharmonic Differential Relay With a Soft Current Limiter for the Protection of Inverter-Based Islanded Microgrids

Inverter-interfaced distributed generators (IIDGs) introduce a major challenge for the protection of inverter-based islanded microgrids (IBIMs) due to their limited fault current contribution. This paper proposes a new strategy for the protection of IBIMs, founded on two novel aspects including a soft current limiter and an interharmonic differential relay. In addition to limiting the fault current, the proposed current limiter performs an amplitude modulation of the output fault current from the IIDG which occurs only during fault conditions. This modulation produces interharmonic currents with specific signatures that flow towards the fault. The process of injecting interharmonics is spontaneously initiated by a subset of the IIDGs upon the fault occurrence without a need for a fault detection scheme. Thereafter, based on the locally detected interharmonics, an interharmonic differential relay requiring low bandwidth communication is proposed to identify and isolate the fault. The proposed strategy has been validated using PSCAD/EMTDC. The results confirm the ability of the proposed protection strategy to protect IBIMs considering different fault types, locations, and values of the fault resistance without the need for additional fault detection components or high-bandwidth communication which may constraint the applicability and reliability of the protection schemes.