Islanded Mode Research Articles

Strengthening distribution systems after earthquakes with a new analytical model

This study highlights that earthquakes can lead to short-circuit faults, power outages, and equipment damage in distribution systems. Based on the observation of the effects of two major earthquakes that occurred in Turkey (February 6, 2023), a new analytical model for strengthening distribution systems against natural disasters is proposed in this study. In this model, the distribution system is analyzed, and the fragility index of the buses is determined. The technical condition of the distribution system is assessed using fragility indexes, and integration and sizing of Distributed Generation (DG) and energy storage units are performed to ensure energy continuity after an earthquake. This fragility index takes into account both the operation of the distribution system in island (microgrid) mode and its operation connected to the main grid. In addition, with the fault index recommended within the scope of the analytical model, the vulnerable points of the distribution system against short circuit faults are analyzed and short circuit fault currents are limited at these points. Thus, the resilience of distribution systems against natural disasters such as earthquakes can be enhanced with the proposed analytical model.

Hierarchical-power-flow-based energy management for alternative/direct current hybrid microgrids

Modern microgrids are systems comprising both Alternative Current (AC) and Direct Current (DC) subgrids, integrated with Distributed Generations (DGs), storage systems, and Electric Vehicles (EVs) parking facilities. Achieving stable and reliable load flow control amidst varying load, generation, and charging/discharging strategies requires a hierarchical control scheme. This paper proposes an hourly power flow (PF) analysis within an Energy Management System (EMS) for AC/DC Hybrid Microgrids interconnected via an Interlinking Converter (IC) in both grid-connected and islanded modes. The framework operates within a two-level hierarchically controlled platform. Tertiary control at the top level optimizes DGs' reference power for generation and consumption, minimizing power purchase costs and load shedding in grid-connected and islanded modes, respectively. DG converters employ current control mode to share their power references as the primary controller. While no secondary controller is adopted in this scheme, the Battery Energy Storage System (BESS) in islanded mode utilizes P/Q droop control to maintain voltage and frequency in the AC subsystem. Power sharing between AC and DC subgrids through IC is determined by the difference between AC grid frequency and DC link voltage. Integration of controlled converters’ buses into PF equations enables solving the unified system using the traditional Newton-Raphson (NR) method. A segment of a real distribution grid planned for installation in Italy under the HYPERRIDE project serves as a case study. Comparison with MATLAB/Simulink results confirms the effectiveness, precision, and convergence speed of the proposed model and control schemes, demonstrating efficient load distribution and voltage/frequency restoration in islanded mode.

Smooth Transition Control Strategy for Operation Modes of Optical Storage Microgrid based on VSG

Addressing the challenge of suppressing voltage and frequency oscillations during mode transitions and achieving smooth transitions between grid-connected and islanded operation modes in peer-to-peer structured optical storage microgrids, is an urgent technical issue. This paper proposes a control strategy for smooth mode transitions based on numerical soft starters, focusing on the dynamic characteristics of voltage and frequency in optical storage microgrids during grid-connected and islanded operation modes. The numerical soft starter smoothens the input reference value of the current inner loop, reducing the impact of transient surge current on the current inner loop during transitions, thus ensuring the oscillations of bus voltage and frequency during microgrid mode transitions. In islanded operation mode, a VSG control strategy is introduced by constructing a small signal model to analyze the frequency response characteristics of the system under different inertia and damping characteristics, setting appropriate control parameters to provide strong frequency support for the system during islanded operation. Finally, an optical storage microgrid system is established using Matlab/Simulink to conduct simulation experiments under conditions such as grid-connected/islanded mode transitions, load fluctuations, and sudden changes in photovoltaic output power, validating the effectiveness of the proposed smoothing strategy.

A Probabilistic Approach to Adaptive Protection in the Smart Grid

Smart grids are critical cyber-physical systems that are vital to our energy future. Smart grids’ fault resilience is dependent on the use of advanced protection systems that can reliably adapt to changing grid conditions. The vast amount of operational data generated and collected in smart grids can be used to develop these protection systems. However, given the safety-criticality of protection, the algorithms used to analyze this data must be stable, transparent, and easily interpretable to ensure the reliability of the protection decisions. Additionally, the protection decisions must be fast, selective, simple, and reliable. To address these challenges, this paper proposes a data-driven protection strategy, based on Gaussian Discriminant Analysis (GDA), for fault detection and isolation. This strategy minimizes the communication requirements for time-inverse relays, facilitates their coordination, and optimizes their settings. The interpretability of the protection decisions is a key focus of this paper. The method is demonstrated by showing how it can protect the medium-voltage CIGRE network as it transitions between islanded and grid-connected modes and radial and mesh topologies.

Current-Controlled Synchronverter: A Grid Fault Tolerant Grid Forming Inverter

A Synchronverter (SV) equipped with an Output Admittance Synthesizer is modified to avoid the usual transient instability of grid forming inverters in presence large grid disturbances, keeping the injected currents at safe values. The proposal consists of changing the control objective of the SV: instead of controlling the injected active and reactive powers, the injected active and reactive currents are controlled (as in a Grid Following Inverter). The resulting system [called here Current Controlled SV, (CCSV)], injects balanced currents, even against asymmetric faults. The CCSV is capable of operating both in island mode and in grid connected mode, and can switch between these modes without prior synchronization. Like the SV, the CCSV does not use a dedicated algorithm for synchronization (such as a PLL).

Stability Region of Grid-Forming Wind Turbine with Variable Parameters Using Bialternate Sum Matrix Approach

Although the stability regions of wind turbines in the islanding mode have been widely researched, small-signal modeling of grid-forming wind turbines (GFWTs) in the islanding mode has yet to be explored. In addition, the state-space matrix of the wind turbine system has yet to be fully represented. Therefore, this paper proposes a small-signal modeling of GFWT and a method for identifying the stabilization region of a system with variable parameters. First, small-signal modeling of a GFWT based on virtual synchronous generator control is developed. Second, the bialternate sum matrix approach is used to determine the system stabilization region. The system matrix with multiple variable parameters is first decomposed into the sum of several matrices in this paper. Furthermore, the rotor-side generator control is simplified. It can reduce the dimensionality of the system matrix model. Finally, the simulation shows that the proposed method for determining the stabilization region of the variable system is accurate.

Heterogeneous microgrids: Centralized control strategy with distributed grid-forming converters

Advanced microgrid (MG) is a likely model for reaching the goal of 100% renewable grid. A complete advanced MG control must steer the power flow in grid-connected mode; regulate voltage/frequency in islanded mode; and perform power sharing among distributed energy resources (DERs) in both modes. Power-based control (PBC) is a well-known centralized control capable of achieving these three targets. A serious disadvantage of usual MG structures using PBC approach is the existence of a single centralized grid-forming converter to form the islanded MG. Centralized grid-forming converter is a single point of failure, which reduces the capability of the MG expansion, and it is an expensive element for the MG. This paper proposes a MG control strategy using an improved centralized control together with a droop-based power-loop in distributed voltage-controlled mode (VCM) converters to allow the MG to operate in both modes, without a centralized grid-forming converter and critical islanding detection. Moreover, the new approach considers single- and three-phase DERs, controlled in VCM or current-controlled mode, and DERs with self-imposed limits, characterizing a heterogeneous MG. Real-time hardware-in-the-loop setup is used to test the proposed control strategy in several operating conditions considering ten heterogeneous DERs. Results show grid power flow control, power sharing, unbalance compensation, voltage/frequency restoration, and smooth transitioning between operating modes without critical islanding detection.

Fault identification, classification, and localization in microgrids using superimposed components and Wigner distribution function

The integration of Distributed Energy Resources (DERs) into distribution grids has become increasingly feasible and sustainable due to the development of microgrids. However, the development of an effective protection strategy remains a challenge in the implementation of microgrids. To address this challenge, this paper presents a simple and novel microgrid protection method based on superimposed components, Wigner distribution function (WDF) and alienation index-based. The proposed method develops a new fault detection index (FDI) by applying the alienation coefficient and WDF on a superimposed current signal to detect faulty events in the microgrid. The scheme is inherently phase segregated because the FDI is obtained for each phase individually. In addition, the proposed strategy introduces a new fault zone identification method based on the superimposed positive sequence reactive power (SPSQ). After obtaining the complete fault information, a relevant trip signal is generated to isolate the faulty section from the rest of the grid. The proposed methodology is evaluated through simulations using MATLAB/SIMULINK software. Various fault types, with varying parameters are simulated to validate the proposed approach. The results indicate that the proposed methodology is capable of recognizing, classifying, and locating all fault types in both grid-connected and islanded modes of operation.

Novel DC-link voltage regulation and seamless transfer control strategy of a battery-based microgrid system

The main challenge of modern microgrids (MGs) is the capability to operate in either grid-connected mode (GC) or islanding (IS) mode with DC-link voltage regulation for all the utilized parallel-connected DC-AC converters in the MG during all operating modes and, at the same time, is capable of transferring seamlessly between these two operating modes. This paper suggests a novel control strategy capable of operating a battery-based MG system in each of the GC and the IS modes and, at the same time, seamlessly transferring it between these two operating modes. A PQ controller is utilized to realize the GC operation, whereas a V/f controller is used for the IS operation. The smooth transfer between the two modes is accomplished by minimizing the transient variations in the MG voltage, current, frequency and phase at the point of common coupling (PCC). Additionally, the suggested strategy is qualified to deliver a novel regulation and a transient-free transition for the DC-link voltage of the used bi-directional DC-AC converter of the considered battery-based MG system. Therefore, the suggested control strategy can potentially improve the reliability of the considered battery-based MG system. The considered battery-based MG system is simulated and controlled by utilizing MATLAB/Simulink. The simulation results demonstrate that the suggested control strategy can effectively act during the operation of the considered battery-based MG system, either in the GC mode or the IS mode, and it can also effectively control the transitions between the two operating modes.

Research on Adaptive Droop Control Strategy for a Solar-Storage DC Microgrid

When the solar-storage DC microgrid operates in islanded mode, the battery needs to stabilize the bus voltage and keep the state of charge (SOC) balanced in order to extend the service life of the battery and the islanded operation time. When there are multiple energy storage units in the DC microgrid, it is necessary to solve the problem of unbalanced circulation and the state of charge between batteries using a reasonable droop control method. In this paper, firstly, the DC–DC charging and discharging circuit of the battery is designed, and the unbalanced SOC of the battery caused by the different impedances of the line is analyzed. Secondly, an adaptive droop control method is proposed to solve the problems of SOC imbalance and current circulation between the batteries. Thirdly, based on MATLAB/SIMULINK R2021b simulation software, the proposed control method is modeled and simulated. Compared with the traditional droop control, the effectiveness of the proposed method is validated.

DC-Link Dynamics Examination of the Parallel-Connected Three-Phase Inverters in Islanded and Grid-Connected Modes

This paper presents a comprehensive analysis of an AC microgrid setup, consisting of parallel three-phase inverters. It introduces a novel controller that integrates the tasks of controlling the DC-link voltage and implementing a current limiting approach. The design is informed by the characteristics of lead compensators and its main objectives are twofold: i) to regulate the DC-link voltage in order to minimize unwanted power exchange among the parallel inverters, and ii) to ensure the current-limiting capabilities of all inverters, regardless of the microgrid's operational state. This includes both standalone and grid-connected modes, voltage sags in the grid side, as well as transitional intervals between the two. In contrast to existing DC-link voltage control methods that do not incorporate a current limiting strategy, the proposed solution effectively restrains the dq axis current values in all operating conditions. This is crucial for preserving system stability, avoiding abrupt changes in DC-link voltage and current during sudden grid modifications and transitions. To validate the efficacy of the proposed controller, a series of simulations were carried out using Matlab/Simulink to evaluate its performance and compare it with an established method. The results explicitly demonstrate that the proposed controller successfully stabilizes the system, minimizes fluctuations in DC-link voltage, and restricts the dq axis current for each inverter without the potential of protection relay tripping.

Design of prototype for the short circuit protection of DC bus using Arduino Uno and DSP controller

Micro-grids (MGs) play a prominent role in the exchange of bulk power between loads and the central grid. MG can run in an islanding mode or grid-connected mode. When the central grid fails to provide power in such situations, MGs serves as power sources for critical facilities such as coordination facilities in railway stations and airports, transportation, university and research facilities. MGs use energy sources such as solar, wind, hydro, geothermal, and biomass to produce electricity along with rechargeable batteries, aircraft applications is continuously increasing. This leads to the development of DC MGs. A model of DC MG is located at the disaster management lab in the Department of Electrical and Electronics Engineering. The MG consists of step-downed and rectified DC power supply from conventional 230 V AC source, a PV array, wind, and a battery bank a source supplying power to a set of nine DC loads. Although DC MGs have many advantages compared to AC MGs, one of the challenges faced is the study of faults and protection of the DC MG. In this paper, a prototype for the existing system in the disaster management lab is designed in hardware and tested.

Grey Wolf Optimized Economic Load Dispatch Including Battery Storage In Microgrid

In last decades, grey wolf optimizer algorithm as a new meta-heuristic optimization technique plays major role in optimization of engineering problems such as load forecasting, controller parameter tuning and job scheduling. In this paper, grey wolf optimization (GWO) is used to optimize the microgrid system for effective dispatching of power to load with economic manner. The model of microgrid system components are developed and investigated in the MATLAB/Simulink platform. The vital objective of the proposed grey wolf algorithm is to minimize overall cost of the microgrid operation. The detailed investigation is carried out on power dispatch optimization and cost minimization for both modes, i.e. island and grid-connected modes of the microgrid system with considering the impact of running costs. From the analysis, the cost of the overall system is optimized effectively, and load sharing is done effectively by means of GWO.

Novel protection method for AC microgrids with multiple distributed generations using Unscented Kalman filter

Traditional protection methods were inadequate in multiple distributed generations (MDG) microgrids due to the bidirectional power flow/high fault current in the grid-connected (GC) mode, and reduced fault current in the islanded (ID) mode. Therefore, a reliable and robust protection method has become of paramount importance for such MDG-based microgrids. This article suggests an Unscented Kalman filter (UKF) to detect, classify, and locate faults in such MDG-based microgrids. Initially, the harmonic content is generated from the UKF-estimated current signal to compute the total harmonic RMS factor (THRF). Then the changes in the THRF of each phase are cross-checked to a pre-specified threshold level to distinguish the fault conditions. Furthermore, the cumulative covariance-dependent reactive energy (CCDRE) is computed from both the estimated state and covariance matrix of voltage and current signals provided by UKF. Finally, the directional trends in the CCDRE are used to identify the fault zone in the microgrids. The effectiveness of the proposed method is validated through ample simulations on CIGRE and IEC 61,850–7–420 microgrid test system via MATLAB® Simulink. The results illustrate that the presented strategy is robust in both the GC mode and the ID mode of operation in meshed and radial topologies with 99.9% accuracy.

Robust control for improving performance of grid-synchronized solar PV-BES-wind based microgrid

An integration of hybrid renewable energy systems (HRESs) to the grid results in power quality issues at the point of interconnection. This paper proposes an approach to investigate and mitigate power quality issues and uncertainties associated with integration of a HRES that includes a solar photovoltaic (PV) array, a wind generator, and battery energy storage (BES). The system functions in grid tied mode, standalone mode and seamless mode transition between these modes. Primary objective of proposed control approach is to ensure stable and reliable operation while mitigating effects of power quality issues. To achieve this objective, proposed control approach employs advanced control strategies, including generalized fourth order filter (GFOF) for grid tied mode and phase angle estimation. An off-grid mode uncertainty and disturbances are overcome by proportional resonant based control. GFOF addresses multiple power quality issues associated with grid-connected mode, including robust performance during voltage unbalance, harmonics suppression, DC offset removal, weak grid conditions, and evaluation of unit templates. GFOF extracts fundamental load currents and voltages for evaluation of reference grid currents and phase angle. This precisely estimates frequency and phase angle for seamless synchronization. It provides adaptive mode transition. Power quality of currents is enhanced in both islanded and grid-connected modes through utilization of GFOF control, ensuring that total harmonic distortions (THDs) of grid currents remain below 5% (simulated THD is 1.77% and experimental THD is 3.80%), as specified by the IEEE-1547and 519 standards. A comparative analysis has justified efficacy of control. Simulated and experimental results confirm effectiveness of proposed controllers in various scenarios, demonstrating their ability to provide a favourable solution for controlling unfavourable grid conditions with technical strength and precision.

Extensive assessment of virtual synchronous generators in intentional island mode

Virtual Synchronous Generators (VSGs) are considered one of the most effective cutting-edge technologies for seamlessly integrating Renewable Energy Systems (RESs) into power grids. While various VSG control schemes have been proposed and analysed for different technical aspects, not many experimental demonstrations have been published about VSGs operating in Intentional Island Mode (IIM). Inversely, the IIM has become increasingly attractive, nowadays, to energize those remote areas of the world where entire districts are served by a small number of RESs operating off-grid. In this scenario, the main objective of this paper is to present an extensive performance assessment of the VSG operating in IIM. Different working conditions are considered, with special focus on: black start capability, grid-connected-to-IIM transition, grid reconnection, feeding of non-linear and asymmetric local loads, load sharing between multiple VSGs operating in parallel. In order to achieve this objective, an improved structure of VSG is implemented on 8kVA three-phase converters and significant experimental results are presented. Algorithms for black start capability and grid synchronization are illustrated in detail as well. Meaningful information is provided for engineers involved in activity of designing and testing islanded VSGs.

The impacts of the transmission line length in an interconnected micro-grid on its performance and protection at different fault levels

Power systems, in recent years, have been experiencing a dynamic rise in the amount of power obtained from distributed renewable energy sources leading to the concept of microgrids to address the distributed power grid integration issues. Microgrids, a promising means of facilitating the green transformation of power systems, allow the union operation of distributed energy resources (DER) such as combined heat and power (CHP), renewables like photovoltaic (PV), wind and fuel cells (FC), energy storage systems, diesel generators, and controllable loads, either individually or in combination. The protection of DERs within microgrids can be considered as one of the main challenges associated with such phenomenon. Short and Long power transmission lines, in case of a fault, both have particular impacts on system parameters and may result into subsequent events threatening the microgrid and renewable generation units. On the other hand, The high penetration of microgrids not only can change the power flow within the power network, but it can also affect the fault current levels and may lead to their islanding in case of a fault. Before investing in microgrids, especially those in far places, this paper develops a tool to be used in investigating the influence of the interconnecting transmission line length as well as the type/severity of fault on the microgrid performance. The toolbox was developed using MATLAB/Simulink Toolbox. The developed tool was then validated on a case study microgrid and results show that the length of the interconnecting transmission line and the fault severity directly impact the microgrid performance (i.e. voltage and power deviations). In that case, interconnection or islanded mode is contingent upon the decision of the utility operator which also depends on the sensitivity of the equipment used in the microgrid.

Distribution State Estimation and Its Impact of Load Modeling

To maintain system security and other essential parameters like reliability and quality, continuous monitoring of the system is very important. Considering the distribution network, state estimation (SE) methods can be adopted. The purpose of the method is to identify and estimate unknown variables based on the online measurements of test data. The primary objectives considered in this paper are: To choose the exact SE method and the artificial neural network (back propagation algorithm), which can be used for determination in the islanding mode of distribution network states, the composite load model is considered for the estimation of states and further enhancement. By adopting the system, state variables in terms of error are measured in the 12-bus distribution network with precise measurements and compared with practical values. The SE proposed includes results with the load flow backward-forward sweep method to satisfy the system state variables. Numerical results indicate that the model performs better for error measurement data with states and in the case of state forecasting.

Multi-objective based Hybrid Artificial Intelligence Controlled Parallel Inverter in Islanded and Grid Connected Operations

Background: The Integration of non-conventional energy systems (NCES), like solar, wind, etc., into the grid with power electronic devices is adapted to meet the demand. Parallel connection of inverters (PCI) is an efficient method to boost power handling capacity, reliability, and system efficiency. However, the main drawback is the unequal power sharing among the inverters while using the conventional droop control technique (CDC). In addition, the circulating currents (CC) flow between these PCI, leading to common mode voltage (CMV), current waveform distortion, and reduction in the system's overall performance. Objectives: This work consists of a photovoltaic system (PVS) and battery energy storage system (BES) as the distributed generation (DG) unit to voltage source inverter (VSI) 1 and 2. The multi- objectives of the suggested work are (a) the equal power/load sharing among two inverters, (b) effective minimization of CC and the CMV, (c) maintaining constant DC-link (DCL) voltage during different solar irradiation and constant temperature, and (d) the reduction in total harmonic distortion (THD) of load current. Methods: A novel approach related to the enhanced droop control (EDC) method with an adaptive neuro-fuzzy hybrid controller (ANFHC) was suggested here to overcome the above issues. The performance analysis of the suggested technique was done in the Matlab/ Simulink platform with islanded and grid-connected modes for different loads. Results: A comparative analysis with the available methods like the proportional integral controller (PIC) and sliding mode controller (SMC) was carried out to exhibit the viability of the developed control technique. Conclusion: This study focused on the operation and control of a PCI with ANFHC in islanded and grid-connected modes to address issues such as uniform power sharing and reduction of CC and CMV.

Stability Analysis of Distributed Multi-Converter System

This paper discusses dynamic stability and performance of current control scheme for parallel connection of pulse-width modulated (PWM) voltage source converters (VSCs) fed by distributed energy recourses (DERs) across the distribution level of power grid. The controller has feedback compensation and limits the current harmonics while featuring fast regulation for multi-converter system. The controller enables islanded and grid-connected operations of DERs, dynamically. The load dynamics will be included in the stable control loop to provide perfect regulation of the voltage at the point of common coupling (PCC). The current controller is tested under the considerations of system delays. In the islanded mode, one of the controllable units provides voltage and frequency control as a master controller and the other units operate based on the grid-connected control strategy. The stability assessment of the system is presented based on a detailed simulation. The simulation results show that the controller for the islanded two-parallel controllable DER can ensure the stability of the system when converters are tied together. In the other hand, the weakness of the current control based method is drawn out while mismatch coupling condition.