Inverter-based Microgrids Research Articles

Survivability of Autonomous Microgrid During Overload Events

Grid-forming sources are voltage sources that draw necessary currents to meet any load changes. A load step can cause part or all of these sources to become overloaded in a microgrid. This paper presents an overload mitigation controller that addresses the two overload issues in a microgrid by actively controlling the sources’ frequency. When part of the sources in a microgrid is overloaded, the controller autonomously transfers the extra load to other sources by rapidly reducing its frequency. The frequency difference between sources during transient results in a change of phase angle, which redistributes the power flow. When all sources in a microgrid are overloaded, each source keeps dropping the frequency. Therefore, under frequency load shedding can be used to trip the non-critical loads resulting in the survival of microgrid. The advantages of these concepts are that communications between sources are not needed during transient, and the robust voltage control is maintained. Simulation and field tests from Consortium for Electric Reliability Technology Solutions/American Electric Power microgrid test site verify that the control strategy is effective in both purely inverter-based microgrids and inverter and generator mixed microgrids.

Mitigation of Power and Frequency Instability to Improve Load Sharing Among Distributed Inverters in Microgrid Systems

This paper presents the design of a power system stabilizer (PSS) for stability improvement of a droop-controlled inverter-based microgrid systems. For faster power sharing, high droop gains are required, but increasing the droop gains degrades the system stability. Hence, there is a tradeoff between the faster load sharing and system stability. An answer to the given problem is presented in this paper by developing a PSS for inverter-based microgrid. The proposed stabilizer provides sufficient damping to the low-frequency oscillation even at higher value of droop gains which are usually unstable otherwise. The PSS designed in this paper is a generalized one and a step-by-step method to select the PSS parameters is also presented. Time domain simulation, experimental results, and eigenvalue analysis are presented for verification of the proposed controller.

A Two-Level Hierarchical Controller to Enhance Stability and Dynamic Performance of Islanded Inverter-Based Microgrids With Static and Dynamic Loads

This paper proposes phasor measurement unit (PMU) supported two-level hierarchical controller to enhance stability and dynamic performance of islanded inverter-based microgrids with static and dynamic loads. The proposed hierarchical controller consists of a local decentralized controller for each inverter-interfaced distributed generation (IIDG) unit at the primary level helped by a multi-input multioutput centralized controller at the secondary level. The primary-level decentralized controller incorporates the addition of auxiliary control terms to the conventional droop controller. The auxiliary control terms are based on the total real and reactive power generation information acquired from wide area measurement system through PMUs and the desired power sharing of each IIDG unit. The secondary-level centralized controller, on the other hand, enhances the performance of each local decentralized controller by compensating the voltage and frequency deviations caused by the load disturbances. The performance of the proposed hierarchical controller has been demonstrated through eigenvalue analysis and time-domain simulation results carried out in MATLAB/SIMULINK environment. Further, in order to have a fair comparison, the performance of the proposed hierarchical controller, based on modal-based extended linear quadratic Gaussian (LQG) approach, has been compared with that of hierarchical controller based on state-based extended LQG approach. Simulation results revealed the capability of the proposed hierarchical controller to mitigate the unstable oscillations as well as to settle quickly at different operating points under the application of small-signal disturbances.

Modelling virtual oscillator‐controlled microgrids

Virtual oscillator control (VOC) is an emerging non-linear technique to control the terminal voltage of inverters in ac micro-grids. This study compares two small-signal models of VOC, obtained from average VOC dynamics over one ac cycle. The first model from previous literature is based on the approximation of real and reactive power in the average dynamics by the corresponding instantaneous values of power. The second model proposed in this study, in contrast to the first, approximates the average power by filtered power from a second-order low-pass filter. The two models are evaluated by eigenvalue and participation analyses. Simulations and experimental results are used to verify the models, by observing transient responses of a three-phase inverter subject to a step change in the load. The proposed linearised model can tracks transients of the output voltage and current more accurately than the existing technique both in the simulations and experimental tests. The proposed linearised VOC model enables a more precise small-signal stability and transient analysis of the average VOC dynamics. In conclusion, this improvement benefits stability studies when system parameters are changed or load disturbances are added to virtual oscillator-controlled inverter-based micro-grids.

Stability analysis and decentralized control of inverter-based ac microgrid

This work considers the problem of decentralized control of inverter-based ac micro-grid in different operation modes. The main objectives are to (i) design decentralized frequency and voltage controllers, to gather with power sharing, without information exchange between microsources (ii) design passive dynamic controllers which ensure stability of the entire microgrid system (iii) capture nonlinear, interconnected and large-scale dynamic of the micro-grid system with meshed topology as a port-Hamiltonian formulation (iv) expand the property of shifted-energy function in the context of decentralized control of ac micro-grid (v) analysis of system stability in large signal point of view. More precisely, to deal with nonlinear, interconnected and large-scale structure of micro-grid systems, the port-Hamiltonian formulation is used to capture the dynamic of micro-grid components including microsource, distribution line and load dynamics as well as interconnection controllers. Furthermore, to deal with large signal stability problem of the microgrid system in the grid-connected and islanded conditions, the shifted-Hamiltonian energy function is served as a storage function to ensure incremental passivity and stability of the microgrid system. Moreover, it is shown that the aggregating of the microgrid dynamic and the decentralized controller dynamics satisfies the incremental passivity. Finally, the effectiveness of the proposed controllers is evaluated through simulation studies. The different scenarios including grid-connected and islanded modes as well as transition between both modes are simulated. The simulation conforms that the decentralized control dynamics are suited to achieve the desired objective of frequency synchronization, voltage control and power sharing in the grid-connected and islanded modes. The simulation results demonstrate the effectiveness of the proposed control strategy.

Small Signal Modeling of Inverter-based Grid-Connected Microgrid to Determine the Zero-Pole Drift Control with Dynamic Power Sharing Controller

This paper presents a small signal state space modeling of three-phase inverter-based microgrid (MG) system with consideration of improved droop control. The complete system matrices for one distribution source-grid connects to the local load have been elaborated by applying high, medium and low-frequency clusters to the system without considering the switching action on the inverter during power-sharing. Moreover, the final matrices will be used to determine the location of the eigenvalues for the control parameters gains due to dynamic effect of the MG, by observing the root locus graph on cluster identification. Sensitivity analysis of all types of frequency cluster showed that power-sharing control parameters such as load current, source current, and inverter voltage are influencing system stability and must be considered when designing the proportional-integral (PI) control when different load scenarios have been applied from the zero-pole drifting. Those eigenvalues of the system model are indicating the frequency and damping oscillatory components when there is sudden changed at the inverter-grid connection. The matrices’ eigenvalues are being plotted using MATLAB/Simulink to identify system stability region and find the PI controller parameters.

Seamless Integration of an Autonomous Induction Generator System into an Inverter-Based Microgrid

A control strategy for an autonomous induction generator (IG) system synchronization and seamless transfer to an inverter-based microgrid (MG) is presented in the current paper. The IG system control in autonomous mode is performed by a combination between a Voltage Source Inverter (VSI) and a Dump Load (DL). The MG consists of an MG leading inverter having on its DC side a supercapacitor-based energy storage system, two MG supporting inverters, and local loads. The paper presents the IG control part for the VSI-DL system, as well as the synchronisation algorithm that enables the smooth interconnection with the MG. An analysis of the IG impact on an islanded MG is also provided. Experimental validations accomplished on a complex laboratory test-bench have focused on the dynamic events associated with the IG system connection/disconnection to/from the MG and also on the MG response to a load being turned on and off when the IG operates connected to the MG. The obtained results have shown that the proposed synchronization algorithm ensures a seamless transfer for the IG system from autonomous to MG connected mode and vice-versa. Moreover, when a significant load transient occurs within the MG operation, the IG presence does not alter the MG stability.

Distributed Load Sharing Under False Data Injection Attack in an Inverter-Based Microgrid

In microgrids, distributed load sharing plays an important role in maintaining the supply–demand balance of power. Because false data injection (FDI) is one of the crucial threats faced by future microgrids, the study of the impact of FDI on distributed load sharing is both of theoretical merit and practical value. In this paper, we consider the distributed load sharing problem of the microgrids operating in autonomous mode under FDI. Each bus is assumed to be equipped with an agent. Under a well-developed distributed load sharing protocol based on multiagent systems, we first construct an FDI attack model, where the attacker is capable of injecting false data into the bus agents. Then, a utilization level is introduced for coordinating generators, and its variation is evaluated in the presence of FDI attacks with given injection strategies. The stable region of the microgrid is defined, and conditions are given to determine stability. Finally, theoretical results are validated on the Canadian urban distribution system.

Optimized Hierarchical Control for an AC Microgrid Under Attack

Context: An inverter-based microgrid working in islanded mode can suffer cyber- attacks, these can be done against either the local controller or the communication links among the inverters. Secondary control is able to reject those attacks, however, a tertiary control action is necessary in order to stabilize the power flow among the microgrid.
 Method: Confidence factor technique allows to reject attacks in a microgrid acting directly over the secondary control, however, this technique omits other factor related to the power available. In this case, secondary control was complemented with a tertiary control that includes optimization criteria.
 Results: An inverter-based microgrid is simulated in Matlab for different scenarios and under cyberattack, this allows checking the correct response of the controller under attacks and the effective powersharing among inverters.
 Conclusions: The tertiary control allows stabilizing the active power of the system after the rejection of a cyber-attack by the secondary control. Each inverter supplies active power according to its máximum power rating without affecting the stability of the whole system.

Development and validation of a generalized modeling approach for islanded inverter-based microgrids with static and dynamic loads

State-space modeling approaches have been extensively used in stability studies of the conventional power system. These stability studies involve the development of state-space models of different components of the power system and then linearizing them around a steady-state or an equilibrium point and combining them on a common DQ reference frame. In this paper, a similar approach has been proposed to develop a generalized modeling approach for islanded inverter-based microgrids (IIMGs) with static and dynamic loads. The proposed generalized modeling approach divides the whole IIMG system, virtually, into three sub-modules such as generation sub-module, network sub-module, and load sub-module. The load sub-module is further divided into passive, active and dynamic load sub-modules. To obtain the complete model of the IIMG system, all the individual sub-modules are modeled in a local dq reference frame and then combined on a common DQ reference frame. The developed model includes both low as well as high-frequency dynamics of IIDG units, network dynamics, and load dynamics. Eigenvalue analysis and time-domain simulations have been performed to validate the linearized and non-linear models, respectively, obtained from the proposed generalized modeling approach. The simulation results revealed the applicability of the proposed generalized modeling approach for stability studies and to find out the causes behind the excitation of a particular mode, which play an important role in the design of a proper controller to mitigate the unstable oscillations.

Transient Response Analysis of Inverter-Based Microgrids Under Unbalanced Conditions Using a Dynamic Phasor Model

Microgrids (MGs) are often unbalanced due to the integration of single-phase generators, unbalanced loads, and asymmetrical faults. To better analyze such MGs, this paper presents an approach to analyze the transient response for an inverter-based MG under the unbalanced condition. The dynamic phasor (DP) concept is used for the MG modeling under the stationary abc reference frame. First, the DP model of the inverter-based distributed generation (DG) is developed. The influence of unbalanced conditions on the inverter including oscillations on the dc side is considered in this paper. Then, the model of the network and loads is developed. Finally, all the submodules are combined on a time-variable system frequency to obtain the complete DP model of the unbalanced MG. To validate the proposed approach, the DP method is applied to an MG test system with three-phase and single-phase DGs. Small-signal analysis is carried out to derive the dominant modes and their influence on the system response. Simulation results from the DP model are compared against the detailed model built in MATLAB/SimPowerSystem. The results from load disturbances and asymmetrical faults are used to verify the DP model.

Power Conversion Technologies for High-Performance AC Micro-grid

Increasing electrical energy demand in tandem with the requirement to ensure good levels of load power quality is a growing need. Traditionally, power electronics-based static uninterruptable power supplies and diesel gensets have been employed to ensure continuity of power supply to tackle power interruptions. AC micro-grids (MG) have attracted significant attention recently owing to their ability to address these issues on a macro-scale with the aid of clusters of on-site heterogeneous micro-sources along with suitable energy storage systems. However, these systems are not necessarily large and complex networks, and it is observed that even simpler systems of smaller ratings functionally qualify as AC micro-grids. To meet the objectives of achieving high performance at low cost, three types of power conversion technologies are proposed for AC micro-grids: (1) reconfigurable battery/grid-tied inverter (RBGTI); (2) dual-mode converter (DMC); and (3) active-phase converter (APC), which enhance power quality and efficiency of energy access. RBGTI and DMC are photo-voltaic (PV) inverter-based MG technologies that ensure efficient solar energy utilization even in the absence of the power grid, unlike conventional PV grid-tied inverters. APC is a motor-drive technology that enables safe interconnection of three-phase motors on single-phase grids or micro-grids while ensuring high levels of efficiency and load power quality when compared to the conventional double-conversion drive systems. These technologies are particularly relevant in the Indian scenario, where there are grid power quality and availability are significantly different from that of other parts of the world. A high-performance General-Purpose Inverter Stack platform developed in the laboratory is employed to realize and verify the performance of the proposed power conversion technologies.

Sliding-Mode Control of AC Voltages and Currents of Dispatchable Distributed Energy Resources in Master-Slave-Organized Inverter-Based Microgrids

This paper proposes a current-controlled voltage-mode control scheme for dispatchable electronically coupled distributed energy resource (DER) units based on sliding-mode control (SMC) strategy. The proposed control strategy provides fast and stable control on the terminal voltage and frequency of DER unit and ensures protection of the power-electronic interface to external faults. In addition it maintains the quality of the output voltage of the host DER unit in spite of unbalanced and/or distorted load currents. Performance of the proposed SMC strategy is demonstrated through time-domain simulation of a single islanded DER unit, starting from black-start and subjected to various operating scenarios, and it is compared to the performance of a proportional-integral (PI) based control strategy. Also a current-mode control strategy is proposed for DER units based on SMC. The performance of both voltage- and current-mode control strategies are demonstrated in a sample master-slave organized three-unit microgrid.

Nonlinear Lyapunov Stability Analysis of Seven Models of a DC/AC Droop Controlled Inverter Connected to an Infinite Bus

The transient stability of inverter-based microgrids is important given the low inertia of microgrids especially in islanded mode. However, as a prerequisite to understanding transient stability of such microgrids, the transient stability of individual inverters requires further analysis. To address inverter stability, this paper presents the 13th-order nonlinear model of a dc/ac droop controlled inverter connected to an infinite bus. The singular perturbation method was used to decompose the nonlinear model into 11th, 9th, 7th, 5th, 3rd, and 1st-order models. The aim of the study is to understand the accuracy and validity of the reduced order models in replicating the performance of the full order nonlinear model. The performance of each model is investigated using two different tools, time domain simulations and Lyapunov functions for the estimation of the domain of attraction. The work aims to present the best model to use for time domain simulations and domain of attraction estimation. Results show that certain reduced order models are capable of accurately reproducing the performance of the full order model while others can be used to gain insights into those areas of study. This will enable future studies to save computational effort and produce the most accurate results according to the needs of the study being performed.

A Reinforcement Learning Approach for Frequency Control of Inverted-Based Microgrids

In this paper, we present a reinforcement learning control scheme for optimal frequency synchronization in a lossy inverter-based microgrid. Compared to the existing methods in the literature, we relax the restrictions on the system, i.e. being a lossless microgrid, and the transmission lines and loads to have constant impedances. The proposed control scheme does not require a priori information about system parameters and can achieve frequency synchronization in the presence of dominantly resistive and/or inductive line and load impedances, model parameter uncertainties, time varying loads and disturbances. First, using Lyapunov theory a feedback control is formulated based on the unknown dynamics of the microgrid. Next, a performance function is defined based on cumulative rewards towards achieving convergence to the nominal frequency. The performance function is approximated by a critic neural network in real-time. An actor network is then simultaneously learning a parameterized approximation of the nonlinear dynamics and optimizing the approximated performance function obtained from the critic network. The performance of our control scheme is validated via simulation on a lossy microgrid case study in the presence of disturbances.

Primary voltage and current control for an autonomous inverter-based microgrid

Primary voltage and current control for an autonomous inverter-based microgrid

Resilient and Immune by Design Microgrids Using Solid State Transformers

Solid State Transformers (SST) may become, in the near future, key technological enablers for decentralized energy supply systems. They have the potential to unleash new technologies and operation strategies of microgrids and prosumers to move faster towards a low carbon-based economy. This work proposes a paradigm change in the hierarchically and distributed operated power systems where SSTs are used to asynchronously connect the many small low voltage (LV) distribution networks, such as clusters of prosumers or LV microgrids, to the bulk power system. The need for asynchronously coupled microgrids requires a design that allows the LV system to operate independently from the bulk grid and to rely on its own control systems. The purpose of this new approach is to achieve immune and resilient by design configurations that allow maximizing the integration of Local Renewable Energy Resources (L-RES). The paper analyses from the stability point of view, through simplified numerical simulations, the way in which SST-interconnected microgrids can become immune to disturbances that occur in the bulk power system and how sudden changes in the microgrid can damp out at the Point of Common Coupling (PCC), thus achieving better reliability and predictability in both systems and enabling strong and healthy distributed energy storage systems (DESSs). Moreover, it is shown that in a fully inverter-based microgrid there is no need for mechanical or synthetic inertia to stabilize the microgrid during power unbalances. This happens because the electrostatic energy stored in the capacitors connected behind the SST inverter can be used for a brief time interval, until automation is activated to address the power unbalance for a longer term.

A simplified stability analysis method for LV inverter-based microgrids

This paper presents a simplified small-signal stability analysis method for low voltage (LV) inverter-based microgrids, in a generalized manner. The simplification is based on a simplified microgrid structure that relies on dominating inverter coupling impedances with respect to interconnecting line impedances of LV distribution networks. The analysis is further simplified by analytically determining equilibrium points of system state variables in terms of known microgrid parameters. And it eliminates the additional analysis required for determination of the equilibrium points of the state variables. Simulation and analysis results show that the proposed method successfully predicts the instability boundaries of microgrid systems for resistive interconnecting lines.

Microgrid relay protection scheme based on harmonic footprint of short-circuit fault

Due to the low overcurrent capacity of the inverter-based microgrid, traditional overcurrent protect is not suitable for the entire power system including microgrid. A fast detecting method and a whole protection scheme is needed to identify the fault and keep the system stable. This paper proposes a microgrid relay protection scheme combining a fast fault detection algorithm and low voltage ride through(LVRT) strategy. The detection algorithm combines a new harmonic analysis method and impedance delay which can complete fault detection within several milliseconds. Under a fault condition, the LVRT strategy utilizes an enhanced phase-locked loop(EPLL) to allow the microgrid to operate under unbalanced conditions for a period of time. In addition, for the dispatch requirements of power system, it also has communication based power compensation functionality. The scheme given in this paper is verified by simulation, verifying feasibility of its application in practice.

Adaptive complex virtual impedance control scheme for accurate reactive power sharing of inverter interfaced autonomous microgrids

The mismatched feeder impedance of the voltage-sourced converter (VSC) units may result in reactive power sharing error in the islanded microgrids. This study presents a reactive power sharing strategy for single-phase inverter-based microgrids that eliminates the circulating currents and reduces the coupling between real and reactive powers. The proposed controller is based on an adaptive complex virtual impedance that aims to equalise the E - Q characteristic output of VSCs. The resistive term regulates the virtual voltage drop using the proposed ΔR v - P droop characteristic, while the inductive one controls the phase angle of VSC current and increases the X/R ratio of the microgrid. The proposed strategy does not require a central controller and knowledge of the microgrid lines and loads data and is applicable for the various microgrid topologies. Furthermore, it employs the unidirectional low bandwidth communication link and is robust against the delay and failure of communication channels. The proposed method is implemented using the conventional droop control scheme and presents the medium plug and play capability. Both simulation and experimental case studies using various microgrid topologies are conducted to demonstrate the merits of the proposed scheme.